5th October 2011, 02:18 PM
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Manipulation Resistance Team
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Join Date: 5th February 2002
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"Glory to you, Lord, showing your unfathomable might in the laws of the universe!"...
... A FAQ on dark energy Dark Energy FAQ
In honor of the Nobel Prize
, here are some questions that are frequently asked about dark energy, or should be. What is dark energy?
It’s what makes the universe accelerate, if indeed there is a “thing” that does that. (See below.) So I guess I should be asking… what does it mean to say the universe is “accelerating”?
First, the universe is expanding
: as shown by Hubble
, distant galaxies are moving away from us with velocities that are roughly proportional to their distance. “Acceleration” means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger. Galaxies are moving away from us at an accelerating rate. But that’s so down-to-Earth and concrete. Isn’t there a more abstract and scientific-sounding way of putting it?
The relative distance between far-flung galaxies can be summed up in a single quantity called the “scale factor,” often written a
) or R
). The scale factor is basically the “size” of the universe, although it’s not really the size because the universe might be infinitely big — more accurately, it’s the relative size of space from moment to moment. The expansion of the universe is the fact that the scale factor is increasing with time. The acceleration of the universe is the fact that it’s increasing at an increasing rate — the second derivative is positive, in calculus-speak. Does that mean the Hubble constant, which measures the expansion rate, is increasing?
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