THERE’S a lot of stuff missing in the Universe. We’ve known that for decades. Most of it is tied up in a mysterious form of dark matter that will forever be invisible, and in an even weirder form called dark energy. Only a tiny fraction of the Universe consists of ordinary matter, the stuff that makes up stars and galaxies.
While we search for clues to the murky nature of dark matter and dark energy, surely we can derive comfort from the knowledge that the ordinary matter we can detect is all present and correct?
Not exactly. We now know that just 4 per cent of the Universe is made of “baryonic” matter, the ordinary atoms that we are made of. Yet decades of observations have revealed an uncomfortable truth: when astronomers add up all the stars, galaxies and gas clouds, the total mass falls well short of this. Only around a quarter of the baryonic mass is tied up in objects we can see.
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Theoretically, up to another quarter or so of the missing baryons may be locked inside objects too faint for our telescopes to pick up, such as burned-out stars, small planets or stars that failed to ignite. But these shadowy entities are still not enough to make up the shortfall. Are more than half the baryons still missing? Or dare we believe that, after years of hunting, astronomers have at last found them, strung like cobwebs throughout the cosmos?
The lost baryons account for just 2 per cent of the stuff in the Universe. That might not sound like much, but it’s a mammoth amount. Around 1080 atoms are unaccounted for, equivalent to 10,000 billion billion Suns.
Just as the enormity of the crisis struck astronomers in the mid-1990s, clues as to the whereabouts of the lost baryons started to emerge. Eight years ago Richard Lieu, then at the University of California in Berkeley, and his colleagues were studying data from NASA’s Extreme Ultraviolet Explorer (EUVE), the first satellite to map the sky at the shortest ultraviolet wavelengths.
The last thing they expected to see at these wavelengths were galaxy clusters – congregations of galaxies filled with gas as hot as 10 million kelvin. At these temperatures hot gas gives off high-energy X-rays, not the lower-energy ultraviolet radiation that EUVE was tuned to detect. So they were astonished when they noticed the Virgo cluster of galaxies beaming out far more extreme-ultraviolet radiation than expected. “At first we couldn’t believe what we saw,” says Lieu, who is now at the University of Alabama in Huntsville.
Lieu and his colleagues suspected that much cooler gas was being sucked into the galaxy cluster from intergalactic space. It was a bold idea: intergalactic space was thought to be empty. But if they were right, it meant the Universe is filled with a wispy gas of baryons.
No one believed them, though. “We were given a hard time,” recalls Lieu. Many astrophysicists questioned the team’s analysis and interpretation of the data. For a start, the cooler gas in the Virgo cluster might not be intergalactic in origin. Critics argued that the gas clumped around Virgo wouldn’t be found in every cluster and would fail to account for all the missing baryonic matter in the Universe.
Medium rare
But Lieu and his team were not alone. At Princeton University, theorists Renyue Cen and Jeremiah Ostriker had been trying to explain what had happened to the lost baryons and had come up with the same idea: that they are held in a tenuous intergalactic medium.
They knew the baryons hadn’t always been missing. Long before galaxies began to form, 3 billion years after the big bang, baryonic matter was spread throughout the Universe. Back then, the gas was dominated by hydrogen at a relatively cool 10,000 kelvin. By studying the spectral “fingerprint” hydrogen leaves in the light from distant quasars that were around before galaxies formed, astronomers have been able to discover how much neutral hydrogen there was in the early Universe. Their observations show that all the baryons were present and correct, in agreement with the predictions of big bang theory.
But in today’s Universe, the clouds of hydrogen are all but gone. Cen and Ostriker decided to test the assumption that most of the gas was eaten up during galaxy formation, a process that started some 10 billion years ago and continues today. Clusters of galaxies start to take shape when regions with slightly more matter than the surrounding space exert their gravitational pull on nearby matter.
Computer simulations developed by theorists over the past 30 years show that the dark matter that dominates the Universe tends to be pulled in a particular direction until it is eventually drawn out into filaments. These strands crisscross each other to form a giant cosmic cobweb. As the dark matter forms this tangled mesh, its gravitational pull takes the baryonic matter along for the ride. Over time, the densest knots in the web turn into the large congregations of galaxies we see today. So did all the baryons end up in galaxy clusters?
According to Cen and Ostriker’s simulations, they did not. Most of them are still in intergalactic space, but they are just too hot to spot easily. Cen and Ostriker showed that the process of galaxy formation sends shock waves through intergalactic space, heating the gas to about 1 million kelvin. They called it “warm-hot intergalactic matter” (WHIM) – baryons spread so thinly throughout the Universe they cannot transfer heat to each other. Insulated by vast expanses of space, the baryon gas cannot cool efficiently. Cen and Ostriker calculated that as many as half of all the baryons produced shortly after the big bang still sit in a web of filaments strung between galaxy clusters.
However, their conclusions remained buried in a little-known theoretical paper published in 1994 until Lieu and his colleagues stumbled across them a year later. Seizing upon the theorists’ work as supporting evidence for their observations, they pointed out that the WHIM would be too hot for optical telescopes to see, yet it would still be much cooler than the fiery gas in galaxy clusters, which emits high-energy X-rays. Instead, they argued, the WHIM would beam out low-energy X-rays and extreme-ultraviolet radiation.
Detecting this radiation is not easy, though, because low-energy X-rays are easily absorbed by hydrogen in the Milky Way, so any signal would be faint. Missions to map ultraviolet light in the Universe, just as EUVE did, might have a better chance. And Lieu is convinced that EUVE did indeed spot the telltale ultraviolet light from the WHIM in the Virgo galaxy cluster. “The detection of the WHIM is the legacy of the EUVE mission,” he says.
It now looks as though he might be right. Over the past few years, the body of evidence in favour of the WHIM has grown. Two years ago, Todd Tripp of Princeton University and Blair Savage, working separately at the University of Wisconsin in Madison, looked at the radiation coming from two quasars over 3 billion light years away. Both teams were interested in the spectral fingerprint of highly ionised oxygen that theory predicts will pepper the WHIM. Oxygen was among the heavy elements produced in the first generation of stars, which later exploded, scattering their contents like confetti throughout the Universe. In the hot conditions of intergalactic space, oxygen atoms shake off five or six of their eight electrons to leave O2+ and O3+ ions.
Lost in space
Tripp and Savage’s teams found that certain ultraviolet wavelengths were missing from the quasars’ spectra. They deduced that radiation from the quasars was being absorbed by oxygen ions on its journey through space. From the way the absorbed light was shifted to longer wavelengths, they worked out how far away the O3+ ions were. This placed the ionised oxygen firmly in intergalactic space. Tripp and Savage had uncovered signs of the WHIM.
More recently, a team led by Fabrizio Nicastro of the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used the same technique to spot O3+ ions in gas clouds close to the Milky Way. Nicastro and his colleagues believe that the clouds are part of the WHIM that has settled in a halo around the Local Group of Galaxies, which takes in 30 galaxies, including the Milky Way.
Last summer, Claude Canizares at the Massachusetts Institute of Technology, and Taotao Fang, now at Carnegie Mellon University in Pittsburgh, observed a distant quasar using the Chandra X-ray Observatory. They found signs that X-rays from the quasar had been absorbed by a cloud of O2+ ions 800 million light years away. And Jelle Kaastra of the Space Research Organization Netherlands in Utrecht and colleagues have searched for the faint X-ray glow given off by O2+ ions using a space-based telescope called XMM-Newton. They claim to have found the telltale glow in the outer regions of five galaxy clusters. Even though XMM-Newton has large mirrors well suited to observing faint and diffuse X-rays, Kaastra believes his team has only identified the densest parts of the WHIM that surround big clusters.
With Chandra and XMM-Newton uncovering independent evidence for the WHIM, astronomers are only now daring to believe that the lost baryons have been found. “None of the observations by themselves are conclusive,” says Ostriker. “But together they are consistent with the theory.”
Of course, not everyone accepts that the lost baryons have been recovered. When a team led by Andrew Rasmussen of Columbia University pointed XMM-Newton at Canizares and Fang’s quasar, they failed to spot the telltale spectral fingerprint of the lost baryons. And sensitive as XMM-Newton is, it still is not responsive enough to detect the tenuous filaments of WHIM between galaxy clusters. The evidence for the WHIM may be tantalising, but we still don’t know if it is caught in a cosmic cobweb as Cen and Ostriker’s theory predicts.
“We really have to find these low-density filaments,” says Joel Bregman, an astronomer at the University of Michigan in Ann Arbor who has searched for the X-ray glow from the WHIM. Only then can we hope to study the characteristics of the lost baryons, such as their distribution, density and mass.
We should know for sure where the lost baryons really are when NASA launches a satellite called SPIDR (Spectroscopy and Photometry of the Intergalactic medium’s Diffuse Radiation) in 2005. As well as looking for ultraviolet sources, such as comets and interplanetary gas, SPIDR will spend about a third of its time mapping the O3+ ions trapped within the cosmic web. Supriya Chakrabarti, an astronomer at Boston University who is leading the mission, is confident that it will succeed even though everyone expects the ultraviolet light to be extremely faint. And looking further ahead, in 2010 NASA hopes to launch Constellation-X, a fleet of four X-ray telescopes that will work together to map the WHIM in great detail.
Although these missions won’t help us to understand what the remaining 96 per cent of the Universe is made of, it is comforting to know that they will solve a critical part of the missing mass enigma – accounting for all the ordinary stuff from which people, planets and stars are built. “This is not a small field,” says Lieu. “After all, we’re talking about 50 per cent of all the atoms in the Universe.”