Dancing With Myself
Page 40
The thing that decides which of these three possibilities will occur is the total amount of mass in the universe, or rather, since we do not care what form the mass takes and mass and energy are totally equivalent, the future of the universe is decided by the total mass-energy content per unit volume.
If the mass-energy is too big, the universe will end in the Big Crunch. If it is too small, the universe will fly apart forever. And only in the Goldilocks situation, where the mass-energy is “just right,” will the universe ultimately reach a “flat” condition. The amount of matter needed to stop the expansion is not large, by terrestrial standards. It calls for only three hydrogen atoms per cubic meter.
Is there that much available?
If we estimate the mass and energy from visible material in stars and galaxies, we find a value nowhere near the “critical density” needed to make the universe finally flat. If we arbitrarily say that the critical mass-energy density has to be equal to unity just to slow the expansion, we observe in visible matter only a value of about 0.01.
There is evidence, though, from the rotation of galaxies, that there is a lot more “dark matter” present there than we see as stars. It is not clear what this dark matter is—black holes, very dim stars, clouds of neutrinos—but when we are examining the future of the universe, we don’t care. All we worry about is the amount. And that amount, from galactic dynamics, could be at least ten times as much as the visible matter. Enough to bring the density to 0.1, or possible even 0.2. But no more than that.
One might say, all right, that’s it. There is not enough matter in the universe to stop the expansion, by a factor of about ten, so we have confirmed that we live in a forever-expanding universe.
Unfortunately, that is not the answer that most cosmologists would really like to hear. The problem comes because the most acceptable cosmological models tell us that if the density is as much as 0.1 today, then in the past it must have been much closer to unity. For example, at one second A.C., the density would have had to be within one part in a million billion of unity, in order for it to be 0.1 today. It would be an amazing coincidence if, by accident, the actual density were so close to the critical density.
Most cosmologists therefore say that, today’s observations notwithstanding, the density of the universe is really exactly equal to the critical value. In this case, the universe will expand forever, but more and more slowly.
The problem, of course, is then to account for the matter that we don’t observe. Where could the “missing matter” be, that makes up the other nine-tenths of the universe?
There are several candidates. At this point it might be good to honor the promise made in Section 1, that I would announce it in advance when we reached the highly speculative science. We are certainly there now, and perhaps I ought to point out that to some workers we got there some time ago. To them, the Big Bang itself is not the preferred cosmological theory.
One suggestion is that the universe is filled with energetic (“hot”) neutrinos, each with a small but non-zero mass (as remarked earlier, the neutrino is for most purposes assumed to be mass-less). Those neutrinos would be left over from the very early days of the universe—so we are forced back to studying the period soon after the Big Bang. However, there are other problems with the Hot Neutrino theory, because if they are the source of the mass that stops the expansion of the universe, the galaxies, according to today’s models, should not have developed as early as they did in the history of the universe.
A few years ago, observations from a short-duration rocket experiment sent up in February 1987 seemed to give evidence that the cosmic background radiation had features in its spectrum that argued against hot neutrinos. However, COBE (the Cosmic Background Explorer) satellite, launched last year, showed a very smooth spectrum for the background radiation, and hot neutrino theories gained a few points.
What about other candidates? Well, the class of theories already alluded to and known as supersymmetry theories require that as-yet undiscovered particles ought to exist.
There are axiom, which are particles that help to preserve certain symmetries (charge, parity, and time-reversal) in elementary particle physics; and there are photinos, gravitinos, and others, based on theoretical supersymmetries between particles and radiation.13 These candidates are slow-moving (and so considered “cold”) but some of them have substantial masses. They too would have been around soon after the Big Bang. These slow-moving particles clump more easily together, so the formation of galaxies could take place earlier than with the hot neutrinos. We seem to have a better candidate for the missing matter—except that no one has yet observed the necessary particles. At least neutrinos are known to exist!
Supersymmetry, in a particular form known as superstring theory, offers at least one other possible source of hidden mass. This one is easily the most speculative. Back at a time, 10-43 seconds A.C., when gravity decoupled from everything else, a second class of matter may have been created that is able to interact with normal matter and radiation, today, only through the gravitational force. We can never observe such matter, in the usual sense, because our observational methods, from ordinary telescopes to radio telescopes to gamma ray detectors, all rely on electromagnetic interaction with matter. The “shadow matter” produced at the time of gravitational decoupling lacks any such interaction with the matter of the familiar universe. We can determine its existence only by the gravitational effects it produces; which, of course, is exactly what we need to “close the universe.” Unfortunately, the invocation of shadow matter takes us back to such an early time that if we are sure of anything, it is that the universe was unrecognizably different then from the way that it is today.
7.ORIGINS
We have discussed the present state of the universe. We have discussed the early state of the universe. What we have not done, any more than Georges Lemaître or George Gamow did, is to ask the most basic question: Where did the universe come from?
The answer, until twenty years ago, was probably, Nobody can say. The most popular modern answer, perhaps not much more satisfying, is: It came from nothing.
That statement may call for a little explanation. Since this article is already becoming rather long and we are nowhere near the end, a little explanation is all I will give. However, I am not sure that a long explanation would be any more persuasive.
We need an idea from quantum theory. One of the best-established concepts of that subject, and one of the most famous, is the Heisenberg Uncertainty Principle. In its most familiar form, this states that one cannot know both the precise position and the velocity of a particle simultaneously. A more general formulation if that one cannot specify the values simultaneously of any pair of “conjugate variables,” such as position and momentum; or, to pick the pair that we want, of energy and time.
This means that there can be a large uncertainty or fluctuation in energy, provided only that the duration of the uncertainty is short enough. Conversely, if the energy fluctuation has zero net energy, then it can be around for an indefinitely long time.
The first person to suggest in print that the whole universe might be nothing more than an energy fluctuation with zero net value was Edward Tryon, in a paper published in Nature in 1973. At the time, his suggestion was cheerfully ignored.
It certainly sounds ridiculous on first hearing. We sit in a universe that absolutely fizzles with energy, everything from gigantic stellar furnaces, like our own sun, pumping innumerable gigawatts into space every second, to supernovas, briefly shining a hundred billion times as bright. How can anyone propose that the universe has zero net energy?
To see that, we have to go back to Lord Kelvin, and his suggestion that the Sun shone because of its own contraction. If solar contraction releases energy, then moving the atoms that compose the Sun farther and farther apart must require energy.
How much energy would it take to move the atoms of the
Sun from very close together, to indefinitely far apart? The answer to that, known for fifty years, is a curious one: the total energy needed is exactly the amount that would be produced were the Sun’s mass totally converted to energy. In the language of the physicist, the rest mass energy of the Sun is equal and opposite to its gravitational potential energy.
Exactly the same argument can be applied to the whole universe, to show that the total material energy (matter plus radiation) is equal and opposite to the total gravitational potential energy. The net energy is thus exactly zero. And a fluctuation of zero energy, according to the Heisenberg Uncertainty Principle, can sustain itself for an indefinitely long time.
The universe was created out of nothing, by a zero energy fluctuation. And one day it may simply disappear, when the vacuum fluctuation that created it pops out of existence.
If this explanation appears unsatisfactory, let me point out that there are alternative theories, all at least as implausible on first hearing as Tryon’s “Vacuum fluctuation” universe. The “steady state universe” of Bondi, Gold, and Hoyle, popular in the early 1950’s but banished from serious consideration later in that decade by observations from radio astronomy, has been revived recently by Hoyle and refurbished with the aid of a concept of cosmology known as inflation. In the steady state universe, the origin of the universe does not have to be described, because the universe has been around forever.
The “no boundary” universe of Stephen Hawking is a more recent idea. It does not posit an eternal universe in the sense of the steady state theory, but instead it suggests that the apparently unique moment of the Big Bang is an artifact, created by the way that we choose to measure things.
The idea proposed by Hawking sounds like a simple change of coordinates, but it is much more than that. Anyone who looks at the early history of the universe, in which significant events occur closer and closer together as we approach the moment of creation, will suspect that maybe we are using the wrong method of measuring time. Our system applies well today, but perhaps it was totally wrong near the Big Bang. To take one example, suppose that we define a transformation of the usual time coordinate, t, as measured from the moment of the Big Bang, and replace it with a new “time” variable, T, defined by: T = log(tN/t)
In this formula, the constant tN is the present age of the universe, which we’ll take to be 15 billion years (the actual present age is not important). As we go backwards in time to the different events of Section 6, Table 1 lists the values we find for t and T:
Table 1
Event
t
T
Today
Birth of solar system
Galaxy formation begins
Atoms form
Helium forms
Electron/positron pairs vanish
Neutrinos decouple
Nuclear matter density
Inflation of universe
Gravity decouples
15 billion years
4.6 billion years
1 billion years
1 million years
3.75 minutes
30 seconds
1 second
0.0001 seconds
10-35 seconds
10-43 seconds
0
0
1
4
15
16
17
21
51
60
.51
.18
.18
.32
.20
.67
.67
.67
.67
The values of T are increasing as t tends to zero, as the Table shows; but they are not outlandish even when t is very close to zero. The crowding of events closer to the Big Bang is no longer evident. However, the singularity at t = 0 is still present in this picture—whereas in Hawking’s model, there is no Big Bang at all. I’m not sure which I like better.
8.THE INCOMPLETE BIOGRAPHER
In order to know what the universe is like today we were forced back, much as we might like to have avoided it, to the earliest times.
However, we are now able to draw a quick sketch of the object whose biography we set out to write.
The moment of birth is shrouded in mystery. But we have been able to describe, in rather definite terms, everything from a fraction of a second after that birth, through the annihilation of electron-positron pairs, through the formation of neutrons, on past the formation of helium nuclei, and then to the production of stable hydrogen and helium atoms. The heavier elements came much later, by high-temperature cooking in the interior of stars and through stellar explosions as supernova, but the mechanics of that process are reasonably clear.
We also believe that we understand fairly well the way in which radiation “decoupled” from matter, so that the universe of matter became almost transparent to radiation when it was about a million years old. We observe today the leftover, or “relict” radiation from that time, as a cosmic background radiation at about 2.7 Kelvins, i.e., 2.7 degrees above absolute zero. It gives us our most direct information about the early days of the universe.
We also see, by direct observation, a universe in which stars are aggregated in the great hundred-billion star groupings we call galaxies; and the galaxies, receding faster and faster at greater distances, are themselves scattered through the whole of visible space.
We believe that there is just enough matter in the universe to slow its final expansion to zero. That belief is based on faith in our cosmological theories, since we cannot detect most of that matter. In fact, we observe directly only one percent, and infer another ten percent by gravitational effects on the rotation of galaxies.
What is missing in the biography? Oddly enough, the biggest historical gap is not at the beginning of the universe, though certainly things there become very speculative. But the big blank spot occurs during the adolescence of the universe, between the time it was a million and a billion years old.
We know surprisingly little about that period. It was then, in all likelihood, that the formation of galaxies began; perhaps also the formation of stars. The problem is, the only way to make direct observation is to peer deep, deep into space, seeking light which set out on its journey when the Big Bang was less than a billion years in the past and is only just reaching us.
That light, on its way for something over ten billion years, has been shifted in wavelength by the expansion of the universe to five or more times its original wavelengths. If we want to observe in visible light, we have to examine sources that originally generated signals in the far ultraviolet region of the spectrum. And signals, originally visible, reach us as shortwave infrared radiation—at wavelengths for which the Earth’s atmosphere is likely to be opaque.
There is one other way in which this brief biography is clearly incomplete. For any biography, unless it is of a dead person, is always incomplete. The universe is anything but dead. It still exists, it is still changing, and we are still observing it.
How far along in its evolution is the universe, today? From a strictly qualitative point of view, one might say that it is by definition halfway. The whole of time can conveniently be divided into three parts: we have the past, and the future, and the moving knife-edge of the present that separates the two.
That seems to me like cheating. I like numbers, and I would like a numerical answer. If the universe is 10 to 20 billion years old now, for how many more years will it exist?
Some may fin
d that a meaningless question. How can the universe possibly not exist? It sounds like a metaphysical issue, the same as asking what was there before the universe.
But there are ways in which the universe can go through a change whereby no information about anything in the universe that we know can possibly survive, including the physical laws.
Suppose that the Universe collapsed back into a Big Crunch. After that happened one could with justice say that at least our universe no longer existed. And yet in another sense we will find that we always have an infinite amount of time available to us.
9.ALTERNATIVE FUTURES
“When I dipped into the Future far as human eye could see,” said Tennyson in Locksley Hall. Writing in 1842 he did pretty well, foreseeing air warfare and universal world government. We can go a long way beyond that.
Let’s start with the “near-term” future. In another five billion years or so our own Sun will run out of fusion fuels and begin to swell up to become a red giant star. In doing so it will expand far enough to include Earth’s orbit within it.
That should not be a problem for humanity. Long before five billion years have passed we will have moved beyond the solar system. We can go, if we like, to sit around a smaller star. It will be less prodigal with its nuclear fuel, and we can enjoy its warmth for maybe a hundred billion years. By that time the needs of our descendants will be quite unknowable.
However, before that time something qualitatively different may have happened to the universe.
We have stated a preference for a “just-closed” Universe that expands forever, but slower and slower. That happens when the mass-energy density is just enough. Suppose, however, that the mass-energy density is more than enough to close the universe. Then the present expansion must be followed by a contraction. At last there will come a Big Crunch, rivaling the Big Bang in its temperatures and pressures. Everything familiar to us, certainly including life, will be destroyed in that final moment of infinite extremes.