Dark Energy Hits Tenth Birthday
So we're just about ten years into the discovery that the universe is probably blowing apart due to a cosmic, anti-gravitational force called dark energy. And how much more do we know about it?
Scientists have made some headway, but there are still some frighteningly large questions to be addressed. They now agree that dark energy makes up 75 percent of the cosmos. Dark matter, another mysterious substance, commands a 21 percent share.
Albert Einstein was the first person to realize that empty space is not the same as nothingness. Space has amazing properties, many of which are just beginning to be understood. The first property of space that Einstein discovered is that it is possible for more space to come into existence.
One version of Einstein's gravity theory makes a second prediction: "empty space" can possess 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.
But why haven't we observed it? Often, we measure differences, not absolute values. When we talk about the height of mountains, we are talking about how high they are above sea level, not the distance between the mountain top and the center of the earth. We assume a "floor" for these purposes. Likewise, we measure differences of energy in the universe – but there could be a "sea-level" for energy in the universe that we cannot yet measure.
This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
What is the nature of this energy? As scientists developed the quantum theory of matter, they realized that "empty space" was full of temporary ("virtual") particles continually forming and destroying themselves. Physicists began to suspect that indeed the vacuum ought to have a distinct form of energy, but they could not predict its magnitude.
While theoretical physicists were trying to come to grips with dark energy, observational astronomers were trying to explain a bizarre result. Theories at that time predicted that the universe's mass should be slowly overcoming the momentum of the big bang, causing the expansion of the universe to slow down. But observations of supernovas in other galaxies were showing that the universe was actually expanding much faster than expected. Something was causing the universe to have another growth spurt!
Theory and observation dovetailed in dark energy. The dark energy has presumably been around since the beginning of the universe, but its effect may become more dominant as the universe expands. We still do not know whether or how the highly accelerated expansion in the early Universe (inflation) and the current accelerated expansion (due to dark energy) are related.
The latest issue of the journal Physics World features reflections and insights from two of the leading dark energy astrophysicists, Eric Linder and Saul Perlmutter, of the University of California, Berkeley. They say that planned and potential space missions - like the probe pictured here - could make the next decade an exciting one for astrophysics.
Who knows, maybe we'll get really lucky and understand the nature of ten or even, dare I say, twelve percent of the universe!
Kemo D. (a.k.a. no.7)