In the last issue of SKY & SPACE (printed DEC/JAN 1998/99), I discussed one of the hottest topics in modern astronomy - dark matter - so-called because, whatever it is, it doesn't give off any detectable light. Dark matter could be comprised of unseen gas clouds, lots of black holes, burned-out stars, itinerant planets or any one of a number of other things that possess mass. Yet if it can't be seen, why do astronomers believe it exists in the first place? Because measurements of our Universe - and, in particular, the motions of galaxies - indicate that there must be a lot more mass out there than we can actually see. Astronomers refer to this as the 'missing mass' problem.
The previous article was concerned mainly with the hunt by astronomers for the dark matter that is thought to exist in a halo surrounding our Galaxy. This dark matter has been given the acronym MACHOs, or Massive Astrophysical Compact Halo Objects. Using the 50-inch MACHO Telescope at Mt Stromlo Observatory near Canberra, an international team has had some outstanding success in detecting these MACHOs, and there is hope that we are well on the way to understanding the nature and extent of the dark matter that affects our Galaxy.
But there is a second missing mass problem, on a somewhat grander scale. It involves the total amount of mass in the Universe - or, more correctly, the total 'mass-energy density' of the Universe.
Since Edwin Hubble's pioneering work in the late 1920s, we have known that the Universe is expanding, and astronomers have been able to measure the rate at which objects, such as galaxies, are moving apart. Theoretical arguments, which explain several observed facts about our Universe, suggest that we reside in what is known as a 'geometrically flat universe'.
There are three possible fates for the Universe. It may continue to expand forever - referred to as an 'open' universe - or, it may gradually cease to expand and begin to contract back in upon itself - known as a 'closed' universe. The third option is when the universe grinds to a halt and remains there - this is in fact the popular model and is referred to as a 'flat' universe. The name reflects the curvature of the associated space, actually space-time, in the universe. While in a 'closed' universe one can keep travelling in the same direction and return to one's point of origin (rather like circum-navigating the globe) this does not happen in a 'flat' universe.
It is the 'mass-energy density' of the Universe which dictates which scenario will unfold. In the absence of a special property known as the 'cosmological constant', a flat universe will neither undergo a Big Crunch (eventual re-collapse) or a Big Chill (eternal expansion) but will forever remain delicately balanced between these two scenarios. In other words, the expansion should eventually come to a halt. (Strictly speaking, a flat universe, whose expansion gradually slows to zero, will also undergo a Big Chill as the stars and galaxies will eventually burn up all their nuclear fuel and fade away.)
However, the amount of matter that we can see is less than 1% of the amount required to provide enough gravity to eventually bring the Universe to a stand-still, and is sometimes referred to as the 'flatness' problem. This suggests that there must be lots of matter out there that we can't yet see. Dark matter.
In the previous article I mentioned that our Galaxy, the Milky Way, contains around 10 times more invisible mass than visible mass. We know this because of the gravitational influence the invisible matter has on the stuff that we can see.
But there is a further missing mass problem, one that works on a scale even larger than that of our Galaxy, that is, out in the realm of galaxy clusters (some containing over 1,000 galaxies) and clusters of clusters ('superclusters').
Why did matter 'clump' into galaxy clusters and superclusters after the Big Bang, and not simply spread itself throughout the cosmos in a uniform way? What was it about the early Universe that caused this to happen? Measurements of the so-called 'cosmic microwave background radiation' show us that the radiation released by the Big Bang was very evenly distributed, ie. not clumpy. Since ordinary ('baryonic') particles of matter and radiation were mixed together ('coupled') until 300,000 years after the Big Bang, this means that the baryonic matter was not clumpy either. In other words, ordinary matter was too smoothly distributed to be the 'seeds' of the localised gravitational collapses that formed the galaxy clusters and large-scale structure we see in the Universe today. Such structures have to form under the influence of gravity, and this takes a long time. Galaxies and galaxy clusters simply would not have had enough time to form after the 'decoupling' of matter and radiation, given the small degree of clumpiness observed in the 'baryonic' matter straight after the Big Bang.
However, if the early Universe had a large proportion of strange and exotic ('non-baryonic') particles of matter, they could have begun clumping together well before the ordinary matter did. Thus, they could have provided the necessary 'seeding' mechanism that helped to accelerate the gravitational clumping and collapse of the ordinary matter. This could explain why the Universe was quite smooth initially, yet very clumpy today.
Alternatively, it is possible that what we think is a high density of mass in galaxy clusters, compared to the low density in the voids between clusters, is actually an exaggeration of the true level of variation. In other words, perhaps the voids aren't really as empty as we think, but are filled with some form of dark matter. Either way, there needs to be some form of additional dark matter.
In recent years, X-ray observations have revealed that hot gas (at temperatures of a few million degrees) within galaxy clusters, can add up to two or three times the mass observed within the galaxies themselves. In addition, there is a growing body of evidence that suggests a large number of galaxy cluster members (referred to as low-surface brightness galaxies) have been overlooked in the past as they have been simply too faint to detect in the usual surveys. However, while low-surface brightness galaxies, hot gas, MACHOs, and even hypothesised cool gas clouds may make up a good fraction of the dark matter, they cannot be the only remaining piece to the missing mass puzzle - Big Bang nucleosynthesis arguments do not allow it. Big Bang nucleosynthesis refers to the production, or synthesis, of the elements at the time of the Big Bang. Measurements of the Universe's initial ('primordial') abundances of the lightest elements (hydrogen, deuterium, helium and lithium) - from which all other ordinary matter has been created - place a strong upper limit on the total amount of ordinary matter that can possibly exist. This upper limit is about 10 to 15 times the amount of matter that we can see, but still far less than is required to account for the missing 99% of the Universe. The figure is also well short of typical estimates derived from other techniques, all of which imply that only around 30% of the mass needed is present."
This, then, is our second missing mass problem. There is not enough ordinary matter to explain the total mass known to exist in galaxy clusters, and nowhere near enough to provide the mass-energy density needed to balance the Universe between eternal expansion and eventual re-collapse. Some new type of mass is needed.
Particle physicists have no problem coming up with potential alternative dark matter candidates. Neutrinos, for instance, have been known to exist (and have also been detected) for years. These tiny sub-atomic particles are believed to be so numerous - there are roughly a billion times as many neutrinos as there are ordinary particles - that if they had even a minuscule amount of mass, they could represent a large fraction of the total mass of the Universe. The problem has been to determine whether neutrinos actually do possess any mass at all, or if, like particles of light (photons), they have no mass. Last year, a team of American and Japanese researchers presented results that suggest the neutrino does indeed possess mass - exactly how much is not yet known, since only a lower limit has been measured. The lower limit is itself too low to account for any significant fraction of the missing mass of the Universe, but the mass of the neutrino may well turn out to be greater.
Other candidates are the light-weight 'axion' and the somewhat heavier 'neutralino'. These and other 'massive' exotic elementary particles, which may have formed within the first few minutes of the Big Bang, are collectively known as WIMPs (Weakly Interacting Massive Particles). Due to their negligible interaction with ordinary matter they are exceedingly difficult objects to study. (The Sun alone produces so many neutrinos that, every second, some sixty thousand million of them pass straight through every square centimetre of your body completely undisturbed. In fact, the overwhelming majority of these particles pass straight through the Earth without any interruption before continuing off into deep space). It should be noted that the existence of MACHOs does not rule out the possible existence of WIMPs - in fact, it appears that both may be needed.
There is, however, a sort of escape clause which might reduce the pressure on our second missing mass problem. In 1915, Einstein included a special term in his famous equations on relativity. Denoted by the Greek letter, lambda (L), it is known as the 'cosmological constant'. At the time, scientists believed that the Universe was static (that is, not expanding), and so there must have been something that was preventing gravity from collapsing the Universe under its own weight. Thus, Einstein included lambda in his equations to act as a long-range repulsive force against gravity.
In 1929, however, Edwin Hubble showed that this was not the case - the Universe is, in fact, expanding. The cosmological constant was no longer needed. Einstein later publicly renounced it, saying that '[it] was the greatest blunder of my life'. Yet today it is once again in vogue, and it has become a very controversial topic. The cosmological constant represents a possible so-called 'vacuum energy', which has never actually been measured or proven to be real, but which nonetheless has been predicted by modern quantum physics. Heisenberg's famous 'uncertainty principle' implies that things known as 'virtual particles' spontaneously pop into existence in empty space and then almost immediately disappear again. This process gives rise to the 'vacuum energy', which would provide empty space with a repulsive force that acts against gravity. If it can be shown to exist, it could solve the 'flatness' problem of our Universe without the need to invoke the existence of roughly 100 times more dark matter than visible matter. Dark matter would still exist - indeed, it has already been detected - but not as much would be needed as presently thought.
Recently, other studies have begun to narrow down the possible value of the cosmological constant. Every third night, on behalf of Mt Stromlo Observatory astronomer, Dr Brian Schmidt and his collaborators, the MACHO Project searches for supernovae (exploding stars) in distant galaxies. (Observations of the most distant of these are made with larger telescopes in Chile and Hawaii). When one is discovered, many of the world's telescopes, including the Hubble Space Telescope, swing into action to catch the supernova's light before it fades. Because these supernovae all have virtually the same intrinsic brightness, measurements of their apparent brightness can be used to work out how far away they are. By combining this information with their redshifts (a measure of how fast an object is moving away from us due to the expansion of the Universe) it is possible to determine not just the past expansion rate of the Universe, but also it's eventual fate.
Dr Schmidt and his team expected their studies to confirm the popular cosmological model, which predicts a cosmological constant of zero and the presence of roughly 100 times as much dark matter as visible matter. But they did not find this. Instead, they found that in order to comply with the standard Big Bang model and maintain a 'flat' Universe, a cosmological constant of 0.7 is required, together with a mass density only 30% of that required to halt the expansion of the Universe on its own.
Yet, as unexpected as this result is, it is in agreement with work done by another group of researchers, led by Dr Saul Perlmutter of Berkeley (USA), who also published their results early in 1998. When both teams adjusted their theoretical models to have a cosmological constant of zero, and abandoned the idea of a flat Universe, they came up with a result that suggested the Universe has a negative amount of mass. This obviously cannot be correct; so the only way the models can work is if the cosmological constant is greater than zero.
The results imply that while we may still live in a spatially flat Universe, the presence of a large, positive cosmological constant demands that the expansion of the Universe is not slowing down as always thought, but is instead accelerating and will continue to do so forever. While the matter content of the Universe spreads more and more thinly as the Universe expands, the vacuum energy will remain constant and continue to push the Universe apart at an ever-increasing rate.
While further work is still to be done, the presence of a cosmological constant has been implied with a statistical certainty greater than 99%. Thus gravitational forces, and hence all matter, may be but a minor player in the growth of the Universe. The dominating energy/force of the Universe may actually be coming from the fleeting existence of particles which disappear into empty space just as quickly as they appear.
It's a humbling thought. The stuff we are made of - protons, neutrons and
electrons - may comprise only a tiny fraction of the Universe. The bulk of
the
Universe may consist of a sea of elementary particles that go about their
business around us (and indeed through us) largely unaware of our existence.
Far from being at the centre of the Universe, we may be something of a minor
by-product of Nature - we are the ones made of unusual stuff.
The MACHO Project is an international collaboration, established in 1991,
between researchers from Mount Stromlo and Siding Spring Observatories in
Australia, the Lawrence Livermore National Laboratory in California, and the
Center for Particle Astrophysics at the University of California, Berkeley.
The programme operates from Mount Stromlo Observatory's 50-inch telescope
and
is run locally by Dr Tim Axelrod, Professor Ken Freeman, Dr Bruce Peterson
and
Dr Jon Smillie. The observational stage of the programme is due to be
completed at the end of December, 1999, but the analysis will continue for
many more years.
Dr Alister Graham recently left the MACHO Project in Canberra to start a new research position in the Spanish Canary Islands (working at the Instituto de Astrofisica de Canarias), where he is studying the motions of galaxies and the large-scale structure of the Universe.