The Cosmic Landscape (24 page)

Perhaps the resolution is that our theory of gravity—the General Theory of Relativity—is just plain wrong. In fact some physicists have jumped to this conclusion. These physicists usually try to make modifications in the theory that will only affect the gravitational force at very great distances. Personally, I don’t find much merit in these schemes. They are usually very contrived, often violate fundamental principles, and in my opinion, are quite unnecessary.

Another possible way out is to suppose that astronomers are taking the precision of their data too seriously. You can make a good living betting against experimental data that contradict the prevailing expectations. Such data are almost always wrong, and further experimentation usually proves it. In this case I would have bet against the astronomical data, not the theory. But it seems I would have lost my bet. As the data have improved over the last few years, they reinforce the fact that observation and theory are at odds with each other. There really is something wrong.

But one possibility lurks just beneath the surface that cannot be easily dismissed. What if there is a small cosmological constant after all? What if Einstein’s greatest blunder was really one of his greatest discoveries? Could that resolve the conflicts?

When we considered whether the observable mass in the universe would be enough to render it flat or closed, we completely ignored the possibility of vacuum energy. That would be a mistake in a world with a cosmological constant. Einstein’s equations say that
all
forms of energy affect the curvature of space. Energy and mass are the same thing, so vacuum energy must be counted as part of the mass density of the universe. The ordinary and dark matter together add up to about 30 percent of the mass needed to flatten or close the universe. The obvious way out of the dilemma is to make up the missing 70 percent in the form of a cosmological constant. This would mean that the vacuum-energy density was a little more than twice the mass of ordinary and dark matter combined, about thirty proton masses per cubic meter.

Because the cosmological constant represents a repulsive force, it would have an effect on the way that the universe expands. The early phase of the expansion would not be much affected, but as the distance grows between galaxies, so too does the repulsive force. Eventually the cosmological constant can accelerate the outward motion of the galaxies, causing the Hubble expansion to pick up speed.

Let’s run it backward. The galaxies are falling inward, but now the extra repulsion slows them down. The initial estimate of their inward velocity (the one we make today) overestimates how fast they will be moving as they grow closer. Failure to account for the vacuum energy will lead us to underestimate the length of time until the galaxies all merge. In other words, if there were a cosmological constant but we didn’t know it, we would find the universe appearing younger than it really is. Indeed, if we include the effects of a vacuum energy equal to about thirty proton masses per cubic meter, the ten-billion-year lifetime of the universe gets stretched to about fourteen billion years. That’s perfect because it makes the universe just a little older than the oldest stars.

These conclusions concerning the existence of a cosmological constant are so important that I want to repeat them. The existence of a small cosmological constant, representing 70 percent of the energy in the universe, solves the two biggest puzzles of cosmology. First, the additional energy is just enough to make the universe flat. This fact removes the awkward discrepancy between the observed flatness of space and the fact that the mass in the universe was insufficient to render it flat.

The second paradox that is eliminated by the cosmological constant is the equally awkward discrepancy that the oldest stars appear older than the universe. In fact, the same vacuum energy—70 percent of the total—remarkably, is exactly what is needed to make the universe a little older than these ancient stars.

Over the last decade the historical accuracy of the universe’s biography has been greatly improved. We now know the history of the expansion in much greater detail. The trick involves a class of distant events called
Type I supernovae.
A supernova is a cataclysmic event in which a dying star collapses under its own weight and becomes a neutron star. The supernova is so unimaginably violent that when it occurs in a galaxy it can outshine the billions of stars that galaxy comprises. Supernovae are easy to spot even in very distant galaxies.

All supernovae are interesting, but something is very special about Type I supernovae. They originate from double star systems in which an ordinary star and a white dwarf are orbiting each other at a relatively close distance. The white dwarf star is a dead star that didn’t have quite enough mass to collapse to a neutron star.

As the two stars revolve around each other, the gravity of the white dwarf gradually sucks matter away from the ordinary star and, in this way, slowly increases its own mass. At some very precise point, when the mass is just right, the white dwarf can no longer support its own weight, and it implodes, creating a Type I supernova. The behavior of the final collapse doesn’t depend on the original mass of the white dwarf, or for that matter, its companion. In fact these events are believed to occur in a unique way and always give the same amount of light. An astronomer would say they all have the same luminosity.
⁷
Astronomers can tell, with a good degree of certainty, how far away they are by how bright they appear.

The velocity of the galaxy in which the supernova is embedded can also be easily determined using the Doppler method. And once we know both the distance and the velocity of the distant galaxy, the Hubble constant is easy to determine. But the special thing about very distant galaxies is that their light was given off long in the past. A galaxy five billion light-years away radiated the light that we now see five billion years ago. When we measure the Hubble parameter on earth today, we are really measuring the value that it had five billion years ago.

By concentrating on galaxies at a variety of different distances, we effectively measure the history of the Hubble parameter. In other words, Type I supernovae allow us to know a great deal about the history of the universe during the various stages of its evolution. And most important, they allow us to compare our real universe with mathematical models, with and without cosmological constants. The results are unambiguous. The expansion of the universe is accelerating under the influence of a cosmological constant, or something very much like it. To theoretical physicists like myself, this is a stunning reversal of fortune that cannot help but change our entire outlook. For so long we were trying to explain why the vacuum energy is exactly zero. Well, it seems that it is not zero. The first 119 decimal places of the cosmological constant cancel, but then, in the 120th, incredibly, a nonzero value results. To make matters even more interesting, its value is just about what Weinberg predicted it would be based on the Anthropic Principle!

Because light travels with a finite velocity, great telescopes that look to tremendous distances are also looking far back into the past. We see the sun as it was eight minutes ago, the nearest star as it was four years ago. Early humans were first beginning to stand straight when the light started its two-million-year journey from the nearest galaxy, Andromeda.

Oldest of all is the light that has been traveling to us for about fourteen billion years. This light started before the earth or even the oldest stars were formed. Indeed, the hydrogen and helium had not yet begun the process of differentiation into galaxies. So hot and dense were these gases that the atoms were all ionized. It was as close to creation as nature will ever allow us to see, at least if the messenger is electromagnetic radiation.

Think of the universe as a series of concentric shells with us at the center. There are, of course, no real shells out there, but nothing prevents us from dividing space up in that way. Each successive shell is farther away than the last. Each shell also represents an earlier (time) epoch than the previous. By looking deeper and deeper, we are, in effect, running the movie of the universe backward.

The deeper we look, the more densely populated the universe appears. In the reverse movie of the universe, the matter gets progressively denser as if some giant piston were squeezing it ever tighter. That piston is, of course, gravity. Moreover, it is a property of matter that as it is compressed, it grows hotter as well as denser. Today, the average temperature of the universe is only about 3 degrees above absolute zero, or -270 degrees centigrade. But as we follow the universe into the past, the temperature rises, first to room temperature, then to the boiling point, and eventually to the temperature on the surface of the sun.

The sun is so hot that the atoms that it is composed of have been torn apart by their violent thermal motion. The nuclei are intact, but the more loosely attached electrons have been torn free and can roam throughout the sun’s hot gases, which are now electrically conducting
plasma.
⁸

Electrical conductors are generally the least transparent of materials. The freely moving electrons easily absorb and scatter light. This scattering of light makes the sun opaque. But as we move outward to the sun’s surface, the temperature and density decrease to the point where it becomes transparent. That is where we see the sun’s surface.

Now let us travel backward in time and outward in space to the last visible shell, where the conditions are similar to the sun’s surface. Again the light comes to us from a surface like the sun’s—a giant shell of hot plasma surrounding us on every side. Astronomers call it the
surface of last scattering
. Sadly, looking through the conducting plasma to an even earlier and more distant shell is no more possible than looking through the sun.

Immediately after the Big Bang, the light from the surface of last scattering was every bit as bright as the sun’s surface. That raises an interesting question: why, when we look at the sky around us, don’t we see the bright glare of ionized hot primordial plasma? To ask it another way, why isn’t the sky uniformly illuminated with the same brightness that we would see if we were to look straight into the sun? Fortunately, the Doppler shift rescues us from that awful prospect. Because of the Hubble expansion, the plasma that originally emitted the primordial light is receding away from us with a large velocity. In fact, using the Hubble Law, we can calculate the velocity of this recession, and the result is only slightly less than the speed of light. This means that the emitted radiation was Doppler redshifted way past the visible and infrared, all the way to the microwave spectrum. Here, one of the earliest discoveries of quantum mechanics plays an important role: the energy of a photon depends on wavelength in such a way that a microwave photon has about one thousand times less energy than a photon of visible light. For this reason, the photons that eventually reach us from the surface of last scattering are not very potent. They have no more effect on our retinas than the radio waves that continually surround us.

There is another way to understand the diminished potency of the cosmic radiation by the time it reaches us. The photons from the surface of last scattering were very hot, about as hot as the sun’s surface. They filled space, forming a kind of photon gas, and like all gases, when they expand, they cool. The expansion of the universe, since the time of the Big Bang, cooled the photon gas to the point where it lost most of its energy. Today, the CMB (cosmic microwave background) radiation is very cold: fewer than 3 degrees above absolute zero. The two explanations of the CMB’s loss of power are mathematically completely equivalent.

George Gamow was the first to have the idea of a Big Bang. Soon after, two of his younger colleagues, Ralph Alpher and Robert Herman, got the idea for CMB as a kind of leftover afterglow. They even estimated the temperature of the radiation today, and got 5 degrees: within two degrees of the right answer. But physicists at that time believed that such weak radiation could never be detected. They were wrong, but it took until 1964 for the CMB to be accidentally discovered.

At that time, the Princeton cosmologist Robert Dicke wanted to test the idea of CMB by measuring the radiation left over from the hot Big Bang. While he was in the process of building a detector, two young Bell Laboratory scientists were doing precisely the kind of experiment that Dicke was aiming for. Arno Penzias and Robert Wilson were scanning the sky for microwave signals, not for the purpose of discovering the birth of the universe, but for communications technology. They couldn’t identify a strange background static that was getting in the way of their real goal. Legend has it that they thought it was bird droppings on the detector.

Princeton University and Bell Labs are close neighbors in central New Jersey. As fate would have it, Dicke found out about the Penzias-Wilson “noise” and realized that it was the CMB from the Big Bang! Dicke got in touch with the Bell Labs scientists and told them what he thought was going on. Subsequently, Penzias and Wilson got the Nobel Prize for the discovery. It is one of those twists of fate that had Princeton and Bell Labs been farther apart, Dicke might have finished his experiment and been the first to make the discovery.

The Penzias-Wilson detector was a crude affair mounted on the roof of Bell Labs. By contrast, the most modern CMB detectors are extremely sophisticated and are mounted in space, high above the atmosphere. The detectors can be pointed in different directions to measure the CMB from each point in the sky. The results are presented as a kind of map of the sky.

One of the most striking features of the CMB is how dull these maps are. To a very high degree of precision, the microwave-sky is a featureless, homogeneous expanse. It seems that, in early times, the universe was almost perfectly homogeneous and isotropic. The microwave radiation coming from the surface of last scattering is almost identical in all directions of the sky. This extraordinary degree of homogeneity is somewhat puzzling and needs an explanation.