January 17, 2007

Dark Energy May Be "Giant Sucking Sound"

Over the past many years, astrophysicists have been involved in a search for the energy that is causing the apparent accelerating expansion of the universe. You see, the observed expansion of the universe created a puzzle, because when you compare the gravitational pull of the matter in the universe, the universe should be slowing its rate of expansion. Instead, it is accelerating. This energy has been dubbed 'dark energy'. There really is no simple, quick way of describing dark energy theory, so I'll let Wikipedia fill you in on the theories to date.

An article on Eurekalert dtd 16 Jan 2007 talks about research being done to measure 'dark energy' to within 10%. Allow me to quote from the article:
In a third paper, led by the Danish team and released this week, the many new theories that have been proposed to explain the acceleration of the universe are critically assessed in the face of this new data. Dr. Jesper Sollerman and Dr. Tamara Davis lead the team who show that despite the increased sophistication in cosmological models over the last century the best model to explain the acceleration remains one that was proposed by Einstein back in 1917

You see, back in 1917, Einstein wrote the General Theory of Relativity. At the time he didn't realize that the universe was expanding, and because experimentation was not agreeing with his theory, he needed a cosmological constant (that he labeled lambda) to explain the differences between theory and experimentation. In other words, he got as far as he could go with the theory and put a label on what was 'left over'. Once it was discovered that the universe was expanding at an increasing rate, he took back his constant, and apologized for the 'biggest blunder' of his career. However, it was not recognized at the time that his cosmological constant would actually become explainable in terms of energy that we know about.
Let's skip ahead in the article:
In modern terms the cosmological constant is viewed as a quantum mechanical phenomenon called the 'energy of the vacuum'. In other words, the energy of empty space. It is this energy that is causing the universe to accelerate. The new data shows that none of the fancy new theories that have been proposed in the last decade are necessary to explain the acceleration. Rather, vacuum energy is the most likely cause and the expansion history of the universe can be explained by simply adding this constant background of acceleration into the normal theory of gravity.

1 comment:

Rich Gautier said...

So I asked:
Although, there is one nagging question: Does your paper mean that the best cosmological fit involves only the cosmological constant, which can be fully compensated for by 'vacuum energy' and does not require the existence of 'dark matter'? Or am I mixing and matching things?
and received this response:
Hi Rich,
Yep, you're mixing and matching things. Dark matter is still needed. The cosmological constant can account for all of the *dark energy* (at least to the accuracy of the current data) but that only makes up about 70% of the energy density of the universe. For the other 30% dark matter, normal matter, radiation, etc... are still neeed.