pic1:Final distribution of baryonic matter and its two peaks in density profile pic2:Photon start to stream/the right panel/ but baryonic matter remain in its place.The graph shows the distribution of baryonic(blue line) matter and photons(red line)
Pictures from personal page of Prof.Martin White
Pictures from personal page of Prof.Martin White
In dark energy model approaches to solve the accelerating universe puzzle,in the first step we confront a problem.The problem is that we measure the effect of dark energy indirectly through its effect on expansion rate of universe H=H(z).
So generally there are three main cosmological tools/probes/ to address this problem.
1)Standard candle:Use to find the relation between luminosity distance and redshift.In process of finding this we need to integrate the inverse of Hubble parameter.
2)Standard rulers:Standard rulers are object that we know their measure in some epoch of universe history,so looking at them through a specific angle give us the opportunity to calculate the angular diameter distance, which like the luminosity distance,once more we are able to calculate the integral of expansion rate of universe.
3)Growth of structure,also permit us to put constraint on the expansion rate and consequently on Dark energy model./This probe is useful for determine the difference between modified gravity models with dark energy models./
Although the cosmological objects in general does not have a specific shape to use them as standard rules but in early universe we will found one.
Baryon acoustic oscillation is a standard ruler from the last scattering epoch.
What is the story?
In early universe when the density and the temperature of universe were high ,there was
a plasma of baryons, electrons, photons, neutrinos and Dark matter.
Baryons and electrons are coupled to photons through Thompson scattering /electromagnetism interaction/ and Dark matter is only coupled by gravitational interaction.Then the universe was expanding and becoming cool until the last scattering epoch /approximately 300000 years after big-bang/ when the photons decoupled from matter.This is because the energy of photons are not enough to ionized the hydrogen atoms, so electrons were bound by protons.
At this epoch the photons free stream to the universe but the baryonic matter distribution was maintain in the same radius of 100 Megaparsec.As the universe continues its expansion, photons
spread uniformly through universe, while there was a potential well constructed in the middle of the 100Mps shell which the baryonic matter is distributed in its shell uniformly .
This is the sound horizon.Now the gravitational potential cause the final configuration of Acoustic peaks as two peaks in mass profile, one in the middle and one in 100Mps.
This imprint remains in large scale structure.
If we compute 2-point correlation function in LSS,we will found the effect of sound horizon.
Baryon Oscillations are well used in order to:
1)For firm prediction of models with baryons
2)prediction of dark matter and baryonic matter density in CMB3)Using in order to determine 3)angular diameter size and put constraints on Dark Energy models.
Anyhow the ruler of BAO is inconveniently large.Probe of gigaparsec volume is required to mark a precision measurement of the BAO.
A nice internet pages on BAO:
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