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Answers
Cosmology (from the Greek κόσμος, kosmos "world" and -λογία, -logia "study of") is a branch of astronomy concerned with the studies of the origin and evolution of the universe, from the Big Bang to today and on into the future. It is the scientific study of the origin, evolution, and eventual fate of the universe.
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Answer:
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Anisotropy of CMB Radiation & Cobe
In this chapter, we will discuss the anisotropy of CMB Radiation and COBE, i.e., Cosmic Background Explorer.....
1). Primary Anisotropies in the CMB
To understand the observations from space and the primary anisotropies in the Cosmic Microwave Background Radiation, let us take the following equations and understand it as shown below...
2). CMB Photon Number Density (nγ,0)
nγ,0=TotalenergydensityCharacteristicenergyofPhotonsnγ,0=TotalenergydensityCharacteristicenergyofPhotons
nγ,0=aT40kBT0nγ,0=aT04kBT0
where kBkB is Boltzmann Constant and T0T0 is the present temperature of the universe.
Using the present temperature (T0)(T0) as 2.7 K, we get the current CMB photon number density as 400 cm−3.
The cosmic stellar photon number density is much smaller (∼= 10−3 cm−3) over large scales...
3)..Baryon to Photon ratio (η)
If the stellar contributions from galaxies, which get mixed with CMB, are negligible, the baryon to proton ratio is −
η=nb,0nγ,0η=nb,0nγ,0
The present value is ∼5 × 10−10. Since both photon and baryon number densities are proportional to a−3, then η doesn’t evolve with time...
4). Energy Density
As opposed to the number density, the matter energy density is more dominated than photon energy density at present.
The Energy density of baryonic matter = ρb,0c2=0.04ρcc2=2×10−9ergcm−3ρb,0c2=0.04ρcc2=2×10−9ergcm−3. While, the Energy density of radiation = aT40=4×10−13ergcm−3aT04=4×10−13ergcm−3....
5).Isotropy of CMB Radiation
Penzias and Wilson found the CMB to be isotropic within the limits of observations. The limits are low angular resolution and sensitivity of instruments. They made observations from earth, due to this, observations cannot be made through all the spectrum as water vapor in the atmosphere absorbs many wavelengths ranging from 1mm to 1m. So, CMB can’t be asserted as a spectrum.
The CMB is thought to be rotationally invariant (isotropic). Since there existed a time when matter and radiation were in equilibrium, then the formation of structures in the universe is unexplainable. Since the distribution of matter is not isotropic but is clumped together like a cosmic web with huge voids in between, CMB is thought to have an extragalactic origin.
But, as the observations from the space began, anisotropies in the CMB were found, which lead to the reasoning that these anisotropies in matter lead to the formation of structures...