What might Dark Energy be?

Dark energy remains one of nature's deepest puzzles.

At a recent meeting, a renowned scientist displayed his six-year-old daughter's drawing of dark energy. "This," he said of the scribbled drawing, "is as good as anyone's guess!" The nature of dark energy remains one of the great mysteries in all of science. However, the search for clues is focusing on several possibilities.

An energy of empty space?
Einstein was the first person to realize that empty space is not nothingness. Space has amazing properties, many of which are just beginning to be understood. The first property of space that Einstein discovered is that more space can actually come into existence. Einstein's gravity theory makes a second prediction: "empty space" can have its own energy. This energy would not be diluted as space expands, because it is a property of space itself; as more space came into existence, more of this energy-of-space would come into existence as well. As a result, this form of energy would cause the universe to expand faster and faster as time passes. Unfortunately, no one understands why space should contain the observed amount of energy and not, say, much more or much less.

A new kind of energy?
Recently, physicists have speculated about a new form of energy, called "quintessence," that might be the dark energy. Quintessence is related to the idea of the energy of space itself, but unlike the energy of space envisioned by Einstein, quintessence would have the property that it could vary from place to place and moment to moment. Existing evidence tends to disfavor quintessence, but does not rule it out.

A new view of gravity?
It is possible that the accelerating universe is not a sign of dark energy at all, but instead signals a new aspect of the law of gravity that Einstein overlooked. Scientists are hesitant to modify the known "laws" of physics – especially something like Einstein's theory of gravity, which has been very well tested over the past century. But with something as perplexing as dark energy, it is important to leave no stone unturned. Several leading scientists are now investigating the possibility that the most cherished laws of physics may need to be modified.

An effect of extra dimensions of space?
Many physicists think it likely that the universe has extra dimensions of space beyond the three dimensions which we see and in which we move. It is possible that one of these extra dimensions of space can mimic the effect of a dark energy by causing the expansion of our three-dimensional space to accelerate. The theory that predicts these extra dimensions, called "string theory," has not been tested experimentally. However, physicists are developing possible ways to detect the influence of these extra dimensions on our observable universe.

Resolving the question
Any of the possibilities above might be ruled out if we knew more precisely how the dark energy is affecting the expansion of the universe over time. NASA, with other federal agencies, is planning a "dark energy mission" that would observe thousands of exploding stars from space, using them as beacons to help pin down the exact effect of the dark energy on the expansion of space. Plans are also underway for similar studies using ground-based telescopes.

Whatever the outcome of the hunt for dark energy, it is clear that a radically new view of the universe is in store for us.

Dark energy must be different from anything we've yet seen.

Dark energy has captured the imagination of scientists because it must be very different from anything we know. In order to understand it, we would have to have a much better understanding of how space, time, and matter are related. This is the kind of challenge that drives revolutions in science.

There are subtle questions as well. What is the purpose of dark energy? What role does it play in the unfolding universe? Such questions may seem unscientific, yet scientists continue to be astonished at the connections and relatedness within the physical world – a kind of "ecology of the universe." It would be surprising indeed if dark energy were an afterthought on nature's part.

One of the most perplexing questions is why the universe we observe today should contain an amount of dark energy comparable to the amount of normal matter. Why not much more or much less? What is special about this near-balance between dark energy and normal matter? No one knows yet.

It seems curious that dark energy should be discovered at just that time in history when we thought we had finally answered the ancient Greek question, "what is the world made of?" It took two thousand years to answer that question. First, we had to invent science along the way. Finally, we have studied and understood all of the chemical elements and many of the fundamental particles of which they are made. We have periodic tables and textbooks crammed with lists fundamental particles. Yet now we find that most of the universe is made of something completely unknown. It is as though we are now the ancients, asking questions of nature that we do not yet know how to answer.

This great dialogue with nature seems to keep going. Where it is going no one really knows yet; the next generations will have a better idea. But what a marvelous journey!