The Cosmos in a Heartbeat 3: The End is Just the Beginning

by Shane L. Larson

The fence we walked between the years
Did bounce us serene.
It was a place half in the sky where
In the green of leaf and promising of peach
We'd reach our hands to touch and almost touch the sky,
If we could reach and touch, we said, 
'Twould teach us, not to ,never to, be dead.
We ached and almost touched that stuff;
Our reach was never quite enough.
If only we had taller been,

Ray Bradbury.

So opens Ray Bradbury’s epic poem, “If Only Taller We Had Been…” He had composed it for a panel discussion on 12 November 1971, just before Mariner 9 arrived at Mars, becoming the first spacecraft to ever orbit another planet (here’s a video of Bradbury reading the poem). Bradbury’s poem is an ode of optimism to the future — smiling gently at our ancestors and their grasping reach which didn’t quite reach us on the first try. It is a grand hope for our posterity that they achieve all that we dare to dream. The future is a murky place, fraught with unknown difficulties and challenges, but illuminated by desire and ambition.

Despite the long years that remain between today and tomorrow, you can image what our future selves might do in our quest to understand the Cosmos. The Universe will not change much between now and then, but our understanding of the Universe will change, and change dramatically.  The specific details we cannot know, but we know where the limits of our knowledge are, and we know what the capabilities of our current science experiments are. Scientists already are planning for the future, imagining, designing, and developing the Great Machines of tomorrow.  Let’s spin the clocks forward 30 years and take a glimpse of what we expect.

How my best friend imagines my future personal telescope might be like. [Image: J. Harmon]

In 2048 it is impossible to know what sorts of telescopes amateurs will have. But if history is a teacher, amateurs will have access to bigger telescopes and better technology for capturing light from the sky, whether it be cameras or spectroscopes, or automated robotic mounts, or telescopes that see in light our eyes cannot see. A few amateurs already have such capabilities, and the future will surely make such technology more accessible.

Professional astronomers are also looking ahead toward their next generation of telescopes. On the ground, the next generation of telescopes will be about thirty meters in diameter, three times larger than the current generation and six times larger than the venerable Hale Telescope at Mount Palomar. Thirty years from now, Hubble will be long gone, but its successor, the James Webb Space Telescope (JWST), will have been lofted in its place. Right now, JWST is expected to launch in 2021. It should cast its gaze on the Unvierse for a decade or more, depending on how long its fuel reserves last. As with Hubble, it will see farther and see more than any telescope in history, though we don’t yet know what new things it will teach us. It is a marvel of engineering, designed to fold up and fit inside a rocket for launch, it will unfold like a flower when it arrives in space — 18 hexagonal segments that together comprises a gigantic 6.5 meter diameter mirror that looks out on the Cosmos, all of it sitting in the shade of a multi-layered sun-shield the size of a tennis court. If JWST has a tenure as long and as lustrous as the Hubble Space Telescope’s, then the discoveries in store for us will be extraordinary and transformative for astronomy.

The James Webb Space Telescope (JWST) will be more than two and half times the diameter of Hubble, it’s mirror comprised of large hexagonal segments. [Image by Paul Kalas]

Space affords many opportunities to expand our view of the Cosmos, and this is certainly the case in gravitatioanl wave astronomy. In the early 2030s, the European Space Agency and NASA are planning to launch a gravitational wave observatory called LISA — the Laser Interferometer Space Antenna. It will operate continuously for a decade or more, well into the 2040s, probing new and exotic phenomena, many of which can only be understood or detected at all with gravitational waves. 

LISA will be a triangular constellation of spacecraft separated by 2.5 million kilometers, following the Earth around in its orbit. [Image: S. Barke]

Like its ground-based cousin, LIGO, LISA is a laser interferometer. It shines lasers back and forth between mirrors, timing how long it takes the laser to make the flight. Small changes in that flight-time, changing in regular undulating patterns, are the hallmark of gravitational waves. So what’s so different about LISA?  LISA is about a million times larger than LIGO, which is why it has to be in space. The reason it is so much larger is that the size of a laser interferometer determines the sources of gravitational waves it can detect.  Whereas LIGO can detect small stellar skeletons in the last moments of their lives, as they whirl around each other hundreds or thousands of times a second before colliding, LISA is sensitive those same stellar skeletons when they are much farther apart, earlier in their lives when they speed around their orbits only once every thousand seconds or so. Whereas LIGO can detect black holes that have a few to a few tens times the mass of the Sun, LISA can detect black holes that are millions of times the mass of the Sun (“massive” black holes, like the one at the center of the Milky Way).  Astronomers say that LISA observes a different part of the gravitational wave spectrum. Just as we have different kinds of “light” telescopes (optical telescopes, radio telescopes, x-ray telescopes), we have different kinds of gravitational-wave “telescopes” (LISA, LIGO, and others).

So what will LISA teach us about the Cosmos? Consider the Milky Way. Like many galaxies, the Milky Way is ancient — 10 billion years old. Comprised of 400 billion stars, many stars in the Milky Way have lived their lives and passed on into the stellar graveyard over the long course of its history. Something you may remember from your astronomy learnings is that many of the stars in the galaxy are not single stars, but binary stars — two stars orbiting around each other the way the Moon orbits the Earth or the planets orbit the Sun. That means when both stars die, their skeletons sometimes stay together, orbiting each other over and over and over and over again. The stellar graveyard is full of not just skeletons, but binary skeletons, and in particular binary white dwarf skeletons.

White dwarfs are the size of the Earth, but the mass of the Sun. When they are separated by roughly half the Earth-Moon separation, they emit gravitational waves LISA can detect. [Image: NASA/Chandra X-Ray Center]

A white dwarf is a particular kind of stellar skeleton, created when an ordinary star reaches the end of its life. Most stars — and our Sun is among them — are not heavy enough to explode upon their deaths. Instead they swell up into a red giant, then compress themselves down into a skeletal remnant of their former selves, about the size of the Earth. These white dwarfs are hot and crystalline, mostly comprised of carbon, and will over the remaining history of the Universe slowly cool and fade. Most stars in the Milky Way are ordinary, average stars like the Sun. That means that most of the stellar skeletons in the Milky Way are white dwarfs, created over the ten billion history of the galaxy. All told, there are some ten to fifty million white dwarf binaries, and all of them are going to be emitting gravitational waves that LISA can see. 

A simulation of the stellar graveyard of the Milky Way. [Data by K. Breivik and S. Larson]

Part of preparing for astronomy in the future is imagining what you might observe and discover. We simulate the entire life history of the Milky Way on the computer and ask what does the stellar graveyard look like. Where are the white dwarfs, where did they come from, and what do they tell us about the life and history of stars in the Milky Way? When faced with a view of our home galaxy like this, and you imagine all the vast cacophony of gravitational waves from the stellar graveyard? We often describe this as the “lunch-room problem” or the “party problem.” Imagine you are hanging out in the cafeteria or a crowded restaurant at lunchtime. Everyone there is talking, and what you hear is a dull rumble of noise coming from every direction in the room. You can tell it is people talking and laughing, but by and large all of the sound is mixed together and most of the conversations are indistinguishable from one another. Astronomers call this “confusion noise.” Of course you can hear some conversations. You can hear people that are close to you, and you can hear loud people, even if they are far away. You can always hear these close or loud people, no matter what the background noise is. Astronomers call signals that stand out above the confusion “resolved sources.”

Your ears, and mostly your brain (based on what your ears are telling it), are fully capable of separating resolved sources from confusion noise — you do it every time you go out to eat!  Our job as future gravitational wave astronomers will be to teach computers how to carefully pour over the LISA data and learn to separate resolved white dwarf binaries that are close or loud. Out of the tens of millions of confused skeletons in the stellar graveyard, tens of of thousands will be resolved and studied by LISA. Encoded in that collection of stars are the tales of how stars like the Sun have lived out their lives, and a deeper understanding of how the birth and death of stars has surged and waned over the long history of the galaxy.

Dead stars aren’t the only thing that LISA will observe; it will also be sensitive to black hole binaries — massive black hole binaries. One of the great astronomical discoveries astronomers have made in the last few decades is that big galaxies harbor massive black holes at their centers, black holes of millions or billions of solar masses. As our telescopes have gotten larger and able to see deeper into the Cosmos, we have also started cataloging galaxies in all their shapes and forms, and discovered that sometimes they collide. So what happens when two galaxies, each harboring a massive black hole, collide? 

There are many known examples of colliding galaxies, but this is a personal favorite — the Rose and Hummingbird (Arp 273). The smaller galaxy is throught to have already passed through the larger galaxy once. [Image: Hubble/STScI]

Their stars swarm and merge like a cloud of angry bees, eventually coalescing due to their mutual gravitational attraction and form a new galaxy. Their big black holes slowly sink to the center, where they find one another and begin a slow, spiraling orbit that grows ever shorter as time goes on. When the orbits take only 10,000 seconds or less, they become observable by LISA.

Today, we know galaxies merge, but we know little about the processes that help massive black holes grow. Do they accrete gas? Do they grow by absorbing stars over and over again? Or do they only grow by merging with other black holes? LISA’s observations of massive black holes, together with where in the Cosmos they are found, will begin to provide answers to those questions.  We don’t know those answers today, because we’ve seen that galaxies collide but have yet to see massive black holes merge. In lieu of LISA data, which is still more than a decade in the future, we simulate different ways to grow galaxies and black holes on super-computers, and simulate what LISA would observe.

One massive simulation, spanning the entire age of the Universe, is called the Illustris Simulation. The movie below is a visualization of the Illustris simulation from the Big Bang to the present day. The simulation accounts for gas and dark matter int he Universe, and tracks the formation of stars, galaxies, and black holes across Cosmic time. We use the simulation as a model for the actual Universe, and “observe it” with a simulation of LISA. What do we learn from this? That the sky is going to be alive with massive black hole binaries, visible to LISA in every direction and all the way to the edge of the Observable Universe. In the movie below, we show all the mergers that would be detectable by LISA if it were flying at the right times (data simulation be Michael Katz and S. Larson).

You can imagine, and rightfully so, that all the massive black holes just add to the confused cacophony of gravitational waves created by the millions of white dwarfs in the galaxy. The Cosmos is full with the gravitational chorus, and our job as astronomers is to pick out all the melodies, and harmonies, and individual instruments and voices that make it up.

The miracle of the modern age is that we are suddenly aware that the Universe is sending us messages with a multitude of signals — light, particles, and gravitational waves. It’s an intricate, interlaced story that we are just now learning to interpret. Modern, instrument based astronomy began with the invention of the telescope some 400 years ago. Particle astronomy is only 100 years old, and gravitational wave astronomy has only been successful in the last five years. Our ability to probe the Universe carefully and precisely has existed for only the bareest fraction of Cosmic time — a heartbeat in the life of the Cosmos. We’ve used our ingenuity, our curiosity, and our creativity to spin that short experience into a complex and increasingly sophisticated understanding of the nature of the Universe, and our place within it.

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This post is the last of three based on a talk I have given many times over the last few years, updating it each time to reflect the latest coolest things. The complete set posts of the series are:

The Cosmos in a Heartbeat 1: A Love Affair with the Cosmos

The Cosmos in a Heartbeat 2: Coming of Age

The Cosmos in a Heartbeat 3: The End is Just the Beginning (this post)

This post was enabled by a new version of the talk done as a Kavli Fulldome Lecture at the Adler Planetarium in Chicago. The talk was captured in full 360, and you can watch it on YouTube here. If you have GoogleCardboard, click on the Cardboard Icon when the movie starts playing; if you watch it on your phone, moving your phone around will let you look at the entire dome!

I would like to thank all my colleagues at Adler who worked so hard to translate what was in my brain into a story told in the immersive cradle of the Grangier Sky Theater. The talk was given on 9 Nov and 10 Nov 2018.

This is just the beginning

by Shane L. Larson

Each morning, I roll out of bed, dutifully feed the three cats that own me, help my fourth-grader get her backpack put together for the day and put my daily secret note in her lunch, enjoy a few brief moments over morning coffee with my spouse, and then it is off to work.

For my day job, I’m a scientist. My friends and I work in a completely new branch of astronomy called gravitational wave astronomy. Our express goal is to detect a phenomenon that was predicted almost a century ago by Einstein: the undulations and propagating ripples in the fabric of spacetime that signify the dynamic motion of matter in the Cosmos.

Gravitational waves are ripples in the fabric of spacetime; propagating disturbances caused by the dynamical motion of heavy masses, like black holes or neutron stars.

Gravitational waves are expected to be a phenomenal probe of the Cosmos because they are readily generated by objects that are otherwise hard to detect by other means. This includes objects of intense interest to astronomers, like neutron stars, stellar mass black holes, white dwarfs, cosmic strings, and supermassive black holes at the hearts of galaxies. Despite their apparent utility in astronomy, the are exceedingly hard to detect. When Einstein first deduced their existence, he famously showed that the waves were so weak he thought we might never be able to measure them. But as is often the case, the future is full of wonders, and with the advent of the Space Age, people began to question that judgement. Maybe, with some cleverness and awesome technology, we could gaze at the Universe with gravity rather than light.

As with many scientific endeavours, gravitational wave detection is a difficult task because we’ve never built machines to do this before. We are learning how to do everything for the first time. You try things out, making your best guess as to how it is all going to work, but when you finally flick the switch to “on” you can debug your experiment because it is right in front of you.  That’s all well and good when your lab is here on planet Earth, but when you shift your experiments to space, it becomes a bit more difficult.

LISA will be a constellation of 3 spacecraft, 5 million kilometers apart, shining lasers at each other. [Image: Astrium]

Someday we want to build a space observatory for measuring gravitational waves, called LISA — the Laser Interferometer Space Antenna. LISA consists of three spacecraft, each about 2 meters in diameter and 50cm deep. They fly in space, 5 million kilometers apart, and shine lasers back and forth between themselves. We time the flight of those lasers (nominally just over 16.6 seconds from one spacecraft to another) and if a gravitational wave blasts through LISA, we see the laser times change.

So how do we go about building new spacecraft for the first time? We take things in stages, just like you and I do when we try to learn something new. When I want to learn to play guitar, I don’t take the stage on Day One with Dr. Brian May; instead I get an old beater guitar out of the basement and I plunk out riffs of “Old Sussanah” until my fingers bleed. Then I work on the guitar solo in “Brighton Rock.”  Building spacecraft is kind of the same thing.

Artists conception of the LISA Pathfinder spacecraft. [Image: European Space Agency]

No space observatory like LISA has ever been built before, so we have to figure out how to do it. How do you build the laser timing system? How do you set up the spacecraft thrusters to respond to external influences like the solar wind? How do you get the whole thing into orbit in one piece, then set it up so it works? How do you control spacecraft temperature to the precision we need?  The best way to answer all of these questions, and to discover all the pitfalls we haven’t imagined, is to build one. This is one of the primary reasons we built a spacecraft called LISA Pathfinder.

LISA Pathfinder is an “almost LISA”. The spacecraft itself is roughly the size and shape of a LISA spacecraft, but it’s guts are slightly different. Deep down inside, it has a linked laser system that is easiest to think of as if it is just an entire LISA arm, shrunk down to fit on a single spacecraft. This is not ideal for doing astrophysical work, but it is perfect for understanding how the spacecraft are going to work in space.

The heart of LISA Pathfinder (the “payload” in spaceflight lingo). A laser system monitors two freely flying “test masses” (2 kg cubes of gold & platinum). [Image: European Space Agency]

Throwing a robot into space is hard. You have to get it to outer space, and get it there in one piece! The usual way you get things into space (so far) is with rockets. Putting aside the fact that they sometimes explode, a rocket ride to space is not the gentlest experience in the world. It’s loud — noise levels in proximity to a typical rocket engine are a million-billion times louder than sound you encounter at home every day. It shakes a LOT — rocket vibrations back and forth across the body of a rocket can be so strong they have led to catastrophic destruction of the rocket itself. The launch forces are enormous — human spaceflight engineers keep launch forces low for crew comfort (the maximum on space shuttle flights was about 3 times Earth gravity), but rockets without human crews regularly reach 5 to 10 times Earth gravity during launch. Add that all together, and the ride to space can be pretty rough. So how do you get a sensitive gravitational wave experiment into space, all in one piece and undamaged, on a rough and tumble rocket ride?

Hucking robots into space is hard, to be sure, but using a robot you threw into space to do science can be even harder. First, everything has to work. When your robot is tens of thousands of kilometers away from the closest space engineer, you can’t tinker with it — there’s no tightening up bolts, no replacing faulty lasers, no kicking stuck gear boxes, nor swapping out new battery packs. Second, the environment of space is harsh — there’s no air, the Sun is constantly blasting and heating one side of your spacecraft while the other side is turned toward the frigid chill darkness of deep space. And all the while, your dedicated space robot is bathing in a constant wash of hard cosmic radiation. Every ultra-sensitive space experiment has to weather through those hardships, while collecting data that would be hard to collect even under controlled laboratory conditions on Earth.

So you take a baby step, and you test everything first on Earth, then in space. This is the purpose of LISA Pathfinder. To teach us how to build a spaceborne gravitational wave detector, then to show we know how to get the thing safely to space, then once we’re in space, we turn it all on to show that we can do the actual experiment we want to do.

LISA Pathfinder launch on a Vega rocket (VV06). [Image: European Space Agency]

On December 2, after many years of design and laboratory work, LISA Pathfinder was launched atop a Vega rocket from Kourou Space Center in French Guiana. It has gone through a series of orbital burns that are sending it to a neutral “Lagrange point” between the Earth and Sun, where it will enter a “halo orbit” to test its lasers, thrusters, and spacecraft guidance systems in the very same way that LISA will have to work. So far, the flight has been flawless.

Just a few of the people who worked on LISA Pathfinder, my colleagues Karsten Danzmann (L), Paul McNamara (C), and Stefano Vitale (R). [Image: Paul McNamara]

What constantly amazes me about the people who build these machines is their diligence and tenacious attention to detail. A robot that we huck into space is not just a dumb hunk of metal. It is an amazing complex machine that is capable of thinking and taking care of itself. It conducts experiments that we tell it to do, stores the results of those experiments and faithfully beams the information back to Earth. At the same time, it is surviving one of the most hostile environments known: the vacuum of space. The influence of the Sun produces drastic temperature shifts across your spacecraft. Cosmic radiation is constantly bathing the spacecraft in a wash of seething, energetic particles. And all the while it has to gather and store energy, and all the zillions of parts and components have to work together, flawlessly and seamlessly.

Your car is also an amazingly complex machine. But if some piece of it stops working and leaves you on the side of US Route 50 in Nevada (the Loneliest Highway in America), a passing motorist will still happen along to help you, or you can make a quick call to the motor club to come tow you. There are no such luxuries in the game of space exploration.

The scientists and engineers who contemplate these things every day are ingenious and clever. The delivery of LISA Pathfinder was the culmination of a decade long effort by an enormous team of scientists and engineers. And all the while they were designing and building LISA Pathfinder, they were teaching classes, and training new students and young scientists who will go on to do new and awesome things in the future. These are the people who make our modern world go ’round. I have nothing but admiration for my colleagues who have built and flown this marvelous machine.

So, at long last, the beginning has arrived. We are all simultaneously exhilarated, relieved, joyous, and eager for the next bit of news and the latest results to get here. Because this is only the beginning, the culmination of decades of hard work, difficult hardships, and anticipation. The BEST stuff — the detection of gravitational waves from the Cosmos — is yet to come.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

In 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

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.

Behind the Curtains of the Cosmos 2: Gravitational Waves

by Shane L. Larson

Nearly every picture you have ever seen of galaxies and nebulae and stars, and virtually everything that we know about the Cosmos, has been obtained with light.  Light is plentiful and easily created, so it is natural to use it as a probe of the Universe. Vision is, for most of us, the primary sense by which we interact with the world, so again light is an obvious probe for us to lean on.

Three dimensional gravitational wave emission from colliding black holes (NASA-Goddard).

But there are other physical phenomena in Nature, and technology enables us to probe those phenomena, whether we can personally sense them or not. One such phenomena, first predicted by Albert Einstein in 1916, are gravitational waves. Gravitational waves are propagating disturbances of gravity — in the modern language of gravity, they are ripples in the fabric of space and time.  If a massive object moves, it takes time for the gravitational field to respond because nothing can travel faster than light.  Heuristically then, gravitational waves are the shifting of gravitational fields, in response to dynamical motion of mass.

When two massive objects (like stars or black holes) orbit one another, the information about how the gravitational field is changing moves outward at the speed of light. These ripples spiraling outward are gravitational waves, and carry detailed information about the source. Click to animate. [Image by S. Larson]

Most of us are familiar with the idea of a spectrum from light, produced by a prism or seen as a rainbow after a storm passes. The “electromagnetic spectrum” encompasses more than just those few colors we can see with our eyes; it also includes kinds of light our eyes can’t see, like gamma rays, ultra-violet light, radio waves, and microwaves.

The electromagnetic spectrum — light in all its varieties, illustrated in the many different ways that scientists describe the properties of a specific kind (or “color”) of light.

Gravitational waves also form a complete spectrum, independent of the electromagnetic spectrum. Since we can’t sense gravitational waves with our bodies, we have no sensory experience to describe them, and have no names for the different parts of the spectrum, so we simply identify the waves by their wavelength (the distance between peaks of the wave) or alternatively their frequency (how often a peak passes by you, if you just stand still and let the waves wash by). As astrophysicists, we have thought hard about the Universe, and can easily imagine Nature creating “high frequency waves” (short wavelength), where thousands of wave peaks pass by you every second, all the way down to “very low frequency waves” (long wavelength), where a wave peak may only pass by once every 30 million years.

There are many different sources of gravitational waves that we expect to see. These include (clockwise from upper left): Colliding neutron stars, merging supermassive black holes, white dwarf binaries, supernova explosions, the Big Bang, and the capture of compact stars by supermassive black holes.

Gravitational waves are generated by all kinds of different astrophysical systems, and as such the waves themselves are as varied as the phenomena that created them. From the simplest viewpoint, the astrophysical system making the waves defines how big they are. Systems that have large spatial extent and large masses tend to make much longer wavelength waves than small, super-compact systems. To develop some intuition about this, consider one of the bread-and-butter source for modern gravitational wave observatories, a binary black hole — two black holes, locked in a mutual gravitational dance, orbiting one another the way the planets orbit the Sun. When the black holes are far apart (larger orbit) they complete their orbits more slowly. As a result the gravitational waves emitted have low frequencies. Suppose the orbit shrinks. What happens? The black holes orbit each other more quickly, so the frequency of the gravitational waves increases.  What we see here is that the size of the orbit influences the size of the gravitational waves. Similar arguments can be made for other physical properties that influence the shape and form of the gravitational waves, which makes them useful for doing astronomy.

A simple example of how the properties of the source change the structure of the gravitational waves. If we can detect and measure the waves, that will tell us something about the sources. [Image by S. Larson]

Astronomy is a game of detection — as I like to say, “astronomy, unlike life, IS a spectator sport!”  Our job as astronomers is to watch the Cosmos and record what we see. We build instruments to help us accomplish that mission, instruments with cool names that you can use to impress your mother or a date: telescope, observatory, detector, bolometer, radiometer, and so on. If you want to build an instrument to look for some form of putative radiation, then you need to know how it interacts with your detector. The basic behaviour we exploit with light is that it bounces off of appropriately designed surfaces, whether the light is radio waves, optical light, or x-rays, it bounces off of surfaces, a fact that humans have exploited to gather light for more in depth experiments.

The effect of a PLUS (+) polarized gravitational wave on a grid of particles. Click to animate. [S. Larson]

The effect of a CROSS (X) polarized gravitational wave on a grid of particles. Click to animate. [S. Larson]

So what do gravitational waves do? Gravitational waves change the proper distance between particles. There is a very nice way to visualize this. Imagine a grid of particles — little marbles or marshmallows, arranged on this screen.  Now imagine a gravitational wave shooting straight through the screen: from your eyes, through the middle of your grid, and out the back.  What happens? The distances between all the marshmallows changes in a very specific way, illustrated in the animated images here.  At the start, in one direction all the marshmallows are stretched farther away from each other, while at the same time marshmallows in the perpendicular direction are pulled closer together. If you wait a while, the gravitational waves move a bit farther on in its cycle and it returns the grid to its original appearance. But it doesn’t stop there! On the “downside” of the wave cycle, the stretching and pulling swap directions!  This pattern repeats itself for as long as the gravitational wave is flying through the grid of marshmallows. There are two different “flavors” of gravitational waves (what astronomers call “polarizations”) that both distort our grid of marshmallows, just in different directions.  One flavor is called “PLUS” because the pattern of deformation looks like a + sign.  The other flavor is called “CROSS” because the pattern of deformation looks like a X sign.

So if we want to detect gravitational waves, we need a way to see this distortion. Like other radiation we encounter in astronomy (particularly electromagnetic radiation — light), gravitational waves carry energy which can affect other objects in the Cosmos. A “detector” is a device which extracts some of that energy to let us know the wave is passing by. But gravitational waves have one big problem —- they interact very WEAKLY with matter! That means it is hard to get them to deposit energy in a detector, and that means they are hard to detect.  This is a fact that Einstein appreciated full-well — he knew if we were ever going to see gravitational waves, technology was going to have to get better — much better. It would not be until the 1960s that any serious effort to detect gravitational waves would begin.

The gravitational wave interferometer concept is to measure the changing proper distance between points using lasers.

If you look at our particle grids, you see the effect is more pronounced for particles that are farther apart, so the bigger the instrument is the better.  Today, the premiere technology for gravitational wave searches is a classic physics instrument called a “laser interferometer” —  a device that times how long it takes a laser to fly in two different directions and very precisely compares the results to tell you if the two directions have different lengths! This is perfect for gravitational wave detection! Laser interferometers are easy to build in the lab (or at home, if you can acquire a few parts — try these instructions), but it is easier to detect gravitational waves over larger distances, because the stretching effect is larger.  So we’ve built HUGE laser interferometers, that stretch 4 kilometers (2.5 miles) from one end to the other! We’ve built two of them here in the United States — they are called LIGO.  The Pictionary style picture of LIGO is three mirrors — one at a corner, and two at the far ends of two long arms. Think of any three marshmallows in our grid — those are the functional locations of the mirrors in LIGO that the lasers are measuring distances between. When gravitational waves change the distances between the mirrors, we can measure those changes with our lasers.

(L) Aerial view of LIGO-Hanford Observatory [top] and in Google Maps [Bottom]. (R) Aerial view of LIGO-Livingston Observatory [top] and in Google Maps [Bottom].

Just as in traditional photon astronomy, we have to build different kinds of detectors to detect different kinds of gravitational waves. As a general rule, when the wavelengths of the gravitational waves we want to see are longer, we have to build bigger detectors. A laser interferometer could be built in space by imagining the mirror locations being free-flying spacecraft. Such a concept exists, known as LISA, where the spacecraft are 5 million kilometers apart (that’s 13 times the distance from the Earth to the Moon; it will take the lasers 16.6 seconds to fly between the spacecraft!).

Spacecraft concept for a gravitational wave detector in space [eLISA Consortium]

Eventually it becomes impractical to imagine building bigger and bigger interferometers. Does that mean there are parts of the gravitational wave spectrum that we simply will never be able to observe? No; astronomers are clever folks and have devised ways to use astrophysical systems as gravitational wave detectors by measuring the imprints the gravitational waves leave behind them.

There are two primary ways we monitor astrophysical systems to detect gravitational waves. One way is we watch pulsars — the spinning dead hulks of massive stars that died in supernova explosions, and now spin relentlessly, periodically flashing the Earth with a bright beam of electromagnetic radiation.  Many pulsars spin in a very stable manner, shining their spinning light on us at very precise intervals, which we can time and write down.  If a gravitational wave passes between us and a pulsar, then it stretches the space between us, and it takes the pulses longer or shorter amounts of time to reach us! We can detect gravitational waves by watching for changes in the arrival time of pulsar pulses!  This is called pulsar timing.

We can detect gravitational waves by monitoring the arrival times of pulses from many pulsars in different directions on the sky.

The second astrophysical method for gravitational wave detection is to look very closely at the Cosmic Microwave Background, and see if gravitational waves left very tiny distortions in the microwaves. There are many different distortions that can and do appear in the Microwave Background radiation, but the signature of gravitational waves is unlike just about every other kind of known distortion. They are of intense interest because if that signature could be detected, it would mean the gravitational waves came from the other side of the Curtain! This is exactly what the BICEP2 experiment was all about, and will be the theme of our discussion in the next and final post of this series.

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This is the second installment in a 3-part series about the March 2014 BICEP2 announcement about the detection of a putative signal from inflation in the Cosmic Microwave Background.  Part 1 can be found here.