sound waves in the early universe

7/29/2019 Sound Waves in the Early Universe

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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.

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Distance

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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

sound waves in the early universe

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