B00B7H7M2E EBOK (37 page)

There’s little point in even trying to get a sense of the vastness described here. Probably we come closest when we are feeling most overwhelmed by our
lack
of ability to conceive of such size and how it compares with familiar distances. The box
here
gives approximate relative size scales in the universe
as
calculated in the mid-1990s, but in truth such numbers are beyond our human capacity to comprehend. Only for the sake of rough comparison, therefore, here are some figures having to do with the larger scales: a typical ‘group’ of galaxies is a few million light years across; the Local Group measures about five million light years; a cluster may be 10 to 20 million light years in diameter, a cloud some 30 million light years, a supercluster 100 to 200 million light years.

The question remains: Has astronomy discovered the ‘top’ of the hierarchy, or does it all go on and on to larger and larger structure? Will the voids turn out to be parts of systems of supervoids? Are the superclusters grouped in clusters of superclusters? Will we ever find a level at which the universe is isotropic and homogeneous? Is there such a level?

There are tentative answers to these questions. Pencil beam experts probing six billion light years of space (one three-billion-light-year-long beam pointing out each side of the Galaxy) have found no structure larger than the superbubbles. At extremely great distances, clusters, superclusters and voids seem to be spread more or less uniformly, with about an equal number in any direction you look. Perhaps it is here, in the ultimate sponge, that there is isotropy and homogeneity.

Astronomy has not been able to reveal an unbroken chronology leading from the era of the cosmic background radiation through to the present. There is a gap in the history of the universe like the lost years in the life of someone with amnesia. In terms of the structure of the universe there is one window (that provided by the cosmic background radiation) into the early universe about 300,000 years after the Big Bang . . . and, the next anyone is able to know, there are the voids and supercluster sheets and walls, much closer to the present in time and space. This is the same universe, but no one would guess that from appearances alone. Finding out what happened in the invisible stretch in between is one of the challenges facing the next generation of physicists and astronomers.

However, the one window to the time before that blackout does suggest homogeneity. Though the most interesting recent news about the cosmic microwave background has been the discovery of a minuscule
lack
of homogeneity, it
is
remarkably homogeneous, and it is a picture from further in the past than any other we have. However, it shows the universe only in its extreme youth. It’s like the smooth-skinned glamour shot that shows what a wrinkled dowager looked like in her late teens but gives us barely a hint of the ‘structure’ in that face today. It isn’t evidence that the universe now is isotropic and homogeneous.

No human being, and no universe, can be captured adequately in a still photograph. The question of what is out there can’t be considered meaningfully without asking, as well, how what is out there is moving. What movement does the Galaxy have, for instance, in relation to the rest of the universe besides Andromeda? One way to find out is to measure its motion against the cosmic microwave background radiation, because this radiation comes from a distance far beyond the remotest galaxies. The reasoning is that as the Galaxy moves through the cosmic background radiation, the radiation will measure warmer in ‘front’ of the Galaxy (in the direction towards which it’s moving) and cooler ‘behind’. If the temperature of the radiation reads the same in all directions, then the Galaxy isn’t moving. George Smoot and colleagues measured these temperature differences in the early 1970s from high-altitude U2 planes. They found that the Milky Way Galaxy is moving at a rate of about 600 kilometres per second in the direction of the Virgo cluster. However, the Virgo cluster meanwhile is moving in the other direction and the distance between us is increasing – though not as rapidly as it would were the expansion of the universe the only movement at work here.

The motion of other galaxies besides our own is more difficult to plot, but one of the more intriguing results of this
effort
has been the discovery that several hundred galaxies, the Milky Way among them, are sidling off in a direction and at a speed that the expansion of the universe does not explain. The Great Attractor was the name Alan Dressler, one of those who first calculated this movement, gave to the huge theoretical mass concentration that must be drawing them, but it was difficult to identify any obvious culprit whose gravitational pull was to blame. The identity of the Great Attractor remained for a time one of the baffling enigmas of science, and the mystery has not yet been completely solved. However, in early 1996 Renee Kraan-Korteweg, at the Observatory of Paris-Meudon, and her colleagues reported sighting a massive galaxy cluster that appears to be located just about where the Great Attractor ought to be. Because dust in the Milky Way blocks much of this cluster’s light, scientists had not realized before how wide and massive it is.

Does the universe as a whole move? Of course the universe is expanding, but what about other types of motion? For instance, does the universe rotate? Is it moving through some larger environment? The reply comes in the form of another question: In relation to what could the universe as a whole be said to rotate or not to rotate, or to move or not to move? We have come almost full circle to questions that date back to antiquity. Some of them have become meaningless. Others seem likely to remain forever unanswered. But some of the answers that our ancestors, even those with minds like Kepler and Newton, probably didn’t think could ever be known by human beings are now, according to modern astronomers, astrophysicists and theoretical physicists,
almost
within our reach.

CHAPTER 8

The Quest for Omega

1930–1999

When I started, cosmology was very much like philosophy. There was very little chance of measuring something precisely. It’s now turning into high precision science.

Alexander Szalay

HOW OLD IS
the universe? What is its future? Much of the work going on in state-of-the-art physics and astrophysics at the turn of the twenty-first century focused on these two fundamental questions. In order to answer them, researchers wanted to know the mass density of the universe – the elusive ‘omega’.

The mass density of the universe means the amount of matter there is per cubic metre, averaged throughout the observable universe. Obviously this matter is unevenly distributed, at least on scales normally accessible to us. One way to find out the average density would be to add up all the matter in the universe and then divide by the number of cubic metres in the universe. On the face of it, that would appear to be a ludicrously difficult undertaking.

Never underestimate modern astrophysicists. It is possible to arrive at some estimates. In one method, called ‘representative
sampling
’, researchers divide the sky into sections of equal size, count the number of galaxies in a section, then multiply the count from that section by the total number of sections. Combined with knowledge about the masses of galaxies, this procedure gives a rough estimate of the total mass of the universe. Studies like the Hubble Deep Field make such sampling increasingly substantive.

Another way to try to find the average mass density is to study the way the universe appears to be working – how fast it’s expanding, whether the expansion is speeding up or slowing down, how gravity seems to be affecting different parts of the universe, what other forces besides gravity come into play, and how the contents of the universe have evolved over time. This sounds considerably more difficult than counting galaxies and multiplying by sections. It is extremely complicated. Nevertheless, theorists have a formula that they believe shows how the mass density of the universe is related to such questions, an equation that allows them to weigh the answers one against the other. It is the so-called ‘equation for omega’.

The equation shows how the mass density, omega, affects the future of the universe. If omega turns out to be more than one (meaning that there is more than an average throughout space of one hydrogen atom per 10 cubic metres), the universe will eventually stop expanding and contract. That would be a ‘closed’ universe. If omega is less than one (less than an average throughout space of one hydrogen atom per 10 cubic metres), the universe will expand forever. That would be an ‘open’ universe. If omega is precisely one, then the universe is at the ‘critical density’ that will allow it to expand at precisely the right rate to avoid recollapse, eternally slowing down its expansion but never completely ceasing to expand. That would be a ‘flat’ universe – the type of universe inflation theory predicts. (See box below.)

Why should there be such a tight connection between the mass density of the universe and the fate of the universe? First,
‘mass’
is the measure of how much matter there is – in a planet or star or galaxy or, in this case, in the universe as a whole. Every particle of matter in the universe is attracting every other by means of gravitational attraction. How greatly objects are influenced by one another’s gravitational attraction depends on how far apart they are. The closer they are the more they ‘feel’ one another’s pull. So when it comes to the question of whether or not the universe will eventually contract or whether it will keep expanding, much hangs on how densely or thinly the matter in the universe is spread out. In fact, the mass density very possibly does dictate the fate of the universe.

Solving the equation for omega requires knowing four numbers, three of which are currently not known with certainty. The four are the speed of light, the cosmological constant (the theoretical constant Einstein suggested and that he hoped would allow the universe
not
to expand or contract), the Hubble constant and the deceleration parameter. The third and fourth need introduction:

The
Hubble constant
or H
_o
(pronounced ‘H nought’) denotes the rate at which the universe is expanding. However, it isn’t a direct indication of the speed at which everything out there is rushing away. To give a specific example: if the Hubble constant
is
50, that indicates that there is an
increase
in the recession velocity of 50 kilometres per second for every megaparsec of distance from the observer doing the measuring. Taking the simplest case and remembering that there
are
no receding galaxies this close to Earth, if the Hubble constant is 50 and Galaxy A is one megaparsec away from Earth, Galaxy A should be receding at a velocity of 50 kilometres per second. If Galaxy B, out beyond Galaxy A, is two megaparsecs away from Earth, Galaxy B should be receding at a velocity of 100 kilometres per second. A galaxy three megaparsecs away . . . 150 kilometres per second, and so forth. This also means that if the Milky Way, Galaxy A and Galaxy B were all lined up in a straight line, and if you and I were in Galaxy A, we would find Galaxy B receding at a rate of 50 kilometres per second in one direction and the Milky Way Galaxy receding at 50 kilometres per second in the other. Notice that this does follow the raisin bread rule of twice as far, twice as fast, no matter what value the Hubble constant has, and no matter where you and I stand to watch other galaxies recede.