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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Value of $H$ at the emission of the CMB

According to the lambda CDM model, $H_0$ is around 67.7. If this value was calculated from a value for $H$ at the emission of the CMB, what was it, or do we have constraints on its value at that time?
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Ejected bodies, dynamical friction and dark energy?

I have a question after reading a couple of papers (https://arxiv.org/abs/1707.06220 and https://www.arxiv-vanity.com/papers/1102.0007/). Here, the authors seem to indicate that when bodies like stars ...
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"Boltzmann" equation for radiation (Reheating period)

These equations are given in many papers as the "Boltzmann equations" (without derivation) governing the reheating period, where $\rho_\phi$ is the energy density of the decaying inflaton ...
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How large was the visible universe at the time of the CMB?

Given that our best models so far suggest that the universe has been expanding a lot ever since the time the CMB was emitted, I'm wondering: How large was the (visible) universe back at the time it ...
70 views

Determining location of Big bang [duplicate]

I am just a high school student. I had already heard that we can't determine the exact location of big bang. So now i have a question can't we approximate it? Like say divide the observable universe ...
1 vote
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What mass do neutrinos have in the early universe?

Since neutrinos interact constantly in the early universe, I assume that they are present as flavor eigenstates. However, they are Fermi-Dirac distributed, f(E, T) = \frac{1}{e^{E/T} +...
1 vote
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Gravitational potential in an expanding universe during the matter era

In the book Cosmology, by Daniel Baumann, there is an exercise to show, using the Poisson equation, that the gravitational potential is a constant during the matter era. As I understand, the Poisson ...
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Does Dark Energy contribute to increase the isothermal temperature of plasma in galaxy clusters?

I have a question about this work called "Dark energy and key physical parameters of clusters of galaxies" There, towards the end, the authors talk about the isothermal velocities and ...
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What is the connection between matter in the universe and the baryon number not being conserved?

Towards the end of "Quarks, the Stuff of Matter", the author discusses the implications of the proton is not stable and ultimately decays. He states, that if the proton decays, then the ...
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Galactic Rotation Curves

While researching rotation curves, I've noticed a variety of velocity behaviors in different galaxies. In some, the velocity decreases, in others, it remains relatively constant, and in some cases, it ...
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The equation of motion for the scalar field

Consider a scalar field ϕ with the following Lagrangian density in the FRW scale factor: $L= (-\frac{1}{2} \partial_{\mu} \phi \partial^{\mu} \phi -V(\phi)) \sqrt{-g}$, the motion for ϕ in this ...
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Size and boundary of the Milky Way Galactic disk

I have been pondering a question that arose while I was reading a research paper that mentions galactic disk stars have been found up to distances as far as 25 kpc from the galactic center and ...
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Confusion about Hubble law vs. objects freezing at the cosmological horizon

I am puzzled about an apparent contradiction. In a universe with a Hubble law, you see that, the farther an object is, the faster it is moving away from you. On the other hand, you also have a ...
1 vote
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“Negative energy” definition of “gravitationally bound” in expanding cosmology

First off, I realize that there is already a number of questions relating to gravitational binding in cosmology: Gravitationally bound systems in an expanding universe Gravitationally bound systems ...
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Effects of dark energy in the kinetic energy of a body?

If I launch a ball into the sky it would reach a distance after which it would return into the ground transforming the potential energy into kinetic energy as it hits the ground This is similar to ...
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Does General Relativity, without Cosmic Inflation, predict a perfect blackbody for the CMB radiation?

My understanding is that the universe did not have enough time to thermalize before the epoch of recombination, so many patches of the sky were not in causal contact with each other, which means they ...
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Clocks at different redshift: does calculating luminosity distance shift the frame of reference?

Luminosity is energy per time. When we calculate a luminosity distance, using angular diameter distance times $(1+z)^2$, I think we are correcting the energy (from redshifted photons) by $(1+z)$ and ...
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Is it possible to take the time derivative of sound speed of an ideal fluid when the background $w=0$?

Context: To determine the evolution for a particular unified dark matter model we use the following equation: \delta''+3(s\delta)'+(\delta'+3s\delta)\left(2+\frac{H'}{H}\right)-\...
1 vote
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How to obtain the continuity and Euler equations by taking moments of the Vlasov equation in Cosmology?

In a set of notes about cosmology, I have found the following claim: The 0th moment of the Vlasov equation yields the continuity equation. For that, upon integrating over the momentum, we have to ...
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In a homogeneous cosmological fluid, which position vector is constant, $\vec{r}$ or $\vec{x}$?

This question occurred to me while trying to prove an equation in the context of Cosmology. When I decompose the Newtonian gravitational potential $\Phi$ as the sum of a potential $\phi_b$ due to the ...
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Cosmological understanding from Penrose diagram of de Sitter spacetime

The conformal diagram of de Sitter spacetime looks like this I think I understand the causal properties of this diagram. Someone who is static in the south pole can send messages only to the upper ...
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Dimensional analysis suggests that $\Lambda R^2 \sim O(1)$, where $\Lambda$ is the cosmological constant and $R$ is the radius of the universe. $\Lambda$ is measured to be around $10^{-52}$ m$^{-2}$, ...