Four hundred years ago, astronomy embraced all that was visible. For Galileo, looking through his primitive telescope, the vistas included jewel-like stars, mountains on the moon, moons orbiting Jupiter and the glow of comet tails.
Today astronomy is often about what cannot be seen. Astronomers have known for decades that stars and galaxies are mere baubles floating on a vast sea of dark matter. More recently, astronomy’s roster of Darth Vaders has expanded to include an even more mysterious force: dark energy, an entity that drives the universe to accelerate its expansion just when gravity’s tug ought to be slowing it down.
On the brighter side, astronomers are beginning to learn more about the complicated processes that formed stars and galaxies, giving the universe its light. The Planck mission (SN: 4/11/09, p. 16) will test the idea that the Big Bang was accompanied by a brief burst of rapid expansion called inflation, which is thought to have created the seeds of matter from which stars and galaxies arose.
On smaller scales, explorations within the solar system, along with the discovery of more than 345 extrasolar planets, pose questions about the possible existence of life beyond Earth. The Kepler mission, launched in March, will provide a head count of Earthlike planets in the nearest reaches of the galaxy. Other new telescopes will examine the composition of these orbs and their potential for life.
Galileo’s successors have pieced together an impressive outline of cosmic history, from the inflationary beginnings of spacetime to the arrival of planets and people. But many details remain to be filled in, and strange new features may be added as astronomers push the limits of current theory and knowledge. New forms of matter, new twists in spacetime and even entire extra universes may emerge from the ongoing efforts to explain and understand the workings of the heavens.
From light to darkness
To understand the points of light that decorate the sky, it has become necessary to embrace the darkness. The brilliant but irascible astronomer Fritz Zwicky first realized that truth more than 75 years ago, when he found that all the visible matter in the Coma galaxy cluster wasn’t nearly enough to keep the cluster intact. And individual galaxies, like our rapidly rotating Milky Way, would fly apart if the only gravitational glue came from visible matter. Something else, something unseen, must be providing the extra gravitational pull, Zwicky and others reasoned.
Over the past few decades, astronomers have come to the conclusion that only about 15 percent of all the matter in the universe is visible. Researchers have deduced that vast halos of dark matter envelop and extend thousands of light-years beyond a galaxy’s visible outlines.
While most astronomers agree that dark matter exists, nobody knows for sure what it is. But last year, several teams of researchers reported finding hints for the existence of one of the leading candidates for dark matter, known as WIMPs, for weakly interacting massive particles. WIMPs respond only to gravity and the weak nuclear force.
Theory predicts that WIMPs would have been forged by the Big Bang. Moreover, their calculated density in the present-day universe would be just right to account for the observations that require the presence of dark matter. Researchers call this cosmic coincidence “the WIMP miracle.”
Like any proposed dark matter particle, WIMPs can’t be seen. But some WIMPs have an odd property: Whenever two collide, they annihilate each other, producing a spray of ordinary, visible elementary particles such as positrons, electrons and neutrinos, along with gamma rays. Two recent experiments found a greater than expected abundance of positrons and electrons in the Milky Way. Scientists say the surplus particles may have been produced by WIMP annihilations (SN: 9/27/08, p. 8; 2/28/09, p. 16).
Other experiments have now joined the WIMP search. NASA’s orbiting Fermi Gamma-ray Space Telescope is looking for an excess of gamma rays, a possible product of WIMP annihilation. IceCube, a telescope at the South Pole, is searching for an excess of neutrinos that might indicate WIMPs’ existence. Some experiments are seeking to directly detect these dark matter particles through the energy they would deposit in underground detectors. Finally, studies at the Large Hadron Collider, the world’s most powerful accelerator (scheduled to reopen this fall), could provide new clues about the identity of dark matter (SN: 7/19/08, p. 16).
“There’s a very good chance in the next two to three years we might find out what dark matter is,” says theorist Carlos Frenk of the University of Durham in England.
Energy of darkness
But even if researchers soon unmask dark matter, a gloomier mystery remains. In 1998, astronomers were astonished to find that the expansion of the universe has been speeding up. Cosmologists call whatever is behind this accelerated expansion dark energy.
At a recent seminar at the Space Telescope Science Institute in Baltimore, Mario Livio did something perfectly ordinary. He threw his car keys up in the air. As expected, the keys rose, slowed down and then fell, landing back in his hand.
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Now, said Livio, a theorist at the institute, imagine if the car keys kept accelerating skyward instead of returning to his hand. “That’s how shocking dark energy is,” he exclaimed.
In fact, Einstein’s theory of relativity does allow gravity to exert a cosmic push as well as the more familiar pull. According to relativity, gravity has two sources: the pressure exerted by a substance as well as its mass. Ordinary pressure contributes to gravitational attraction, but dark energy exerts negative pressure, which pushes space apart. If the push is strong enough, the needle on the gravity meter swings from attraction to repulsion.
Dark energy seems to resemble the cosmological constant, a space-filling energy represented by a term that Einstein inserted into his equations to keep the universe balanced between expansion and collapse. The most likely source of this constant would be the energy associated with the vacuum of space.
On the subatomic scale, the vacuum seethes with pairs of particles and antiparticles popping in and out of existence. But calculations of the expected vacuum energy predict an amount of dark energy 10120 times larger than observations allow, notes Robert Caldwell of Dartmouth College. So despite thousands of papers written about dark energy, there’s no convincing explanation of what it actually is.
It may even turn out that dark energy isn’t real. Some physicists suggest that the observed cosmic acceleration might be a sign that Einstein’s beloved relativity theory needs revision.
New ways to chart the expansion of the universe may help determine whether dark energy is real and whether it’s truly constant over time. And that will give astronomers new insight into the fate of the universe — whether cosmic expansion will slow down, continue to accelerate at its current rate or speed up even more, ultimately ripping apart the universe and every galaxy within it.
When it comes to understanding star and galaxy formation, astronomers must straddle the boundary between darkness and light. On the one hand, without dark matter, there would be no stars, galaxies, planets or people, says theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. On the other hand, once stars and galaxies begin to coalesce within halos of dark matter, the messy “gastrophysics” involving visible matter such as stellar winds and gas particles become just as important in shaping galaxies and their pattern on the sky.
Dark matter, unlike visible matter, can’t be pushed around by photons, so it was able to collapse earlier than the visible material. Clumps of dark matter eventually pulled in the visible matter. Because it doesn’t interact with photons, the dark matter retained the memory of the original cosmic blueprint — the lumps laid down and amplified during the first tiny fraction of a second after the birth of the universe.
According to the dark matter theory, the first stars to coalesce and ignite within dark matter halos were about 100 times heftier than the sun and appeared about 30 million years after the Big Bang. Large collections of those stars became the universe’s first galaxies about 200 million years after the Big Bang, the theory holds.
With dark matter’s role firmly in place, “we were told by theorists that we had a pretty good picture of galaxy formation,” says Richard Ellis of the California Institute of Technology in Pasadena. According to the model, small galaxies would form first and grow larger. Only later, as small dark matter halos coalesced to form bigger halos, would bigger galaxies emerge.
But beginning in the mid-1990s, as telescopes became more powerful, astronomers stumbled upon a puzzle: They discovered massive galaxies that existed when the universe was only half its current age, about 7 billion years after the Big Bang. Soon a slew of new observations revealed that massive galaxies finished forming their stars early, while smaller galaxies began making stars later, seemingly the opposite of what dark matter theory dictated. Even more surprising, astronomers found “big, well-fed galaxies when the universe was just a billion years old,” Ellis notes. “There hasn’t been enough time in the universe for them to have got there by merging,” as the theory had predicted, he says (SN: 4/25/09, p. 5).
Finding young, big galaxies forced astronomers to the realization that although dark matter plays a critical role in galaxy formation, other factors also come into play. For instance, supermassive black holes that develop at the centers of galaxies may generate jets and winds that push gas away or heat it so that it cannot coalesce and form stars. Because smaller galaxies have less gravity, these jets and winds may be more effective in temporarily halting or delaying the onset of star formation in smaller systems.
It’s also possible, says Ellis, that dark matter theory needs some revision, although not a major overhaul. For instance, if massive dark matter halos grow faster than theorists have calculated, it could explain the production of massive galaxies early in the universe.
The discovery of these massive galaxies has spurred researchers to search for starlit bodies even farther back in time. A new infrared spectrograph scheduled to be installed at the Keck Observatory on Hawaii’s Mauna Kea next year, along with a powerful new infrared camera that astronauts are set to install on the Hubble Space Telescope, “will enable us to systematically start charting the universe” when it was less than 800 million years old, Ellis says.
Astronomers have begun to pinpoint the era when spiral galaxies like the Milky Way began taking on their distinctive appearance. Observers have caught glimpses of some of the first galaxies with rotating disks — the earmark of a spiral galaxy — that began taking shape when the cosmos was between 2 billion and 3 billion years old.
Using bigger telescopes to study such galaxies in detail “would allow us to start to really understand what state young galaxies are in and how we can link them to galaxies today,” says Ellis.
C’est la vie
Where there are galaxies, there are planets — some perhaps with life. For seekers of life beyond Earth, the solar system harbors an abundance of possibilities. There’s Saturn’s largest moon, Titan, a frigid world shrouded in an organic haze with pools of liquid methane on its surface. In 2005, astronomers discovered that a much tinier Saturnian moon, Enceladus, spews geysers of water vapor from its south pole and have since found hints that the moon’s interior may contain a reservoir of salty water. The fractured surface of Jupiter’s icy moon Europa suggests that it may have an underground ocean that occasionally wells up, heated by the internal flexing that the gravitational tug-of-war between the moon’s siblings and Jupiter generates. And then of course, there’s Mars, the desiccated reddish world crisscrossed by channels that might once, at least briefly, have carried water.
“Mars clearly has got to be the top place” to look for life, “and that’s exactly what NASA is [focusing] on,” says Alan Boss of the Carnegie Institution for Science in Washington, D.C. One new wrinkle, he notes, is the seasonal detection of methane on the planet (SN: 2/14/09, p. 10).
Although plenty of nonbiological processes produce methane, the gas could be a signature of the decay of biological material. “NASA’s mantra should not be just ‘follow the water’ but ‘follow the water and follow the methane,’” says Boss. “We would really like to find the locations of the methane; it really focuses the search.”
But Phil Christensen, a Mars researcher at Arizona State University in Tempe, says it isn’t clear that the Red Planet was ever warm and wet long enough for life to gain a foothold. Some evidence suggests that the era of flowing water lasted for only hundreds to thousands of years — and was confined to a few specific places on Mars. Finding out whether that era lasted long enough to support life could speak volumes about the conditions required to support life elsewhere, says Christensen.
Another planetary scientist, Jonathan Lunine of the University of Arizona in Tucson, thinks that Saturn’s moon Titan may offer a more promising venue for life. Pools of liquid methane on Titan, Lunine says, may play the same role that liquid water does on Earth.
“If I have to identify a place where one might find, right on the surface, a self-organizing chemical system, even if it’s not life as we know it, I would say go and look at the hydrocarbon seas of Titan,” says Lunine. “The most dangerous thing we can do is to define life so narrowly that the only place we’re going to find it is Earth.”
Beyond the solar system, researchers have now found more than 345 planets. While most of these extrasolar orbs are blisteringly hot, hugging their parent stars tightly, some lie in the habitable zone, where water would be cool enough to be liquid. NASA’s Kepler mission will soon begin its hunt for Earth-sized planets around 100,000 sunlike stars.
“I’m a wild-eyed optimist that the Kepler mission is going to find hundreds and hundreds of Earths,” says Boss. The mission, however, can reveal only the size of the planet and how far away it lies from its star, not its mass, chemical composition or whether life exists there.
To measure the mass of such a planet will require a space-based mission that can monitor stars for telltale wobbles induced by the planet’s tiny gravitational tug. Such space technology is already available, and a mission could be launched in only a few years, if funding were available. Then, another space mission could examine the starlight filtering through the atmosphere of some of these Earthlike planets to look for possible signatures of biological activity, such as carbon dioxide and ozone, or oxygen in combination with methane. Though not a biomarker in itself, water would also be required for a planet to support life — as least as it is known on Earth, notes Lisa Kaltenegger of Harvard-Smithsonian.
A mission capable of detecting these chemical fingerprints in a planet’s atmosphere wouldn’t be ready for launch for another decade. Such a mission might also manage to take a blurry picture of an Earthlike planet by using advanced techniques to blot out the blinding light from the parent star.
An exoplanet task force that included Boss and Lunine recently noted, however, that there could be a shortcut to looking for habitable exoplanets. Instead of looking for Earth-mass planets orbiting sunlike stars, scientists could focus on superEarths —planets five to 10 times as massive as Earth — orbiting lightweight, cooler stars called M dwarfs. Because M dwarfs aren’t as hot as sunlike stars, the habitable zone lies relatively closer to these low-mass stars. That makes a habitable planet easier to detect. And it’s more likely that such a close-in planet will pass in front of its star as seen from Earth, allowing the starlight to filter through the distant planet’s atmosphere and reveal whether the composition might be compatible with life. The James Webb Space Telescope, scheduled for launch in 2013, could examine superEarths, determining which might be the best candidates for the search for extraterrestrial life.
The universe has no dearth of oddball objects. But the JWST and a slew of other powerful telescopes now in the planning stages are likely to reveal even more exotic beasts that the cosmos has kept under wraps.
Astronomers have long known about neutron stars, the ultracompact cinders left behind by supernovas. These cinders are so dense that they squeeze electrons and protons into giant balls of neutrons. And for more than two centuries researchers have theorized about black holes, which capture all matter and light that enter them.
But even stranger stuff may exist. For instance, particularly massive neutron stars may squeeze neutrons so tightly that they break down into quarks. By measuring the size and radius of neutron stars, researchers are attempting to find evidence for such “strange stars” or “quark stars.”
Another novel space oddity would be a wormhole — a black hole’s distant cousin. In 1935, Einstein and Nathan Rosen realized that general relativity allows such tunnels, which would directly connect two vastly distant regions of spacetime, or even locales in different universes.
Theorists once believed that these proposed portals could exist only for a fraction of a second. But calculations suggest that “exotic matter” — material endowed with a special property called negative energy — could prop a wormhole open much longer.
“There are mathematical solutions, but whether or not they correspond to something in reality remains to be seen,” says cosmologist Michael Turner of the University of Chicago.
Perhaps the strangest notion about the cosmos is that the observed universe is only one among many other universes, each residing in a pocket disconnected from the others.
Inflation — the early epoch of rapid expansion — could allow for an infinity of separate bubble universes. And string theory, which envisions each elementary particle as a string rather than a point, also suggests the existence of a vast ensemble of different universes, each with its own physical laws.
The notion of such a multiverse is the ultimate in the revolution begun by Copernicus nearly five centuries ago, Turner says. Not only is Earth not the center of the solar system or the galaxy, but our universe may be just one of many.
Four hundred years from now, says Turner, inflation may be remembered as the theory that drastically changed people’s view of the cosmos. “It may be infinitely bigger than we imagined,” he says. Much bigger than Galileo could have realized when he first peered at the sky through a crude set of magnifying lenses in 1609.