Knocking on Heaven's Door (54 page)

We really don’t know what exists beyond the
horizon
—the boundary of the observable universe. The limits to our observations allow for the possibility of new and exotic phenomena beyond. Different structures, different dimensions, and even different laws of physics can in principle apply so long as they don’t contradict anything that has been observed. That doesn’t mean every possibility is realized in nature, as my astrophysics colleague Max Tegmark sometimes asserts. However, it does mean there are many possibilities for what can be out there.

We don’t yet know if other dimensions or other universes exist. Really, we can’t even say with certainty whether the universe as a whole is finite or infinite, though most of us think it’s likely to be the latter. No measurement shows any sign of its ending, but measurements only reach so far. In principle, the universe could end, or even have the shape of a ball or balloon. But no theoretical or experimental clue leads us in that direction at present.

Most physicists prefer not to think too much about the regime beyond the visible universe, since we are unlikely to ever know what is there. However, any theory of gravity or quantum gravity gives us the mathematical tools to contemplate the geometry of what might exist. Based on theoretical methods and ideas about extra dimensions of space, physicists sometimes consider exotic other universes, which are not in contact with us over the lifetime of our universe or are only in contact via gravity. As discussed in Chapter 18, string theorists and others contemplate the existence of a multiverse that contains many disconnected independent universes that are consistent with string theory’s equations, sometimes combining these ideas with the anthropic principle that exploits the possible riches of universes that might exist. Some even try to find observable signatures of such multiverses for the future. As we saw in Chapter 17, in one distinct scenario, a two-brane “multiverse” might even help us understand questions in particle physics and in that case have testable consequences. But most additional universes, though conceivable and maybe even likely, will probably remain beyond the realm of experimental testability for the foreseeable future. They will then remain theoretical abstract possibilities.

THE BIG BANG: FROM SMALL TO LARGE THROUGH TIME

Now that we’ve ventured out to the largest sizes we can observe or discuss in the context of the observable universe, and reached the outer limits as to what we can see (and contemplate with our imagination), let’s explore how the universe we do live in and observe evolved over time to create the enormous structures we see today. The Big Bang theory tells us how the universe grew during its 13.75-billion-year life span from its small initial size to the current extent, 100 billion light-years across. Fred Hoyle facetiously (and skeptically) named the theory after the initial explosion when a hot dense fireball began to expand into the massive extent of stars and structures we now observe: growing, diluting matter, and cooling as it evolved.

However, the one thing we certainly don’t know is what banged in the beginning and how it appened—or even the precise size it had been when it did. Despite our understanding of the universe’s late evolution, its beginnings remain shrouded in mystery. Nonetheless, although the Big Bang theory does not tell us anything about the universe’s initial moment, it is a very successful theory that tells us much about its subsequent history. Current observations combined with the Big Bang theory teach us quite a lot about how the universe has evolved.

No one knew the universe was expanding when the twentieth century began. At the time that Edwin Hubble first peered into the sky, very little was known. Harlow Shapley had measured the size of the Milky Way to be 300,000 light-years across, but he was convinced that the Milky Way was all that the universe contained. In the 1920s, Hubble realized that some of the nebula that Shapley had thought were clouds of dust—which did indeed merit this uninspiring name—were in fact galaxies, millions of light-years away.

Once he identified galaxies, Hubble made his second stunning discovery—the universe’s expansion. In 1929, he observed that galaxies red-shifted, which is to say there was a Doppler effect in which light waves shifted to longer wavelengths for more distant objects. This red shift demonstrated that galaxies were receding, much as the high-pitched wail of a siren decreases in frequency as an ambulance speeds away. (See Figure 71.) The galaxies he had identified were not stationary with respect to our location, but were all expanding away from us. This was evidence that we live in an expanding universe, in which galaxies are growing farther apart.

[
FIGURE 71
]
The light from an object moving away from us is shifted to lower frequencies—or shifted toward the red end of the spectrum—whereas light from objects moving away is shifted to higher frequencies, or blue shifted. This is analogous to the noise from a siren that is lower pitched when an ambulance moves away and higher pitched when it approaches.

The universe’s expansion is different from the pictures we might first imagine since the universe doesn’t expand into some preexisting space. The universe is all there is. Nothing is present for it to expand into. The universe, as well as space itself, expands. Any two points within it grow farther apart as time progresses. Other galaxies move farther away from us, but our location is not special—they move farther from each other as well.

One way to picture this is to imagine the universe as the surface of a balloon. Suppose you had marked two points on the balloon’s surface. As the balloon blows up, the surface becomes stretched and those two points grow farther apart. (See Figure 72.) This is in fact what happens to any two points in the universe as it expands. The distance between any two points—or any two galaxies—increases.

[
FIGURE 72
]
The “ballooniverse” illustrates how all points move away from one another as the balloon (universe) expands.

Notice in our analogy that the points themselves don’t necessarily expand—just the space between them. This is in fact what happens in the expanding universe as well. Atoms, for example, are tightly bound together via electromagnetic forces. They don’t get any bigger. Neither do relatively dense strongly bound structures such as galaxies. The force driving the expansion acts on them too, but because other force contributions are at work, the galaxies don’t themselves grow with the overall expansion of the universe. They feel such strong attractive forces that they remain the same size while their relative distance from each other gets bigger.

Of course, this balloon analogy is not perfect. The universe has three spatial dimensions, not two. Furthermore, the universe is large and probably infinite in size, and not small and curved like the balloon’s surface. On top of that, the balloon exists in our universe and expands into existing space, unlike the universe, which permeates space and doesn’t expand into something else. But even with these caveats, the surface of a balloon illustrates quite nicely what it means for space to expand. Every point moves away from every other point at the same time.

A balloon analogy—this time referring to the interior—is also helpful for understanding how the universe cooled from its initial hot dense fireball existence. Imagine an extremely hot balloon that you allow to expand to a very big size. Though it might have been too hot to handle at first, the expanded balloon will contain much cooler air that would no longer be alien to human contact. The Big Bang theory predicts that the initial hot dense universe expanded, all the while cooling as it did so.

Einstein had actually derived an expanding universe from his equations of general relativity. At that time, however, no one had yet measured the universe’s expansion, so he didn’t trust his prediction. Einstein introduced a new source of energy in an attempt to reconcile his theory with a static universe. After Hubble’s measurements, Einstein dispensed with the fudge he had made, calling it “his biggest blunder.” This modification was not entirely erroneous, however. We will soon see that more recent measurements show that the cosmological constant term he added is actually necessary to account for recent observations—although the measured magnitude, which accounts for the recently established acceleration of its expansion, is about an order of magnitude bigger than the one Einstein proposed to merely stall it.

The expansion of the universe was a nice example of a convergence of top-down and bottom-up physics. Einstein’s theory of gravity implies that the universe expands, yet only with the discovery of the expansion did physicists feel confident they were on the right track.

Today, we refer to the number that determines the rate at which the universe expands at present as the
Hubble constant
. It is a constant in the sense that the fractional expansion everywhere in space is the same. However, the Hubble parameter is not constant over time. At an earlier time, when the universe was hotter and denser and gravitational effects were stronger, it expanded at a far more rapid rate.

Measuring the Hubble constant precisely is difficult, since we face exactly the problem we raised earlier of disentangling the past from the present. We need to know how far away the red-shifting galaxies are, since the red shift depends both on the Hubble parameter and distance. This imprecise measurement was the source of the factor-of-two uncertainty in the age of the universe that I mentioned at the beginning of this chapter. If the Hubble parameter measurements were uncertain by a factor of two, so too would be the universe’s age.

That controversy is now pretty much resolved. The Hubble parameter has been measured by Wendy Freedman of the Smithsonian Astronomical Observatories and her collaborators and others, and the expansion rate is about 22 kilometers per second for a galaxy a million light-years away. Based on this value, we now know the universe is about 13.75 billion years old. This might under- or overestimate the age by 200 million years, but not by a factor of two. Although this might still sound like a good deal of uncertainty, the range is too small to make any great difference in our understanding today.

Two other key observations that agreed nicely with predictions further confirmed the Big Bang theory. One class of measurement that relied on both particle physics and general relativity predictions and therefore confirmed both was the density of various elements in the cosmos, such as helium and lithium. The amount of these elements that the Big Bang theory predicts agrees with measurements. This is in some respects indirect proof, and detailed calculations based on nuclear physics and cosmology are required to compute these values. Even so, this agreement of many different element abundances with predictions would be an unlikely coincidence unless physicists and astronomers were on the right track.

When the American Robert Wilson and the German-born Arno Penzias accidentally discovered the 2.7-degree microwave background in 1964, it was further confirmation of the Big Bang theory. To put this temperature in perspective, nothing is colder than absolute zero, which is zero degrees kelvin. The universe’s radiation is less than three degrees warmer than this absolute limit to how cold anything can be.

The collaboration and adventure of Robert Wilson and Arno Penzias (for which they won the 1978 Nobel Prize) was a superb example of how science and technology sometimes work in concert to achieve results beyond what anyone had imagined. Back when AT&T was a phone monopoly, it did something rather wonderful, which was to create Bell Laboratories, a spectacular research environment where pure and applied research proceeded side by side.

Robert Wilson, who was a detail-oriented gadget technology geek, and Arno Penzias, who was more of a big picture scientist, both worked there, and together used and developed radio telescopes. Wilson and Penzias were interested in science and technology, while AT&T was understandably interested in communications, so radio waves in the sky were important to everyone involved.

While pursuing a specific radio astronomy goal, Wilson and Penzias found what they initially considered a mysterious nuisance that they simply couldn’t explain. It seemed to be uniform background noise—essentially static. It wasn’t coming from the Sun, and it wasn’t related to a nuclear test from the previous year. They tried every explanation they could think of, most famously pigeon droppings, in their nine-month attempt to figure out what was going on. After considering all imaginable possibilities, cleaning out the pigeon droppings (or “white dielectric material” as Penzias called it), and even shooting the pigeons, the noise still didn’t go away.