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Disclaimer The question below is based on a vague knowledge, and hence, statements can be potentially wrong or misleading. An important quantity in Cosmology is the density fluctuation of matter quan …

The polarization of CMB requires a quadrupole anisotropy of the incident radiation field acting on the plasma. How did such a special pattern (i.e. hot and cold in two orthogonal directions) generate …

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation. If we probe the present Universe, it ap …

The particle horizon is defined as the maximum distance up to which light can travel between two given times $t_i$ (often taken to be zero with $a(t_i)=0$) and $t$. This is the farthest distance in t …

After the inflationary era the Universe became radiation dominated. The era of radiation domination is defined as the phase during which the temperature of the Universe was so high that the kinetic en …

To the best of my knowledge, the CMB power spectrum is obtained from a statistical analysis of the observed temperature anisotropies of the CMB sky. Is there a way of getting the power spectrum theore …

Edited version From Hubble's measurement, the only thing that he can conclude, in my opinion, is that the galaxies move away from Earth with their speeds proportional to their distances from the Earth …

How does one measure the curvature parameter $k$ in the FLRW metric? $$ds^2=-c^2dt^2+a^2(t)[\frac{dr^2}{1-kr^2}+r^2d\theta^2+r^2\sin^2\theta d\phi^2]$$ In particular, what is the convenient equation ( …

Our cosmology course instructor said that At a given cosmic time $t=\tau$, the comoving observers define a spacelike hypersurface $\Sigma$ in which the universe looks homogenous and isotropic. An …

The flatness problem in a nutshell One of the Friedman equation is given by $$ H^2\equiv\Big(\frac{\dot{a}}{a}\Big)^2=\frac{8\pi G}{3}\sum\limits_{i}\rho_i-\frac{k}{a^2}.\tag{1} $$ In terms of the den …

According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark m …

Matter makes up roughly $5\%$ of the Universe while dark matter makes up roughly $25\%$. For matter and radiation, we have a complicated Standard model with many generations of fermions and various fo …

Question edited on 27.06.2018 for clarity The Cosmic Microwave Background (CMB) is often called the radiation leftover from big bang, or more precisely, the photons decoupled from the thermal bath whe …

We say the Earth is in relative motion with respect to the cosmic microwave background (CMB), causing anisotropies in the CMB spectrum. I have four very simple questions about this. How is it possib …