Knocking on Heaven's Door (57 page)

Perhaps the strongest evidence to date that dark matter, rather than a modified gravitational theory, explains such phenomena comes from the
Bullet cluster
, which involved two colliding clusters of galaxies. (See Figure 76.) Their collision demonstrated that the clusters contain stars, gas, and dark matter. The hot gas in the cluster interacts strongly—so strongly that the gas remains concentrated in the central collision region. Dark matter, on the other hand, doesn’t interact—at least not very much. So the dark matter just passed through. Lensing measurements showed that the dark matter was indeed separated from the hot gas in just the way implied by a model of very weakly interacting dark and strongly interacting ordinary matter.

[
FIGURE 76
]
The Bullet cluster indicates that clusters of galaxies contain dark matter, and that their dynamics are unlikely to be explained by modified gravitational laws. That’s because we can see a separation between the more strongly interacting ordinary matter that gets trapped in the middle when two clusters collide and the far more weakly interacting dark matter, which is detected by gravitational lensing, and evidently just passes through.

We have further evidence for dark matter’s existence from the cosmic microwave background discussed earlier. Unlike lensing, the radiation measurements don’t tell us anything about the distribution of dark matter. Instead they tell us the net energy content carried by dark matter—how big a piece of the cosmic pie is constituted by the energy it carries.

CMB measurements tell us a great deal about the early universe and give us detailed information about its properties. These measurements argue not only for dark matter. They also support the existence of dark energy. According to Einstein’s equations of general relativity, the universe could only be flat with just the right amount of energy. Matter, even accounting for dark matter, simply didn’t suffice to account for the flatness measured by WMAP and balloon-based detectors. Other energy had to exist. Dark energy is the only way to account for the universe’s flatness—with no measurable curvature of three-dimensional space and agree with all other measurements to date.

Dark energy, which carries the bulk of the universe’s energy—approximately 70 percent—is even more puzzling than dark matter. The evidence that convinced the physics community of dark energy’s existence was the discovery that the expansion of the universe is currently accelerating—much as it did during inflation earlier on but at a very much slower rate. In the late 1990s, two independent research teams, the Supernova Cosmology Project and the High-z Supernova Team, surprised the physics community when they discovered that the rate of expansion of the universe is no longer slowing down, but is actually increasing.

Before the supernova measurements, a few hints had pointed to the existence of missing energy, but the evidence had been weak. But careful measurements in the 1990s showed that distant supernova were dimmer than expected. Since this particular type of supernova has fairly uniform and predictable emission, this could only be explained by something new. And that something new seems to be an accelerated expansion of the universe—that is, it is expanding at an increasingly faster rate.

This acceleration would not arise from ordinary matter, whose gravitational attraction would slow the universe’s expansion. The only explanation could be a universe that acts like one that is inflating, but with far smaller energy than during the inflationary phase the universe had undergone much earlier on. This acceleration could be due only to something that acted like the cosmological constant that Einstein had introduced, or dark energy, as it has become known.

Unlike matter, dark energy exerts negative pressure on its environment. Ordinary positive pressure favors inward collapse, whereas negative pressure leads to accelerated expansion.
⁶⁸
The most obvious candidate for negative pressure—one that agrees with measurements so far—is Einstein’s cosmological constant, representing an energy and pressure that permeates the universe but is not carried by matter. Dark energy is the more general term we now use to allow for the possibility that the cosmological constant’s assumed relationship between energy and pressure isn’t precisely true but is only approximate.

Today dark energy is the dominant component of the universe’s energy. This is all the more remarkable because the amount of dark energy density turns out to be extraordinarily small. Dark energy has dominated only for the last few billion years. Earlier in the universe’s evolution, first radiation and then matter were dominant. But radiation and matter, which are shared over the volume of an ever-increasing universe, dilute. Dark energy density, on the other hand, remained constant, even when the universe grew. By the time the universe had lasted so long as it has, the energy density in radiation and matter had decreased so enormously that dark energy, which doesn’t dissipate, eventually took over. Despite dark energy’s incredibly tiny size, it was bound to eventually dominate. After 10 billion years of expanding at an increasingly slower rate, the impact of dark energy was finally felt and the universe sped up its expansion. Eventually, the universe will end up with nothing in it but vacuum energy and its expansion will accelerate accordingly. (See Figure 77.) The meek energy might not inherit the Earth, but it is in the process of inheriting the universe.

[
FIGURE 77
]
The universe has expanded differently over time. During the inflationary phase it quickly expanded exponentially. The conventional Big Bang expansion took over when inflation ended. Dark energy now makes the expansion rate accelerate again.

FURTHER MYSTERIES

The necessity for dark energy and dark matter tells us that we can’t be as smug about our understanding of the evolution of the universe as the incredible agreement of cosmological theory with cosmological data might suggest. Most of the universe is stuff whose identity remains a mystery. Twenty years from now, people might smile at our ignorance.

And these are not the only puzzles evoked by the energy of the universe. The value of the dark energy, in particular, is actually the tail end of a much larger mystery: why is the energy that pervades the universe so small? Had the amount of dark energy been greater, it would have dominated matter and radiation much earlier in the evolution of the universe, and structure (and life) would not have had time to form. On top of that, no one knows what was responsible earlier on for the large energy density that triggered and fueled inflation. But the biggest problem with the energy of the universe is the
cosmological constant problem
.

Based on quantum mechanics, we would have expected a much larger value for dark energy—both during inflation and now. Quantum mechanics tells us that the vacuum—the state with no permanent particles present—is actually filled with ephemeral particles that pop in and out of existence. These short-lived particles can have any energy. They sometimes can have energy so large that gravitational effects can no longer be neglected. These highly energetic particles contribute an extremely large energy to the vacuum—much larger than the long evolution of our universe would allow. In order for the universe to look like the one we see, the value of the vacuum energy has to be an astonishing 120 orders of magnitude smaller than the energy that quantum mechanics would lead us to expect.

And there is yet a further challenge associated with this problem. Why do we happen to live in the time when the energy densities of matter, dark matter, and dark energy are comparable? Certainly dark energy dominates over matter, but it’s by less than a factor of three. Given that these energies in principle have entirely different origins and any one of them could have overwhelmed the others, the fact that their densities are so close seems very mysterious. The peculiarity of this coincidence is especially notable because it is only (roughly speaking) in our time that this coincidence is true. Earlier in the universe, dark energy was a much smaller fraction of the whole. And later on it will be a much greater fraction. Only today are the three components—ordinary matter, dark matter, and dark energy—comparable.

The questions of why the energy density is so extraordinarily tiny and why these different energy sources contribute similar amounts today are entirely unsolved. In fact, some physicists believe that there is no true explanation. They think we live in a universe with such an incredibly unlikely value for the vacuum energy because any larger value would have prevented the formation of galaxies and structure—and us—in the universe. We wouldn’t be here to ask about the value of the energy in any universe with a somewhat larger value of the cosmological constant. Those physicists believe that there are many universes, and each of these universes contains a different value of the dark energy. Out of the many possible universes, only the ones that could give rise to structure could possibly contain us. The value of the energy in this universe is ridiculously small, but we could exist only in a universe with just such a small value. This reasoning is the
anthropic principle
we considered in Chapter 18. As I said then, I’m not convinced. Nonetheless, neither I nor anyone else has a better answer. The explanation for the value of the dark energy is perhaps the most major mystery particle physicists and cosmologists face today.

In addition to puzzles about energy, we also have a further cosmological mystery about matter: Why is there matter in the universe at all? Our equations treat matter and antimatter on the same footing. They annihilate when they find each other, and both disappear. Neither matter nor antimatter should remain when the universe has cooled.

Whereas dark matter doesn’t interact very much and therefore sticks around, ordinary matter interacts quite a lot through the strong nuclear force. Without an exotic addition to the Standard Model, almost all of our usual matter would have disappeared by the time the universe had cooled to its current temperature. The only reason matter can be left is that there is a predominance of matter over antimatter. This isn’t built into the simplest versions of our theories. We need to find reasons that protons exist but can’t find antiprotons with which they can annihilate. Somewhere a matter-antimatter asymmetry must be built in.

The amount of leftover matter is smaller than the amount of dark matter, but it is still a sizable chunk of the universe—not to mention the source of everything we know and love. How and when this matter-antimatter asymmetry was created is another big question that particle physicists and cosmologists very much want to tackle.

The question of what constitutes the dark matter of course remains critical as well. Perhaps eventually we will find that the underlying model connects the dark matter density to that of matter, as recent research suggests. In any case, we hope to soon learn a lot more about the dark matter question from experiments—a sampling of which we’ll now explore.

VISITORS FROM THE DARK SIDE

When the LHC’s chief engineer, Lyn Evans, spoke at the California LHC/Dark Matter conference in January 2010, he closed by teasing the audience about how for the last couple of decades, “You theorists have been thrashing around in the dark (sector).” He added the caveat, “Now I understand why I spent the last fifteen years building the LHC.” Lyn’s comments referred to the paucity of high-energy data over the previous years. But they were also hints about the possibility that LHC discoveries might shed light on dark matter.

Many connections exist between particle physics and cosmology, but one of the most intriguing is that dark matter might actually be made at the energies explored by the LHC. The remarkable fact is that if a stable particle species with weak scale mass exists, the amount of energy carried by particles of this type that survived from the early universe to today would be about right to account for dark matter. The result of calculations about the amount of dark matter that is left over by an initially hot—but cooling—universe demonstrate that this might be the case. That means that not only is dark matter literally right under our noses, its identity might prove to be too. If dark matter is indeed composed of such a weak mass particle, the LHC might not only give us insight into particle physics questions, it might also provide clues to what is out there in the universe and how it all began—questions that are incorporated into the science of cosmology.