B00B7H7M2E EBOK (38 page)

The
deceleration parameter
measures the rate at which the expansion is slowing down due to the gravitational attraction among all the clusters of galaxies. It might seem astrophysicists ought already to know what that is. If it’s true that when we observe very distant galaxies and galaxy clusters we are seeing them as they were billions of years ago, why isn’t it possible to compare the rate at which they are receding with the rate at which nearer galaxies are receding, find out whether the expansion has slowed down and, if so, how much? Astrophysicists are trying to do just that, but it isn’t easy. Some have suggested that one reason why it’s difficult may be that the universe
is
perfectly balanced between the mass density that would allow it to expand forever and the mass density that would cause it to collapse to a ‘Big Crunch’. In other words, perhaps the very difficulty of making that determination is a clue that omega equals one and the universe is expanding at
precisely
the right rate to go on forever, always slowing down its expansion but never stopping the expansion and collapsing.

One source of complication is that the mass density of the universe is changing over time. Unless new matter or energy is appearing on the scene (which the Steady State theory proposed but most physicists don’t believe is happening) things inevitably grow less and less dense in an expanding universe. They thin out.

It is the interrelatedness of these numbers, or values, that’s laid out concisely in the equation for omega. It doesn’t take much expertise to see that there are relationships, that one thing depends on another. The equation shows precisely in what manner and to what degree they are related. At the risk of sending a great many readers running for cover, here (
Figure 8.1
) is the formula for omega. Consider it a souvenir, something a patient reader is owed for having made it so far with this book! We will
not
proceed to solve it.

How far have researchers got in the process of discovering the unknown numbers in that equation? They know the speed of light. What a pleasure to be able to plug in one actual number here! No one yet knows the value for the deceleration parameter or the cosmological constant. There is disagreement over
the
Hubble constant. Modern scholars find themselves in very much the same situation their forebears were in when they had Kepler’s laws but not Cassini’s and Flamsteed’s measurements of the distance to Mars. Here is a formula – but not all the numbers to put into it.

Figure 8.1 The Formula for Omega

One serious problem in estimating how much matter there is in the universe is that there actually doesn’t seem to be enough of it around. In the 1930s, Swiss astronomer Fritz Zwicky discovered that galaxies in the ‘great cluster’ in the constellation Coma Berenices were moving too rapidly, relative to one another, to be bound together by their mutual gravitational attraction. Given the way gravity works, and what can be seen of this cluster of galaxies, the arrangement should be flying apart. Searching for an explanation, Zwicky thought of two possibilities. What appeared to be a cluster might instead be a short-term random grouping of galaxies;
or
there might be more to these galaxies than met the eye or the telescope. In order to provide the amount of gravitational attraction required to bind the cluster together, it would have to contain much more matter than we observe. No one was willing to consider the third possibility that physicists might have made an egregious error in figuring out how gravity operates, or in assuming that it operates the same everywhere.

With Zwicky’s discovery was born the puzzling notion that it may be impossible to observe more than a tiny fraction of all the matter in the universe. In the years since he first speculated about it, plenty of support has emerged for the existence of ‘dark matter’. That support has been both observational and theoretical. For everything to work as it appears to do, there has got to be much more matter in the universe than present technology is able to detect. By some calculations, from 90 to 99 per cent of the matter in the universe is not radiating at any wavelength in the entire electromagnetic spectrum. While other pieces of the Big Bang picture fell into place, the missing matter remained a mystery.

There is an example much closer to home than the constellation Coma Berenices: the mass and distribution of observable matter in the Milky Way Galaxy isn’t sufficient to account for the way the Galaxy rotates. What would it take to cause the Milky Way to rotate as it does? The matter should be mostly outside the visible disc of the Galaxy, it ought to extend well beyond the edge of the observable disc, and much of it should not be level with the disc but ‘above’ and ‘below’ it. If all that were the case, then the rotation would make sense. The suspicion is that the Galaxy must be surrounded by a halo of dark matter that is much larger than the observable mass of the Galaxy. The total diameter of the Galaxy might be four or five times what it is possible to observe in any range of the spectrum. Dark matter might also provide an explanation for the ‘hat brim’ tilt of the Galaxy’s thin gas disc.

There is no way to investigate dark matter directly, only by watching how it affects other things – that is, what its gravitational effect is on other matter and radiation. Sometimes it gives its presence away by the manner in which it bends the paths of light. Such paths through spacetime are bent by the presence of massive objects (‘benders’) such as stars, planets, galaxies and galaxy clusters. This happens regardless of whether or not the benders are themselves detectable at any wavelength. When the distortion is too great to be caused by the observable matter in the bender, or when there is no observable bender at all, researchers know they are not observing everything that’s out there between them and the background. They suspect the presence of dark matter.

The mystery of dark matter lies at the heart of the problem of measuring the age and the future of the universe. Calculating roughly whether there is sufficient observable matter in the universe to produce the gravitational attraction necessary to keep the universe at critical density, omega-equals-one, shows that the amount of matter observed directly with present technology falls far short. But the discussion doesn’t end there,
because
dark matter does exist and because no one is yet certain how much there is or what it is.

Big Bang theory in its most straightforward form has it that even a microscopic deviation from omega-equals-one would have caused the universe very early on to recollapse or made it expand so rapidly that stars could never have formed. Inflation theory has proposed a solution to that fine-tuning problem, but the question right now is:
Is
the universe all that fine-tuned? It seems things would be dramatically different if it weren’t, but it isn’t actually obvious how it
is
. No measurement of existing mass density comes anywhere near critical density, which means that the universe should have expanded too fast for stars to form. It didn’t. What is it we don’t know yet?

Candidates for dark matter range from still-hypothetical mysterious exotic particles to black holes a billion times more massive than the sun. Primordial black holes (tiny ones formed in the early universe), planets, dwarf stars too dim to have been observed, massive cold gas clouds, comets and asteroids, and an assortment of dead or failed stars make up a broad middle ground of possibilities. Some physicists insist on tossing in a few copies of the
Astrophysical Journal
.

In 1998, hard-to-detect particles known as neutrinos moved to the short list. The existence of neutrinos is not a new idea. They were first suggested in 1930 by Wolfgang Pauli as a way to explain a mysterious loss of energy in some nuclear reactions, but it was not until 1956 that observations by Frederick Reines and Clyde Cowan at the Los Alamos National Laboratory in New Mexico confirmed their existence outside of theory.

No one now questions the existence of neutrinos, but they remain notoriously difficult to study. They rarely interact with any kind of matter. A typical neutrino can pass through a piece of lead a light year thick without hindrance. Clues to their existence come on those rare occasions when a neutrino does happen to collide with an atom, but even then the evidence is indirect.

Whether neutrinos have any mass at all has been in question, and of course if they have no mass they cannot be contributing to the mass density of the universe. There have been a number of claims in the last few years of the discovery of neutrino mass, but much stronger evidence came in June of 1998, from a team of Japanese and American physicists at an observatory in Takayama, Japan.

Their detector was a tank the size of a cathedral containing 12.5 million gallons of ultra-pure water, inside a deep zinc mine one mile inside a mountain. The rationale for the experiment ran like this: one of the ways neutrinos are produced is when cosmic ray particles from deep space slam into the Earth’s upper atmosphere. Experimenters hoped to compare neutrinos that came from the upper atmosphere directly over the detector (a short distance) with those that were coming up from under the detector after having passed through the Earth (a long distance). Neutrinos from both sources, moving through the water, would occasionally collide with an atom. The result of such a collision is a scattering of debris, and the particles of that debris race through the water creating cone-shaped flashes of blue light called Cherenkov radiation. The light is recorded by 11,200 20-inch light amplifiers that line the inside of the tank. Researchers analyse the cones of light, finding the proportions of different sorts of neutrinos coming from each direction, and attempt to determine whether the neutrinos, which come in three types, change type on their journey from the upper atmosphere. If neutrinos can make this change, that means they must have mass.

Dr Yoji Tkotsuka, leader of the team and director of the Kamioka Neutrino Observatory, the site of the detector, announced that the evidence was strong for neutrino mass. ‘We have investigated all other possible causes of the effects we have measured,’ he reported, ‘and only neutrino mass remains.’

Calculations based on these findings show neutrinos might (not everyone agrees they do) make up a significant part of the
mass
of the universe. Not that a single neutrino amounts to much. The mass of a neutrino turns out to be almost infinitesimal – about
of the mass of an electron. But neutrinos nevertheless pack considerable clout by dint of their numbers, for there are about 300 of them in every teaspoonful of space. They outnumber other particles in the universe by a billion to one. In fact, the discovery of that tiny mass by the team in Takayama adds considerably to the mass density of the universe, by some calculations more than doubling it at a stroke.

As promising as this might appear, the discovery that neutrinos have mass can’t account for all the missing matter that calculations show ought to exist. And though the combined mass of all those neutrinos may be enough to slow the expansion of the universe, it isn’t likely to be enough to stop it or turn it around. The puzzle still has not been solved. The discovery of neutrino mass hasn’t revealed the future of the universe.

In the mid-1980s, a panel of astronomers reviewing plans for use of the Hubble Space Telescope decided that the determination of an absolute distance scale outside the Galaxy and the discovery of the expansion rate of the universe – the Hubble constant – should be among the highest-priority projects undertaken by the telescope. A team of astronomers from the United States, Canada, Great Britain and Australia, led by Wendy Freedman of the Carnegie Observatories in Pasadena, California, received the largest allocation of time on the Hubble Space Telescope for a period of five years. The Extragalactic Distance Scale Key Project, as they call their work, involves trying to determine distances to nearby galaxies more accurately than ever before. These galaxy distances will then form the underlying basis for a number of other methods that can be applied at more remote distances, making possible several independent measurements of the Hubble constant.