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The Question

(Submitted December 14, 2011)

My question involves the theoretical basis for the existence of Dark Energy. My understanding is that dark energy is theorized because researchers using the Doppler effect believe the galaxies at the edge of the visible universe are accelerating away from us, when it was expected they would begin to slow. But I feel this is incorrect. These galaxies, being 13.5 to 14 billion light years away, would be showing red shift from 13.5 to 14 billion years out of date, and of course they would appear to be accelerating. We are seeing them as the existed shortly (within half a billion years) after the big bang, NOT as they really exist today, and therefore the entire basis for thew existence of 'dark energy' should be called into question. Unless the red shift is instantaneous?

The Answer

You are correct that when we look at very distant objects, we are seeing light that left them a long time ago, so we are seeing them in the past so to speak. What we have is a velocity and a distance from that time. We can also look at somewhat closer objects, but still quite distant, and get a velocity and a distance of those objects (at a later time in universal terms). So we can construct a plot of distance and velocity, where distance is also related to time before present. We expect some slowing of the initial expansion due to gravitational attraction. The observed curve on that plot departs from the expected curve. In the first several billion years, the Universe slowed about as expected, but several billion years ago it apparently began to accelerate. So actually, it is galaxies that are relatively close to us (still billions of light years) that are moving away faster than expected.

Jay and Jeff
for Ask an Astrophysicist

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