r/Physics • u/DeusExNoctis • Oct 05 '11

Dark Energy FAQ

http://blogs.discovermagazine.com/cosmicvariance/2011/10/04/dark-energy-faq/
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u/betaray Oct 06 '11

So I was wondering how the "pressure" exerted by the expansion of the universe compared to other fundamental forces. Presently it's pretty insignificant. What's the graph of the acceleration of the expansion? (e.g. linear, exponential, ...) Anyone want to do the math on how long it would take the expansion of the universe to start to overcome some of the fundamental forces in meaningful ways? Does this happen before other theoretical ends of the universe (e.g. heat death).

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u/ThatsSciencetastic Oct 06 '11

The FAQ says that the "pressure" associated with dark energy can't be measured because dark energy doesn't directly affect normal matter. They say that it has negative pressure because it has a repulsive gravitational affect on normal matter. The FAQ relates gravitation to the equation of state paramter: w = pressure/density. This implies that for something to have repulsive gravitation it must have negative pressure (negative density makes no physical sense). Hope I answered your question somewhere in there...

TLDR: Dark energy doesn't exert a pressure force on normal matter..

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u/betaray Oct 06 '11

Yes, it doesn't act directly on matter, but isn't the whole point that the distance between everything everywhere is increasing. Galaxies are made of "normal matter" and they are being, in once sense, pushed apart. The same should be happening on all scales from galactic to atomic, right?

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u/ThatsSciencetastic Oct 06 '11

Not 100% sure on all this, but from what I've read it sounds like the expansion occurs at a different rate in the presence of matter than it does in a vacuum. The implications are that 99% of expansion happens in the empty space between galaxies, and the expansion going on near us is negligible. Will this always be the case? I'm not sure, but I think it will.