Tag Archives: gravitational waves

New Astronomy at the New Year (GW170104)

by Shane L. Larson

Newton’s portrait.

January 4 holds a special place in the hearts of scientists — it is Isaac Newton’s birthday (*). Newton stood at the crossroads that led to modern science, and astronomy in particular. He was the first person to build a workable reflecting telescope, a design that now bears his name and for the past 4 centuries has been the dominant type of telescope used by amateurs and professionals alike. Newtonian telescopes have revealed much about the Cosmos to our wondering minds. Newton was also responsible for the first formulation of a physical law that describes the working of gravity, called the Universal Law of Gravitation. Today we use the Universal Law to launch satellites, send astronauts into orbit, convert the force of your feet on the bathroom scale into your “weight“, and a thousand other applications.  There is much to celebrate every January 4.

(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].

But on January 4, 2017 the Cosmos celebrated with us, singing in the faint whispers of gravity itself. On January 4, the signal of two black holes catastrophically merging to form a new bigger black hole washed quietly across the shores of Earth, carried on undulating vibrations of space and time. You were very likely unaware of this cosmic event — it happened at 4:11:58.6 am in Chicago. It was a Wednesday morning, and I imagine most people were blissfully asleep. But two of the grandest pieces of experimental apparatus ever built by humans were paying attention – the twin LIGO detectors in the United States.  For only the third time in history, a gravitational wave signal from the deep Cosmos was detected here on Earth.

The signal was the signature of two black holes (a “black hole binary,” in the lingo of the astrophysicists) merging to form a new, bigger black hole. The black holes, by definition, emit no light themselves. However, astronomers know that black holes can often be surrounded by swaths of interstellar gas. The intense gravity and motion of the black holes can stir the gas into a violent froth that can emit light. At the time of the event, the LIGO team sent out alerts to astronomers around the world, who turned their telescopes skyward looking for a tell-tale signature of light bursting from the energized gas. Our best estimate of the location of the event was canvased by 30 groups, in many different kinds of light ranging from radio waves, to optical light, to gamma rays. No tell-tale emissions of light were seen. The only way we were aware of this event is from the LIGO detectors themselves.

An artist’s impression of two black holes insprialling, near merger. [Image by Aurore Simonnet, SSU E/PO]

The Gravitational Wave Signal. We call the event GW170104, named for the date it was detected. The signal from the black holes registered first in the LIGO detector outside Hanford, Washington, and 3 milliseconds later registered at the LIGO detector outside Livingston, Louisiana. All told, it only lasted about 0.3 seconds. The signal exhibited the characteristic chirp shape expected of compact binaries that spiral together and merge — a long sequences of wave peaks that slowly grow in strength and get closer and closer together as the black holes spiral together.

Comparison of the chirp waveforms from the first 3 detected gravitational wave events. LVT151012 was a very quiet event that was not strong enough for LIGO scientists to be confident it was a pair of black holes. [Image: LIGO Collaboration]

During the early inspiral phase of GW170104, where the black holes are independent and distinct, the heavier black hole of the pair was 31 times the mass of the Sun, and the smaller black hole was 19 times the mass of the Sun. Ultimately, they reached a minimum stable distance (in astrophysics lingo: the “innermost stable circular orbit“) and plunged together to form a new bigger black hole. When that plunge happened, the gravitational wave signal peaked in strength, and then rang down and faded to nothing as the black hole pulled itself into the stable shape of single, isolated black hole. For GW170104, this final black hole was 49 times the mass of the Sun.

All of this happened 3 billion lightyears away, twice as far as the most distant LIGO detection to date. Perhaps these numbers impress you (they should) — they tell the story  of events that happened billions of years ago and in a place in the Cosmos that neither you, nor I, nor our descendants will ever visit. We add them today to a very short list of astronomical knowledge: the Gravitational Wave Event Catalogue, the complete list of gravitational wave signals ever detected by human beings. There are only three.

The current Gravitational Wave Catalogue, of all known events [click to make larger].

Take a close look at the list. There are interesting similarities and interesting differences between the three events. They are all black hole binaries. They are all at least a billion light years away from Earth. Some of the black holes are heavier than 20 times the mass of the Sun, and some are lighter than 20 times the mass of the Sun. Astronomers use those comparisons to understand what the Universe does to make black holes and how often.

This is the most important thing about GW170104 — it is a small but significant expansion to this very new, and currently, very limited body of knowledge we have about the Cosmos. These three events are completely changing the way we think about black holes in the Cosmos, forcing us to rethink long held prejudices we have about their masses and origins. We shouldn’t feel bad about that — evolving our knowledge is the purpose of science. LIGO is helping us do exactly what we wanted it to do: it is helping us learn.

What do we know? There are many things we are trying to learn from the meager data contained in these three signals. The new signal from GW170104 in particular has tantalizing evidence for the spin of the black holes, and some neat assessments of how close these astrophysical black holes are to what is predicted by general relativity. But I think the most important thing about the event from the perspective of astronomy is this: the black holes are, once again, heavy. GW170104 is the second most massive stellar mass binary black hole ever observed (GW150904 was the heaviest).

The masses of known black holes. The purple entries are observed by x-ray telescopes, and represent what we knew about the size of black holes before LIGO started making detections. [Image: LIGO Collaboration]

With the first two events we had one pair of heavy black holes (GW150914), and one pair of lighter black holes (GW151226). There is a great mystery hiding there: where do the heavy black holes come from, and how many are there in the Cosmos? Perhaps they are just a fluke, a random creation of Nature that is possibly unique in the Cosmos. But the detection of GW170104 suggests that this is not the case; we’ve once again detected heavy black holes. The race is on to decide how the Cosmos makes them. The answers to those questions are encoded in the properties of the black holes themselves. How many are there? Are they spinning or not? Are they spinning the same direction as one another? How do their masses compare to one another? GW170104 is another piece of the puzzle, and future detections will help solidify what we know.

How can you help? If you’d like to help the LIGO project out, let me direct your attention to one of our Citizen Science projects: GravitySpy. Your brain is capable of doing remarkable things that are difficult to teach a computer. One of those things is recognizing patterns in images. The LIGO detectors are among the most sensitive scientific instruments ever built; they are making measurements at the limit of our capabilities, and there are all kinds of random signals that show up in one detector or the other — we call them glitches.  It is very hard to teach a computer to tell the difference between glitches and interesting astrophysical events, so we have citizens just like you look at glitches and identify them, then we use that information to train the computer. So far citizens like you have helped LIGO classify more than two million glitches, and they put more on the pile every day.

If you’d like to help out too, head over to http://gravityspy.org/ and try it out; you can do it in your web-browser, or on your phone while you’re sitting on the train to work. We have citizens from kids to retirees helping us out. If gravitational waves aren’t your thing, there are more than 50 other projects in science, arts, history and more at http://zooniverse.org/ you can try out!

A representation of the GW170104 signal, from the scientific paper. These are the kinds of images citizens can classify easily, whereas computers sometimes have trouble. [Image: LIGO Collaboration]

PS: For all of you super-nerds out there, let me point something out if you haven’t already noticed. Suppose you were to parse the name of the signal in the following way: 1701 04. Look familiar? The 4th incarnation of 1701; for the cognoscenti, this event shares the designation of the Enterprise-D. 🙂  Until next time, my friends. Live long, and prosper.

(*) When Newton was born, England had not yet switched to the new Gregorian Calendar, which we use today. They were still using the older Julian Calendar, by which Newton was born on December 25; when converted Newton’s birthday falls on January 4 on the Gregorian Calendar.

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You can read about the previous LIGO detections in my previous posts here:

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Many of my colleagues in the LIGO Virgo Collaboration have also written excellent blog posts about the GW170104 event, and the work we do to make gravitational wave astronomy a reality. You should visit their blogs!

My Brain is Melting — GW150914 (Part 2)

by Shane L. Larson

It has been just more than a week since we told the world about our great discovery. It was a cold winter morning in Washington DC, the temperature hovering just below freezing. In a room at the National Press Club, the world press had gathered, and at the behest of NSF Director, Frances Córdova, LIGO Executive Director, Dave Reitze, took to the podium.

“Ladies and gentlemen. We have detected gravitational waves. We did it!” Mic drop. (Well, he should have; in the movie dramatization, he will. You can watch the moment here on YouTube, or the full press conference.)

Dave Reitze makes the announcement to the world that LIGO had detected gravitational waves.

Dave Reitze makes the announcement to the world that LIGO had detected gravitational waves. “We did it!”

So began a ninety minute press conference delivering the news of the first gravitational wave detection to the world. In the days that followed, social media and press outlets exploded in a veritable tidal wave of excitement and awestruck wonder. On twitter, the hashtags #gravitationalwaves, #LIGO, and #EinsteinWasRight have accumulated more than 70 million tweets in just one week.

Everyone has the same sense that we scientists have — this is a doorway, now open, to a Universe we have only imagined. Beyond the threshold are certainly things we have predicted and speculated about, but also many wonders yet to be found or understood.

We have done our best to explain what we are doing with LIGO, and how it works. We have made a Herculean effort to describe the astrophysical significance of the discovery. We have tried mightily to explain what Einstein’s ideas about spacetime and gravity are all about.

But this is hard stuff to think about, it is hard stuff to understand, and it is hard stuff to explain. It is well outside our normal everyday experience, so it is easy to feel like your brain is melting.

brainMelt

You shouldn’t worry that these things are hard to understand. It took physicists 41 years to even decide gravitational waves were real, and then another 59 years to build an experiment capable of detecting them. There is no doubt these are hard, brain melting matters. But the beauty of the discovery of gravitational waves is that this can be understood!

A large number of my colleagues in LIGO (and myself) have spent the last week collecting and responding to questions emailed to us, asked in public forums, and delivered on social media (if you have more questions, ask in the comments below, or please email question@ligo.org). All of them are thoughtful, genuine, and demonstrate a pleasing curiosity and wonder about the nature and workings of the Cosmos. I am constantly amazed by the questions people ask.

Here are a few of the more common brain-melters we have been asked, and some meager attempt to answer them. The questions are marked in red, to make them easy to find. Some responses are more complicated than others, and you may or may not want to read them all. They are here to help stem the meltdown, if you find your brain is still reeling. 🙂

What does this mean for ordinary folks?  Far and away, this is the most common question I’ve been asked, particularly from the press. What does this mean for the world? How will this help my golf game?

LIGO’s discovery is what we call “fundamental physics.” It is a discovery that tells us something about how the Universe works and why it behaves the way it does. Figuring out how to use knowledge like that to make your life better or turning it into a gadget that’s useful in your kitchen or garage takes time — we’ve only just now made the first detection of gravitational waves, and are trying to wrap our brains around it.  Scientists and engineers will have to think a long time, maybe decades, before they can make this knowledge “useful for everyday life.”  That’s always how it works with scientific discoveries. How it will impact our everyday lives is not for us to know — that is for the future.

In the modern era, many of us navigate using GPS technology, built directly into our smartphones.

In the modern era, many of us navigate using GPS technology, built directly into our smartphones.

That is not to say that there isn’t some amazing future application. We only have to look at the history of general relativity itself to know the truth of this. Einstein worked out general relativity between 1905 and 1915. This was an age before cars and electricity were mainstays in everyday life. Yet Einstein had the where-with-all to understand that gravity could be thought of as the warpage of spacetime, and that one consequence of that warpage is clocks tick at different speeds depending on how strong the gravity is.

Did you know that little, obscure fact of general relativity is used by you and most other people every, single day? It is an essential part of how the GPS in your phone works. It took nearly a hundred years for the “fundamental physics” we call general relativity to be turned into an essential piece of technology that now gets millions of us from place to place in the world every day. Without GPS and general relativity, you’d still be navigating using paper maps. Einstein had to rely on his neighbor to tell him where to find a pub; you have a smartphone.

Is it really that important? I think this is one of the most important discoveries in astronomy in the last 100 years. It is as important as discovering that there are other galaxies beyond the Milky Way, it is as important as discovering the expansion of the Universe, it is as important as discovering the Cosmic Microwave Background.

The reason I think this is just about everything you’ve ever heard about the Universe, or seen a picture of, has been discovered using LIGHT. Telescopes are just instruments that do what your eyes do (collect light), though telescopes collect much more light than your eye or collect light that your eye cannot see (like infrared or ultraviolet light).

Gravitational waves are different — none of us have a “gravitational wave detector” as part of our bodies. Gravitational waves are something that we predicted should exist, and we built an experiment that showed us our ideas were right.  The beginning of gravitational wave observations will change how we see the Universe in ways that we cannot yet imagine.

Dr. France Córdova, Director of the National Science Foundation.

Dr. France Córdova, Director of the National Science Foundation.

As a scientist and a teacher, I can appreciate the importance and utility of the collection of knowledge. But LIGO’s discovery goes far beyond the mere acquisition of yet another fact to post on Wikipedia. What the scientists and engineers working on LIGO have done was often regarded as impossible to do. But as Dr. Córdova intoned at the LIGO press conference, we took a big risk. Through a judicious application of sweat, brains, and stubbornness, we endured a decades long effort to design a machine to do the impossible. We encountered countless challenges and obstacles, and diligently overcame every single one of them to arrive at this day. That should make every person sit up a little bit straighter and prouder. That should make every single person aware that whatever challenges or problems we face on our small world, we have the means to overcome them, if we have the will to commit our time and brains and resources to them.

The black hole collision LIGO observed was more than 50 times brighter than all the stars in the Universe. How can that be?  The comparison is “the gravitational energy released by the merger is about 50 times the energy released by all the stars in the Universe during the same time.”  This is an example of a “Fermi problem” which astrophysicists use all the time to figure out if our numbers are right when we are doing complex calculations.

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

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, bathed in the light of the stars of the Milky Way. [Image: Shane L. Larson]

Astrophysicists measure brightness in watts, just like you are used to expressing the brightness of a light bulb in watts — the “wattage” tells you how much energy is released in a fixed amount of time. The higher the wattage, the more energy is released in a given moment, so the brighter the star (or bulb). Astronomers call this the “luminosity.” We can estimate the luminosity of all the stars in the Universe and compare it to what LIGO measured from the black holes. [[I’m going to use some scientific notation here to write some mind-bogglingly big numbers; a number like 106 means a 1 followed by 6 zeroes: 106 = 1,000,000. ]]

If you express the luminosity of the black holes (3 solar masses in just about 20 milliseconds) as a “wattage,” the brightness is about 3.6 x 1049 watts, or about 1023 times brighter than the Sun.

The Hubble Extreme Deep Field (XDF).

The Hubble Extreme Deep Field (XDF). We can use images like this to estimate the total number of stars in the Cosmos.

Now suppose we make the assumption that all the stars in the Universe are just like the Sun. This isn’t true, of course — some are brighter, some are dimmer, but on the average this is a good starting guess. There are about 100 billion stars in a galaxy like the Milky Way, and if you look at an image like the Hubble Extreme Deep Field, there are on order 100 billion galaxies in the Universe. So there are 100 billion x 100 billion = 1022 stars in the Universe. If each one of them is the brightness of the Sun, the total brightness of stars in the Universe is 1022 times the brightness of the Sun.

But we said the black hole merger seen by LIGO was 1023 times brighter than the Sun, so: 1023/1022 = 10. The black hole merger was 10x brighter than all the stars in the Cosmos. With a careful calculation, we could get the 50 number you hear from LIGO, but 10 is pretty close. This is the nature of Fermi problems — they don’t give you the exact number, but they quickly get you close to the exact number so you can understand the Universe.

What do you mean “spacetime is stretching LIGO’s arms?” What is spacetime? Spacetime is the substrate, the matrix upon which everything in the Universe is built — as we like to say, spacetime is the “fabric of the Cosmos.” It is, of course, easy to say that, but difficult to wrap your brain around. We’re used to not thinking about space at all; it is the nothing between everything. But it is exactly that nothing of which we speak — if we were not here, if nothing were here, there is still space.

Imagine a gravitational wave shooting through LIGO, directly out of the screen at you. (A) When there are no waves, the arms are at their fixed lengths. (B) When the wave first hits LIGO, the spacetime in one arm stretches and in the other arm compresses. This changes how long it takes light to go from the corner to the end of the arms and back again. (C) As the wave passes by, the arms change back and forth between stretching and compressing.

Imagine a gravitational wave shooting through LIGO, directly out of the screen at you. (A) When there are no waves, the arms are at their fixed lengths. (B) When the wave first hits LIGO, the spacetime in one arm stretches and in the other arm compresses. This changes how long it takes light to go from the corner to the end of the arms and back again. (C) As the wave passes by, the arms change back and forth between stretching and compressing.

How do you measure the length of something in space? Most of the time we use a ruler or a tape measure. You lay it down along the thing you are interested in, like LIGO’s arms, and you see how many it takes. Imagine that you put down kilometer markers along LIGOs arms, just like you see on the highway — one at 0km, 1km, 2km, 3km and 4km. When spacetime between the ends of LIGO changes, the entire arm stretches. You still think the arm is 4 kilometers long, because the markers are still evenly spaced (the spacing is just larger than it was before, though you may not be aware of it). We need a way to measure the stretching without relying on the kilometer markers.

Visualization of LIGO interferometry. (A) When no gravitational wave is present, the laser timing is set up to make a "dark fringe" at the output [square panel on the right]. (B) At the output, the light is like waves canceling each other out. (C) When a wave stretches or compresses the arms, it changes how the light is added together at the output. [Frames from video by Caltech/LIGO]

Visualization of LIGO interferometry. (A) When no gravitational wave is present, the laser timing is set up to make a “dark fringe” at the output [square panel on the right]. (B) At the output, the light is like waves canceling each other out. (C) When a wave stretches or compresses the arms, it changes how the light is added together at the output. [Frames from video by Caltech/LIGO]

A reliable way to measure the distance in a space that is changing and stretching, is to time a beam of light as it makes its way through the space you are trying to monitor. In LIGO, we use laser light. Imagine two photons, injected into LIGO at the corner, with a photon traveling down each of the two arms (in terms of the the optics, there is an element at the corner called a “beamsplitter” that splits a laser beam and sends part of it down each of the two arms). When there are no gravitational waves distorting LIGO, the two photons arrive back at the beam splitter and are combined to make an interference pattern, which is a brightness pattern that depends on how the photons arrive together. We set it up so the pattern is a “dark fringe” — the two photons cancel each other out (what physicists call “destructive interference”).

A simple demonstration of how sensitive interference can be to small shifts in space. These interference patterns are made with regular circles (as opposed to moving waves) and create Moiré patterns. The difference between the left and right image is a shift of only 0.05 inches, but the pattern difference is easy to see, indicating the shift happened.

A simple demonstration of how sensitive interference can be to small shifts in space. These interference patterns are made with overlapping patterns of regular circles (as opposed to moving waves) and create Moiré patterns. Here the horizontal dark region in the left image is analogous to LIGO’s “dark fringe.” The difference between the left and right image is a shift of only 0.05 inches, but the pattern difference is easy to see. What was a “dark fringe” now has a sliver of white, indicating the shift happened. [Image: S. Larson]

When a gravitational wave goes through LIGO it stretches the spacetime in one arm, and compresses the spacetime in the other arm. That means the photon in the stretched arm arrives back at the beam splitter LATE (it had farther to travel) and the photon in the compressed arm arrives at the beam splitter EARLY (it had less distance to travel). The result is the brightness pattern CHANGES. The changing pattern of brightness is exactly in tandem with the passing gravitational wave, telling us about the shape of the wave as it passes by.

They said the stretching that LIGO measured was a fraction of the width of a proton. But I remember from Chemistry that atoms are always moving, so how can you make such a precise measurement? Remember that LIGO is not measuring the distance shift in single atoms — it is watching the mirror, which is comprised of many atoms, each of which is moving exactly as you remember from Chemistry.

Everyone on a boat is doing their own thing, but they are all moving together as the boat moves on the waves.

Everyone on a boat is doing their own thing, but they are all moving together as the boat moves on the waves.

When we make our measurements, we are looking at the behaviour of many, many photons that have travelled down the arm together, hit the mirrors, and made the return journey. Sure — some of the atoms are going one way, and some are going some other way, but overall they are all moving together, going wherever gravity is pushing the center of mass of the mirror. When we read out the light, we are looking at all of those photons that hit the mirror at the same time and using that information to determine where the mirror is.

It’s a bit like having a big gravitational wave discovery party on a boat. If you are on the shore, watching all the physicists and engineers having a good time, you see they are all going every which way on the deck. But they are all on the boat, which moves them all together in response to the underlying waves of the sea.

Will this help with time travel? quantum gravity?  Einstein’s great discovery with general relativity was the idea that gravity can be described as the interaction of mass with the shape and warpage of spacetime. The unification of space and time into a single entity — spacetime — is a huge conceptual leap that is sometimes hard to come to grips with because of the way we think about space and time.

In our everyday lives we measure space with rulers and car odometers, and we measure time with wristwatches and calendars. If they are the same thing, why don’t we measure them the same way? The idea that the two are connected takes some getting used to, though as I like to remind people: when you go somewhere, you are usually comfortable saying your destination is “25 minutes” away or saying “20 miles” away!

You think about travel as travelling through space or travelling through time without even noticing! You are used to being a "spacetime traveller."

You think about travel as travelling through space or travelling through time without even noticing! You are used to being a “spacetime traveller.”

Since gravitational waves are moving ripples, propagating warpage in spacetime, it is natural to ask: can this discovery can help us understand space and time? Can we understand “time travel” and “warp drive?

Time itself, despite being part of general relativity, is still a great mystery to us. But what we often forget is that we are time travelers. Even as you are reading this, you are traveling through time from this moment, heading toward next Tuesday. It is not possible, so far as we know, to go backward toward last Friday, and that is a great mystery. It appears to be true based on experimental evidence, but we don’t yet understand how the laws of Nature — general relativity — tell us that. So in as much as gravitational waves will dramatically improve our understanding of how spacetime works and behaves, that deeper understanding could lead us down a path of thinking that will ultimately give us more insight into the mystery of time.

In a similar way, the LIGO detection does not address the enduring questions about the microscopic, quantum nature of gravity. The gravitational waves are a “big world” phenomenon, created by strongly gravitating astrophysical objects. But based on our experience with other quantum physics, we expect that there will be a clear (though not now obvious) connection between quantum gravity and general relativity. The more we expand our understanding of general relativity, it becomes more likely we will stumble on the deep connections that would lead to ultimately understanding quantum gravity.

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I encourage you to continue asking your favorite physicist your questions and share what you learn. Also, send questions to: question@ligo.org

Remember: no question is a dumb question. If you are wondering something, I’ll bet you a jelly donut someone else has the exact same question!

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]

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) Collaboratoin]

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].

(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. [Image: S. Larson & LIGO Collaboration]

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]

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]

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]

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]

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:

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:

 

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.

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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.

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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.

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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.

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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.

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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.

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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.

awesomeLISAThe 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Gravity 11: Ripples in Spacetime

by Shane L. Larson

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

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

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

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

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

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

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

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

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

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

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

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

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

The International Prototype Kilogram (IPK).

The International Prototype Kilogram (IPK).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Behind the Curtains of the Cosmos 3: Keys to the Cosmos

by Shane L. Larson

The fact that the curtain of the Cosmic Microwave Background exists is a huge boon to astronomers, giving us confidence in our understanding of Big Bang cosmology. But is that all? We have all this microwave light coming at us from every direction in the Cosmos — what can we do with it?  Is there anything else we can learn?

As it turns out, there is more to the light than meets the eye. Astronomers excel at taking information from the Cosmos and digging deeper, to tease out secrets and information that at first glance are not easy to see.  If you look closely at the Cosmic Microwave Background you discover some Very Important Things. One thing is that it is not absolutely, perfectly smooth. Imagine picking up a ball off a pool table — it looks and feels very smooth; that is analogous to the uniform temperature of the Cosmic Microwave Background looking the same no matter where we look on the sky.  But if you take that same pool ball and look at the surface closely with a magnifying glass, you find that it has some scratches and roughness to it — they are so small as to not be detectable by your fingertips, nor to your naked eye.

(L) A billiard ball looks and feels smooth to your eye and your finger tips. (R) But if you can look at it very carefully under a microscope, you find there are minuscule imperfections that are only detectable with excellent technology.

(L) A billiard ball looks and feels smooth to your eye and your finger tips. (R) But if you can look at it very carefully under a microscope, you find there are minuscule imperfections that are only detectable with excellent technology.

The Cosmic Microwave Background is very similar. If we use a highly accurate microwave telescope to look, we find that there is a small scale  “roughness” to the sky; astronomers think this roughness is an indicator of clumpiness at the time atoms formed, ultimately leading to what you and I see today — galaxies and clusters of galaxies in the Universe. This roughness was first detected by the COBE satellite in 1992. It was later mapped at even finer precision by the WMAP mission, and by the Planck mission.The spots on this map can be thought of as hot and cold spots on the sky. The red spots are the hottest, at only about 2.7 Kelvin (1 Kelvin is 1 degree Celsius) above absolute zero. The blue spots are the coolest, about 0.00001 Kelvin cooler than 2.7 Kelvin — a difference almost so small as to be undetectable except with the most exquisite astronomical instruments! Before the microwave background broke free from matter, the small variations in the density of matter were spread across the Cosmos. When the matter condensed to form neutral atoms, these variations imprinted themselves in the Cosmic Microwave Background. The spotted map of the Cosmos seen by COBE, WMAP and Planck is a message from the other side of the curtain!

The Planck All-Sky Map of the Cosmic Microwave Background variations. [Planck Collaboration]

The Planck All-Sky Map of the Cosmic Microwave Background variations. [Planck Collaboration]

Is there any other property of the Cosmic Microwave Background that might be measured? Is there any other physical imprint that might tell us something about the early days of the Cosmos?  Of course there is!  It is called polarization. Light is known to scientists by its proper name, “electromagnetic radiation.” As the name suggests, it has an electric part and a magnetic part, called “fields.” The way we visualize light is as a wave of electric fields and magnetic fields travelling together, at right angles to each other. For any light you receive, the direction the electric field is waving defines the polarization of the wave.

Light is a propagating "electromagnetic field," a waving electric field travelling together with a waving magnetic field. The direction the electric field waves defines the polarization of the light.

Light is a propagating “electromagnetic field,” a waving electric field travelling together with a waving magnetic field. The direction the electric field waves defines the polarization of the light.

Polarization encodes many kinds of information, related to how light and matter interact with one another. The most common way you encounter this every day is with sunglasses. Reflected light is polarized, so we build sunglasses that block polarized light, cutting down glare off of reflected surfaces. You might have noticed that the display of your smartphone is polarized too — you can’t see it in certain orientations through your polarized sunglasses.

Light reflected off of surfaces is polarized, like these mud flats. (L) The waves and the sand look bright when polarized light is seen. (R) When polarized light is blocked, as with polarized sunglasses, the appearance changes dramatically. [Image from Wikimedia Commons]

Light reflected off of surfaces is polarized, like these mud flats. (L) The waves and the sand look bright when polarized light is seen. (R) When polarized light is blocked, as with polarized sunglasses, the appearance changes dramatically. [Image from Wikimedia Commons]

Light from the Cosmic Microwave Background is expected to be polarized, imprinted with patterns because during decoupling (the time when electrons were binding to nuclei to form atoms) the light bounced off of the free electrons (in physics-speak: it “scattered”) giving it a definite polarization. Astronomers call these polarization patterns “E-modes” and they are recognizable because they make symmetric patterns in the sky — the polarization pattern looks exactly like itself if you look at a reflection of the pattern in a mirror. This kind of pattern was discovered by the DASI experiment in 2002.

Polarization of the Cosmic Microwave Background detected in 2002 by the DASI Collaboration. These are "E-mode" polarizations, caused by scattering.

Polarization of the Cosmic Microwave Background detected in 2002 by the DASI Collaboration. These are “E-mode” polarizations, caused by scattering.

So what does this all have to do with inflation? Inflation happened shortly after the Big Bang, before anything that you and I might recognize as a particle had formed. The forces of Nature spontaneously appear as the Universe cools, enabling different kinds of physical interactions to appear. Gravity had appeared sometime before inflation, during a time cosmologists call the “Planck epoch.” Inflation was the sudden, rapid expansion of everything — the energy soup, and spacetime itself — from the incredibly tiny point that expanded to become the Observable Universe. Think of spacetime like a balled up bundle of wrapping paper being unfolded, flattened and smoothed out by inflation.  On the smallest scales, spacetime is changing — expanding, stretching, flexing of the folds in our bundle of paper. Physicists call these happenings “quantum fluctuations.” The stretching and unfolding of spacetime means the gravitational field is changing. But that’s exactly what we said causes gravitational waves! As the Universe inflates, the quantum fluctuations in spacetime itself generate gravitational waves. This has physicists really excited, because detecting these gravitational waves would be the first hint of the quantum nature of gravity itself.

As we noted last time, gravitational waves interact with matter very weakly, so they propagate through the slowly evolving soup of the Cosmos, pushing matter here and their until the formation of the Cosmic Microwave Background.  What is the net result? The net result is that gravitational waves leave a polarization imprint in the microwave light. Just as with the expected polarization from electron scattering, there is a pattern to the polarization made by the gravitational waves. Astronomers call this pattern “B-modes” — they have a twist to them.  You can recognize a B-mode pattern, a twist, because in a mirror the pattern appears reversed.

(L) E-mode polarization patterns look identical if viewed in a mirror.  (R) B-mode polarization has a "twist." If the twist is clockwise, then when viewed in a mirror the twist is counter-clockwise, and vice versa.

(L) E-mode polarization patterns look identical if viewed in a mirror. (R) B-mode polarization has a “twist.” If the twist is clockwise, then when viewed in a mirror the twist is counter-clockwise, and vice versa.

Which brings us back to the story of BICEP2. Astronomers have been looking for the tell-tale twist of polarization in the Cosmic Microwave Background for some time. Major experiments have been slowly gearing up to look for and characterize the unique, gravitational-wave signature of inflation. The science team at BICEP2 won the race. What has all of us so excited is the measured value is larger than we anticipated, indicating the relative importance of the gravitational waves is large.  This has provided sudden and unexpected guidance for theoretical physicists trying to model inflation, and for future experimenters attempting to build new experiments to probe the early Universe.

The BICEP2 polarization map, showing B-mode ("twist") patterns.

The BICEP2 polarization map, showing B-mode (“twist”) patterns.

So what now?  Astronomy is a spectator sport — we keep looking!  Now that our colleagues at BICEP2 have made the initial detection, we are gearing up to look more closely at the result, and to dissect it for clues that confirm we are on the right track to understanding the Cosmos.  New papers are appearing rapidly (I counted about 100 at the time of this writing). The result doesn’t precisely agree with other results we already have (there is “tension” between the results, in physics-speak), and some of us still have reserved skepticsm.  The field has been thrown into a big mess, and now we have to figure out what happened. It’s a bit like coming home to find your living room in a disastrous state, and trying to figure out if it was the kids, the cat, the dog, or some combination thereof that made the mess! But astronomers don’t mind — it’s the figuring out of the mess that is so rewarding. But make no mistake — it was an outstanding achievement, a triumph of the human intellect and human ingenuity. And onward we go.

The BICEP2 Telescope at the South Pole. [Image: National Science Foundation]

The BICEP2 Telescope at the South Pole. [Image: National Science Foundation]

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This is the last 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.