The Cosmic Classroom on Boxing Day

by Shane L. Larson

The seas of the Cosmos are vast and deep. From our vantage point here on the shores of Earth, we have seen much that is beautiful, awe-inspiring, frightening, humbling, confusing, and enigmatic. The simple truth of astronomy is that it is a spectator sport. The only thing we can do, is watch the skies and wait for the next Big Thing to happen. We collect events, like bottle-caps or flowers, and add them to our collection. Each new addition is a mystery, a new piece of a puzzle that takes shape ever-so-slowly over time.

On 14 September 2015, the LIGO-Virgo collaboration announced that they had detected the first gravitational waves ever, and that those waves had been created by a pair of merging black holes far across the Cosmos.

Today, we have some more news: LIGO has detected the second gravitational wave event ever, and those waves were also created by a pair of merging black holes far across the Cosmos. But as is often the case with astronomy, we know what we’ve observed, but we still don’t know what it means.

The name of the event is GW151226 (the date of the event), but within the collaboration, we call it “The Boxing Day Event.” On 26 December 2015 (Boxing Day in Europe), the two LIGO detectors responded to the faint ripple of gravitational energy washing across the Earth, the signature of two black holes merging to form a new larger black hole.

LIGO detected the black holes merging at 3:53 UTC in the morning on Boxing Day (it was late in the evening on Christmas Day in the United States, 9:53pm Central Standard Time). The event happened 440 Megaparsecs away — almost 1.4 billion lightyears! As with GW150914 before it, this titanic merger of black holes happened long, long ago, in a galaxy far, far away. It happened before multi-cellular life had ever arisen on Earth, and for a billion years that information has been sailing through the void, until it washed across our shores.

Learning to do astronomy: We can’t do experiments in astronomy, not the way we all learned to do them in middle school. Experiment. Observe. Fail. Learn. Repeat.

The timeline of LIGO’s first Observing run (called O1). The first detection (GW150914) and the second detection (GW151226) are marked. There was also a candidate that looked like a gravitational wave, but was not strong enough for astronomers to confidently say a detection was made. [Image: LIGO Collaboration]

In astronomy, all we can do is observe, and hope that when we see something interesting happen, it happens again. Or something similar happens again, so we can start trying to make connections. Since the first LIGO detection, we have been patiently waiting for more detections. It could have been anything: merging neutron stars, a gamma-ray burst with an associated gravitational wave signal, a supernova explosion in the Milky Way, or perhaps other pair of black holes similar to GW150914.  As it turns out, it was the merger of black holes, but somewhat different than the one we observed before. Excellent! A chance to learn something new about the Cosmos!

When you look at the pile of gravitational wave events we’ve seen before (it’s a very small pile — there is only one event there, GW150914), we do the most obvious thing you can imagine: we start to compare them.

Strictly in terms numbers, you see that the Boxing Day black holes are less massive than the GW150914 black holes, by a substantial amount. This tells astronomers something very important: black holes can and do come in a variety of masses. That certainly did not have to be the case; there are many instances in the Cosmos where almost every example of an object is similar to every other object. People are all roughly the same height; grains of sand are almost all roughly the same size; yellow-green stars like the Sun (“Type G2” in astronomer speak) are all roughly the same mass. Though we did not expect it to be true, it could have been the case that all black holes were about the same mass; LIGO is happy to report that black holes come in many different masses.

But this, in and of itself, inspires new questions and new mysteries. The question for astronomers now is where do black holes of different sizes come from? The Boxing Day black holes are “normal size” — we think we understand how black holes in this mass range are made in supernovae explosions. The GW150914 black holes are a much grander mystery — they are larger (by a factor of 2 or 3) than any black holes that we expect to form from stars today. We have some interesting ideas about where they may come from, but those ideas can only be tested with more gravitational wave observations.

Comparison of the size of black holes observed by LIGO, as well as other candidates detected with conventional telescopes. (L) The physical size of the black holes overlaid on a map of the eastern United States. (R) The same image showing the masses on the vertical axis, and the black holes that combined to make larger black holes. [Image: LIGO Collaboration]

Gravitational wave astronomy: Every observation is different, because every source is different. Every set of waves is a unique fingerprint that encodes the physical properties of the objects that made the waves: their masses, how fast they are spinning, what kind of object they are,  how physically big they are, the distance to them, and so on. It’s like looking at the pictures in your high school yearbook — every picture is the same size, and is what we all call a “picture,” but each one uniquely identifies you or your friends. It encodes the color of your hair and eyes, whether you were smiling and wearing braces, the sweater you wore on picture day, and so on.

A typical visualization of a black hole binary. They emit no light, so there are no pictures! [Image: SXS Collaboration]

When we look at our data, we don’t usually show pictures. LIGO is not a telescope, so it does not generate images like we are used to seeing from the Hubble Space Telescope. Most “pictures” you see are simulations or realizations of the data. Instead, we show our data as graphs and plots that represent our data in ways that tell astronomers what LIGO is measuring and how that relates to quantities in physics we understand, like orbit size or energy.

A stereo equalizer display.

One common picture we use is something called a “spectragram” — you may have encountered something like a spectragram on a stereo. The equalizers on your stereo tell you how loud the music in terms of whether it is more treble sounding or bass sounding.  In LIGO, we look at our data by looking a spectragram and how it changes over time.  The fact that the Boxing Day black holes and GW150914 are different is immediately obvious when comparing their spectragrams — the fine details of the shape and duration is different in the two cases, but they have the same basic swoopy shape to them. Think about your high school yearbook: the pictures are all kind of the same, but different in the details.

The comparison of spectragrams from GW150914 (top) and the Boxing Day event (bottom). The blue swoop is the gravitational wave signal as it evolves in time (early in the event on the left, and the final merger in the tall swoop on the right). [Images: LIGO Collaboration]

The difference in the gravitational waves LIGO detected is even more obvious if you look at the waveforms themselves. Imagine you are standing on the beach watching waves roll in and crash on the sand. In between waves, the water is calm and relatively low, but at the moment the wave is washing ashore, the height of the water increases subtantially; if you happen to be standing in the wave as it washes by, you might not be able to stand up because the energy carried by the wave is enough to knock you over. In a very similar way, the waveforms illustrate the strength of the gravitational waves as they wash past the Earth. The size of the “up and down” in the waveforms we plot tells us how strong the waves are.  If you compare the Boxing Day black hole waveforms with the GW150914 waveforms, you see they both have a lot of up and down (a measure of strength — they were strong enough for LIGO to detect!), but their overall shape and duration is different.

Comparison of the “waveforms” for GW150914 (top) and the Boxing Day black holes (bottom). The signals are considerably different, and longer in the case of the Boxing Day event. [Images: LIGO Collaboration]

Gravitational wave astronomers at LIGO are most excited about the long chain of up-and-downs in the Boxing Day waveforms. This is a part of the black hole evolution we call the insprial — the long, slow time where the orbit is shrinking, the black holes drawing inexorably closer, creeping toward their ultimate fate: the coalesence into a new, single, spinning black hole. The longer the inspiral is visible to LIGO, the longer we can study the black holes with gravitational waves. Once they merge to form a new black hole, they very quickly become quiet, much like a bell fading into silence after being struck by a hammer. The inspiral, and the merger, are the only chance we have to take the measure of these tremendous astrophysical entities.

What now? LIGO has now made two detections of gravitational waves, both during our first observing run (what we call “O1”). In mid-January 2016, we turned LIGO off and have spent the ensuing months combing over the machine and addressing all the problems and difficulties we encountered in O1. In late summer 2016, we’ll start up for “O2.” We’ll turn up the lasers a little bit, and LIGO will be able to see a bit farther into the Cosmos. If our first stint as gravitational wave astronomers is any indication, we will likely see something new; we don’t know, all we can do is observe.  After a few months, we’ll shut down again, tune things up, think hard about how we are working with the machine, and in 2017 expect to come back online with everything at full design specifications.  We are like toddlers, learning to walk. We’ve taken our first few steps, and have discovered there is a tremendous world just waiting to be explored. We’re learning to keep our balance and do things right, but in the not too distant future will be confident and excited in our new found ability to observe and discover a Cosmos that up to now, has been completely hidden from us.  Carpe infinitum!

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Many of my colleagues in the LIGO Virgo Collaboration have also written excellent blog posts about the Boxing Day event, and the work we do to make gravitational wave astronomy a reality. You should visit their blogs!

• Christopher Berry [Birmingham] – The Boxing Day Event
• Mark Hannam [Cardiff] – Black holes rule!
• Matthew Pitkin [Glasgow] – Yes, we’ve found another one
• Amber Stuver [LIGO] – Merry Christmas, LIGO: Another Gravitational Wave!
• Daniel Williams [Glasgow] – The wave that stole Christmas

The Harmonies of Spacetime — GW150914

by Shane L. Larson

I have a good friend, Tyson, whom I don’t get to see nearly often enough. We are both privileged to be among the first generation of scientists who will know the Universe by observing the faint whisper of spacetime, bending under the influence of massive astrophysical systems. We are “gravitational wave astronomers.”

Picking crab with Tyson (far right) and family. [Image: Sabrina Savage]

A while back we were sitting on his back porch late into the evening, picking crab and talking about everything. It was the kind of common, easy conversation among friends that ranges over movies, politics, family, childhood memories, inside jokes, and so on. But at one point, the conversation drifted back to science and to the near future. Tyson said something that really just kind of made us all stop in shocked silence: “If we’re really going to detect gravitational waves in the next 3 or 4 years, they are already closer than Alpha Centauri and heading right for us.”

Whoa.

Little did we know then how prescient that observation was. We are both part of a project called LIGO — the Laser Interferometer Gravitational-wave Observatory. And this morning our collaboration made the big announcement.

Frame from a visualization of the binary black hole merger seen by LIGO [Visualization by “Simulating Extreme Spacetime” (SXS) Collaboration]

On 14 September 2015, the two LIGO observatories detected a very loud gravitational wave event. Our analysis since that day has told us that it was the merger of two black holes — one 29 times the mass of the Sun, the other 36 times the mass of the Sun. The two black holes merged, forming a new, bigger black hole 62 times the mass of the Sun. We named the event after the date: GW150914.

All of this happened about 400 Megaparsecs from Earth (1.3 billion lightyears). If you are adding up the numbers, you see that there are 3 solar masses missing. That is the equivalent mass that was radiating away from the system in the energy of the gravitational waves.

Make no doubt about it — this is one of the most momentous discoveries in the history of astronomy. It will be up to historians of science to place this within context, but I would rank it right up there with the discovery of the nature of the spiral nebulae and the discovery of the Cosmic Microwave Background.

There are many important and stunning parts of this story. Let’s me tell you just a small slice of how we got to today.

LIGO: LIGO is two gravitational wave observatories that work together as a single experiment. The are located 3002 kilometers apart, with one in Hanford, Washington and the other in Livingston, Louisiana. They are enormous, 4 kilometers to a side — so large, they can be seen in satellite photos.

(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].

The observatories are “laser interferometers” — laser light is injected into the the detector, and split so it flies up and down each of the two arms. When the light returns back to the splitter, it is recombined. When you combine laser light in this way, it can be combined such that the beams cancel out (making what we call a “dark fringe”) or they combine to make a bright spot (making what we call a “bright fringe”); in between combinations have a full range between bright and dark. We sit on a “dark fringe.”

Schematic of the LIGO interferometers, showing the basic layout of the lasers and optics locations. The lasers travel up and down the two 4 kilometer long arms, and are recombined and detected at the photodetector. [Image: S. Larson & LIGO Collaboration]

When a gravitational wave hits LIGO, it stretches and compresses the arms. The result is that it changes how long it takes the lasers to travel from the splitter to the end mirror and back. If that happens, when the lasers are recombined the brightness of the fringe changes.

What Happened? Both the LIGO detectors run more or less continuously, and we get our primary science data when they are on at the same time. In the early morning hours of 14 September 2015, at 4:50:45am Central Daylight Time, a signal was detected in the Livingston detector. 7 milliseconds later, a signal was also detected in the Hanford detector. These detections are sensed automatically by sophisticated software that looks for things that are “out of the ordinary.” Notable events are logged, and then humans can take a look at them. In this case, we knew almost immediately it was significant because it was in BOTH detectors, and it was a strong signal (we use words like “loud” and “bright” to mean strong, but we don’t really “hear” or “see” the signals in the usual sense; these are descriptive adjectives that are helpful because of the analogy they make with our normal senses).

Spectrograms of the event at Hanford and Livingston. The darker areas are what a “typical” spectrogram might look like; the bright swoops are the (very noticeable) signal! [Image: LIGO Collaboration]

One of the easiest ways to see the signal is in a diagram called a “spectrogram” which shows how the signal in the detector changes in time. Once we had the first spectrograms, the emails began to fly.

Finding Out: We all get LOTS of email, so it took a while before everyone in the collaboration actually realized what was going on. I didn’t hear until the night of September 15. AT 9:35pm CST I got an email from Vicky Kalogera, the leader of our group, that said “have you caught any of what’s going on within LIGO?” We had a round of email with unbearably long delays between them, but by 11:35pm, I had our initial understanding/guesses in my hands. That was enough to do what we all do in science — we make some calculations and extrapolations to understand what we have seen, and to plan what we should do next. We want to figure out what the new result might mean! Here’s the page out of my Moleskine, where I started to compute what a detector in space, like LISA, might be able to see from a source like this.

My journal page from the hour after I first found out about the event. [Image: S. Larson]

The Importance: There are all kinds of reasons why this discovery is important. If you take your favorite gravitational physicist out for pizza, they’ll talk your ear off for hours about exactly why this is important. But let me tell you the two I think the most about.

First, this is the first direct detection of gravitational waves. It is the first time we have built an experiment (LIGO) and that experiment has responded because a gravitational wave passed through it. This is the beginning of gravitational wave astronomy — the study of the Cosmos using gravity, not light.

Second, this is the first time that we have directly detected black holes, not observed their effects on other objects in the Universe (stars or gas).

The Astrophysics: The two black holes, caught in a mutual gravitational embrace, had spent perhaps a million years slowing sliding ever closer together, a long and lonely inspiral that ended with their merger into a single, bigger black hole. This is the first time we know conclusively of the existence of black holes that are tens of solar masses in size. Such black holes have been predicted in theoretical calculations, but never seen in the Cosmos before.

A more technical simulation of the binary black hole merger; gravitational physicsists and astronomers will be comparing the data to their simulations to examine how well we understand “real” black holes. [Image: SXS Collaboration]

Our next big question is “how often does this happen?” If it happens a lot, that is a potential clue pointing to where such black holes come from. If it is a rare event, that also tells us something. So now, we wait — this is just the beginning of LIGO observations, and after a few years of listening for more, we’ll know how common these are.

The People: Science is a way of thinking about the Universe, and so often when we talk about science we talk about Nature — all the wonder, all the mystery, the rules of the Cosmos. But science is a uniquely human endeavour and every momentous discovery is the culmination of countless hours of sweat, uncountable failures, and equally uncountable tiny moments of success that culminate at a profound moment of knowing something new. It would not be possible without the dedication of enormous numbers of people. The world gravitational wave community has been working toward this day for decades. More than 1000 authors appear on the discovery paper, and there are thousands of others who have worked and are working on the project, who are not in that list of authors. It has been a heroic effort on the part of physicists, astronomers, optical engineers, data and computer scientists, technical and support staff, professors and students.

Just some of the thousands of people who have made LIGO a reality and the detection of GW150914 possible. [Images from the LIGO Collaboration]

Teasing out the secrets of Nature is hard. Since before recorded history began, our distant ancestors  have plumbed the mysteries of the Cosmos using tools that Nature gave us — our five senses. Astronomer Edwin Hubble once opined “Equipped with his five senses, man [sic] explores the universe around him and calls the adventure Science.” (Harper’s Magazine 158: 737 [May, 1929]).

Today, we add a new sense to our quest to understand the Cosmos. TODAY the Era of Gravitational Wave Astronomy opens. Within the next few years, we will no longer live in a world where our view of the Cosmos is limited to what light alone can tell us. TODAY, we see the Cosmos anew, with senses attuned to the fabric of space and time itself!

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I’ve written about gravitational waves here at WriteScience before. In many of those I’ve explored what the physical description and meaning of gravitational waves are, and what the endeavour to detect them is all about. If you’d like to take a stroll down memory lane, here are links to those old posts:

• An early post about what it takes to build a machine like LIGO. “Knowing something about everything”
• When the BICEP2 results were first announced, I wrote a three part series about what they were looking for, what gravitational waves are all about, and how scientists were going to move forward after their announcement.
  • “Behind the Curtains of the Cosmos 1: Inflation and the Microwave Background”
  • “Behind the Curtains of the Cosmos 2: Gravitational Waves”
  • “Behind the Curtains of the Cosmos 3: Keys to the Cosmos”
• I wrote a long, 13 part series for the GR Centennial year. As part of that series, I wrote two posts about gravitational waves. At the bottom of each of these posts are links to little 3 minute YouTube videos I did to accompany them.
  • “Gravity 11: Ripples in Spacetime”
  • “Gravity 12: Listening for the Whispers of Gravity”
• I work on another gravitational wave experiment called LISA, and we just launched a spacecraft to test our ideas called LISA Pathfinder, so I wrote a bit about that too. “This is just the beginning”

Many of my colleagues in LIGO are also blogging about this momentous discovery. I will add their links here as they appear, so you can read their accounts as well:

• Christopher Berry (Birmingham)
• Rebecca Douglas (Glasgow)
• Sean Leavey (Glasgow)
• Matt Pitkin (Glasgow)
• Brynley Pearlstone (Glasgow)
• Amber Stuver (LIGO)
• Daniel Williams (Glasgow)
• Roy Williams (Caltech LIGO)
• Andrew Williamson (Cardiff)