Feb_19 - Why is the universe lumpy? Tuesday, February 19...

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Why is the universe Why is the universe lumpy? lumpy? Tuesday, February 19 Tuesday, February 19
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The average density of the universe is 9 × 10-27 kg/m3. Some of the universe is much denser much denser than average (stars, white dwarfs, black holes…) However, most of the universe is slightly less dense less dense than average (intergalactic voids).
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Dark energy Dark energy : apparently uniform density, with no lumps. Evidence: speeding up of expansion seems to be the same everywhere.
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Dark matter Dark matter : large lumps, about 1 megaparsec across. Evidence: “dark halos” around galaxies and clusters of galaxies.
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Ordinary matter Ordinary matter (protons, neutrons, electrons): small, but extremely dense, lumps. Evidence: Some of these lumps (that is, stars stars ) glow in the dark!
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(1) Why is the average density so close to the critical density? (2) Why isn’t the density equal to the average density everywhere? Why is the universe flat flat on large scales? Why is the universe lumpy lumpy on small scales?
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The answers to these two questions are actually linked!! By explaining why the average density is close to critical, we can also explain why there are deviations from the average. 1st question first…
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Why should the average density of the universe (ρ) be so close to the theoretical critical density (ρ crit )? There’s no law of nature that says Ω Ω ( = ρ/ρ crit ) must be equal to one. Why not Ω =0.001 or Ω = 1000?
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Maybe it’s just a coincidence that Ω is close to one, and space is nearly flat. The coincidence becomes
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This note was uploaded on 07/17/2008 for the course ASTRO 294 taught by Professor Ryden during the Winter '08 term at Ohio State.

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Feb_19 - Why is the universe lumpy? Tuesday, February 19...

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