Tag Archives: singularity

Black Holes 4: Singularities, Tunnels, and Other Spacetime Weirdness

by Shane L. Larson

I think one of the great things about the modern world is the propensity of information. Information is free and easy to come by, and it possible to learn about anything you want. More-or-less, the total knowledge of our civilization has been written down in books and documents, and disbursed to libraries, websites, and other mediums of communication. It is not always easy to discern what is authentic and what is not, as is clearly the case when one looks at the wild, apocalyptic wastelands of modern social media. But none-the-less, it is easy to indulge your desire to simply learn. We consume books and podcasts and documentaries, sacrificing time we could spend on woodworking or yardwork or binging TV shows in favor of trying to recapture how we felt in 2nd and 3rd grade, before school became about exams and homework and was just about how awesome all the far flung corners of the world and Nature could be. 

I think deep down all of us are lifelong learners; I’ve met many of you at public lectures or here at the blog. Some of you are quiet, and sit in the back with contemplative furrows on your brow; others of you are more exuberant and can barely contain your questions. Either way, you all show up, because you remember how cool it was when you were first learning. But I’ve noticed something interesting in my years talking to all of you: as a rough rule of thumb, I can usually triple the attendance of any talk if it is about sharks, volcanoes, dinosaurs, or black holes. Vast numbers of you succumb to your curious inner child if I talk about the right things. 

People’s minds, young or old, can be captured if we talk about science in ways that stimulate their interest and imaginations. [Image: Bill Watterson]

What is it about these topics that inspires deep interest in people? I think, at the heart, they are very real examples of the Universe’s ability to put you in mortal danger with implacable indifference. Never mind that it is unlikely you will encounter any of these dangers in your life. Pondering being faced with a highly improbably danger in the Universe allows us to ask ourselves, “what would I do?” in much the same way we watch super-hero films and imagine ourselves in the fray. 

Black holes are notable in this list because not only do they have the mystique of danger about them, but they are suffused with a long list of exotic, mind-bending phenomena that add to their mysterious nature.  

Let’s talk about some of the exotic things you have hard about black holes, and I often get asked about.

“Is everything is going to get sucked into black holes?” 

This is probably the most common question I get about black holes! The simple answer is “no” — black holes are not little Hoovers running around the Cosmos sucking stuff up. I’ve thought a lot about where this idea comes from, and I think it is a mis-extrapolation of the inescapability of a black hole. When you are far from a black hole, its gravity is exactly the same as the gravity produced by ordinary objects of the same mass. If you are orbiting far away from a billion-solar mass black hole, the gravity you feel is exactly the same as if you are orbiting around a dwarf galaxy that has a billion sun-like stars in it!  If we could magically replace the Sun with a one solar mass black hole, the Earth would continue along in its orbit as if nothing had happened because the gravitational influence is exactly the same! 

Far from the black hole, you cannot tell if you are orbiting a black hole or a star of the same mass — their gravity is identical unless you get close. [Image: S. Larson]

If you are silly (or unfortunate) and fall into a black hole, you are never going to get out. The gravity of a black hole is so strong that it can trap anything inside it; that is true. But it is not infinitely strong and able to influence everything outside it. 

“What does it mean to get spaghettified?” 

When you get close to a black hole, the gravity can become more intense than anywhere else in the Cosmos. Imagine you are jumping in feet first. The gravity is strongest close to the black hole, so your feet are pulled on more strongly than your head, which is farther away. The result of this dichotomy of gravitational strength is the black hole tries to pull you apart, much as you stretch a rubber band by pulling on opposite ends. Physicists call this difference in force a tidal force, and the process of pulling you apart is called tidal disruption. Stephen Hawking, in his famous book “A Brief History of Time” called this effect “spaghettification.”  

Some ways of falling into a black hole will feel less painful than others. [Image: S. Larson]

Somewhat paradoxically, the spaghettification effect is strongest near the event horizon of small black holes, and weaker near the event horizon of larger black holes. The spaghettification effect is also stronger when your head is farther away from your feet (so tall people will suffer more than us short people). The two rules of thumb for surviving spaghettification when you are jumping into black holes are this: 

  1. Jump into the biggest black hole you can find; million solar mass black holes are much more fun to jump into than solar mass black holes.
  2. Belly flopping into black holes is safer than jumping in feet first.

“Do black holes really bend time?” 

The movie Interstellar has revived broad interest in black holes and inspired wide-ranging conversations about what black holes are really like. One of the most common conversations we have is about time, which usually begins with “what was the deal with the guy who got old when he didn’t visit the black hole?” This plot device could just be accepted at face value, like we do with so much science fiction, but in this case it is rooted in the physics of the real world. General relativity predicts that the closer you are to a source of gravity, the slower your clock ticks compared to someone very far from the source of gravity. Here I use the word “clock” in the physics sense: anything that keeps regular time, whether it is a digital watch, a wind-up pocket-watch from your grandparents’ day, an hour-glass, or the steady beat of your heart.  

Consider two people, one close to the black hole and one farther from the black hole. Every clock ticks slower when you are close to the black hole — this could mean an actual clock that tells time, but it can also mean a regular biological clock, like your heartbeat. [Image: S. Larson]

The bending of time is definitely one of those counter-intuitive predictions of general relativity, but if space and time are one entity (“spacetime”), then bending space very strongly must necessarily also bend time. It doesn’t take much to bend time by a measurable amount — the bending of time is the central physical effect behind the Global Positioning System, which you use everyday on your phone to navigate to the nearest ice cream shop (or coffee shop — whatever). The difference between the bending of time around the Earth and the bending of time around black holes is the strong gravity near the black hole makes the effect much more pronounced. 

“Are black holes are infinitely dense? What does that mean?” 

Anything labeled infinity is, generally, an anathema to scientists. “Infinity” is a perfectly good concept in mathematics, but with respect to the natural world, it seems that the Cosmos is only filled by things that are finite and measurable. That is not to say there aren’t enormous, gigantic, mind-bogglingly huuuuuuge numbers, but they are all tiny compared to “infinity.” In the natural sciences, we have often encountered “infinity” in the mathematical ways we describe Nature, but we’ve found most of them were simply artifacts of our early poor understanding of how the world works, particularly on the microscopic scales of fundamental particles. Gravity is the last frontier in this regard, and there are many persistent “infinities” we encounter, and they often manifest themselves in the study of black holes. 

An example of your common experience with density. This cube of tungsten and this clown nose are about the same size, but the tungsten is significantly heavier. Why? because more stuff is packed into roughly the same amount of space. [Image: S. Larson]

To think carefully about this, let us be precise about what we mean. “Density” is a common concept in physics and chemistry. It is how much stuff (mass) is squeezed into a given amount of space (volume). Dense objects feel heavy in your hand, while less dense objects feel lighter.  As a matter of practical everyday experience, you most often encounter the notion of density when thinking about things floating or sinking in water (objects more dense than water, like rocks, sink; objects less dense than water, like styrofoam, float). 

So let us define the “density of a black hole” the way we define the density of any other object in the Universe: the mass of the black hole, divided by the volume of the black hole. Those of you who are practicing gravitational physicists will recognize that we should be careful when computing the “volume”, but for practical purposes here let us use the ordinary formula for the volume of a sphere where the radius of the sphere is the radius of the event horizon of the black hole. This is practical and intuitive, and will illustrate our point effectively.  

The first picture of the black hole at the heart of M87, formed by light being bent around the inner most regions of space outside the event horizon. This black hole has a diameter larger than the diameter of our solar system! [Image: Event Horizon Telescope Collaboration]

Imagine two black holes: one that is the mass of the Sun, and one that is one billion times the mass of the Sun (a bit smaller than the black hole in M87 that was the subject of the Event Horizon Telescope picture). The solar mass black hole only has a radius of about 3 kilometers, and a density of about 18 quadrillion times more the density of water (1.8 x 1019 kg/m3, for those calculating themselves). By comparison, a 1 billion solar mass black hole has a radius just under 3 billion kilometers (about the radius of Uranus’ orbit); it would have density of only 2% the density of water (numerical value: 18 kg/m3; slightly less than the density of styrofoam). If you could somehow drop it in a gigantic cosmic bathtub, its density suggests it should float. 

If black holes were solid objects and could interact with the world like ordinary “things,” a calculation of their density suggests some are less dense than water and could float in a cosmic wading pool. [Image: S. Larson]

It can’t float, of course — the event horizon is not a hard surface that water can act on and thus provide buoyancy in a ginormous cosmic pond. Water would flow right through the event horizon and disappear, so all the water in the cosmic pond would essentially flow into the black hole like some kind of drain.  But that’s not the point in the floating analogy. As a general rule, we think we understand the physics of things that have densities less than the density of water, so the idea that the density of a black hole is the same as materials that do float is a strange and discomfiting result. And it should be! Just remember your discomfiture is related to the odd nature of black holes — density defined in the classical way really doesn’t apply to black holes the way we’ve done it here, because as we’ve noted before, they are mostly empty space! This odd result has little to do with their overall size, and more with what lies at their heart… the singularity. 

“The Singularity” 

The real mystery of black holes lies at their heart, in the center of the space defined by the boundary of the event horizon. All the gravity of the black hole is concentrated there. All the matter that collapses to form the black hole is still being drawn together even after it falls through the area we call the event horizon. Gravity keeps pulling it inward, inexorably inward, squeezing it smaller and smaller with a force so great no known force in Nature can stop it. Everything that fell inward to create the black hole gets squeezed down smaller and smaller, becoming more and more dense. Eventually it gets squeezed into a space that is vanishingly small, or so general relativity predicts. This point of zero size with everything squeezed into it is infinitely dense, and is called the singularity. The laws of physics as we understand them break down before you ever really reach the singularity, at a distance away from it called the Planck length, about 10-35 meters (0.00000000000000000000000000000000001 meters). This is the length where we expect the physics is governed by quantum gravity, a description of gravity, space, and time on the tiniest scales. We have searched for such a mathematical description of Nature for many years, but so far have been unsuccessful. 

How do physicists think about the singularity? It is an infinity, and infinities are anathemas to physicists. More often than not we are trying to understand what is happening far away from the singularity when thinking about the Cosmos. This is, more or less, what astronomers do because they are observing the Cosmos outside the event horizon, which is far from the singularity. Easy peasy — you don’t even have to waste one brain cell on the singularity if you don’t want to! Sometimes physicists pretend they are okay with the singularity being infinitely dense, and use the classical laws of physics (general relativity in particular) to understand the influence of the singularity around it. Gravitational physicists often do this, in particular because they are trying to understand how the world behaves under the influence of strong gravity. All the tales and imaginings you have heard about the inside of black holes are figured out by scientists thinking about the singularity this way. The last prominent group that thinks about the singularity are the quantum gravity squad. There are many ideas about what a complete description of quantum gravity will look like — all of them are clever, and elegant, and exotic. But we don’t yet have a way of experimentally testing any of them. Someday we will be able to test them. The day we understand quantum gravity, it will tell us something about the true nature of the singularity. 

“Are Black Holes Spacetime Tunnels?”

The last and most famous example of spacetime weirdness and black holes is the astonishing idea that for some kinds of black holes, if you jump in, they may in fact be tunnels. For the movie nerds out there, this is the central plot device in many science fiction stories. For perfectly spherical black holes, there are no tunnels — if you jump in a black hole, the singularity lies in your future; you will be crushed ruthlessly and mercilessly.  But for black holes that happen to have electric charge on them (expected to be few) or are spinning (most black holes found in Nature are expected to be spinning) there are trajectories inside the event horizon that do not end at the singularity. They end… somewhere.   

Scientists struggle to visualize black holes just as much as ordinary people, so we have developed a special map called a Penrose diagram. As you go up the diagram from bottom to top, time increases from the past to the future. As you go left or right on the map, you change where you are in space. Here the white areas are the ordinary Universe, and the yellow areas are inside a black hole. The one on the left is a Schwarzschild black hole that has no tunnel; if you are inside, the singularity lies to your future and there is no ordinary Universe you can get to. The one on the right is a charged black hole, which might have a tunnel. If you are inside, you can avoid the singularities on the left and right, and possibly emerge at the top of the diagram. [Image: S. Larson]

In these kinds of black holes, if you avoid the singularity you come out of something that looks mathematically similar to a horizon. The difference is you come out of this horizon, emerging from the inside to the outside. Such things are variously called “wormholes” or more commonly “white holes.” They are, in essence, the other end of the black hole, like it is some kind of giant culvert or tunnel that connects one place to another. 

Tunnels to where? you quite astutely ask. The truth is we don’t know, but there are several possibilities. One possibility is the tunnel emerges somewhere else in our observable Universe. As an astronomer this is a very intriguing possibility, because it suggests there may be something that could be observed with telescopes. Sadly, to date, we have not seen anything exotic and unexplained that might be a white hole.  

Another possibility is that it may emerge somewhere in our Universe, but outside the observable part of the Universe. This idea is a bit harder to wrap your brain around, because it hinges on understanding that the Universe can be larger than what we can observe and could, in principle, go on forever. The “Observable Universe” are just those parts of the Universe that are close enough for us to observe in a telescope because light has had time to reach us in the time since the Universe was born. An easy way to think about this is to think about your home state where you live. Most of us can walk only a few miles per hour — let’s say 3 miles per hour. That means in one day, you could only walk 3 miles per hour x 24 hours = 72 miles. If you started walking right now, by this time tomorrow you could be anywhere in the state within 72 miles. Does that mean the rest of the state isn’t there? Of course not — it just means the parts of the state you could get to at the fastest pace you could walk (the “Observable State”) is only 72 miles away in any direction. 

If I start at Northwestern University and walk for 24 hours at 3 miles per hour (average walking speed of a human) I can reach anywhere inside the red circle. I can get no farther, but that doesn’t mean there aren’t more places outside the circle! The Universe is the same, only the time is not 1 day, but the age of the Universe, and the speed is not walking speed, but the speed of light. Inside the circle is what we call the Observable Universe, but it is not the Entire Universe. [Image: S. Larson, Map by Google]

A third exotic possibility is that the white hole may emerge not in our Universe, but in some other Universe. A Universe that is not our own, but is somehow parallel to our own. It is an interesting possibility to ponder and imagine because it opens up all kinds of possibilities. Are the laws of physics the same there, or is the other Universe some weird place that doesn’t have stars and planets and galaxies? Do all of our black holes emerge as white holes in the other Universe? Where do black holes from the other Universe go? Do they emerge in our Universe, or do all white holes in all the other Universes emerge in only one of the other Universes? Some things we can imagine within the realm of science and do calculations and simulations about, but others are mere speculation that we have yet to ponder and consider seriously. It makes your head spin, but these are the things that great speculative science fiction about black holes are made of.  

The last possibility is that tunnels through black holes simply do not exist at all, that Nature somehow closes them off, or we have not fully understood the mysteries of black hole insides completely yet. There is much we still have to learn.

Which of course is the point. Black holes, on any given day, seem completely unfathomable, especially in the context of the weird implications about what they do to the world around them. But is precisely that mystery that draws our attention time and again. Partly because we like that feeling of being completely baffled by Nature, but also because some deep part of us knows that these inscrutable mysteries hide deep and precious secrets, secrets that lie at the core of how Nature and the Cosmos work. 

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This post is the fourth in a series about black holes.

Black Holes 01: Imaging the Shadow of Darkness

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff

Black Holes 04: Singularities, Tunnels, and Other Spacetime Weirdness (this post)

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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.