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10-fluctuations in the CMBR (Horizon Distance (1 radian in sky means…
10-fluctuations in the CMBR
anisotropy short causes
1 part in every 10^5 of the CMBR temperature
due to the interaction of photons with matter, since matter is not smoothly distributed
photon in region of high density at decoupling gets redshifted
photon in region of low density at decoupling is blueshifted
angular size is key feature, and what sets it
Horizon Distance
1 radian in sky means separation of 12.8 Mpc at time CMB was produced
however at this time light hadn't had time to travel this far
not causally connected
use benchmark model to calculate the max separation that was causally connected, also known as the horizon distance
calculate the angular size of the horizon distance
Statistical properties are important
how different parts of CMB correlate with each other on different scales
tells how different regions of the universe related to one another at the time when the CMB was produced
Causes of wiggles in plot
strength of correlation at different scales have different causes
First peak
before time of last scattering, photons and baryons were coupled, and formed a photon-baryon fluid
Dark matter dominated gPE, so fluid moved under the influence of gravity
as fluid flows into high dm density regions the density of the fluid increase
as density increased the temperature increase producing higher energy bb photons, pushing back on the baryons in the fluid
Therefore attracting force and restoring force in proportion to the compression of the fluid, so SHO created
creates sound waves
At decoupling the soundwave has travelled a distance
by measuring the angular scale the first peak corresponds to and comparing that expected from a flat universe, cosmologists measure curvature
larger than expected = positively curved
smaller than expected = negatively curved
if we also know the matter and photons densities it can tell us about the density of DE
Second peak
amplitude relative to the first is sensitive to the density of baryons relative to photons at the time of recombination
increased baryon density increases the depths of the troughs of the sound wave relative to the heights of the peaks
power spectrum means odd peaks represent troughs
so amplitude of second peak relative to the amplitude of the first tells us about the relative densities of photons and baryonic matter in the universe
increased baryonic density causes 1st and 3rd peak to be stengthened
Higher order peaks
higher order means smaller scales, which means oscillations started at earlier times
At earlier times the photon energy density was increased relative to the matter energy density (from a scaling)
matter is also needed to produce the oscillations, so higher order peaks are suppressed relative to lower order peaks
scale of suppression is depends on relative matter photon densities
higher matter densities require looking to smaller scales to probe when universe became radiation dominated
very high densities, only smallest scales, are suppressed because universe will have been matter dominated since very early on
therefore sensitive to relative densities of total matter to radiation