Using general relativity to magnify the cosmos

One of the most powerful known magnifying lenses isn’t found on Earth. The lens is built from stars, gas and dark matter and lies about 4 billion light-years away. As astronomers peer through it, they are finding the seeds of galaxies that were scattered around the universe more than 13 billion years ago.

The lens is known as Abell 2744, a cosmic pileup where four groups of galaxies are colliding to create one gargantuan gathering with the mass of about 2 quadrillion suns (SN: 6/13/15, p. 32). The gravity from all that mass redirects any light that tries to sneak past, bending and focusing it, creating bigger and brighter images of galaxies far beyond the cluster.

Abell 2744 is useful as an astronomical tool because the universe obeys Albert Einstein’s general theory of relativity. That theory describes how gravity, mass, space and time work together to build a universe. It forms the bedrock of science’s understanding of the cosmos. And for astronomers today, two primary consequences of general relativity — mass’s power to focus light plus the ripples in spacetime generated when masses accelerate — provide robust tools for investigating the cosmos. Giant lenses in space are at the forefront of efforts to explore the origins of galaxies. Elusive gravitational waves, meanwhile, can reveal unseen collisions between stellar corpses, such as black holes and neutron stars.

Gravitational lenses and waves are not new ideas. Einstein knew that his theory implied that both exist. In 1937, Caltech astrophysicist Fritz Zwicky proposed that lenses should be found around some massive galaxies. Decades passed before astronomical technology verified that idea: It wasn’t until 1979 that astronomers detected a real-life example of a gravitational lens in the double image of a quasar — side-by-side glimpses of a galaxy’s blazing heart, resembling a pair of oncoming headlights.  

Einstein calculated how the gravity of one star could amplify the light of another more distant star, but he also reasoned that the odds of seeing it are abysmally low. In recent years, the Optical Gravitational Lensing Experiment, one of several efforts to detect celestial bodies wandering in front of stars in the galaxy, has recorded about 2,000 possible events annually.

“It’s amusing how today lensing is so respected,” says Richard Ellis, an astrophysicist at the European Southern Observatory in Garching, Germany. “I’m old enough to remember when it was regarded as a bit wacky.”

Over the last couple of decades, lensing has been used to study all manner of things. Some nearby lenses forged from single stars have revealed planets in our own galaxy, including a few orphans that drift through the Milky Way without a sun to call home (SN: 4/4/15, p. 22). Other lenses, like Abell 2744, let astronomers peer across the cosmos to see galaxies growing up in the early universe.

Seeds of modern galaxies

Telescopes look back in time; light from the most distant locales travels for nearly the entire 13.8-billion-year history of the universe. As astronomers poke around for galaxies so far away (and so far back in time), they hope to find the seeds of what eventually became modern galaxies. Only abnormally bright galaxies, however, can typically be spotted across such distances.

Everything seen so far at the edge of the universe is the brightest, biggest, craziest at that time,” says Jennifer Lotz, an astrophysicist at the Space Telescope Science Institute in Baltimore. Our galaxy, though, “is not big and crazy; it’s more typical.” To find those more classic, less showy protogalaxies requires a really big magnifying glass.

Lotz is leading a three-year effort, known as the Frontier Fields project, to stare at six massive clusters with the Hubble Space Telescope and hunt for the seeds of galaxies similar to our own. Four clusters have been analyzed; the remaining two are now coming under scrutiny.

While peering through one of the clusters, Abell 2744, astronomers recently found a candidate for one of the most distant galaxies known, a toddler growing up about 500 million years after the Big Bang. The galaxy appears as a faint red smudge — or rather, three smudges — as its light traverses multiple paths through the cluster. This remote galaxy is tiny and dense, squeezing the mass of about 40 million suns into a ball just several hundred light-years across. It’s a pale dot compared with the Milky Way. Images such as these add to astronomers’ scrapbook of how galaxies grew over the history of the universe.

The building blocks of galaxies aren’t the only things lurking behind these lenses. In March, researchers announced that they saw the same supernova explode not once but four times (SN Online: 3/5/15).

“I just did not expect to see that at all,” Lotz says. “We got so lucky. The timing was perfect.”

The light from the exploding star, which took 9.4 billion years to reach Earth, fell squarely on one galaxy sitting in one of the Frontier Fields clusters. That galaxy’s gravity steered the light along four different paths, creating a quadruple replay, with each additional flash appearing days to weeks after its predecessor.

“The story’s not done,” she says. “We expect yet another one to show up in the next year or two.” By studying how the lens warps the light from background galaxies, researchers have calculated that there’s a fifth road for the light to travel along. Astronomers now have a rare opportunity to know about a supernova before it appears. “It’s an amazing example of gravitational lensing,” Lotz says.

Expansion ramped up

Strong gravitational lenses built by massive clusters are powerful tools. But they’re not that common. The light from most galaxies doesn’t pass near a cluster such as Abell 2744 on its way to Earth. But there are plenty of smaller clusters and long rivers of galaxies, known as galaxy filaments, that fiddle with the light and create weak lenses. “Every distant object has its image distorted by a small amount,” says Joshua Frieman, an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill.

That subtle distortion could be a key to unraveling one of the thorniest mysteries in modern astronomy: what’s causing the expansion of the universe to speed up?

Supernovas in other galaxies appear farther away than would be expected from a gradually expanding universe. Around 7 billion years ago, something stepped on the cosmic accelerator and picked up the pace of the expansion.

Researchers call this repulsive force “dark energy” (SN: 5/5/12, p. 17). They don’t know exactly what it is, but one idea is that it is some intrinsic property of space that has always been there, lurking in the background. At some point, as the universe stretched out, the density of matter and energy dropped enough for dark energy to become dominant.

The idea started with Einstein when he realized that his theory described an unstable universe, one in which gravity could pull all its stars inward in a massive collapse. That clearly hadn’t happened, so he fudged his equations and added in a “cosmological constant” to set things right.

“In order to arrive at this consistent view,” Einstein wrote in 1917, “we admittedly had to introduce an extension of the field equations of gravitation which is not justified by our actual knowledge of gravitation.”

He dropped the idea after Edwin Hubble reported in 1929 that galaxies appeared to recede from each other at ever greater speeds the farther away they were — a discovery that implied the universe was expanding. But Einstein’s creative accounting has come back into vogue. Today his cosmological constant might be the parameter that describes how dark energy inflates the universe.

Astronomers need to know a few more things about dark energy, though. For example, is dark energy truly constant, Ellis asks, or has it changed over time? “Until we measure it as a function of time,” he says, “we don’t know.”

Dark energy competes with dark matter — an elusive substance that holds together galaxies and their clusters — to erect the scaffolding for the universe, the places where atoms can get together and form stars and planets. Dark matter pulls things together and dark energy tries to pry it all apart. “It’s an epic struggle,” Frieman says.

Frieman leads a project called the Dark Energy Survey, one part of which is spending five years tracking how this tug-of-war has changed over time. The survey is looking for weak gravitational lenses created by that scaffolding. Hidden caches of dark matter slightly skew images of thousands of galaxies that share the same patch of sky. By measuring the very subtle distortions of about 200 million galaxies, researchers are mapping dark matter clumps back to a time when the universe was about half its current size (SN: 5/16/15, p. 9). Knowing how the cosmic clumpiness changed since then will help researchers get a sense of how, or if, dark energy changed as well.

The Dark Energy team is in its third year and is beginning to analyze the data from its first season. Frieman expects that the combined data from the first two years should start to rule out some ideas about what dark energy is.

Ripples in space

Even with gravitational lenses, some things are just too far or too faint to be seen. Einstein’s universe, fortunately, has a work-around: gravitational waves. Gravity is caused when mass puckers the fabric of spacetime. Like a ball bouncing off a rubber sheet, any accelerating mass should send out gravitational waves, ripples that cause space itself to stretch and squeeze.

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Creating detectable flutters requires cataclysmic events. Colliding black holes, merging neutron stars and even the Big Bang itself (SN: 2/21/15, p. 13) should send out ripples in space that echo across the cosmos. If there were a way to sense these spacetime swells, astronomers could investigate entities whipping around the universe that might otherwise remain unseen.

Searches for such signals have been under way at the Laser Interferometer Gravitational-Wave Observatory, or LIGO, twin facilities in Louisiana and Washington state. Should a wave wash over the Earth, the precise distance between pairs of mirrors suspended at the ends of perpendicular 4-kilometer-long tubes will oscillate as the space between the mirrors expands and contracts. Lasers that ricochet within these tubes can sense changes in distance far less than a thousandth of the width of a proton.

Astronomers have already detected gravitational waves indirectly. In 1974, Joseph Taylor (SN: 7/11/15, p. 4) and Russell Hulse, then at the University of Massachusetts Amherst, discovered the first binary pulsar, a rapidly spinning neutron star orbiting a companion. Over the next several years, the pulsar drifted toward its unseen partner at the rate of 3.5 meters per year — an orbital tightening predicted by general relativity if the duo is radiating gravitational waves. The discovery netted Taylor and Hulse the 1993 Nobel Prize in physics.

The ripples from the Hulse-Taylor binary are too subtle to be seen directly. But as the two stars snuggle up, the waves will get stronger. In the final milliseconds before the stars collide, spacetime will ring loud enough for LIGO to hear. That collision won’t happen for another 300 million years, though.

“We don’t want to wait that long,” says Martin Hendry, an astrophysicist at the University of Glasgow in Scotland. “What we’re banking on is that there are many such systems in our galaxy and beyond, and that’s what we’re waiting to detect.”

LIGO’s first eight-year search wrapped up in 2010 with nothing to show. In September, LIGO began another go at hunting its elusive quarry. The second attempt, dubbed Advanced LIGO, uses better instruments, and mission scientists are confident that they will see something in the next few years.

“The real astrophysics begins just after that,” Hendry says. Once researchers have a handful of detections, then LIGO and other similar facilities become just another astronomical tool, but one that is sensitive to changes in gravity rather than light. And unlike telescopes, which typically look at only one place at a time, gravitational wave detectors can listen to the entire sky.

Cosmic metronomes

LIGO should be able to pick up the relatively high frequencies of any neutron stars or black holes spiraling together within about 600 million light-years of Earth. Collisions between supermassive black holes (SN Online: 8/31/15) can be heard from much farther away, but they send out long, undulating waves to which LIGO is deaf. To sense these enormous waves — the peak-to-peak distances are measured in light-years — researchers are turning to pulsars.

Race toward a pulsar, and the tempo of radio bursts will appear to pick up as you run more quickly into successive pulses. Pull away from a pulsar, and the beat appears to slow. As Earth bobs on the spacetime ocean, it pulls away from some pulsars and moves toward others. By monitoring the pulses from dozens of these cosmic metronomes, researchers will know when Earth is riding the wave from a super­massive black hole collision.

“It’s like you’re detecting waves on the ocean by being able to measure the movement of a boat,” says Ryan Lynch, an astronomer at McGill University in Montreal.

The change in distance between Earth and one of these pulsars is staggeringly small: about one part in a quadrillion. That’s like trying to measure a one-kilometer change across roughly 100 light-years.

Three projects known as pulsar timing arrays, in North America, Europe and Australia, are using some of the largest radio telescopes to identify pulsars and look for these waves. The first thing they’ll probably pick up, Lynch says, is not a single event, but the background hum of many supermassive black holes colliding across the universe. Only the closest and biggest will rise above the noise.

Should LIGO or the pulsar timing arrays not detect anything, that wouldn’t necessarily mean there’s something wrong with general relativity, Hendry says. It could just mean the assumptions about these collisions are incorrect (SN Online: 9/24/15). That’s one reason some researchers are trying to persuade the European Space Agency to launch a space-based version of LIGO known as eLISA (for evolved Laser Interferometer Space Antenna) in 2028. In the stillness of space, far removed from the shaky ground, eLISA should hear what LIGO cannot: the buzz from a wide variety of tightly coupled binary stars that litter the Milky Way.

“We’ll see hundreds or thousands of them, and they’re virtually guaranteed,” says Guido Mueller, a physicist at the University of Florida in Gainesville.

These snuggling stars, which are already well studied, will test both eLISA’s capabilities and predictions from general relativity. eLISA will also listen for binary supermassive black holes in other galaxies, a population that astronomers know very little about. And for eLISA, the sky is quite literally the limit.

“eLISA should basically see [the black holes] out as far as they exist,” Hendry says. The orbiting watchtower will sense collisions clear to the edge of the visible universe, back to the dawn of time. “There will eventually come a point where there aren’t any more black holes because they haven’t had time to form yet,” Hendry says. And putting together a census of binary supermassive black holes from the early universe, he adds, might help researchers understand what role (if any) these dark duos had in shaping galaxies during the billion or so years following the Big Bang.

General relativity came on the scene before anyone knew that the universe is expanding, a time when astronomers could not be certain that those fuzzy splotches of light in the sky were actually other galaxies. Now astronomers are ready to start poking at some fundamental truths about the universe, from the formation of the first stars and galaxies to what makes the cosmos tick. One hundred years after its publication, Einstein’s theory is poised to peel back the cosmic curtain even farther.

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This article appears in the October 17, 2015, Science News with the headline, “Magnifying the cosmos: Using general relativity to see deep into space.”