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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. Explore
the implications in the next
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