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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Is there any measurement of all the observable angular momentum in the Universe?
Has the sum of all observable angular momentum in the Universe ever been evaluated? There has been a lot of efforts dedicated to the missing mass and energy problem, but has any experimental work ever ...
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Dark energy contributing to, or modifying, mass estimates?
I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a ...
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The linear growth factor of perturbations today should be 1 but I am unable to derive it
The definition of the linear growth factor $D(z)$ of perturbations in a cosmological setting is usually normalized to unity at redshift $z=0$. So,
$$\delta(z) = D(z)\times \delta(z=0) \tag{1}$$
where $...
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What is the number density of CMB at higher temperature? [closed]
What was the approximate number density of CMB photons when the CMB had a higher temperature of 897 K? (Hint: You can assume that photons in a thermal distribution have a mean energy of 3kT). Answer ...
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Why isn't the curvature scale in Robertson-Walker metric dynamic?
$$ds^2=-c^2dt^2+a(t)^2 \left[ {dr^2\over1-k{r^2\over R_0^2}}+r^2d\Omega^2 \right]$$
This is the FRW metric, here k=0 for flat space, k=1 for spherical space, k=-1 for hyperbolic space. $R_0$ is the ...
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Cosmology: Covariance between Gaussian distributions for complex spherical harmonics coeficients
In the context of the computation of a variance about $a_{\ell m}$ spherical coefficients of Legendre, I am faced to an issue :
There is a term $\langle \text{Re}(a)\text{Im}(a)\rangle$ that appears ...
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Textbooks/papers to learn about the physics of the CMB
I am a PhD student working on observational cosmology. My research is based on the experimental side of it (receiver design, etc.) but I would like to get deep into the CMB and CMB statistics. I have ...
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Can either of LISA, NanoGrav or LIGO measure the polarization of gravitational wave background(GWB)?
Polarization in GWB should carry as much important information as in CMB. However, I've done some superfluous literature research and found little discussion. Is there any planned project for ...
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How does dark energy affect the dynamics of galaxy clusters?
Galaxies interact with each other gravitationally (just as every other celestial object) and in many cases they form groups or clusters.
Does the expansion of the universe (or dark energy) affect the ...
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What mathematics indicate that the vacuum of the universe is metastable?
As I understand it, the meta stability of the universe's vacuum depends on the electroweak force, which depends on the mass of the top quark and the Higgs boson. But I don't understand this fully. Can ...
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How do I interpret the unit of [1/dex]?
I am trying to reproduce the mass function graph from a paper. I have calculated the mass function $\frac{\rm{d}f}{\rm{d}\log M}$, where $f$ is the mass fraction with respect to the total mass in the ...
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What would the universe look like if it had undergone a false vacuum decay in the past?
Inspired by "if a metastable de Sitter space lasting for cosmological durations really is impossible in string theory, then dark energy needs to be explained in some other way, e.g. via ...
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How you would you detect weak negative spatial curvature that only existed in cosmic intergalactic voids?
If the large space voids between galaxies had uniformly-distributed "negative" gravitational lensing, would its presence be obvious from photos in the same way that the presence of Einstein ...
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Poisson noise on $a_{\ell m}$ complex number: real or complex?
In a cosmology context, when I add a centered Poisson noise on $a_{\ell m}$ and I take the definition of a $C_{\ell}$ this way :
$C_{\ell}=\dfrac{1}{2\ell+1} \sum_{m=-\ell}^{+\ell} \left(a_{\ell m}+\...
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Poincaré recurrence in a closed universe? [duplicate]
Is it possible that the Poincaré recurrence applies to a closed universe (with a finite spacetime)? If it is, would this mean that a closed universe could eventually reach the same state as its ...
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Are perturbed FLRW solutions stably causal?
Consider the most general perturbed FLRW metric, as one typically sees in cosmology textbooks:
$$ds^2 = a^2 (\tau) \left( -(1-2\psi) d\tau^2 + 2 w_i d\tau dx^i + \left[ (1-2\phi)\gamma_{ij} + 2h_{ij} \...
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Do Boltzmann brain thought experiments suggest literally anything can form randomly?
Do Boltzmann brain thought experiments suggest literally anything can form randomly?
What are the limitations to what random fluctuations can form? Literally any physical, material object?
Lastly, I ...
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Degrees of freedom in the early universe with MSSM?
As nicely summarized on P4 in On effective degrees of freedom in the early universe here; at high temperatures where all the particles of the Standard Model are present, we have 28 bosonic and 90 ...
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Space expansion or generation [duplicate]
When physicists assert that space is expanding, does this imply the creation of new space? If so, why do they use the term "expansion" instead of "generation"?
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Is it possible that if the universe collapses, it reaches the same state as in its beginning? [closed]
Suppose the universe were to eventually collapse in a Big Crunch. How closely could the universe's final moments resemble those at the beginning of the universe? Could the universe return to its ...
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Does NFW profile work for any galaxy?
We use Navarro–Frenk–White (NFW) to calculate Dark Matter (DM) density. Can we use it for DM halo in any galaxy or is it used only for Milky Way (MW)?
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How would red (frequency) shift work for sound waves on the surface of a balloon?
Imagine an expanding balloon with a source of vibrations, in the fabric of the balloon, on one side (pole), and a microphone on the other side (pole). We would expect the sound to be 'red shifted' ...
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In Cyclical Cosmology (Big Bounce) is it possible the new universe will be different or the same? [closed]
Could it be a universe with similar laws to ours but a different configuration of matter, so there may be another earth like planet in this new universe?
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Vector perturbations in an expanding universe
Why vector perturbations in an expanding universe decay while the scalar and tensor perturbations don't?
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Zel'dovich pancake derivation
I am looking for a derivation of the Zel'dovich pancake. Does anyone have a reference to the derivation or a link to the original paper?
Y. Zel'dovich, Gravitational instability: An Approximate theory ...
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What exists in the world according to the special relativity? [closed]
Before I learned about special relativity, I thought that only one 3-dimensional state of the world exists. Then, like in game of chess, in one "turn" previous state is destroyed - and the ...
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Integral over rapidly randomly fluctuating function [closed]
As a part of my bachelor thesis on cosmic structure formation I have been dealing with a sum of many randomly distributed phase factors, so in principle with a pearson random walk. If there are $N$ ...
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Is there an upper limit to temperature in the universe?
The Sun's core temperature has been modelled to be $\approx 1.57 \times 10^7$ K
In supernovae:
"In lower mass cores the collapse is stopped and the newly formed neutron core has an initial ...
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Why easiest path from Big Bang to Heat Death is not followed by nature? [closed]
Why after Big Bang it is not going to Heat Death in the easiest path, actually the energy can get distributed uniformly to all directions. Instead we see the energy in Big Bang is being converted to ...
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Cosmological Constant Problem calculations involving energy densities
I am following Timo Weigand lecutre notes on QFT, on page 28, he breifly touches on the Cosmological Constant Problem.
But I am a little confused.
He begins with a Lagrangian and include a nonzero $V_{...
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Momentum in terms of comoving velocity in cosmology
I want to understand why the following relation holds in the context of Cosmology, considering that $m$ is the mass of a particle of the fluid that fills the universe:
$$\vec{p}=am\vec{u}$$
where this ...
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Does the current size of the cosmological sound horizon play a relevant role in the universe?
I am doing some interactive plots about cosmological horizons and in my research I stumbled upon the sound horizon, the baryonic acoustic oscillations and how it had an impact on the formation of the ...
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The 'core-cusp' problem for dark matter halos in larger galaxies
TLDR: Do observations of larger galaxies favour 'cuspy' dark matter halo distributions, as predicted by N-body simulations?
I've been trying to understand the 'core-cusp' problem for dark matter halos ...
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Is the 2-sphere always useful for intuition and visualized learning in cosmology?
The unit 2-sphere is often used, pedagogically, to help provide some visual intuition about topology, differential geometry and geometric objects and properties like curvature, geodesic, smoothness, ...
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Has the age of the universe changed in 2023?
I teach high school physics and physical science. I was going through the definitions of theory and law when a couple of my students (of different periods) asked about some recent development that ...
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How does one calculate the Sachs-Wolfe effect?
I start from $$\Delta T/T_{0} = - \frac{1}{3c^2}\Phi$$ where $\Phi$ is the gravitational potential which can be written in its Fourier expression as $$\Phi_k = \frac{-3}{2}\left(\frac{H_0}{k}\right)^2 ...
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What is the state of matter-energy inside Black Holes? [closed]
Do we know absolutely nothing about what might be the state of matter-energy inside the black hole?
Are there existing theories or research that explore the possibility of a metastable vacuum inside ...
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Contradiction of propagation speed of gravitational waves with non-locality of gravity?
Although LIGO at 2016 verified the speed of a traverse gravitational wave being the speed of light in a vacuum $c$ there is also the fact that gravity is a non-local phenomenon, meaning that it is not ...
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Solve the perturbed Boltzmann Einstein equations in cosmology using RK4 method
I'm trying to learn to numerically solve the perturbed Boltzmann-Einstein equations in cosmology using the RK4 method.
These are the equations:
\begin{align}
\dot{\Theta}_{r,0}+k\Theta_{r,1}&=-\...
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Minimum angular size galaxy
I've heard a mention in a lecture that galaxies have a Minimum angular size. Naively, the angular size should drop as $1/d^2$. The effect is supposed to stem from the expansion of the universe.
What ...
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In theory, shouldn't there be multiple BAO peaks and even troughs?
Shouldn't acoustic oscillations created by primordial matter anisotropies create multiple peaks and troughs when those oscillations are frozen by decoupling ? It's not unthinkable to imagine that the ...
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What do the authors of the paper mean here exactly by path integral?
First of all, please forgive me if i am asking a dumb question. I don't have a physics background. I was reading this paper by Hawking & Hertog on populating string theory landscape and came ...
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Sound speed of Baryonic Acoustic Oscillations
I've read in scientific papers, that at the time of decoupling the baryonic sound speed was 5 km/s. Can anyone tell me how this is deduced or point to textbook/papers that do a derivation? I want to ...
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Are we surrounded by Big Bang? [duplicate]
Maybe the questions is too stupid to be asked or I do not know the technical words, but I could not find any answer to this question.
Here is how I started to think the title:
First I thought of if we ...
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Theoretical: what is the meaning of nothing? [duplicate]
Before the big bang, there was a point surrounded by nothing (no space or anything). Then the big bang happened and the universe expanded. so beyond the universe's limits, there is nothing? I don't ...
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Numerical Approximation of the CMB Anisotropy Monopole $\Theta_0$ and the Scalar Metric Perturbation $\psi$
TL;DR:
How can $\Theta_0+\psi$ be modelled, what are viable Ansätze for this, including numerical factors?
$\Theta_0$ being the temperature fluctuation monopole and $\psi$ being the scalar metric ...
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Dark matter, MOND or flattened gravitational fields? [closed]
Could there not be a third variant to explain why e.g. long-distance multistar systems rotate faster than Newton's law of gravity suggests?
In addition to the Dark matter hypothesis and MOND then, ...
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What was the size of the observable universe at the matter-dominated era and its mass density?
For instance, at the scale factor $a=0.5$ and matter dominated era, what was the size of the observable universe and matter density?
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What is the gravitational field intensity of a uniformly distributed mass content in Newtonian gravity?
In an infinite universe composed of single point masses which can be simplified as a uniformly distributed mass density, what is the equation for the gravitational field intensity in Newtonian gravity?...
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Are black holes the edge of our universe?
Are black holes the actual edge of the universe? Because spacetime is another dimension, I would assume the universe doesn’t have perceived corners or edges. At least humans cannot perceive it.
The ...
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