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The Question

(Submitted April 01, 2008)

I've read that dark energy is the energy driving the accelerated expansion of the universe. Its density doesn't decrease as space increases. So, does this mean that it has existed with equal density from the beginning of the universe? Have scientists found that the accelerationed expansion of the universe was first decresing, and has started to increase more "recently"?

The Answer

According to current theories dark energy has always been around and has always exerted the same force. However, in the early universe, matter was closer together and the effects of gravity caused the expansion of the universe to slow down. About 5 billion years ago, the density of matter (both normal matter and dark matter), which had been decreasing since the Big Bang, became low enough that the dark energy was able to start accelerating the expansion of the universe.

More information can be found at:
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html
http://physicsworld.com/cws/article/print/19419
http://en.wikipedia.org/wiki/Dark_energy

Hope this helps,
Mike and Georgia
for "Ask an Astrophysicist"

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