Physics & Math / Cosmology

5. WHY IS THE EXPANSION OF THE UNIVERSE

SPEEDING UP AND NOT SLOWING DOWN?

Answering this one is my personal quest. After Hubble found evidence that the universe is expanding, astronomers expected that the attractive force of gravity would continuously slow down the expansion and that by measuring that rate of slowing they could determine the shape and fate of the universe. According to Einstein, if the universe has a high density of matter, space curves back on itself like a ball, and the slowing would someday halt the expansion and lead to a recollapse. But if the density of matter is low, space would curve away from itself like the surface of a saddle, and the expansion would continue forever. If the density is poised precisely between those extremes, at what is called the critical density, space would be uncurved—flat—and the slowing would continue forever.

It all seems simple, but there is a twist. In 1998 two teams of astrophysicists—one led by Saul Perlmutter and the other led by Brian Schmidt—measured the change in the universe’s expansion rate by using distant supernova explosions as mileposts. They found, and others have since confirmed, that the expansion of the universe has been speeding up, not slowing down, over the past 6 billion years or so.

Cosmic acceleration seems to fly in the face of everything we know about gravity, but Einstein’s theory actually predicts that gravity can be repulsive. And while the discovery of the cosmic speedup was a surprise to most, some theorists and I anticipated it. In trying to save the theory of inflation, which predicts that the universe is flat, we suggested that the gap between the critical density and the actual amount of matter is filled with something whose gravity is repulsive—something similar to what Einstein dubbed the cosmological constant, though in a new guise.

Strangely enough, the simplest example of energy with repulsive gravity involves Einstein’s nemesis, quantum mechanics, which holds that even a perfect vacuum is not empty. It is instead filled with so-called virtual particles, popping in and out of existence and living on borrowed time and borrowed energy in accordance with Heisenberg’s famous uncertainty principle. This is more than theory: In the late 1940s, physicist Willis Lamb detected the effect of virtual particles on hydrogen atoms in his Columbia University laboratory.

According to relativity theory, the energy associated with quantum nothingness—seemingly empty space—has re- pulsive rather than attractive gravity. Just as the strength of gravity around an object depends upon the object’s mass, the repulsive power of nothing depends upon how much “nothing” weighs. It has weight because empty space is actually teeming with virtual particles.

Therein lies the rub and the reason our prediction of cosmic speedup wasn’t a slam dunk. All attempts to compute how much nothing weighs have arrived at absurdly large numbers, more than 50 orders of magnitude larger than what is needed to account for the acceleration of the expansion. If space contained that much energy, the universe would be accelerating a great deal faster than it is. This mystery is tantalizing because it seems to involve a connection between quantum mechanics and gravity and could provide a clue to uniting general relativity with quantum mechanics. String theory may have something to say about how much quantum nothingness weighs because it actually permits the calculation of vacuum energy without pesky infinities cropping up. But it has yet to produce a definitive answer.

When it does, we might find that even quantum nothingness weighs nothing. If so, what is causing the expansion of the universe to speed up? It must be an energy form even more exotic than quantum vacuum energy. I coined the term “dark energy” for the whatever-it-is that is causing the universe to speed up. The possibilities being discussed for dark energy range from quantum vacuum energy to the influence of the unseen extra dimensions predicted by string theory.

Perhaps the most radical idea, and the one I am pursuing now, is that there’s no dark energy at all. (Remember, a foolish consistency is the hobgoblin of little minds.) Instead, our incomplete understanding of gravity is at fault, and when we understand it better, we’ll no longer need to invoke dark energy. Maybe a new principle is involved; for example, perhaps empty space naturally expands at an accelerating rate (while certainly not empty today, the density of the universe has diminished by more than 100 orders of magnitude since it began).

6. WHAT IS THE ULTIMATE FATE OF THE UNIVERSE?

Before we discovered that the universe’s expansion was accelerating, predicting our cosmic fate seemed straightforward: Measure the slowing of the expansion, the average density of matter and energy, or the shape of the universe. Any one or all three should give the answer. We now know the shape of the universe (flat) and the average density of matter and energy (critical density), and yet we don’t know what will happen. That’s because the connection between the three is severed when we allow for dark energy. Until we understand why the universe’s expansion is speeding up, we cannot know our cosmic destiny.

So the possibilities remain wide open. Continued acceleration would lead to a lonely universe in 100 billion years, when all but a few hundred galaxies become too red to see. On the other hand, cosmic speedup could be a passing fad, with dark energy dissipating and a cosmic slowdown returning. Or there could be a rapid slowdown and recollapse, or even a hyperacceleration that leads to the ripping apart of galaxies, stars, and eventually atoms.

In 1919 Arthur Eddington was told, “You must be one of three persons in the world who understand general relativity.” He responded by saying, “I am trying to think who the third person is!” After nearly a century, the depths of the theory are now well plumbed. While there are still parts to be tested with greater precision and aspects to be fully exploited (such as using gravity waves to detect the formation of black holes and events that occurred during the earliest moments of creation), physicists are ready and eager to go beyond Einstein in their understanding of gravity.

This is part of the natural progression in our quest to understand nature. Each successive theory deepens our knowledge of the physical world and at the same time raises new questions that it can phrase but not answer. Einstein’s theory of gravity transformed our concept of space, time, matter, and energy. It also provided us with the strong foundation needed to ask the next set of profound questions whose answers will further our knowledge of the universe, the laws that govern it, and probably even our place within it. Guided by Einstein’s wisdom for nearly 100 years, we are ready to move beyond him and answer the next questions on our own.

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