Previous in thread
Next in thread

Hi folks--

Just a little bit of background on dark energy and on the recent MAP results...

First, MAP is satellite measuring the Cosmic Microwave Background (CMB). The CMB is a relic radiation left over from the early universe; it is the signature of a once young, hot, dense Universe. The CMB formed when the Universe was about 300,000 years old, long before the first stars and galaxies formed.

Superimposed on a nearly uniform background are tiny variations in the CMB of about 10 parts per million. These variations in the density of the early Universe look like hot and cold spots through a microwave telescope. These small variations eventually grew into galaxies and clusters of galaxies. The variations in the CMB were first discovered by the COBE satellite in 1991 and we have been conducting experiments to measure it with greater precision ever since. MAP has measured the variations with about the same resolution as other recent experiments (like Boomerang which I worked on) but has mapped the whole sky, rather than just a small region.

So what can the CMB tell us about the universe? The pattern of the variations (hot and cold spots) will look different depending on certain properties of the Universe, such as its geometry, the amount of regular matter, the amount and nature of dark matter, the smount and nature of dark energy, the expansion rate of the Universe, etc. So, we make a database of models with different combinations of these cosmological "parameters" and then do a statsitical analysis comparing the observations of our actual Universe with the possible models. This is how we get values for say, the amount of dark energy in the Universe.

From this type of modelling, most recent experiments are getting a value for the proportion of dark energy in the universe being about 65% of the total matter and energy (with about 30% cold dark matter and 5% regular matter).
Other astrophysical evidence for dark energy comes from looking at the relation between redshift and distance from the supernovae in far away galaxies-- the model that best fits that data points to a Universe that's not only expanding, but expanding ever more rapidly (accelerating expansion). One thing that could cause the acceleration of the expansion is dark energy.

Other lines of evidence for dark energy come from galaxy surveys and gravitational lensing and especially from the concordance (all the data considered together) of the CMB, supernova, galaxy survey and lensing data.
So now people are busy coming up with models of what the dark energy could be. Some models include a cosmological constant, vacuum energy, quintessence (an exotic particle that decays into other particles), decaying dark matter (cold dark matter that turns into hot dark matter after galaxy formation), or the result of possibly living on a 4-dimensional surface of a higher dimensional space. The jury is definitely still out on the nature of the dark energy. It's a topic that's been getting a lot of attention lately, though the idea has been invoked periodically for decades to explain cosmological data.

For more information and links, see my home page, especially my professional interests page and my list of cosmology links:
http://astro.uchicago.edu/~coble/professional.html
http://astro.uchicago.edu/~coble/cosmolinks.html

Kim