Dark Energy

Talking of AstronomyNow, that reminds me of another story about dark energy in the local universe. What is dark energy? That's a good question. Basically, nobody knows but most astronomers think it is there. No, honestly. In fact it looks as though it actually makes up more than two thirds of the stuff in the universe.

A few years ago, astronomers thought that the universe was made up of matter (what you, me and this computer are made of) and dark matter. Hang on, dark matter? Oh yeah, dark matter is a form of matter that is 'invisible'; it doesn't interact with light but it still somehow knows about gravity. Dark matter is indirectly 'observed' via the way it makes galaxies rotate. There have been lots of suggestions for what particles could be the dark matter such as neutrinos with mass, weakly interacting massive particles (WIMPS), black holes, massive astrophysical compact halo objects (MACHOS)... the list goes on. So far nobody knows which if any it is, but there are lots of people searching.

So back to dark energy. As hopefully you know, gravity is an attractive force which means that it likes to pull the matter together. We know that the universe is expanding and gravity acts to try to slow it down. However, in recent years, observations seem to have shown that the universe started to speed up again about 5 billion years ago. It is this speed up that is attributed to dark energy. Another good question is why would it choose then to kick in. Again, nobody knows the answer. This sort of thing can keep astronomers awake at night if they aren't awake anyway.

So, the current best guess for the ingredient list of the universe would go something like: matter (baryons) 5%, dark matter (anyone's guess) 25% and dark energy (we're getting desperate here) 70%.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (12:17 UTC) | Permalink
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