Dark energy sees the light

March 31, 2006
Dark energy has been proposed as the source of the universe’s accelerating expansion. However, no one knows what dark energy is, which allows particle physicists and cosmologists to stay up late dreaming up wild and untamed physics. Eventually these ideas must emerge from the dark, suffering the humiliation of peer review and presentation at a conference. Recently the Institute of Physics released the proceedings of “The Third 21COE Symposium: Astrophysics as Interdisciplinary Science” conference. Dark energy featured quite heavily in the conference proceedings, but in a gesture of self sacrifice I have waded through the psychedelic flowers of wild speculation and picked the three that looked most interesting.

In a paper by Varun Sahni, dark energy as a cosmological constant is examined. It turns out that if the cosmological constant actually is constant, then it must be fine tuned to get the universe we observe today. However, if the constant is replaced by a term that is allowed to evolve in time then a much wider range of starting values can be used. The physical laws that determine such a variation are not yet known; however, cosmologists have taken a stab at it, making models which are inspired by those used by inflation. This is a tenuous way to do science since inflation itself has much empirical evidence, but little is resolved from a theoretical point of view. One justification for linking the two is that our universe appears to have undergone two periods of inflation: one near the beginning of the universe and one in the present. The two have very different properties making it hard to create a single theory to encompass them both. Enter string theory, which through the addition of extra dimensions can accommodate such different scales.

In a second paper, a researcher considered looking for signs of dark energy in the laboratory. All dark energy models involve some modification to the laws of physics; however some of the most likely models involve fluctuations of the vacuum. These may be detectable using laboratory experiments. Previous work has shown that some vacuum fluctuations can be measured using a Josephson junction (though some theorists dispute these measurements). However, the frequency range of these measurements did not reach the range necessary to measure vacuum fluctuations that might be due to those associated with dark energy models.

It is interesting to note from these papers that dark energy in some form is accepted as necessary for cosmological reasons. However, there will need to be some solid theoretical ideas for what dark energy might be before it find acceptance outside of cosmological circles.

http://arstechnica.com/journals/science.ars/2006/3/31/3415