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Sound Waves in the Early Universe
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The Early Universe
Hot, dense, very smooth (variations are small)
Baryonic matter ionized
Mean time for photons to be scattered off of freeelectrons very small
Universe comprised of baryon-photon fluid withsome very small density differences
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Sound Waves
In baryon-photon fluid of the early universe, thereare regions of greater than average density.
Matter flowed from these regions to the less dense
regions surrounding it, and a spherical sound wavepropagated through the early universe
Shown here: superpositionof sound waves
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Decoupling
When the universe was 380,000 years old, thetemperature started to reach 3000 K and lower.
Protons, neutrons, and electrons could form neutral
atoms Average time for photon electron scattering
becomes greater than the age of the universe
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Decoupling
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Acoustic Peak
Find acoustic peak by expressing temperaturevariations as a sum of spherical harmonics
From this, the temperature fluctuations vs. multipolenumber l (which corresponds to an angular size) can
be plotted
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Acoustic Peak
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Acoustic Peak
Peak occurs at ~0.6 deg., which corresponds to480 Mly
This means the preferred separation for galaxies
will be 480 Mly
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Dark Matter
Observations of galaxies lead to the belief that mostof the mass would be concentrated at the center.
A quick working of basic mechanics would show
that the radial velocity of objects in the arms wouldfall off as r-1/2.
2 4 6 8 10
Distance
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Radial Velocity
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Dark Matter
Instead, relatively flat radial velocity curves arefound outside the central bulge of a galaxy.
The most prominent explanation for this is a
spherical halo of dark matter surrounding thegalaxy.
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Dark Matter
The relative heights of acoustic peaks give someindication of dark matter densities
Lower dark matter density increases peaks while
increasednormal matterdensity increasesodd peaks
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Dark Energy
First evidence from Type IaSupernovae (SNe from whitedwarfs gaining mass to exceedChandrasekhar limit)
Type Ia Supernovae arestandard candles, andobservations showed them to
be fainter than expected
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Dark Energy and Acoustic Peak
Dark energy affected distance
Combining the acoustic peak of 480 Mly withredshift and optical measurements of a baryon
acoustic peak for nearby galaxies allows for a moreaccurate measurement of distance on cosmologicalscales
Can then be used to constrain some of the propertiesof dark energy