IT HAS become firmly established over the past few years, at least in the minds of cosmologists, that most of the matter in the Universe is invisible. But while theorists delight in playing with mathematical models that include such exotica as cold dark matter, hot dark matter, weakly interacting massive particles (WIMPs) and mixed dark matter, observers have slowly been uncovering an unpalatable truth: there seems to be much less invisible matter in the Universe than cosmologists like to think.
In December 1993, this 鈥渃hallenge to cosmological orthodoxy鈥� was highlighted in Nature (vol 366, p 429) by Simon White of the University of Cambridge, Julio Navarro and Carlos Frenk of Durham University and August Evrard of the University of Michigan, Ann Arbor. Since then the challenge has intensified.
The need for large amounts of invisible matter in the Universe comes mainly from ideas about events in the first split second of its existence. The standard big bang model includes the concept of inflation, first promoted by Alan Guth in the early 1980s, which describes a phase of extremely rapid expansion just 1O 鈭�35 seconds after the Universe began. This 鈥渟mooths鈥� out irregularities in the large-scale structure of space, leading to a so-called 鈥渇lat鈥� Universe. In the simplest inflation models, this is equivalent to a 鈥渃ritical鈥� state in which there is just enough matter in the Universe to prevent it expanding forever.
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Meanwhile, for the past thirty years or so, cosmologists have had what they think is a good description of how primordial hydrogen was processed into the lightest elements 鈥� deuterium, helium and lithium 鈥� during the big bang itself. The snag is that this otherwise highly successful model for the generation of the lightest elements limits the density of ordinary matter (protons, neutrons and other 鈥渂aryonic鈥� matter) to about one-twentieth of the critical density needed to make inflation work and stop the Universe expanding forever.
So, reasoned the cosmologists, the remaining matter needed to make the sums add up 鈥� the vast majority of the Universe 鈥� must consist of some kind of invisible 鈥渆xotic鈥� particle such as neutrinos, photinos or axions. Unlike baryons, none of these particles carries electrical charge so they cannot interact via the electromagnetic interaction. This means that they cannot radiate electromagnetic waves, including light, and so are dark.
But proving the existence of this exotic dark matter has been extremely difficult. Although neutrinos have been detected, it is still not clear whether or not they possess mass, and hence whether they can solve the missing mass problem. And, although their existence is predicted by the standard theories of particle physics, photinos and axions have never been detected.
Does this mean cosmologists have been on a wild-goose chase for the past thirty years? Not necessarily. The evidence for dark matter comes from a range of observations. For example, the motion of visible stars around the centre of our own galaxy, the Milky Way, suggests it contains at least as much unseen as visible matter 鈥� probably even more. But recent observations of the way objects in our Galaxy bend light from more distant stars in the Magellanic clouds, a phenomenon known as microlensing, suggest that this missing matter need not be exotic after all 鈥� it could consist of dark baryonic matter locked up either in large planets or in faint, low-mass stars (鈥淭he Galaxy鈥檚 dark secrets鈥�, 快猫短视频, 9 April 1994). And evidence for extensive haloes of dark matter surrounding spiral galaxies has been gleaned from the speed at which gas clouds orbit their outer regions. But once again these could be baryonic; observations on a galactic scale give no direct evidence for exotic matter.
Galaxy clusters
But there is no reason to suppose that the contents of galaxies are representative of the Universe as a whole. In fact, when galaxies are first created, baryonic matter may be concentrated in their interior. A new galaxy is believed to form when material 鈥� the universal mix of baryonic and dark matter 鈥� collapses under the influence of gravity. In the standard picture, both the baryonic and the dark matter should pick up plenty of kinetic energy during the collapse. This means that the individual particles should move rapidly, and bounce off one another in high-speed collisions, producing sufficient pressure to keep them spread out over a large volume of space. Baryons can lose this energy by radiating it as light so they cool very quickly and sink to the centre of the young galactic halo to form the galaxy we see today. This leaves the dark matter spread out over a much larger volume, unable to cool because it does not radiate light.
To find a more typical mixture of material astronomers must look at larger structures 鈥� clusters of galaxies. The hope is that these are big enough to contain the universal mix of dark matter, spread throughout the cluster, and baryonic matter, concentrated in the individual galaxies. A large cluster may contain around a thousand galaxies, prevented from collapsing inwards under the force of their own gravity by their extremely rapid speeds, which can be more than a thousand kilometres per second. Astronomers measure the speed of galaxies by taking advantage of the Doppler effect, which shifts the frequency of the light emitted by the galaxies by an amount that depends on how fast they are travelling (this is in addition to the red shift produced by the expansion of the Universe).
When they have calculated the speeds of the galaxies astronomers can then work out their kinetic energy. This must exactly balance their gravitational potential energy otherwise they would fly off in all directions, or collapse in on themselves. Gravity is a function of mass, so astronomers can deduce the mass of the cluster from the total potential energy of the galaxies it contains.
The Swiss astronomer Fritz Zwicky was the first to do this in the 1930s. He made a surprising discovery: there was far more mass in the Virgo and Coma Berenices clusters than could be accounted for by the visible stars (whose mass he inferred from their brightness). This result was so unexpected that for several decades many astronomers simply ignored Zwicky鈥檚 findings.
Invisible gas
Although it didn鈥檛 occur to Zwicky at the time, it has since turned out that much of the 鈥渕issing鈥� mass in the clusters is in the form of hot baryonic gas. In the 1970s, X-ray satellites launched into orbits above the Earth鈥檚 atmosphere identified an X-ray signature from galaxy clusters, which scientists interpreted as radiation from hot, ionised gas.
As the observations improved in accuracy and resolution, it became possible to pin down the amount and location of the gas, and in 1993 White and his colleagues dropped their bombshell. In their paper for Nature, they added up the different contributions to the total mass of the Coma Berenices cluster, which lies roughly 325 light years away from the Earth and contains more than a thousand bright galaxies within a volume some 20 light years across. They concluded that the gas could not account for all of the 鈥渕issing鈥� mass 鈥� some of it still had to come from dark matter.
But, much more worryingly for the cosmological models, when they added up the masses of the galaxies and the gas in the Coma cluster to deduce the total amount of baryonic matter, they found there were still far too many baryons 鈥� a larger fraction, by a factor of at least three, than the standard model can accommodate. This discrepancy came to be known among those working in the field as the 鈥渂aryonic catastrophe鈥�.
However, this and other early studies dealt with only a couple of clusters, and there was always a possibility that these might be exceptions to the rule. Such hopes have been dashed by new, more extensive studies. In March this year, the Monthly Notices of the Royal Astronomical Society (vol 273 p 72) published a paper by David White and Andy Fabian of the Institute of Astronomy in Cambridge examining data from observations by the Einstein Observatory satellite of 19 bright clusters of galaxies. They conclude that gas accounts for between 10 and 22 per cent of the total cluster mass, with an average value of about 15 per cent. Adding in the mass of the galaxies gives a total baryon fraction in the clusters of between 11 and 27 per cent 鈥� far higher than the 5 per cent predicted by combining big bang nucleosynthesis and inflation theories.
So something has to give. Either the models of big bang nucleosynthesis are wrong, or the Universe contains no more than about 25 per cent of its 鈥渃ritical鈥� mass density, implying that at least the simplest inflation theories are wrong.
Is there a way out of this problem? There are various uncertainties in all the models but these are unlikely to alter the conclusions greatly. One major unknown, however, is the rate at which the Universe has expanded from the big bang to its present size. There is a lively debate among astronomers about the exact value of the parameter that measures this expansion rate, the so-called Hubble constant. Recent observations by Wendy Freedman and her colleagues suggest a large value for the Hubble constant (快猫短视频, Science, 20 August 1994) but this has the unfortunate effect of making the Universe, at least in the standard model, younger than the oldest stars it contains.
The distance to the clusters of galaxies used in working out the baryonic fraction has to be inferred from the Hubble constant, and this affects the methods of determining both the amount of gas they contain and their total mass. The upshot is that the lower the value used for the Hubble constant, the higher the calculated baryonic fraction in the clusters.
However, predictions of the baryonic fraction from the time of the big bang depend even more sensitively on the Hubble constant and so the discrepancy between observation and theory could be reduced if the Hubble constant turned out to be very low. But the only way to solve the baryonic problem this way would be to have a Hubble constant of about 6. Few, if any, astronomers would countenance going to such extremes. Moreover, if the Hubble constant dropped below 14 that would imply that baryonic matter makes up more than 100 per cent of the total mass in galaxy clusters, which is obviously impossible.
So it would seem that one of the cherished foundations of the standard model must be relinquished. As Gary Steigman of Ohio State University and James Felten of the NASA Goddard Space Flight Center in Maryland put it, 鈥渢his 鈥榗risis鈥� forces us to consider other ways of mitigating it, including the politically incorrect possibility that the density is less than the critical value鈥�. Perhaps the shakiest assumption in the standard model is that dark matter must be cold, in other words moving slowly relative to the speed of light. Particles of hot dark matter, such as neutrinos, which emerged from the big bang with speeds close to that of light, are unable to cluster efficiently because of their large random motions 鈥� the faster they move, the harder it is for gravity to pull them together. So instead of gathering in galaxy clusters, particles of hot dark matter might populate the empty space in between, balancing the equations.
What could this hot dark matter be? Although the evidence is tentative as yet, physicists at the Los Alamos National Laboratory in New Mexico claim to have measured a mass for the electron neutrino of between 0.5 and 5 electron-volts 鈥� one to ten-millionths of the mass of an electron 鈥� which would imply that hot neutrinos contribute up to 20 per cent of the critical density (快猫短视频, Science, 11 February and 15 April). In fact, hot dark matter cannot comprise more than about 30 per cent of the critical density because interactions between the hot stuff and ordinary baryonic matter would slow the development of structures such as galaxies and clusters, which does not square with the observed number of distant, old galaxies and quasars. Even if neutrinos and other exotic particles make up the maximum 30 per cent of the critical density this would still be nowhere near enough to top up the Universe鈥檚 mass to the critical value.
Improved observations are also unlikely to help. As measurements are made for larger volumes around the clusters, they are likely to show an even greater proportion of mass in the form of gas, which is more extended than either the galaxies or the dark matter (the galaxies themselves congregate in the centres of the clusters). Supernovae or active galaxies can also heat the gas, expelling it from the clusters and exacerbating the baryon discrepancy.
Cosmic voids
A completely different model was proposed by Jerry Ostriker from Princeton University and Len Cowie, now at the University of Hawaii, in the 1970s. They suggested that clusters do not form by the gravitational collapse of large clouds of material, but by the aggregation of mass swept up at the edges of voids produced by huge cosmic explosions. This model naturally concentrates baryons in clusters. Unfortunately it has been ruled out because it would produce large distortions in the cosmic microwave background, the ubiquitous and very uniform radiation that is a relic of the big bang.
The astrophysicist Craig Hogan from the University of Washington in Seattle and his colleagues have toyed with the idea of nonstandard nucleosynthesis, which would, for example, allow the amount of baryonic matter to vary from place to place. This allows some relaxation of the upper limit on the baryon fraction, but the models are rather contrived and, anyway, they do not work as well as the standard one.
Recently, however, Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Dalia Goldwirth of Tel Aviv University in Israel have suggested that all previous estimates of the amount of helium manufactured in the big bang may have been too small. If they are correct, then the alterations to the standard model needed to accommodate a little more helium would automatically increase standard estimates of the amount of baryonic material in the Universe to 20 per cent of the critical density (快猫短视频, Science, 11 March). Naturally, this claim is highly controversial, and most astronomers want to see more evidence before they accept it. But if this idea proves correct, the overall density of the Universe, including dark matter and hot gas in clusters, will equal the critical density, satisfying the theorists.
The alternative is Steigman and Felten鈥檚 鈥減olitically incorrect鈥� suggestion that the mass density of the Universe is much less than the critical density. If 鈥渨hat you see is what you get鈥�, the Universe could contain as much as 20 per cent baryonic material, and have between a quarter and a third of the critical density, with the baryons themselves mostly in the form of hot cluster gas and only about a third in the form of galaxies. The other 80 per cent of the stuff of the Universe would be mainly cold dark matter, perhaps with a smattering of hot dark matter. The Hubble constant could then be rather higher than 50, as the recent observations discussed earlier seem to suggest. But the price cosmologists might have to pay would be abandoning inflation 鈥� at least, in its purest form.
Useful blunder
If cosmologists want to hang onto the idea of a flat Universe, then they may have to reintroduce the idea of a so-called cosmological constant, an addition to the general theory of relativity first suggested by Albert Einstein himself, but which he later described as his 鈥済reatest blunder鈥�. Despite this damning retraction, the cosmological constant is again growing in popularity because it can in effect 鈥渢op up鈥� the density of the Universe to make space flat, and also because it helps to solve the problem of the Universe appearing to be younger than some of the stars it contains. For a given value of the Hubble constant, the Universe is older if a cosmological constant is present. This has recently been given some theoretical justification by Anupam Singh at Carnegie Mellon University in Pittsburgh (快猫短视频, Science, 21 January 1995).
Meanwhile, cosmologists have begun to find ways in which inflation can be tweaked to produce a Universe in which the density of the entire observable volume of space can have any value between 0 and 100 per cent of the critical density (快猫短视频, Science, 7 January 1995). However the 鈥渂aryon catastrophe鈥� is overcome, it seems sure to lead to a fundamental reshaping of cosmological ideas.