An attentive ear to the echoes of the big bang / Ross D. Andersen

Scientists are preparing to capture the first gravitational waves, so attention is being directed to instruments that will allow astronomers to peer into the invisible innards of black holes and watch the ancient and hidden history of time * The article was published in Scientific American even before this week's discovery of evidence of gravitational waves.

Let's say you want to gain a glimpse into the beginning of time, into the very first moments of cosmic creation. Your first step may be to build a perfect telescope, a device so powerful that it can peer into the far reaches of the visible universe. You will locate the peak of a barren mountain, far from the glow of the lights of civilization that overshadows the light of the stars. Level the ground near its summit to establish a comfortable position there and place on top of it the finest observatory that can be built. Place in the observatory a telescope with a huge mirror, much too large to be launched into space, and equip it with an array of sophisticated detectors. You will invest in this operation several years, and also several billions of dollars, so that no photon escapes your gaze. But what can you see with it? Suppose that night has come, that one and only night among the astronomer's thousand nights, when the moon hides below the horizon and the dome of the sky rises above, dark and clear. What gems will sparkle from the black velvet display case of heavenly visions?

It turns out quite a lot. In the foreground, you'll see a handful of scattered planets, their orbits intersecting the steady spinning circles of the constellations. Beyond them, local stars will shine brightly against a background of fainter white sparkles. In the darker corners of the sky, galaxies will glow, some of them hundreds of millions of light years away. If you aim your perfect telescope at just the right spot, it can reveal even deeper recesses of the cosmos. He will be able to take you to the first stars: the huge spheres of helium and hydrogen, whose fiery faces illuminated the young universe.

More of the topic in Hayadan:

• First direct evidence of the existence of gravitational waves that caused the universe to swell immediately after the big bang
• How does the telescope at the South Pole that discovered the gravitational waves created by the Big Bang work?
• What are gravitational waves?

However, light has limits, it cannot show you the entire universe. Even if you look through a telescope all night, every night, you still won't be able to peer into the center of a black hole or into the wound of time itself. Immediately after the Big Bang, for several hundred thousand years, the photons of the infant universe remained trapped in a dense soup of light-suffocating particles, like fireflies trapped in mud. 380,000 years had to pass from the beginning of the big bang for the universe to cool down and become something transparent, and for that matter, also legible; Emptiness through which you can see the flash of creation. We call this flash the cosmic background radiation (CMB), and it is the main text of modern cosmology. It also functions as a wall, a barrier in time, beyond which the ruler of darkness.

For centuries, the careful collection of primordial lights has been the primary way to observe the universe, the key to the most ambitious experiments in cosmology. But light cannot shine on the beginning of time, no matter how big and sophisticated our telescopes are. To see beyond the background radiation and gaze into the dawn of the universe, cosmologists must turn to gravity, a force that leaves its own echoes scattered throughout space, echoes we call gravitational waves. To detect these echoes, we will need a new type of device, very different from a telescope.

The first detectors

The quest to build a device capable of detecting gravitational waves began decades ago, but as of now,

Has not yet borne fruit. As of the time of writing this article, the Laser Interferometry Gravitational Wave Observatory (LIGO) represents the most serious attempt made in the field. This observatory consists of three instruments, two in Washington State and one in Louisiana. Each is an engineering marvel, a laser-based ruler capable of detecting an atom's width of oscillation. LIGO fires laser beams down two arms perpendicular to each other, and measures the difference in their lengths, a method known as laser interferometry [see box at lower left on page 39]. If a large enough gravitational wave passes through the environment, it will push and pull the arms back and forth, thus changing their length relative to each other. Essentially, LIGO is a celestial earpiece, a giant microphone listening to the dying symphony of the hidden cosmos.

Similar to many exotic physical phenomena, the source of our knowledge about gravitational waves is in theoretical concepts, equations, and not in sensory experience. Albert Einstein was the first to notice that his general theory of relativity predicted the existence of gravitational waves. He realized that certain objects have such a large mass and move at such a high speed that they twist the fabric of space-time itself, sending tiny ripples along it.

How tiny are they? So tiny that Einstein thought it would never be possible to observe them. But in 1974, two astronomers, Russell Hales and Joseph Taylor, concluded that gravitational waves exist through an ingenious experiment: a careful study of an astronomical object known as a binary pulsar. Pulsars are the spinning, flashing cores of stars that exploded a long time ago. Their rate of spinning and flashing is remarkably regular, a feature that has made them very popular with astronomers because they act as cosmic clocks for them. In a binary pulsar system, a pulsar and another object (in this case, a supercompact neutron star) orbit each other. Hals and Taylor realized that if Einstein described relativity correctly, then the spiral pair would produce gravitational waves that would drain the orbital energy from the system, and as a result the radius of the orbit would gradually shrink and the speed of the pair would increase. The two astronomers plotted the pulsar's apparent orbit and then watched it for years to see if the shrinking orbit would show up in the data. Not only did the contraction appear, it matched Halles and Taylor's predictions a perfect match, sitting on the graph without any deviation and giving Einstein such complete confirmation that the two won the 1993 Nobel Prize in Physics.

The problem with LIGO is that it can only hear these binary pulsars in their final moments, when their stellar spirals accelerate and emit a rhythmic series of powerful waves that travel through space like invisible cosmic death rattles. Our universe may be big and full of stars, but binary collapses are rare. If we wish to hear them with any degree of regularity, we will have to listen to a huge segment of the cosmos. Until recently, LIGO's range was limited to a region of space where hundreds of years could pass without any binary collapse occurring within its boundaries.

However, LIGO's first operation was a "dry run", a method to discover the engineering flaws that accompany the coordination of an instrument that stretches over kilometers. Now that LIGO engineers know they can make the complicated detector work, they are upgrading its sensitivity so that it will soon be able to detect a collapsing binary 500 million light-years away, an improvement that could allow it to hear hundreds of such events each year. Indeed, most astrophysicists expect LIGO to make the first direct detection of gravitational waves within a few months of its relaunch in 2016, the centenary of Einstein's prediction.

Atomic wave

Despite LIGO's considerable cost, its ambitions are limited. In some ways, his mission is to prove the principle, a necessary first step before gravitational wave science steps up to its most natural environment: space. Our planet is a terrible place for a gravitational wave observatory because the Earth's crust is constantly bathed in waves of seismic noise, the product of the thunderous collisions of the tectonic plates beneath the Earth's surface and the surging oceans on its surface. All these vibrations and jolts can easily drown out the light fluttering of matter resulting from a gravitational wave. To hear a wider variety of them, we would need to place a detector outside the atmosphere, in the depths of space, where the environmental conditions are much calmer.

At NASA's Goddard Space Flight Center, two teams of engineers plan to be the first to place gravitational wave detectors in space. The older team of the two has been perfecting its mission, the Laser Interferometry Space Antenna (LISA), for decades. The LISA mission is an audacious engineering project, requiring such a high degree of precision that LIGO looks like a toy made of Lego by comparison. The project requires the launch of three spacecraft that will circle the sun in the structure of an equilateral triangle whose side is five million kilometers long. Once the spacecraft are in place, the distance between them will be measured, continuously, using lasers. If a gravitational wave passes in the environment, it will disturb the spacecraft, distort the triangle and the lasers will capture it.

The basic design of LISA hasn't changed much since some pioneers of gravitational wave science sketched it out on a napkin at a NASA physics conference more than thirty years ago. But it was perfected over time thanks to the stubborn struggle of the engineers in the practical challenge: to breathe life into this ambitious design. In the late 90s and early 20s, LISA was one of the early candidates to become the flagship of NASA's astrophysical missions, right after the James Webb Space Telescope (JWST). However, in the years since then, JWST has eaten up most of NASA's astrophysics budget, and since astronomers had no LIGO detections, they had trouble finding arguments in favor of investing billions of dollars in building a gravitational wave detector. It may be more than ten years before a LISA-style mission receives a "green light".

These delays cleared space on NASA's desk for innovative ideas on how to detect gravitational waves in space. A small team in the agency's Advanced Ideas Department recently began developing a new type of gravity detector, based on an emerging technology called atom interferometry. The structure of this team is not rigid, and as of now, it is very difficult to say that the work they have done makes it possible to build a mission from A to Z. The main team leaders, Babak Seif, an interferometry engineer on the JWST project, and Mark Kasevich, a professor of applied physics at Stanford University, are both preoccupied with other pressing issues. For them, this is a side project, something to have fun with and dream about on the sidelines of their daily toil.

In February 2013, I visited Saif in one of the laser labs at the Goddard Center, where he is slowly starting to build an atom interferometer, a technology he expects will be the basis for a smaller, faster gravitational wave detector. Since it is one of the most prestigious space research laboratories in the world, one can find there large groups of scientists who enjoy an eye-popping academic pedigree, but Seif's roots are more modest. After immigrating with his family to the United States from Iran when he was 17, he lived in Northern Virginia, where he began taking science and math classes at a local community college and helping support the family by working nights at a gas station. Saif turned out to be a quick-witted student. In 1981 he transferred to the Catholic University of America on a full scholarship, and in the years since then he has received two doctorates. Before coming to the Goddard Center, Seif spent ten years at the Space Telescope Science Institute, where he designed the interferometer that would test JWST's mirrors. Saif's interferometer will make sure the mirrors are accurate down to the nanometer level, so that we don't have to deal with an embarrassing malfunction again like what happened to the Hubble Space Telescope, which went into orbit with one of its mirrors misaligned.

Saif explained that his and Kasevich's mission idea is similar to LISA's idea, that is, it involves measuring the distances between orbiting spacecraft. However, unlike LISA, whose program name is to measure changes in distance by combining light from laser beams fired from one spacecraft to another, Saif Vaksevich's mission will use atoms that reside just outside the spacecraft [see box on the top left on the next page]. Because the atomic interferometer measures distances between atomic clouds, rather than between spacecraft, it can be much smaller. Its current design requires arms that are 5,000 times shorter than LISA's arms.

The strength of the method lies in its accuracy. A gravitational wave might shift the distance between the spacecraft by less than a trillionth of a millimeter, and even then an atom interferometer would be able to detect the difference.

But not everyone is enthusiastic about the idea. The limited budget available for space science caused tensions between the SAIF atom interferometry team and the LISA team. The ideas behind the two tasks are similar in some ways. Both missions require the spacecraft to be coordinated in their level of precision, and both use interferometry to make precise measurements. But according to Seif, thanks to the transition from light interferometry to atom interferometry, we will be able to build more sensitive and cheaper detectors, and reduce the huge distance between spacecraft; Critics of the LISA project have long seen the issue of distance as a problem that could derail the mission.

But the LISA guys fire back: they attribute the cost savings of atom interferometry to its innovation. They say that enthusiastic advocates of new technologies tend to underestimate the heavy costs of development. According to them, you can only know what the real price tag of any design is after the task is already ready, because only then can you begin to see the more complicated engineering challenges that accompany the integration of the systems.

The problem of light

At the Goddard Center, I asked Saif what motivates him to spend his spare time on a mission that is highly unlikely to ever be launched. He told me that it was the possibility of new physics that fascinated him, that he expected the next decades to bring about a fundamental change in the field of astronomy - a transition from the use of photons to the use of gravitons.

Indeed, gravitational waves can help compensate for several scientific failures that light presents us with, apart from its inability to tell us about the beginning of time. There are additional limitations as far as carrying information is concerned. First, light is a product of interactions between particles. When light appears in the universe, it announces the occurrence of tiny events, such as the creation of helium by the fusion of hydrogen atoms within stars. It records the smallest events. If we are interested in learning how large objects move through space-time, we must collect light from many such tiny events and use it to draw conclusions. We must assemble a mosaic of the surface.

And if that's not enough, the light creates a bias in the way we see the cosmos because it tends to come from very thermodynamically active environments. In astronomy, the large bursts of light worthy of the name "signal" are the products of fiery events, such as stars that go supernova in their death flickers. When we think of the universe, the structure that comes to mind will usually include hot and chaotic places.

Also, light signals are fragile. They often dwindle or disappear altogether as they make their way through the cosmos. Some are engulfed by huge gas clouds standing in their way. Others scatter or fall into deep gravity wells never to return. The deepest pits of this type are supermassive black holes, the pillars of cosmic structures around which entire galaxies move. Scientists want to know more about these black holes, especially what happens when two of them merge with each other. But not a single drop of light ever reaches our eyes or the eye of our telescopes from a black hole, because photons, despite their enormous speed, cannot escape the suction of a black hole's core.

Cosmologists are forced to settle instead for the light that the black hole does not devour, light that emerges from the region of its book, from matter trapped in the turbulent space-time distortions around it. Happily, gravitational-wave signals are much less susceptible to influence than light. They do not dissipate or diminish, but create ripples that spread uninterrupted throughout the universe, indifferent to the astrophysical giants that stand in their way.

Genesis echoes

A few weeks after my trip to the Goddard Center, I visited David Spergel, head of the astrophysics department at Princeton University and one of the most senior cosmologists in the world. Spergel is chairman of the US National Research Council's Ten-Year Survey Committee on Cosmology and Fundamental Physics, whose reports play an important role in determining long-term research priorities in cosmology. It is known that NASA takes the committee's recommendations very seriously, and this means that Spergel's words carry a lot of weight in the question of which scientific missions the agency decides to launch.

When we sat down in Spergel's office, he began detailing the benefits of gravitational waves. Unlike what happens with light, he explained, the universe has always been transparent to gravitational waves. There was no primordial age during which these waves were obscured by strange cosmic conditions. Indeed, there should be no problem for gravitational waves to flutter towards us right from the first moments after the big bang. But how can we know if such waves existed in those times?

"To create gravitational waves, you have to move a lot of matter back and forth at a very high speed, and one of the ways to do this is through a phase transition," Spergel told me. A phase transition occurs when a physical system moves from one state to another. The classic example is water that freezes into ice, but there are also phase transitions on cosmic scales, some of which occurred shortly after the big bang, for example: quarks. Most quarks today are bound inside atomic nuclei, but in the first microseconds of the universe, they bounced freely in any direction in a state cosmologists call a quark-gluon plasma. At some point, the universe went from a state of quark-gluon plasma to a new state populated by protons and neutrons.

"In the case of a first-order phase transition of this type, bubbles will form inside the plasma, which will cause a lot of matter to move wildly in all directions," Spergel said. First-order phase transitions occur abruptly, as weeks of a new phase form in the core of the old phase. These bubbles expand and collide until the old phase disappears completely and the phase transition ends. The chaos of the process should create sets of powerful gravitational waves, which may be passing us right now. If we can detect them, we may get our first glimpse of the universe's infancy.

And even older gravitational waves may exist. In some inflationary models of the universe, the first burst of exponential cosmic expansion occurred simultaneously with quantum fluctuations of spacetime, ripples that caused some regions of the universe to expand faster than others. These fluctuations could produce a special type of gravitational waves, called stochastic gravitational waves, which would have been created when the age of the universe was less than a trillionth of a trillionth of a trillionth of a second.

"Most inflationary models of the universe predict that this background radiation, of stochastic gravitational waves, comes from very early parts of the universe," Spergel told me. "If we can observe them, they can show us fundamental physics. They will show us what the universe looks like at energies on scales 1013 times larger than what we get at the Large Hadron Collider," he said.

Searching for stochastic gravitational waves is a scientific gamble. Their identification will be very difficult. It will require extremely sensitive instrumentation and painstaking data analysis to sift out the precious primordial waves from gravitational wave signal surges that will bombard every detector stationed in space. If we can collect this signal from every corner of the sky and clean it of all the remnants of stray signals, we will get background radiation of stochastic gravitational waves, a sky-spanning map of gravitational waves. We will receive a fundamental text in cosmology that we can delve into.

The mission design designed to realize LISA's ideas, as well as that of the Seif Atomic Interferometer, are both designed to detect gravitational waves from more conservative targets, such as black hole mergers. In more adventurous days, LISA's planners dreamed of building a Big Bang Observatory, a mission that would come after LISA and that would specifically target the detection of stochastic gravitational waves. But such an expectation has always been a wild bet, and it will take decades before it is actually realized. Seif told me that he would like to reverse the mission order and start looking for stochastic gravitational waves, but his plans are aimed at the same signals that LISA plans to look for. The conservative approach is a diplomatic move designed to please the wider community of astrophysicists, who are indeed intrigued by gravitational wave science, but would prefer to start small and target objects that are already known to exist.

"Collisions between supermassive black holes are the 'bread and butter' of the work on gravitational wave experiments," says Spergel. According to him, "If we launch one of these spacecraft and we don't hear giant black holes colliding, it means that there is something very wrong with the picture of the universe we have, but the big prize is cosmology."

It is possible that at some point the Spergel Ten-Year Survey Committee will find itself forced to choose between black holes and cosmology, and perhaps also between atom interferometry and light interferometry. The committee is supposed to reconvene in the middle of the decade to assess the course it set in 2010 and introduce amendments. The next survey of this type will be conducted after JWST is already launched, and it is possible that this will free up funds for the purpose of an ambitious space science mission.

As I walked out of Spergel's office, as he walked me out, I asked him if he had a favorite contender, if he thought Saif's mission would defeat LISA in the long run. He told me that he is not convinced that the idea of ​​atom interferometry will win in the end, but he is convinced that it is an interesting idea that warrants a lot of thought. Then he told me a story: "Many years ago, long before Stephen Chu won the Nobel Prize, I talked with him about the question of what is the way to achieve great scientific achievements, and Chu told me something that is etched in my memory, he said that what is needed is to take a position that allows perform experiments that may be important. I believe that these two experiments meet this condition."

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in brief

Astronomers are on the brink of a new era. Soon they will be able to observe the universe not only through light waves but also through gravitational waves.

Gravitational waves allow a look into the universe that was hidden from our eyes until now. They can reveal what is hidden within the event horizon of a black hole and allow a glimpse into the earliest moments of the universe.

Gravitational wave observatories on Earth should make their first discoveries in the coming years. Beyond that, a cockfight is taking place between two different technologies that may be integrated into a gravitational wave observatory in space.

About the author

Ross D. Andersen is a senior editor at the online journal Aeon. He has written extensively on science and philosophy in several journals, including the Atlantic and the Economist.

Gravitational wave observatories on Earth should make their first discoveries in the coming years. Beyond that, a cockfight is taking place between two different technologies that may be integrated into a gravitational wave observatory in space.

how it works

The universe according to gravitational waves

Albert Einstein taught us that matter and energy can bend the fabric of space-time itself. If you set enough mass in motion, the motion will create ripples in space-time that ripple throughout the universe. Such gravitational waves are the only way we can observe events that cannot be seen with light: the collision of two black holes, for example, or the chaos of quantum fluctuations that took place in the nanoseconds after the Big Bang.

However, identifying echoes from the Big Bang would be incredibly difficult to do; Only a space observatory could handle such a task. Both ideas presented on this page would have the ability to listen to the first echoes of the universe.

Atom interferometer

A new approach to measuring gravitational waves will make use of clouds of very cold atoms that lie just outside two spacecraft 1,000 kilometers apart. In the first stage, laser beams introduce each cloud into a superposition of two parts, with two different speeds. After 10 seconds, another laser beam reverses the process, so the two parts begin to reconnect. When the clouds of atoms overlap again, they are measured again by more lasers. If, during the 20 seconds required for this process, a gravitational wave passes through the space between the spacecraft, it will change the distance between the pairs of clouds by a tiny amount, and cause a measurable change in the final state of the atoms.

Laser interferometer

Conventional gravitational wave observatories, such as the ground-based LIGO observatory currently being upgraded in an effort to find its first gravitational waves, and LISA, an idea for a future platform to be placed in space, work by combining laser beams. LIGO splits a beam into two parts (A and B), reverses the phase of one, then sends the beams out and back through vertical arms. (LISA works on more or less the same principle, but it uses an equilateral triangle instead of perpendicular arms.) When the beams recombine (in yellow), the waves should cancel each other out, so the resulting beam will be dark. But if a gravitational wave changes the relative length of the arms (in blue), the waves will not match each other, and the combined rays will show pulsations that will reveal the secret of the waves. But the effect of gravitational waves is tiny: a nearby neutron star collision, for example, would change the length of LIGO's four-kilometer arms by less than the diameter of a proton. With the help of LISA's arms, which are five million kilometers long, it will be easier to listen to even smaller signals.

Cosmic horizons

What do we hope to find?

Gravitational waves are able to cross boundaries that light cannot cross. For example, they can convey information about what is happening beyond the event horizon of a black hole. Gravitational waves can also emerge beyond the "wall" of the cosmic background radiation (CMB), the barrier of light that will forever prevent us from seeing the universe before it is 380,000 years old. They will give us ears capable of listening to every corner of the cosmos.

More on the subject

Einstein's Unfinished Symphony: Listening to the Sounds of Space-Time. Marcia Bartusiak. Berkley Books, Penguin Putnam, 2000.

Gravitational Wave Detection with Atom Interferometry. Savas Dimopoulos et al. in Physics Letters B, Vol. 678, no. 1, pages 37-40; July 6, 2009.

LISA Project Office: http://lisa.nasa.gov

Watch a video demonstrating how an atom interferometer detects gravitational waves:

 

The article was published with the permission of Scientific American Israel