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.

One supernova appears four times (arrows) as its light follows multiple paths around an intervening galaxy. NASA, ESA, S. RODNEY/JHU, THE FRONTIERSN TEAM; T. TREU/UCLA, P. KELLY/UC BERKELEY, THE GLASS TEAM; J. LOTZ/STSCI, THE FRONTIER FIELDS TEAM; M. POSTMAN/STSCI, THE CLASH TEAM; Z. LEVAY/STSCI

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.

Story continues after graphic

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.

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

12:38PM, OCTOBER 6, 2015
Full article:

Source: Using general relativity to magnify the cosmos | Science News

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