Gravity 13: Frontiers

by Shane L. Larson

I grew up in the American West, where our self-identity is inexorably melded with a deep romanticism about the Frontier. My family homesteaded near Briggsdale, Colorado, where the Rocky Mountains fade into the vast expanse of the Great Plains of North America. You can still visit the old homestead site, now on my family’s cattle ranch, and see the foundations that were laid down many generations ago. I can remember crawling around those walls as a young boy, trying to imagine the world in those long forgotten days — before cars, before computers, before rockets.  What did my old-time relatives think about every morning when they got up? What did they work on every day? Did they stare at the sky, virtually identical to the sky I stare at (except theirs was probably much darker), and wonder?

At the ruins of my family's homestead, near Briggsdale, CO. Left to right: my maternal grandfather, my younger brother, my mother, my youngest brother, me, and my dad.

At the ruins of my family’s homestead (circa 1982), near Briggsdale, CO. Left to right: my maternal grandfather, my younger brother, my mother, my youngest brother, me, and my dad.

We portray the Frontier as a place full of adventure, discovery, and possibly undying fame. But Frontiers are in a very real sense the boundary between all the hard-won knowledge of our species, and our ignorance. This is perhaps captured no where better to my mind than in Ortelius’ 1570 Map of the World, known as Theatrum Orbis Terrarum.

Ortelius' 1570 Map of the World, known as Theatrum Orbis Terrarum.

Ortelius’ 1570 Map of the World, known as Theatrum Orbis Terrarum.

Made in the last decades before the dawn of the Age of Enlightenment, the map was one of the first to capture the knowledge of the true sphericity of the World. The map very clearly shows the boundaries of what was known and what was unknown in western culture. Consider: in 1520, Magellan’s expedition passed through what today is known as the Straights of Magellan, between the mainland of South America and the Isla Grande of Tierra del Feugo. Ortelius’ map captures that Frontier of exploration explicitly, showing the path around South America, but also showing complete ignorance of the nature of the Isla Grande. In fact, the entire lower part of this map shows the boundary of European knowledge of this part of the world, showing the farthest south on the globe any explorer had ever been. Even closer to Europe there are boundaries between knowing and ignorance that can be seen as well: the northern fringe of the Scandinavian Peninsula is clearly not correct, nor is the shape of the Caspian Sea.

Frontiers define those regions where explorers for the first time are discovering and documenting what has only been suspected or imagined before. Frontiers are more often than not enabled by technology. In Magellan’s day, access to the Frontier was enabled by great sailing vessels. When my family homesteaded in the plains of Colorado, their journey had been enabled by Conestoga wagons. Technology is almost always helping us push the boundaries of the Frontier outward. It is as true today as it was in the past.

Technology enables discovery. In the Age of Exploration, the development of great sailing vessels allowed European explorers to cross the oceans of the world.

Technology enables discovery. In the Age of Exploration, the development of great sailing vessels allowed European explorers to cross the oceans of the world.

Today, there are frontiers in science, both in terms of our knowledge, as well as in terms of what our technology is capable of. On both fronts, gravity is at the frontier. In the 100 years since the birth of general relativity, our understanding of the Cosmos has grown dramatically, and at each step, gravity has played a role. Einstein showed us how gravity can explain Mercury’s lagging orbit, and suggested it could bend the trajectory of light and change its color — effects that had never been measured. Since then, the frontiers have expanded well beyond those initial speculations. Modern cosmology was born less than 15 years after Einstein’s initial presentation of general relativity, and even today challenges our understanding of the Cosmos. We have explored the gravitational collapse and death of stars, and discovered the skeletons that survive the throes of death. Closer to home, we have harnessed gravity to allow us to navigate and map the world to exquisite precision. Our satellites have measured the gentle warp of the Earth’s gravity to map out the world in ways Ortelius never imagined.

For the past 100 years, gravity has been a major player at the frontiers of physics and astronomy. (L Top) Our understanding of the expansion of the Universe derives from general relativity. (L Bottom) The gravity of the Earth tells a tale of the movement of water and changing climate of our planet. (R Top) The evolution of stars, and their ultimate death, are consequences of gravity. (R Bottom) High energy astrophysical phenomena like black holes are staples of astronomy knowledge today.

For the past 100 years, gravity has been a major player at the frontiers of physics and astronomy. (L Top) Our understanding of the expansion of the Universe derives from general relativity. (L Bottom) The gravity of the Earth tells a tale of the movement of water and changing climate of our planet. (R Top) The evolution of stars, and their ultimate death, are consequences of gravity. (R Bottom) High energy astrophysical phenomena like black holes are staples of astronomy knowledge today.

Despite all these discoveries, there is still much to learn. Gravity is right on the boundary between our most exquisite triumphs and the precipice of our deep ignorance about the Cosmos. Science is about looking over that precipice and wondering what is at the bottom; we know there are still great mysteries Nature is hiding behind the facade that we call “gravity.” We have come a long way from the frontier Einstein imagined. What are the frontiers of gravity today?

Consider the interiors of black holes. A black hole has gravity so strong, not even light can escape. It’s boundary, the event horizon, forever hides the inside from the external Universe.  If you could somehow peer past the event horizon, deep down inside you would find a point of infinite density and infinitely strong gravity called the singularity.

The structure of a black hole is relatively simple to sketch out: the "surface" is the Event Horizon, and shrouded beneath it is the singularity.

The structure of a black hole is relatively simple to sketch out: the “surface” is the Event Horizon, and shrouded beneath it is the singularity.

Perhaps the greatest enigma, the greatest failing of general relativity, is the existence of the singularity. From a classic perspective, gravity is a purely attractive force that can grow without bound when matter is compressed into a small enough space. The limitless growth in its strength means if you squeeze hard enough, it can grow so large than no other known force can oppose it. When nothing can oppose it, everything collapses in a dramatic collapse not unlike the collapse at the end of a star’s lifeBut nothing can stop the collapse, and mathematically, everything falls into an infinitely small, infinitely dense point that we call “the singularity.”

Singularities — “infinities” — are perfectly fine in mathematics. They are less desirable in physics. There is a strong, prevailing belief that in the physical world, nothing can be “infinite.” Objects and phenomena can be ridiculously large or ridiculously small when compared to the scale of human experience, but never infinite.

focus-italy_singularity-outtake1The prevailing belief is that the singularity is an indicator that general relativity is a classical theory — it is good for large scale descriptions of the world, not for the microscopic landscape of the Cosmos. For that, we will need a new idea, an extension of general relativity into the quantum regime — “quantum gravity.” Where does the realm of quantum gravity become relevant? At distances separated by the Planck length (10^-35 meters = 0.000 000 000 000 000 000 000 000 000 000 000 01 meters).

What is quantum gravity? Fundamentally it is expected to be a theory that describes the nature of space and time itself at the Planck scale; many believe that using quantum gravity to describe the interior of a black hole will obviate the need for a singularity, but no one really knows how that will happen because we don’t have any working models that make predictions testable with observations. But there are many, many seductive and enticing ideas that are waiting for us to attain a state of understanding sophisticated enough to put them to the test.

Fritz Zwicky

Fritz Zwicky

There are also challenges for gravity on scales that are enormously large, spanning the size of the Cosmos. Some of these challenges are recent, some have been known for the better part of a century, but they are all unresolved. Part of the story begins in the 1930s with astronomer Fritz Zwicky.  In 1933 he was observing the Coma Cluster of galaxies, a group of about 1000 galaxies whose center lies 320 million lightyears away, in the direction of the constellation Coma Berenices. This was less than 10 years after the discovery that galaxies were in fact like the Milky Way, but enormously far away. Astronomers were still trying to learn all they could about galaxies, and studying their behaviour.

The Coma Cluster contains about 1000 galaxies (the yellow objects in this image), and is 320 million lightyears away.

The Coma Cluster contains about 1000 galaxies (the yellow objects in this image), and is 320 million lightyears away.

Zwicky made a very reasonable assumption: the light of the galaxy is made by all the stars in a galaxy, and since most of the mass is contained in stars measuring the light is a way to get a handle on how much a galaxy masses. If you could measure the mass of all the galaxies, then you can use gravitational theory to explain their motions. But when Zwicky measured the motion of the galaxies, he found they were moving faster than expected — given the speeds they were moving, the cluster should have flown apart long ago. The only explanation is there was missing matter he could not see — more matter would simultaneously make the galaxies move faster, but also provide enough gravitational attraction to hold the cluster together.

Vera Rubin

Vera Rubin

By the 1960s, the missing matter problem had yet to be resolved. Astronomer Vera Rubin was studying the rotation of individual galaxies. Stars orbiting the center of a galaxy obey Kepler’s Laws of Orbital Motion, just like planets orbiting the Sun. Kepler’s laws say that the farther you are from the center of gravity, the slower your orbital speed should be. What Rubin found was that the outer reaches of galaxies did not slow in their rotation; in fact they rotated just as fast as stars that were closer to the center. This is known as the “galaxy rotation problem” and the plot of the rotation speed versus distance from the center of the galaxy is described as a “flat rotation curve.” Just as was the case with the Coma Cluster, the galaxy should have flown apart. The only explanation is that there is unseen mass — more matter would simultaneously make the stars move faster, but provide enough gravitational attraction to hold the galaxy together.

The "galaxy rotation problem" is that the speed a galaxy rotates with is NOT what we would expect. We expect it to rotate slowly near the edges, but observations show galaxies rotate too fast near the edges.

The “galaxy rotation problem” is that the speed a galaxy rotates with is NOT what we would expect. We expect it to rotate slowly near the edges, but observations show galaxies rotate too fast near the edges.

Rubin began her investigation with the Andromeda Galaxy, but in surveys of many more galaxies found that it was always true — all galaxies appear to have enormous amounts of unseen matter. Today, we call this dark matter.

This has enormous implications for cosmology. If the Universe is expanding, then the rate it expands, and the ultimate fate as a consequence of expansion, depends on the amount of matter in the Universe. This begs some important questions, like “is there enough matter to slow the expansion?” and “is there enough matter to cause the expansion to reverse?” Gravitational physicists classify the possible futures of the Universe in three ways:

  • OPEN: There is not enough matter to slow the expansion of the Universe down at all; it expands forever.
  • FLAT: There is just enough matter in the Universe that the expansion is slowing, but it will never halt, instead coasting forever.
  • CLOSED: There is enough matter to eventually stop the expansion, and cause the Universe to recollapse in a backward version of the Big Bang that is often called the Big Crunch.
One way astronomers measure the expansion scenario of the Universe is looking at the spots on the Cosmic Microwave Background. The direction light travels to us from opposite sides of the spot depends on the expansion geometry of the Universe. (L) In a Closed Universe, the light is bent to make the spots appear larger. (C) In a flat Universe, the spots are seen at their true size. (R) In an open Universe, the spots appear smaller.

One way astronomers measure the expansion scenario of the Universe is looking at the spots on the Cosmic Microwave Background. The direction light travels to us from opposite sides of the spot depends on the expansion geometry of the Universe. (L) In a Closed Universe, the light is bent to make the spots appear larger. (C) In a flat Universe, the spots are seen at their true size. (R) In an open Universe, the spots appear smaller.

Each of these scenarios has particular signatures in observational data, and astronomers have found strong evidence that the Universe is indeed in the FLAT mode. That being the case, this has spawned a multi-decade quest to make a census of all the stuff in the Cosmos and characterize not only its gravitational influence, but also figure out what it all is!

We are aware of dark matter because of its gravitational influence on the rest of the Cosmos, but we have no idea what it is. And there is a LOT of it. Current estimates suggest that the Cosmos is 27% composed of this dark matter. The stuff you and I and planets and stars are made of — atoms — only make up about 5% of the total amount of stuff in the Universe.

So what is the other 68% of the Universe? Astronomers were perplexed by this for a long time, and began to doubt that the Cosmos was put together the way we thought it was. Maybe the Cosmos wasn’t FLAT but was instead OPEN and our observations were wrong in some way.

Supernovae, for a time, shine very brightly compared to other stars in the parent galaxy.

Supernovae, for a time, shine very brightly compared to other stars in the parent galaxy.

But in the late 1990s, there was a breakthrough. Mulitple teams of astronomers were using supernovae to measure the size and expansion of the Universe. Certain supernovae (Type Ia supernovae) are standard candles — they all explode with the same brightness. This means that the brightness of the supernova gives you a way to measure distance — the dimmer the supernova, the farther away it is. But cosmology gives us another way to measure distance, using Hubble’s law — redshift is also a measure of distance. The larger redshift an object has, the farther away it is.

But in 1998, the Supernova Cosmology Project and the High-Z Supernova Search Team discovered that these two methods of measuring the distance to supernovae did not agree — distant supernovae were dimmer than expected given the redshift distance. How can that be? The only explanation seems to be that the expansion of the Universe is accelerating.  An unknown something is accelerating the expansion of the Universe, ever so slightly, on the largest scales. Today, we call that something dark energy. Dark energy, whatever it is, makes up the remaining 68% of the expected stuff in the Universe.

A simple demonstration of the energy content of the Cosmos. Atoms are colored; all the unknown things (dark matter and dark energy) are black.

A simple demonstration of the energy content of the Cosmos. Atoms are colored; all the unknown things (dark matter and dark energy) are black.

At long last, astronomers and physicists have discovered all the stuff we expected to find in the Universe. But we still don’t know what it is. We call this stuff “dark matter” and “dark energy”, but we don’t know anything about their behaviour and properties beyond their gravitational influence. Maybe they are some new, exotic bit of particle physics we have never seen before. Maybe they are some new, exotic behaviour of gravity on large scales. Or maybe they are something completely new, completely unexpected, and completely unexplained. Whatever they are, dark matter and dark energy are clearly at the frontiers of our understanding of gravity and cosmology. The future lies on the other side.

What these discoveries will mean and how they will change the course of human history is not for us to know, just as it was not for Einstein to know how general relativity would change the world. Those are questions for our posterity, our future children, who will have moved on from the simple mysteries that confound us today, and will be challenging their own new frontiers.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

This post concludes this long series for the GR Centennial. Thanks to everyone who read, commented, and supported this effort! We will certainly talk about gravity again at this blog… :-) This post was completed while in residence at the Aspen Center for Physics.

Gravity 12: Listening for the Whispers of Gravity

by Shane L. Larson

The Cosmos is alive with energetic happenings.  Planets barrel along their orbits, unstoppable by anything short of a collision with another planet.  There is a cluster orbiting the black hole at the center of the Milky Way, with stars being flung and slingshot around their orbits like they were nothing more than ping-pong balls. Massive stars, in a last desperate gasp for attention, explode and spew their guts all around the galaxy, leaving a dark, compact skeleton behind. Billions of light years away, the shredded remains of galaxies slowly coalesce to make a larger elliptical galaxy and their central black holes dance together in a deadly inspiral, spewing jets of energetic material outward to mark their titanic struggle.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

All of these examples have one thing in common: huge masses moving in dynamic ways.  The changing gravitational structure in these systems will manifest itself as gravitational waves propagating across the Cosmos, whispering ripples in the structure of space and time.  Encoded in those waves, if we could detect them, is a previously unheard story for the reading.

The “sticky bead experiment,” worked out at the 1957 Chapel Hill conference, taught us the effect of gravitational waves on the world: they change the distance between points in spacetime. Once we knew what physical effect to look for, physicists began to ask “how do we detect it?”  It was straight-forward to compute the size of the distance change caused by gravitational waves, and it was tiny. But seemingly impossible measurements have never stopped physicists and astronomers from trying to imagine clever and imaginative ways to probe Nature’s secrets.

One of the first people to seriously consider how to measure the extremely tiny stretching effect of gravitational waves was Joseph Weber at the University of Maryland. After the Chapel Hill conference he began to think seriously about the problem of gravitational wave detection, and settled on a clever and imaginative idea: if gravitational waves change the distance between any two points in spacetime, it should stretch a physical object as they pass through it. Once the wave goes by, the inter-atomic forces that hold the object together take over, and try to snap it back into its original shape. This kind of snapback motion would set up acoustic waves — sound waves — in the object. If you could detect those tiny, faint sound waves, it would be an indicator of the passage of a gravitational wave.  Weber fashioned such an experiment from a 0.61 meter diameter, 1.5 meter long cylinder of aluminum that massed 1.5 tons. Such a device is now called a Weber Bar.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960's. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960’s. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

There are, of course, many influences and physical effects that can set off acoustic vibrations in a large aluminum bar. Random acoustic vibrations could be mistaken for a gravitational wave, or more likely, hide the putative effect of a passing gravitational wave. Random signals like this are called noise; filtering noise is one of the foremost problems in any experiment. The solution to this difficulty is to have more than one bar; you set them up and wait to see if both bars ring off at the same time. Since noise is random, it is unlikely to influence both bars identically at the same time, so a common signal is most likely a gravitational wave. Weber’s detection program grew to include a second bar at Argonne National Laboratory that operated in coincidence with the bar he had built in Maryland.

By the late 1960’s, Weber’s analysis of his bar data convinced him he was seeing coincident events, which he dutifully reported to the scientific community.  The ensuing debate has been roundly documented (e.g. in Harry Collin’s book “Gravity’s Shadow”), but that tale is not germane to our discussion here. The important point is this: the scientific community suddenly became cognizant of the idea that gravitational waves could be detected through clever, high precision experiments, and Joe Weber set us on that path.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

In the years following the construction of the Maryland experiment, many other Weber bars were built around the world. These included ALLEGRO at Louisiana State University; EXPLORER at CERN; NAUTILUS in Frascati, Italy; AURIGA at the INFN in Legnaro, Italy; and Niobe in Perth, Australia.  While most of the classic bars have gone offline, new efforts in bar detection technology have turned to spherical detectors, of which MiniGRAIL at Leiden University is the archetype. But still, no gravitational wave signal has been confirmed by any bar.

Given the steadfast absence of confirmed signals in our detectors, why are physicists so confident in the existence of gravitational waves? The answer lies in traditional, telescopic observations of the Cosmos.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

In 1974, radio astronomers Joseph Taylor and Russel Hulse were observing on the 305 meter diameter Arecibo Radio Telescope in Puerto Rico. They were looking for new pulsars, and discovered one in the constellation of Aquila. Pulsing every 59 milliseconds, the pulsar rotates at a staggering 17 times per second. After studying it for some time, Hulse and Taylor noticed that the pulses varied regularly every 7.75 hours. The explanation? The pulsar was orbiting another neutron star (that was not pulsing)!  Masquerading under the scientific name PSR B1913+16, this remarkable system is more readily known by its common name: the Hulse-Taylor binary pulsar, or usually “THE Binary Pulsar.” We can track the arrival time of the pulses from the pulsar in the system, and precisely determine the size and shape of the orbit over time. After 40 years of observations, it is clear that the orbit of the binary pulsar is shrinking, by an amount of roughly 3.5 meters per year. This is exactly the amount of orbital decay astronomers expect to see if gravitational waves were carrying energy away from the system, sucking the energy out of the orbit. If all goes according to Nature’s plan, the orbit will decay to the point of collision in 300 million years (mark your calendars!).

The system has a neutron star that orbits with a pulsar -- the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

The system has a neutron star that orbits with a pulsar — the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

We now know of many systems like the Hulse-Taylor binary pulsar, giving astronomers confidence that gravitational waves do, without question, exist. So why haven’t we seen them?  The problem with Weber bars is they are “narrow band” — they are most sensitive to gravitational waves that are close matches to the sound waves that are made in the bar (a condition physicists call “resonant” — the gravitational waves closely match the shape and vibration time of the sound waves, so they reinforce each other). Since it is  unlikely a gravitational wave source will exactly match your bar’s vibration frequency, and because many phenomena generate gravitational waves at all kinds of different frequencies, an ideal detector should be “broad band” — sensitive to a wide range of gravitational waves. One solution is to build a laser interferometer.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Interferometers have a storied history with relativity and astronomy. The earliest scientific interferometers were made in the 1880’s by Albert A. Michelson, and used by Michelson and his collaborator Edward Morley to examine the propagation of light. The results of their experiments demonstrated to the scientific community that light was not propagated by a “luminiferous aether,” and was in fact able to propagate in pure vacuum. Their conclusions also support the founding postulates of special relativity, namely that all observers measure the speed of light in vacuum to be a constant, irrespective of their state of motion.

In the decades that followed, interferometry became a recognized technique for making precise measurements that could not be obtained in any other way. By the time the first results from Weber bars were being reported, people were thinking about other ways to make precision distance measurements, and laser interferometry was a prime candidate technology. The first laser interferometer designed for gravitational wave detection was a table-top experiment built in 1971 at Hughes Aircraft by Robert Forward, who was a student of Weber’s.

(L) Bob Forward's first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss' initial sketch of the components and operation of a laser interferometer like LIGO.

(L) Bob Forward’s first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss’ initial sketch of the components and operation of a laser interferometer like LIGO.

A year later, Rai Weiss at MIT published a report outlining in great detail the basic considerations for building what would evolve into modern day gravitational wave interferometers. Those initial musings came to fruition in the 1990s, when kilometer scale interferometers began to be constructed around the world with one intention: to observe the Cosmos in gravitational waves.

In the United States, there are two observatories that are called LIGO: one is in Hanford, Washington and the other is in Livingston, Louisiana. In Europe, a 600 meter interferometer called GEO-600 was built outside Hannover, Germany, and a 3 kilometer interferometer called VIRGO was built outside of Pisa, Italy. The Japanese built a 300 meter prototype in Tokyo called TAMA, but have now embarked on a much more ambitious instrument built underground in the Kamioka Observatory called KAGRA. These instruments are enormous endeavours, on the scale of large particle accelerators in terms of their physical size and in terms of the number of people required to bring the project to fruition. All of them can be seen from space (just fire up Google Earth or Google Maps: LIGO-Hanford from space, LIGO-Livingston from space, VIRGO from space, and GEO-600 from space).

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

For the first time, these observatories will show us a view of the Cosmos seen not with light, but with the whisper of gravity. The bread-and-butter source, the thing we expect to detect most often, are the merger of two neutron stars. Viewed from the right seats, such collisions generate tremendous explosions known as gamma ray bursts, but we only see a small fraction of the gamma ray bursts in the Universe because they aren’t all pointing toward us. LIGO and its fellow observatories will have no such difficulties — gravitational waves are emitted in every direction from these cataclysmic mergers.

What will we learn from these events? We hope to learn what the skeletons of exploded stars are like — what is their size and what are they made of? What is the matter at their cores like, and what do they become when they merge? Every detected neutron star merger is a clue in the story of stellar lives, which of course, is part of our story too, because we are all of us descended from the exploded ashes of ancient stars.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

Where do all these stellar skeletons come from? It’s a curious thing, looking out at the sky. The thing we see the most of are stars, and over the course of a human life, they change little if at all. Night after night, the stars wheel overhead, distant points of light that no human has ever visited, and no human is likely to visit in my and your lifetimes. But over the last few centuries, through a careful application of technology smothered under an insatiable desire to know, we have figured out their story. Like shrewd protégés of Jane Marple, we have pieced together many parts of the the puzzle to discover how stars are born, how they live, and ultimately how they die. Gravitational wave astronomy investigates these final end-states of stellar life. But when we see the stars, we are seeing the snapshot of the stars alive today — where are all the stars that have gone before?

They litter the galaxy — the Milky Way is a vast graveyard of stellar remnants, the burned out stellar husks of those stars that came before. Since only the largest stars produce neutron stars and black holes, and most stars are lighter-weight, like the Sun, astronomers think most of that stellar graveyard is full of white dwarf stars — tens of millions of them.

LIGO can’t see white dwarf stars because they are too big — they never shrink to small enough orbits to make gravitational waves that LIGO can detect. If we want to study this part of the stellar life story, we have to build something new.

lisa_astriumIn the next decade, NASA and ESA hope to fly laser interferometers in space. The LISA gravitational wave observatory will consist of three free flying spacecraft 5 million kilometers apart, using lasers to measure the distance between the three spacecraft. The first step toward flying LISA is a mission called LISA-Pathfinder that will launch in October 2015.

LISA will listen in on the gentle gravitational whispers of tens of millions of white dwarf stars — so many whispers that the galaxy will actually sound like racous party. Like any rowdy party, there will be loud contributors that can always be heard above the noise, perhaps as many as 20,000 that shout out above the cacophony.  These systems are called “ultra-compact binaries”, and orbit each other on orbits so small they would fit between the Earth and the Moon. We think of LISA’s view of the Cosmos as being complementary to LIGO’s — with observations from both observatories, we will be able to construct our first complete picture of the “decomposition phase” of stellar evolution.

But perhaps the most interesting thing LISA will detect are the supermassive black holes at the centers of galaxies. Some of the most fantastic pictures we have taken of the Cosmos show galaxies in collision. Occurring over billions of years, the graceful and delicate spirals are shredded, giving birth to a new, transformed galaxy. How often does this happen? Do all galaxies experience this at some point in their lives, or is it rare? How does it change the kinds of galaxies we see? Does it change the shapes of galaxies irrevocably, or do they return to their whirling spirals of arms?  And perhaps most interesting, what happens to the black holes that once lurked in their cores?

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

If astronomers are correct, those black holes will sink to the core of the new galaxy that forms, and eventually merge together. When they do, they will emit a wailing burst of gravitational waves that will be visible to LISA all the way to the edge of the Observable Universe. Encoded in that cry will be the birth announcement of a new, bigger black hole, as well as the threads of the story that led to its birth — where they were born, when they were born, and what the Cosmos was like at that time.

These stories and more are contained in the faint whispers of gravity that even now are washing across the shores of Earth. As you are reading this, astronomers and physicists are tuning up our technology to listen closely to those faint messages, and when we finally hear them, they will transform the way we think about the Cosmos.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Gravity 11: Ripples in Spacetime

by Shane L. Larson

We have travelled far in our journey to explore gravity, far from home and into the deep reaches of the Cosmos. But all that we know, all that we have learned, has been discovered from our home here, on the shores of the Cosmic Ocean. Today, let us return home.  In the words of the space poet Rhysling,

We pray for one last landing
On the globe that gave us birth
Let us rest our eyes on the fleecy skies
And the cool, green hills of Earth.

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Imagine yourself in a soft green meadow, far from the hub-bub of everyday life. What do you hear? What do you see? The gentle rustle of the trees, and the whisper of the long grass. The tall flowers of spring rocking gently back and forth, and the dark shadows of a bird of prey soaring effortlessly against the blue sky. All these sights and sounds are the signature of something unseen — the atmosphere of the Earth, the blanket of air that protects us and supports all the life around us.

How do we know the air is there? We can’t see it. All of these observations, infer the existence of the air by recognizing its influence on other things. If we want to measure the air directly, to detect it, then we need to construct controlled experiments where we understand the physical effect of the air and how it interacts with the experiment we design to elucidate its presence. Consider a simple experiment you can do right at home.

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out.  (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way -- air!

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out. (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way — air!

Take a drinking straw and a glass of water.  Dip the straw in the water, then place your thumb over the top of the straw, and remove it from the water.  If you take your thumb off the straw, you find that you had trapped some water in the straw.  Now do a slightly different experiment. Put your thumb over the end of the straw first, then put it in the water. If you take the straw out of the water and remove your thumb, you find that there is no water in the straw!  Why didn’t water go in the straw? There must have been something in the way, something invisible you couldn’t see. It is, of course, the air. This seems completely obvious to us now, thinking about it with 21st century brains, but two millenia ago, when we were just beginning to speculate on the nature of the world, this was a remarkable and marvelous observation of the world.

Today, astronomers find themselves in a similar brain loop with respect to gravity. One can “measure the force of gravity” through experiment. But when Einstein developed general relativity, he did away with gravitational forces in favor of motion on the curvature of spacetime. We can use this idea to describe everything we see in Newtonian gravity — objects freely falling to the ground, orbits of astrophysical bodies, and the weightlessness of astronauts in space. There have been exquisite tests of general relativity confirming its unique predictions beyond Newtonian gravity, and we rely on it every single day.

But is there a way to directly measure spacetime? Can we confirm that gravity is no more than the curvature of spacetime itself?  This is a question that has occupied the minds of gravitational physicists for a century now, and many ideas have been proposed and successfully carried out.

The most ambitious idea to directly measure spacetime curvature was first proposed by Einstein himself, and has taken a century to come to fruition. One of the motivations to develop general relativity was famously to incorporate into gravitational theory the fact that there is an ultimate speed limit in the Cosmos. If the gravitational field changes (for instance, due to the dynamical motion of large, massive objects like stars), that information must propagate to distant observers at the speed of light or less. If gravity is no more than the curvature of spacetime, then changes in the gravitational field must must be encoded in changing spacetime curvature that propagates from one place to another. We call such changes gravitational waves.

The opening pages of Einstein's first two papers on gravitational waves in 1916 (L) and 1918 (R).

The opening pages of Einstein’s first two papers on gravitational waves in 1916 (L) and 1918 (R).

If you want to build an experiment to detect an effect in Nature, you need a way to interact with the phenomenon that you can unambiguously associate with the effect. For the first 40 years after Einstein proposed the idea of gravitational waves, physicists were vexed by the detection question because they were confused as to whether the phenomenon existed at all!  The problem, we now know, was our inexperience with thinking about spacetime.

The International Prototype Kilogram (IPK).

The International Prototype Kilogram (IPK).

Scientists spend their lives quantifying the world, describing it precisely and carefully without ambiguity, as much as is possible. To this end, we use numbers, and so need a way of agreeing on what certain numbers mean. For example, we measure mass using “kilograms.” What’s a kilogram? It is the mass of a reference body, made of iridium (10%) and platinum (90%), called the “International Prototype Kilogram” (IPK). The IPK, and six sister copies, are stored at the International Bureau of Weights and Measures in Paris, France. Scientists around the world agree that the IPK is the kilogram, and can base numbers off of it. Nature doesn’t care what the IPK is; the Sun certainly has a mass, expressible in kilograms, but it doesn’t care one whit what the IPK is. The kilogram is something humans invented to quantify and express their knowledge of the Cosmos in a way other humans could understand.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

In a similar way, when spacetime physicists describe spacetime, we have to have a way of identifying locations in spacetime, so we make up coordinates. Like the kilogram, coordinates are something we humans create to enable us to talk with each other; Nature cares nothing, Nature knows nothing about coordinates. But sometimes we get so used to think about Nature in terms of coordinates, that we begin to ascribe physical importance to them! This was the case during the early decades of thinking about gravitational waves. Physicists were confused about whether or not the coordinates were waving back and forth, or if spacetime itself was waving back and forth.  Arthur Eddington, who had led the 1919 Eclipse Expedition to measure general relativity’s prediction of the deflection of starlight, famously had convinced himself that the waves were not real, but only an artifact of the coordinates.

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

Sometimes coordinates behave badly, giving results that might seem wrong or unphysical. For instance, you can see one example of badly behaving coordinates at the top of a sphere — if you are standing on the North Pole of the Earth, what is your longitude? You can’t tell! Longitude is a badly behaving coordinate there! There is nothing wrong with the sphere, only our coordinates.

And so it was with spacetime. In the early 1930s, Einstein and a collaborator, Nathan Rosen, had discovered a gravitational wave solution that appeared unphysical and claimed this as a proof that gravitational waves did not exist. Their result was later shown to be coordinates behaving badly, and Einstein pivoted away from denying gravitational waves exist, though Rosen never did.

The argument of the reality of the waves persisted for decades; in the end, the questions were resolved by a brilliant deduction about how to measure gravitational waves. As with all things in science, the road to understanding is a slow and steady plod, ultimately culminating in a moment of  understanding. In the early 1950s, our thinking was progressing rapidly (or so we know now, with 20/20 hindsight). The watershed came in January of 1957 at Chapel Hill, North Carolina, at a now famous conference known as “The Role of Gravitation in Physics.” There were 44 attendees who had gathered to discuss and ponder the state of gravitational physics. It was barely 19 months after Einstein’s death, and the question of the existence of gravitational waves had not yet been resolved.

The community had slowly been converging on an important and central issue in experimental physics: if you want to detect something in Nature, then you have to know what the phenomenon does to the world around it. You then need to design an experiment that focuses on that effect, isolating it in some unambiguous way. At the Chapel Hill Conference, the realization of what to do was finally put forward by Felix Pirani. Pirani had settled on the notion that an observable effect of a passing gravitational wave is the undulating separation between two test masses in space (something gravitational physicists called “geodesic deviation” or “tidal deviation”). This idea hearkens back to the idea that the trajectories of particles is a way to measure the underlying shape of gravity, which was one of the original notions we had about thinking of gravity in the context of curvature.

The Sticky Bead argument was a thought experiment that convinced physicists that gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. There is a small amount of friction that keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

The Sticky Bead argument was a thought experiment that convinced physicists gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. A small amount of friction keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

Also present at the conference was Richard Feynman, by then a professor at the California Institute of Technology. Feynman took Pirani’s notion and extended it into what we now call “the sticky bead argument.” He imagined a smooth rod with two beads on it. The beads were a little bit sticky, unable to slide along the rod without being pushed. When the motion of the beads was analyzed under the influence of gravitational waves, they moved back and forth, but their motion was arrested by the friction between the beads and the rod. Friction is a dissipative force, and causes the rod to heat up, just like your hands do if you rub them together. In the sticky bead case, what is the origin of the heat? The heat energy originated from the gravitational waves and was deposited in the system by the motion of the beads.

This idea was picked up by Herman Bondi, who expanded the idea, fleshing it out and publishing it in one of the leading scientific journals of the day. As a result, Bondi is generally credited with this argument.

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

Confirming that the beads move validated the idea that gravitational waves not only carry energy, but can deposit it in systems they interact with. This was the genesis of the notion that an observational programme to detect them could be mounted.  That challenge would be taken up by another person present at the Chapel Hill conference, named Joseph Weber. Weber had spent the previous academic year on sabbatical, studying gravitational waves at Princeton, and left Chapel Hill inspired to begin a serious search. Weber’s entrance to gravitational wave astronomy happened in the early 1960s with the introduction of the first gravitational wave bar detector.  This was the foundation that led to the great experimental gravitational wave experiments of today; we will start our story there in our next chat.

I am indebted to my colleague Peter Saulson (Syracuse) who first made me aware of Pirani’s talk at the 1957 Chapel Hill Conference. That Conference is part of the folklore if our discipline, though details are often glossed over usually going directly to the Bondi Bead story. I am also indebted to Carl Sagan, who introduced me to the idea that one can detect the air with water experiments (in “The Backbone of Night,” episode 7 of Cosmos: A Personal Voyage).

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Gravity 10: Signatures of the Big Bang

by Shane L. Larson

Science has two interlocking pieces that always work together. One part is “describing the world to predict the future and explain the past.” This is the part that many of us remember from science class, involving pen and paper and mathematics and the Laws of Nature. Another part is “observing the world and seeing what Nature is up to.” This is also a part that many of us remember from science class, involving doing experiments and recording numbers and making graphs. These two pieces are called theory and experiment respectively, and they constantly validate and reinforce each other in a never-ending cycle of upgrading and refining our knowledge of the Cosmos.

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

Cosmology has seldom had both theory and experiment walking hand in hand. Instead one or the other has been out in front — sometimes way out in front — waiting for the other to catch up. This was certainly the case when general relativity was announced to the world — as a description of the machinery of the Cosmos, it was perfectly capable of making predictions that were so far beyond our ability to observe and verify that we didn’t recognize the truth for what it was. This famously happened early on, in 1917 when Einstein published one of the earliest papers about relativistic cosmology entitled “Cosmological Considerations in the General Theory of Relativity.”

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

At that time, we were profoundly ignorant about the nature of the Universe. The prevailing view was that the Cosmos was full of stars, and that the Universe was static. It’s the most natural assumption in the world based on your experiences when you step out the door every night — the sky is full of stars and they change little, if at all, night to night.

Einstein considered a Universe simply filled with stars, and asked what general relativity predicted. He found it only predicted one thing: the Universe must collapse.  Preconceptions are a powerful force in science, and Einstein believed strongly in the static Universe, so much so that he supposed maybe he had not gotten general relativity completely correct. So he introduced a mathematical addition to general relativity that pushed back against the collapse, called the Cosmological Constant.

Before 1924, the nature of galaxies was unknown. They were grouped with the "nebulae" -- wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

Before 1924, the nature of galaxies was unknown. They were grouped with the “nebulae” — wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

But science is always on the move. Telescopes in that day and age were getting larger. In 1917 the 100-inch Hooker telescope first saw starlight on its mirror, as it embarked on a long and storied history of astronomical discovery. Telescopes gave us the capability to probe deeper into the Cosmos, and we had started to discover vast diaphanous complexes of light that showed no stellar qualities. These were called nebulae, Latin for “cloud.”  A few of the nebulae, the “spiral nebulae,” perplexed astronomers — some thought they were simply odd nebulae (vast complexes of gas and dust), and others thought they were island Universes (other galaxies, like the Milky Way).

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The discovery of the distances to the spiral nebulae, using Leavitt’s Cepheid variable method, was a watershed moment in the history of cosmology. It put on the table two  important facts: first, the Universe was vast — enormously vast — with distances far beyond the boundaries of our own galaxy.  Second, the major constituent especially on large scales, was not stars, but galaxies (which are agglomerations of stars). These two simple facts suddenly and irrevocably changed the way we thought about the Universe. In the first years after the discovery of the nature of galaxies, Friedmann and Lemaître used general relativity to imagine a Universe filled with galaxies, and discovered the idea that the Universe was expanding. This notion, discovered on paper, was bourne out in 1929 when Humason and Hubble, once again using the 100-inch telescope on Mount Wilson, found all the galaxies in the Universe were receding from one another.

At that time, Lemaître made a great leap of imagination — it was a thought that was well outside the comfort zone of astronomers of the day, though today we may view his leap as completely obvious. From the mindset of the future, it is difficult to imagine just how hard it was to think different. Lemaître supposed that if the Universe was expanding, then in the past it would have been smaller, and hotter. The idea was met with incredulity and derision, sparking enormous debate for decades to come. But science is the blend of ideas on paper with observations of the Universe. If Lemaître’s ideas were right OR wrong, the evidence could be found by looking into the Cosmos.

There are many lines of evidence that confirm the basic idea of the Big Bang, but there are three major pillars of support. The first, is the expansion itself. The Universe is not like a sports car, starting and stopping on a whim, braking and accelerating at random. Its evolution is driven by the Laws of Nature, in a smooth and predictable fashion. If we see expansion today, that expansion started in the past. The rate and trends in the expansion are a function of the amount of matter in the Universe, and the initial conditions of the expansion. These quantities can be determined from astronomical observations, and are consistent with the Big Bang picture.

A popular T-shirt meme about atoms, a clever science pun!

A popular T-shirt meme about atoms, a clever science pun!

The second and third pillars of evidence for the Big Bang have to do with what happens to the Universe as it expands and cools, and the consequences for matter.  Everything you and I see around us here on Earth — rocks, trees, candy bars, platypuses — is made of atoms. Atoms are a composite structure. The center is a compact heavy core called the nucleus comprised of protons and neutrons. It is surrounded by a cloud of electrons, equal in number to the protons in the nucleus.

The fact that the atom holds together is a manifestation of the forces at work. The electrons are held to the atom by virtue of attractive electrical force between them and the nucleus. If you bang two atoms together hard enough, they break apart into free nuclei and free electrons.

The nuclei, built of protons and neutrons, are held together by a very strong force that acts over short distances called the nuclear force. The nuclear force is tremendously strong, but if you bang to nuclei together hard enough, they too can be broken up into free protons and free neutrons.

In the distant past, shortly after the Big Bang, the Universe was very compact: everything in it was closer together, and extraordinarily hot. As the Universe gets smaller, it’s a bit like being squished together in the mosh pit at a concert — you can’t really move anywhere without crashing into something else. The enormous temperatures mean that everything was moving extremely fast — the hotter the temperatures, the faster the motion, the harder the crashes. If you go far enough back in time, the Universe gets so hot you can’t have atoms. If you go even farther back, it gets hotter and the Universe can’t even have atomic nuclei.

The basic constituents made during Big Bang nucleosynthesis.

The basic constituents made during Big Bang nucleosynthesis. Protons (red) and neutrons (green) bind to form the simplest atomic nuclei.

The second pillar can be understood by going back to a time in the first minute after the Big Bang. Up to this point, the Cosmos was a primordial soup of free electrons, free neutrons, and free protons, all swirling around in a maelstrom of churning energy. The Universe had started its inexorable expansion, and was cooling as a result.  By the time the Cosmos was 10 seconds old, it had cooled from its hot beginnings down to a temperature of about 2 billion degrees Celsius. At this temperature, protons and neutrons begin to stick together to form the first atomic nuclei. This process of formation is called primordial nucleosynthesis — it makes hydrogen, deuterium, helium, and small amounts of lithium and beryllium.

Big Bang theory predicts how much of each of these was synthesized in the first 15-20 minutes, a delicate balance astronomers call the primordial abundances. What astronomers can see agrees with the predictions of Big Bang nucleosynthesis.

But perhaps the most important observational signature of the Big Bang has to do with light. After the formation of the atomic nuclei, the Cosmos was still too hot to form proper atoms. Every time a nucleus tried to bind with an electron, a collision would knock the electron free. So, for the next 400,000 years, the Universe remained a seething fluid of atomic nuclei, free electrons, and energy.

When light is packed in so tightly with charged particles, like the electrons and atomic nuclei, it is not free to travel about of its own free will. It travels only a short distance before it encounters an electron, and it scatters.  Light simply can’t go very far.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

But, after 400,000 years, the Universe cools to a balmy 3000 degrees Celsius, cool enough that each time an electron bumps into a nucleus, it binds together to form an atom. This process is called recombination. From the point of view of the light, all of the charged particles suddenly disappear (atoms are neutral, having no overall electric charge) and the Universe becomes transparent. The light can travel anywhere it wants without being impeded by the matter; astronomers call this decoupling.  The Cosmos is full of freely streaming light.

A recreation of the 1965 Cosmic Microwave Background map, covering the entire sky (Penzias and Wilson could not see the entire sky from Bell Labs). The band of stronger microwave light is the signature of the Milky Way Galaxy.

A map of the Cosmic Microwave Background across the whole sky. First detected by Penzias and Wilson in 1965, this light is the signature of ever cooling Universe after the Big Bang. The band of stronger microwave light along the center of the map is the signature of the Milky Way Galaxy.

For the next 13 billion years, the Universe continued to expand to the present day. All the while the streaming light — the signature from the birth of atoms — surfed right along.  Spacetime is stretching as the Cosmos expands, and the light from that early, hot, dense state had to give up energy to fight against the expansion, shifting to longer and longer wavelengths as time progressed. By the time light reaches the Earth today, it should appear as microwave light.  And indeed, it is. In every direction we look on the sky, we see a uniform background of microwave light called the Cosmic Microwave Background. This is the third, observational pillar, the evidence, that tells us our thinking about the Big Bang is on the right track.

This is the basic picture of the Big Bang that was developed since the late 1920’s — decades of careful comparison of observations with theoretical calculations. The refinements and developments — of both theory and experiment — continue to this day.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Getting there from here…

John C. Armstrong:

This is a post I made for my HNRS2030 students, reblogging to Write Science

Originally posted on HNRS 2030 - Our place in the Cosmos:

“The surface of the Earth is the shore of the cosmic ocean. On this shore, we’ve learned most of what we know. Recently, we’ve waded a little way out, maybe ankle-deep, and the water seems inviting. Some part of our being knows this is where we came from…We are a way for the cosmos to know itself.”

― Carl Sagan, Cosmos

As Carl says, most of what we know we’ve learned from the surface of the planet. All of what we know has been gleaned from instruments sent by humans to the near and far reaches of our solar system. The sun is 93 million miles from Earth. α Centauri is 4.4 light years further. Andromeda, our nearest large galactic neighbor, is a 2.5 million light years away.

But only careful application of the scientific method to observations of the solar system, the galaxy, and our Universe has allowed us to deduce the…

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Gravity 9: The Evolving Universe

by Shane L. Larson

Of all the fundamental forces in Nature, gravity is the weakest. What do we mean by that? Let’s forego our usual thought experiments, and do something real to demonstrate this idea.

pizzaFirst, go eat a piece of pizza (or any other food you enjoy). This is the process by which you accumulate the eenrgy needed to make your body go. Without pizza, you wouldn’t be able to do anything.  Second, go stand in the middle of the room (where you won’t hurt yourself) and jump straight up in the air, as high as you can.

Using the chemical energy from some Dr. Pepper, I can overcome the gravitational pull of the entire planet.

Energy from pizza, gives me the power to defy the gravitational pull of the entire planet.

What happened? Since I’m pretty sure most of you reading this aren’t superheroes and can’t fly, you probably ascended up in the air a bit, and then came back down to the floor. It’s an everyday sort of thing, completely ordinary. But this is science, and there are remarkable and deep truths hiding in the simplest of circumstances. So consider this:

Using some simple chemical energy, which your body gleaned by breaking down some food you ate, you were able to (momentarily) overcome the gravitational pull of the ENTIRE EARTH.

This is what we mean when we say gravity is weak. But despite this fact, it is fundamentally the most important force of Nature if we want to think about the Cosmos as a whole. It has no competitor on the largest scales imaginable, meaning that even with its weak ability, gravity is able to change the Cosmos over the long, inexorable flow of time. It made sense that general relativity could and should be used to consider the past, present and future of the Universe itself.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

In 1915, when Einstein first presented general relativity to the Prussian Academy of Sciences, there was precious little we knew about the Universe, though perhaps we didn’t realize it at the time. The only objects outside the solar system that we knew a lot about were stars, and many scientists (including Einstein) supposed the Universe was comprised entirely of stars. Einstein himself made one of the first attempts to use general relativity to describe the Universe. He considered the case where the Universe was uniformly filled by stars, and found a result that disturbed him — no matter what he tried, general relativity predicted the Universe would collapse. To counteract this, Einstein modified general relativity through the introduction of a “Cosmological Constant” that made the Universe slightly repulsive. The result was precisely what Einstein hoped to find, what he and most scientists thought the Universe was: static, unchanging in time. But great changes were afoot, being driven by our ability to see the Universe better than ever before.

Henrietta Swan Leavitt, in her office at the Harvard College Observatory. She made one of the most important discoveries in the history of astronomy: how to measure distances to a common type of star, known as a Cepheid variable.

Henrietta Swan Leavitt, in her office at the Harvard College Observatory. She made one of the most important discoveries in the history of astronomy: how to measure distances to a common type of star, known as a Cepheid variable.

In 1915, the largest telescope of the day was the 60-inch reflector on Mount Wilson, though it would be eclipsed two years later by the 100-inch Hooker Telescope, also on Mount Wilson. Enormous telescopes such as these were enabling us to probe the size of the Cosmos for the first time. The key to making those measurements was discovered by a pioneering astronomer at the Harvard College Observatory, Henrietta Swan Leavitt.

Delta Cephei is a naked eye star, in the lower corner of the constellation of Cepheus ("The King").

Delta Cephei is a naked eye star, in the lower corner of the constellation of Cepheus.

Leavitt was studying a class of stars known as Cepheid variables. Named for the archetype, delta Cephei, these stars are “radial pulsators” — they grow and shrink over time in a regular pattern over the course of many days. The observational consequence, if you are watching, is the brightness and the temperature the star changes. What Leavitt discovered was a regular pattern between the time it took a Cepheid star to change its appearance (its “period”), and its true brightness (its “luminosity”).

The brightness of Cepheid variables goes up and down over the course of many days. The curve here is for Delta Cephei, which changes brightness every 5.4 days. (TOP) The observed change in brightness is a direct result of the stars size pulsating in time.

The brightness of Cepheid variables goes up and down over the course of many days. The curve here is for Delta Cephei, which changes brightness every 5.4 days. (TOP) The observed change in brightness is a direct result of the stars size pulsating in time.

How does that help you measure distances? Let’s imagine a simple example here on Earth. Suppose you have a 100 Watt lightbulb and a 10 Watt lightbulb side by side.  The 100 Watt bulb looks brighter — way brighter. This is the intrinsic brightness of the bulb — it is clearly putting out more energy than its smaller, 10 Watt companion, which you can easily discern because they are right next to each other.  This intrinsic brightness at a known fixed distance is what astronomers call absolute luminosity or absolute magnitude.

Is there any way to make the 100 Watt bulb look dimmer? Yes! You can move it farther away — the farther you move it, the dimmer it appears. In fact, you could move it so far away that the 10 Watt bulb you leave behind looks brighter! By a similar token, you can make it look even brighter by moving it closer! How bright something looks when you look at it is what astronomers call apparent luminosity or apparent magnitude.

Disentangling distance and brightness is one of the most difficult problems in astronomy. Observing how bright an object is depends on two things: its intrinsic brightness, and its distance. (TOP) A bright light and a dim light are side by side at a fixed distance; one is obviously brighter than the other. (BOTTOM) If the brighter light is farther away, it can look dimmer than the closer light!

Disentangling distance and brightness is one of the most difficult problems in astronomy. Observing how bright an object is depends on two things: its intrinsic brightness, and its distance. (TOP) A bright light and a dim light are side by side at a fixed distance; one is obviously brighter than the other. (BOTTOM) If the brighter light is farther away, it can look dimmer than the closer light!

By comparing apparent brightness (how bright something looks in a telescope) to absolute brightness (how bright something would look from a fixed distance away) you can measure distance. The biggest problem in astronomy is we don’t know what the absolute brightness of objects are.

What Leavitt discovered was if you measure how long it takes a Cepheid to change its brightness, then you know its absolute brightness. Comparing that to what you see in the telescope then let’s you calculate the distance to the star! This discovery was a watershed, arguably the most important discovery in modern astronomy: Leavitt showed us how to use telescopes and clocks to lay a ruler down on the Universe. Leavitt died of cancer at the age of 53, in 1921.

Harlow Shapley.

Harlow Shapley.

Despite her untimely death, astronomers rapidly understood the power of her discovery, and began to use it to probe the size of the Cosmos. Already by 1920 Harlow Shapley had used the Mount Wilson 60-inch telescope to measure Cepheids in the globular clusters in the Milky Way. What he discovered was that the globular clusters were not centered on the Earth, as had long been assumed, but rather at some point more than 20,000 lightyears away. Shapley argued quite reasonably that the globular clusters are probably orbiting the center of the galaxy. This was the first indication that the Copernican principle extended far beyond the Solar System.

In 1924, Edwin Hubble, who Shapley had hired at Mount Wilson Observatory, made a stunning announcement — he had measured Cepheid variables in the Andromeda Nebula, and it was far away. At 2.5 million lightyears away, the Andromeda Nebula was the farthest object astronomers had ever measured the distance to. In fact, it wasn’t a nebula at all — it was a galaxy. Here, for the first time, some of the long held, cherished beliefs about Cosmology that were prevalent when Einstein introduced general relativity began to unravel. (Historical Note: Hubble’s original distance to the Andromeda Galaxy was only 1.5 million lightyears. Why? Because there are two different kinds of Cepheids, both of which can be used to measure distances, but calibrated differently! Astronomers didn’t know that at the time, so Hubble was mixing and matching unknowingly. Eventually we learned more about the Cosmos and arrived at the current known distance — science is always on the move.)

The Hubble Ultra Deep Field (UDF), showing what is unseen but can be found if you stare at an empty part of the sky for long enough.

The Hubble Ultra Deep Field (UDF), showing what can be found if you stare at an “empty” part of the sky for long enough. Virtually every object in this image is a distant galaxy.

The Universe was not full of stars…. it was full of galaxies, and those galaxies were further away than we had ever imagined. This was a dramatic discovery that shook astronomers deeply. But it was only the beginning. A scant five years later, Milton Humason and Hubble, using the 100-inch telescope at Mount Wilson, made another astonishing discovery: every galaxy they looked at was receeding away from the Milky Way, in every direction. Furthermore, the farther away the galaxy was, the faster it was receding from us.  This result is now known as “Hubble’s Law.” Humason and Hubble had stumbled on one of the great secrets Nature — the Universe was not static, as a casual comparison of the night sky from one year to the next may suggest.  But what was going on? Why were all the galaxies flying away from us, in every direction we looked? This would seem to contradict the Copernican principle that we weren’t the center of everything!

(L) Alexander Friedmann. (R) Georges Lemaître.

(L) Alexander Friedmann. (R) Georges Lemaître.

As it turns out, the answer was already in hand. It had been discovered several years before Humason and Hubble by two scientists who had sought to use general relativity to describe the Cosmos: Alexander Friedmann, a Russian physicist, and Georges Lemaître, a Belgian priest. Friedmann had used general relativity to describe a Universe that was homogeneous (the same everywhere) and isotropic (looks the same in every direction). The “Friedmann Equations,” as they are now known, describe the evolution of such a Universe as a function of time. Lemaître derived the same result in 1927, two years after Friedmann’s death. In the mid 1930’s, American physicist H. P. Robertson and UK physicist A. G. Walker showed that the only solution in general relativity describing a homogeneous and isotropic Universe as that of Friedmann and Lemaître. This is now called the FLRW (“Friedmann-Lemaître-Robertson-Walker”) Cosmology.

What the FLRW cosmology tells us is that the galaxies aren’t really flying apart from one another — if the Universe is homogeneous and isotropic, then spacetime itself is changing, stretching and deforming. The reason the galaxies are receding from one another is the spacetime between them is expanding — the Universe is getting larger, expanding all the time.

Imagine the galaxies floating in spacetime, unmoving with respect to one anther (they are stapled down to their location in spacetime). General relativity predicts that the galaxies don't move, but that spacetime itself expands, it stretches, so the distance you measure between galaxies increases.

Imagine the galaxies floating in spacetime, unmoving with respect to one anther (they are stapled down to their location in spacetime). General relativity predicts that the galaxies don’t move, but that spacetime itself expands, it stretches, so the distance you measure between galaxies increases.

Lemaître was the first person to think differently about this problem. He had the presence of mind to ask, “We see the Universe is expanding, but what if I run time backward? What did the Universe look like in the past?” In 1931, he argued that the expansion seen in every direction suggested that the Universe had expanded from some initial point, which he called the “primeval atom.” If today we see everything expanding away, and you look backward in time, it must have all been much more compressed and compact, a state which would have made it hot, and dense. Lemaître didn’t know what might have initially caused the expansion of this primeval atom into the Cosmos we see today, but he did not see that as a reason to suppose the idea was invalid.

Lemaître with Einstein in California, 1933.

Lemaître with Einstein in California, 1933.

Change in science is hard, especially when data is new and our ideas are undergoing a dramatic evolution from past modicums of thought. Einstein is widely known to have critically panned both Friedmann’s and Lemaître’s work before the discovery of the expansion, still believing in the notion of a static Universe. Once the scientific community had come to understand and accept the expansion data, it required another great leap of faith to contemplate Lemaître’s notion of a hot dense initial state. Einstein again was skeptical, as was Arthur Stanley Eddington. For more than a decade, the arguments about the idea raged, and in 1949 during a BBC radio broadcast, astronomer Fred Hoyle coined the term by which Lemaître’s “primeval atom” idea would forever be known as: the Big Bang.

All ideas in science stand on equal ground — they are valid for consideration until they are proven wrong by observations. If the Universe did indeed begin in a Big Bang, then the obvious question to ask is what signatures of that dramatic event would be observable today? As it turns out, there are many observational consequences of the Big Bang, and they all have been observed and measured by astronomers, lending confidence to Lemaître’s initial insight.  This will be the topic of our next chat.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Gravity 8: Black Holes in the Cosmos

by Shane L. Larson

When I give talks about black holes, I usually lead with a question for the crowd: “You’ve all heard about black holes. What do you know about them?”  The responses are varied, but can be succinctly summarized as this: black holes mess things up!

This little chat captures the essential truth about black holes: if you fall inside, you are without question doomed.  That notion is a bit horrifying, and one of the reasons why these enigmatic objects are so fascinating to us — there exist objects in the Cosmos that have the ability to utterly destroy anything. No amount of human ingenuity or heroics by Bruce Willis can ever spare your fate if you fall down the throat of a black hole.

People’s intuitions are all (more or less) based on solid science, and can help us understand how astronomers find and study black holes. One of the classic thought experiments is often posited to me as a question: what would happen to Earth if you replaced the Sun with a black hole (of equal mass)?  The answer is simple: absolutely nothing!

Oh sure, 8 minutes after the transformation it would get dark on Earth because there would be no more sunlight, and eventually Earth would turn into a snowball and all life as we know it would die. But in terms of the orbit nothing would change! The Earth would continue to happily speed along its appointed path, obeying Kepler’s laws of orbital motion, with nary a concern that it is orbiting a black hole instead of a friendly star. Far from a black hole, the gravity is not extreme at all.

That doesn’t sound very interesting, but as is often the case in the Cosmos, the most innocuous of ideas are often hiding a deeper, more profound notion, if you open your mind to it. This is the case here.

A binary star is a pair of stars that orbit one another, just like a planet orbits our Sun. They are often roughly the same mass, so they both move around a common center that astronomers call the "center of mass." The stars more or less continue with their lives as if they lived alone, but if they are close enough together their interactions can have profound consequences for their evolution.

A binary star is a pair of stars that orbit one another, just like a planet orbits our Sun. They are often roughly the same mass, so they both move around a common center that astronomers call the “center of mass.” The stars more or less continue with their lives as if they lived alone, but if they are close enough together their interactions can have profound consequences for their evolution.

We know that a large fraction of stars in the galaxy are actually binary stars — two stars mutually orbiting one another the way planets orbit the Sun. So what would happen if we replaced one star in a binary with a black hole? This is eminently reasonable because we think black holes are one of the possible skeletons of dead stars.

In terms of the binary orbit, if the star and it’s black hole companion are far apart, nothing would change! The star that remains a star would continue to happily speed along its appointed path, obeying Kepler’s laws of orbital motion, with nary a concern that it is orbiting a black hole instead of the friendly star that was once its gravitational partner in the Cosmos.

Even though the orbit of the companion star is not dramatically affected by the presence of a black hole, there is an important consequence for astronomers: if they are watching this star system they will see the single star apparently orbiting … nothing! The star will continue to trace out its orbital path, appearing in our telescopes to wobble back and forth for no discernible reason.  This is something we have looked for, and it is something we have found!

Cygnus, the Swan, is a constellation in the northern sky. Three bright stars (Deneb in Cygnus, Vega in Lyra, and Altair in Aquila) make up "The Summer Triangle." The black hole system, Cygnus X-1, lies near the center of the Triangle, in the neck of Cygnus.

Cygnus, the Swan, is a constellation in the northern sky. Three bright stars (Deneb in Cygnus, Vega in Lyra, and Altair in Aquila) make up “The Summer Triangle.” The black hole system, Cygnus X-1, lies near the center of the Triangle, in the neck of Cygnus.

In the northern sky, the Milky Way can be seen high in the sky on clear summer evenings. Prominent along the faint, diaphanous band is the constellation of Cygnus, the Swan, flying south along the great river of the galaxy. In the neck of Cygnus, near the naked eye star Eta Cygni, astronomers have found a bright blue-supergiant known as V1357 Cygni (also known as HD 226868 — there are a lot of stars, so astronomer names for them are not always the most pleasing for idle conversation!). It is bright enough to see in a telescope from your backyard, but there is little else you or I can discern. But in 1964, astronomers flew an x-ray detector on a rocket to the edge of space, and discovered this star is one of the strongest sources of x-rays in the sky. We now call it Cygnus X-1.  Since then, astronomers have watched this star closely, and note that ever so slightly it is wobbling back and forth once every 5.6 days, suggesting its unseen companion is about 14.8 times the mass of our Sun; the orbit between the two is about half the size of Mercury’s orbit.

An artist's impression of Cygnus X-1. The strong stellar wind blowing off the supergiant is captured by the black hole and pulled down to form an accretion disk. [ESA/Hubble image]

An artist’s impression of Cygnus X-1. The strong stellar wind blowing off the supergiant is captured by the black hole and pulled down to form an accretion disk. [ESA/Hubble image]

But what about the x-rays? Ordinary binary stars don’t spew off as many x-rays as Cygnus X-1. What gives? This is another clue pointing toward the companion being a black hole. The blue supergiant blows off a strong stellar wind, much like the solar wind from our own Sun, but stronger. That material is captured by the gravitational pull of the companion and pulled down onto a turbulent maelstrom of material called an accretion disk. The accretion disk swirls just above the black hole, and is subject to intense gravity. Heuristically, the picture is this: the intense gravity makes the gas move very fast. When gas moves fast, it gets hot. When gas gets hot, it emits light. The faster it moves, the hotter it gets, and the more energetic the light. X-rays are very energetic, so the gas must be moving very fast. Why? The extreme gravity of a black hole.

So black holes can do crazy stuff to gas that streams down close to them. But what will the extreme gravity do to a solid object that gets too close? Imagine you (unwisely) decide to jump into a black hole; not being much of a diver, you jump in feet first. As expected, far from the black hole you don’t notice anything; the gravitational field looks perfectly normal, like any Newtonian gravitational field. Space and time are only distorted and stretched by noticeable amounts when you get close.

Tidal forces are a difference in the strength of gravity across your body. In the extreme gravity near a black hole, the side closest to the black hole is pulled on more strongly than the far side. As  you get closer and closer to the black hole the effect is to stretch you out ("spaghettify" you) until you are pulled apart ("tidally disrupted").

Tidal forces are a difference in the strength of gravity across your body. In the extreme gravity near a black hole, the side closest to the black hole is pulled on more strongly than the far side. As you get closer and closer to the black hole the effect is to stretch you out (“spaghettify” you) until you are pulled apart (“tidally disrupted”).

As you get closer, the strength of gravity increases — general relativity tells us the curvature, the warpage of spacetime is increasing. As you approach, the black hole pulls more strongly on your feet than your head. As you get closer and closer, this difference in force (what your physicist friends call a “tidal force”) can become quite strong! The net result — it stretches you out — provided you can withstand the strain, you’ll stay together, but get longer, like a rubber band.

Stephen Hawking has dubbed this effect “spaghettification” — the turning of you into a long piece of spaghetti. It is more extreme if your head is farther from your feet — short people have a better survival probability than tall people!  If you really want to survive the dive into a black hole, your best choice is to belly flop or cannonball — both greatly reduce the distance between the side of you close to the black hole, and the side of you farther from the black hole.

Astronomers observe tidal disruption flares. Here is an artists conception (top) and telescope observations (bottom) of a star being tidally disrupted by a 100 million solar mass black hole in galaxy RXJ1242 in 2004. [NASA]

Astronomers observe tidal disruption flares. Here is an artists conception (top) and telescope observations (bottom) of a star being tidally disrupted by a 100 million solar mass black hole in galaxy RXJ1242 in 2004. [NASA]

Imagine now it wasn’t you diving into a black hole, but a star.  The exact same effects occur. Imagine a star falling toward a black hole. As it closes the distance, the strength of gravity grows inexorably stronger. The side of the star closest to the black hole feels the tug of the black hole more strongly than the far side. Despite the fact that it’s own self-gravity is strong enough to keep it together, as the influence of the black hole grows, it begins to overcome the self-identity of the star, and distorts it into a oblong caricature of its former self.  If the star strays too close, the black hole’s gravity will overcome the star’s gravity, and tear it apart. The star will be tidally disrupted.

When this happens, the guts of the star are violently exposed in an energetic event called a tidal disruption flare. Generally, the remains of the star, now a seething, turbulent cloud of gaseous debris, collapses down toward the black hole, forming an accretion disk that heats up and, for a time, becomes very bright. Slowly, the gas falls down the throat of the black hole, vanishing forever, and all evidence of the star is erased.

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

So what are these black holes that eat stars? They are the great monsters of the Cosmos. Lurking at the centers of spiral galaxies, like Charybdis in the Straits of Messina, these “supermassive black holes” have grown on a steady diet of stars and gas to enormous sizes. Our own Milky Way harbors a massive black hole that is 4 million times heavier than the Sun; even though it is millions of times more massive, the horizon is only about 17 solar radii across. But the consequences of its existence are profound. For the last two decades or so, astronomers have been watching a small cluster of stars in the center of the galaxy. We’ve been watching them long enough now, that they have traced out significant pieces of their orbits, and in some cases completed an entire orbit, allowing us to measure the mass of the black hole.

Despite being 4 million times more massive than our Sun, the black hole at the center of the Milky Way has an event horizon diameter only 17x larger than the Sun's diameter!

Despite being 4 million times more massive than our Sun, the black hole at the center of the Milky Way has an event horizon diameter only 17x larger than the Sun’s diameter!

Astronomers have looked for and found supermassive black holes in many other galaxies. In the course of those observations, we have discovered a tantalizing and interesting connection between galaxies and the massive black holes they host. Galaxies often have a part of them astronomers call “the bulge.” In the Milky Way, and other spiral type galaxies, the bulge is the large spherical bubble of stars that sits over the center of the galaxy. Some galaxies, like elliptical galaxies, are “all bulge.”  Astronomers have discovered an interesting relationship: the bigger a bulge, the bigger the black hole that lies at the center of it.

The black hole in the center of M87 powers an enormous, energetic jet of material spewing out from the galactic core. (L) I was one of the first amateurs image this jet in 2001. (R) HST image of the jet, for comparison. :-)

The black hole in the center of M87 powers an enormous, energetic jet of material spewing out from the galactic core. (L) I was one of the first amateurs image this jet in 2001. (R) HST image of the jet, for comparison. :-)

An example of galaxies that are “all bulge” are ellipticals, like M87 in Virgo. M87 has a 2 BILLION solar mass black hole in its core that has launched an enormous jet that shoots out of the galaxy, extending nearly 5000 light years out from the core. No one knows exactly how black holes launch jets, but the best observations and models lead astronomers to believe that a spinning black hole can twist up magnetic fields into galactic sized magnetic tornadoes. Hot gas is very easy to convince to follow strong magnetic fields, and as it plummets toward the black hole, some of it is redirected up the jets.

But even among galaxies, some black holes are larger than others. In the northern sky, just below the Big Dipper is a smattering of faint stars known as Coma Berenices — “Bernice’s Hair.”  The stars of Coma Berenices are in our own Milky Way galaxy, but behind them, across 320 million lightyears of the void, lies the Coma Cluster of galaxies. A group of about 1000 galaxies, the center of the cluster is ruled by two super-giant elliptical galaxies known as NGC 4874 and NGC 4889 (both of which can be seen with backyard telescopes; NGC 4889 is easier than NGC 4874!). Both show strong evidence for massive central black holes, including enormous jets emanating from the centers. But astronomers have attempted to mass the black hole in NGC 4889 and found the black hole could be as massive as 37 billion solar masses. If true, the event horizon would be 24 times larger than Neptune’s orbit. That size boggles the mind — a void of nothing, almost 25 times larger than the solar system; anything that goes in is lost. Forever.

Coma Berenices is a pretty splatter of stars beneath the Big Dipper (which is part of Ursa Major). The Coma Cluster of galaxies, and NGC 4889, lies 320 million lightyears behind the stars of Coma Berenices.

Coma Berenices is a pretty splatter of stars beneath the Big Dipper (which is part of Ursa Major). The Coma Cluster of galaxies, and NGC 4889, lies 320 million lightyears behind the stars of Coma Berenices.

The idea that black holes and galaxy bulges are related is a new one in astronomy, only having been proposed in 1999.  A diligent padawan of the Cosmos would ask the obvious question: if a galaxy has no bulge, does it then have no super-massive black hole? The answer may be “yes.” A classic example of this is the Triangulum Galaxy (M33), right here in our own Local Group. A beautiful, classic spiral galaxy, M33 is only marginally tipped to our line of sight and can be easily seen and studied with a backyard telescope. Curiously, M33 has no bulge; so far, no massive black hole has been found.

M33, the Great Galaxy in Triangulum. There is almost no bulge surrounding the bright core seen here; astronomers have yet to find any evidence of a supermassive black hole there.

M33, the Great Galaxy in Triangulum. There is almost no bulge surrounding the bright core seen here; astronomers have yet to find any evidence of a supermassive black hole there.

And so the search continues. The number of galaxies for which we know the bulge-black hole relation works is still small — we have seen enough to understand the implications and possibility, but we still haven’t seen so many that we are confident stating, without equivocation, that “all bulgy galaxies have black holes.” Time and diligent observations of new galaxies will help resolve this question.

The fact that you and I can have conversations like this about black holes, dealing with what astronomers see and not (too much) about what we speculate is a mark of how far astronomy has come. When general relativity was first penned, black holes started as a curious, if somewhat suspect mathematical solution to the equations of gravity. Repeated, careful observations of the Cosmos have, however, led astronomers to the inescapable conclusion that black holes do in fact exist. They are part of our understanding of the machinery of the Universe. Now, the questions are different than what they were a century ago. Instead of asking “do they exist?” and “are they real?” we instead noodle our brains on the questions of “how many are there?” and “how big are they?” and “what are they doing to the Cosmos around them?

And a lot of us still wonder, “what would happen if I jump in one?

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.