# Questions tagged [cosmology]

The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.

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A particle (say neutrino) decouples from the plasma in the early universe when its interaction rate $\Gamma$, with the plasma, is slower than the expansion rate $=H = \frac{\dot{a}}{a}$, i.e. $\Gamma &... 1answer 104 views ### Relationship between time derivatives at fixed physical r and at fixed comoving x coordinates I am currently studying Newtonian Perturbation Theory in cosmology. We have introduced the relation between the physical coordinates r and the comoving coordinates x in an expanding universe:$\bf r$... 2answers 3k views ### Do we need dark matter and dark energy, if the behaviour of the universe in its initial stages was similar to that of the Sun? [closed] According to CMBR the universe was a cloud of plasma and was a perfect black body,$380,\!000$years after big bang. But the Sun in our solar system also is in the state of plasma, thus making it a ... 0answers 53 views ### How accurate is the De Sitter solution of the Einstein field equations of general relativity? The De sitter solution models the universe as spatially flat, neglecting ordinary matter, such that it is dominated by the cosmological constant. The cosmological constant notion arguably suggests the ... 1answer 47 views ### How do you calculate conformal time? I've got several references that indicate that conformal time is the integral of the scale factor: $$\eta=\int_0^t \frac{1}{a(t)}\mathrm dt.$$ So I tried to calculating this over a range of {0, ... 0answers 15 views ### If the universe is expanding, then the distance between the atoms must increase, which should result in decrease in density of everything [duplicate] In the expansion of universe theory, it is already written that the scale of universe is itself expanding, which means the consequence for expansion of universe is not that atoms are growing in size, ... 1answer 58 views ### Are objects repelled by the cosmological horizon in Lambda-CDM cosmology? When an observer watches a distant object, is there a gravitational influence of the horizon on that object? ("Distant" here is meant to mean: at a distance that is a sizeable fraction of the Hubble ... 0answers 35 views ### Proving that universe is a blackbody How can you theoretically explain why cosmic microwave background seems to be coming from a perfect blackbody given that the universe is no longer at thermal equilibrium? 2answers 42 views ### Physically acceptable energy-momentum tensor From a problem the line element is: $$ds^2 = -c^2e^{-2ax}dt^2 + dx^2+ dy^2+ dz^2$$ I found energy-momentum tensor ($T_{\mu\nu}$) from Einstein field equation by using the above line element. Only T$...
The Schwarzschild radius involves an expression in terms of Newton's constant $G$, the mass $M$ inside a radius $r$, and the speed of light squared $c^2$. Current estimates of the universe's matter ...