Expert: Steve Nelson - 9/5/2010

Question
Dear expert,

I was reading a New Scientist article (http://www.newscientist.com/article/mg20727764.100-void-that-is-truly-empty-solv...), and decided to clarify by looking up the cosmological constant on Wikipedia (http://en.wikipedia.org/wiki/Cosmological_constant). Now, on that page, just above the Omega Lambda headline in the "Equation" section, it says:

"A positive vacuum energy density resulting from a cosmological constant implies a negative pressure, and vice versa. If the energy density is positive, the associated negative pressure will drive an accelerated expansion of empty space."

I just want to be sure that I am understanding this correctly. Doesn't it mean that the accelerated expansion of the universe should result in a higher and higher energy density? And that this would be generally equivalent to the universe heating up, and not cooling down? I understand that people are not making that conclusion, but doesn't the evidence seem to suggest something like that? Namely that the expansion is increasing the energy density of the universe, in turn causing the expansion to speed up at an increasing rate?

Thank you for you time,
~Soren

Answer
OK, this isn't directly my field of expertise, but I'll give you my quick opinion (you should ask a cosmology expert).  The expansion doesn't cause the higher energy density, the energy density causes the acceleration of the expansion rate.  Objects are already flying apart, this just causes an increase in their speed.  As the universe expands, it cools.  In general, avoid New Scientist magazine and try not to rely on wikipedia for truly advanced topics like the nature of dark energy.