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Questions tagged [cosmology]

Questions about the origin, history, evolution and fate of the Universe.

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Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection

I would like to know how I can estimate the tangential and cross component of the galaxy's shear using Gnomonic projection of the right ascension and declination the galaxies relate to the center of ...
Dalek's user avatar
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8 votes
1 answer
186 views

What is the history of the average pressure, density, and temperature of the matter in the universe over time?

This question is inspired by this more specific question where Cerelic wanted to know if conditions were suitable for liquid water to exist during an epoch when the characteristic temperature of the ...
Mark Foskey's user avatar
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7 votes
0 answers
254 views

Get the expression of probed volume between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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5 votes
0 answers
127 views

Explain as simply as possible how the Sunyaev-Zeldovich (SZ) effect is used to estimate the Hubble constant

The Sunyaev–Zeldovich effect (SZ effect) is useful in determining the Hubble constant because it is independent of the cosmic distance ladder. This effect occurs when CMB (cosmic microwave background) ...
Astroturf's user avatar
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5 votes
0 answers
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How did pressure evolve in the (early) universe?

I am trying to derive how the cosmological pressure $p(t)$ evolved over time in the universe, especially in the radiation and matter dominated epochs. There are some very nice explanations how $H(t)$ ...
NeStack's user avatar
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5 votes
0 answers
87 views

Significantly blueshifted Lyman-$\alpha$

Lyman-$\alpha$ line in galaxies is notably known to show a double peaked profile, mainly due to its scattering in a moving medium, see e.g., the very recent Matthee et al. 2021, The X-SHOOTER Lyman-α ...
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5 votes
0 answers
103 views

How to place an observer in a cosmological simulation

Suppose I have a cosmological simulation with points in xyz space at a certain redshift in a cube like this: And I have these for many cubes of different redshifts say from z=0 to z=2. Now I want to ...
kauii8's user avatar
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5 votes
0 answers
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Does the cosmological principle apply to the entire universe?

Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If ...
UFO's user avatar
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4 votes
0 answers
68 views

What does it exactly mean for spacetime to have no global symmetries?

Are there spacetimes or metrics with no global symmetries? Spacetimes/metrics with no global Poincaré, Lorentz, diffeomorphism, CPT, translational and gauge invariances? And if there are, what does it ...
vengaq's user avatar
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4 votes
0 answers
96 views

Why is the latest Webb discovery of early-age universe considered so important?

The past week, news were flooding of an unexpected discovery of James Webb Space Telescope. In many articles it is mentioned that cosmologists cannot explain the formation of galaxies of this ...
Demis's user avatar
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4 votes
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If the universe is a closed spacetime structure, can we assign a center of mass to it?

If we envision the universe like a balloon but in 3D, is it possible, in general, to assign a of global center of mass?
Felicia's user avatar
4 votes
0 answers
107 views

Reasons for extremely high number density of galaxies at low redshift

I am currently analyzing the SDSS data especially in the extremely low redshift area, in order to measure the cosmological statistics. What I mean by extremely low is z<0.1. One peculiarity that I ...
Jun-Young Lee's user avatar
4 votes
0 answers
101 views

From what distances do the atoms in you come from?

If the atoms of a human come from stars, comets, nebulas and magnetrons, then what is the greatest distance that two atoms of a human can possibly have been away from each other previously? Perhaps ...
bandybabboon's user avatar
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4 votes
0 answers
49 views

Cross-correlations between Lagrange and Fourier synthesis

In the context of forecast for large surveys, I have to make cross-correlations between 2D (with angular coordinates of Lagrange transformation for GC photometric and Weak Lensing) and 3D (Fourier ...
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4 votes
0 answers
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Cosmology : Formula for the bias of galaxies

first I have posted originally on the physics stackexchange but I have not had answers about my question, so I try my luck on this specific forum (if forums leaders want to delete the original post on ...
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3 votes
0 answers
125 views

What are the loopholes in recent claims that dark energy is not necessary?

I read an article on spacechatter about recent work that eliminates the need for dark energy in explaining observations. The pertinent quote from that article is “The study’s findings confirm that ...
tnknepp's user avatar
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3 votes
0 answers
65 views

Critical density and Hubble parameter

Is it possible for the Hubble parameter of an /expanding/ universe made of radiation, matter and cosmological constant to be increasing with time? I'm trying to figure out if any form of scale factor $...
ABC's user avatar
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3 votes
0 answers
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Deviations of conservation laws in the context of cosmological evolution?

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an accelerating expanding spacetime (like it happens in our Universe), are ...
vengaq's user avatar
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3 votes
0 answers
84 views

Cosmic microwave background the same as the first visible light at 379,000 yrs after the Big Bang? How do these measure the age of the universe?

Was the CMB emitted at the same time as visible light at 379,000 yrs after the BB ? Was this a one time event ? If headed away from us, how does it measure the age of the universe?
Jack Farley's user avatar
3 votes
1 answer
316 views

Redshift distance proportionality at high Z and need for "mighty mouse" galaxies?

Allegedly supported by some evidence from the new James Webb space telescope physicist Eric Lerner has written an article that have garnered some attention. He writes that: "Put another way, the ...
Agerhell's user avatar
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3 votes
0 answers
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Why are CMB peak heights sensitive to the physical densities

A very simple question: why should it be that the CMB power spectrum allows constraints to be placed on the combination of parameters $$\omega_c = \Omega_c h^2$$ $$\omega_b = \Omega_b h^2$$ as opposed ...
Astroguy1234's user avatar
3 votes
0 answers
97 views

spectral index of primordial power spectrum

According to eq (7.62) here, the dimensionless primordial power spectrum takes a power law form: $$\mathcal P(k)=A_s\left(\frac{k}{k_0}\right)^{n_s-1},$$ with $n_s\approx 1$, i.e. $\mathcal P(k)$ is ...
Salmonella mayonnaise's user avatar
3 votes
1 answer
246 views

Mass of the universe

How do we calculate the mass of the universe taking into consideration dark matter? Mass of visible matter can be computed by stellar method, but how do we calculate mass of dark matter which we don'...
user25951's user avatar
3 votes
0 answers
322 views

Total number density of galaxies and problematic expression in practise

This post comes from physics exchange forum, I have transfered it here, maybe I will be luckier since I have not received answers on the other forum. Here is the issue : I am asked to give the ...
user avatar
3 votes
0 answers
120 views

What is the difference between Steinhardt's and Neil Turok's cyclic cosmology and Guth's inflationary model and Hartle, Hawking no boundry cosmology?

I have been reading some cosmology and need to ask the experts at this point some specific points. I must admit I am a little confused. There are a few models of the universe and may be necessary at ...
user17197's user avatar
3 votes
0 answers
187 views

What is the spectral power output of a typical galaxy?

I'm looking for a function which gives the typical isotropic spectral power of light emitted by an entire galaxy on the whole electromagnetic spectrum (in watts per frequency unit), as a function of ...
Cham's user avatar
  • 273
3 votes
0 answers
61 views

Why is the E-mode polarization spectrum out of phase with the Temperature spectrum?

The E-mode polarization power spectrum of the Cosmic Microwave Background displays the same acoustic peaks that can be seen in the (more famed) temperature power spectrum. However, they are out of ...
user1991's user avatar
  • 844
3 votes
0 answers
69 views

Available data on the Milky way around 1920

In a book by Alexander Moszkowski, there is an Einstein quote about a hypothetical size of the universe (100 million Light years). Moszkowski claims that Einstein had deduced that from the ...
ClassicalPhysicist's user avatar
3 votes
0 answers
75 views

How exactly can the hypothetical conformal invariance of the CMB spectrum be established by analyzing tensor modes?

In the introduction of this paper at the top of p11, it is mentioned that a hypothetical enhancement of the scale invariance of the CMB spectrum to conformal invariance could potentially be ...
Dilaton's user avatar
  • 809
2 votes
0 answers
75 views

Would we know if the universe was rotating?

I was wondering what we would observe if the universe had a small rotation. My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than ...
KDP's user avatar
  • 373
2 votes
0 answers
151 views

What happens at the edge of a finite and "bounded" universe?

So far, we know only about two types of universes: Infinite and finite universes (Unbounded edition). In an infinite universe, there is no edge. Because well, an infinite space doesn't have an edge. ...
Alastor's user avatar
  • 2,668
2 votes
0 answers
34 views

Local anisotropies in the expansion of the universe and tidal forces...?

In the context of the large-scale structure of the universe, there can be local anisotropies (https://www.aanda.org/articles/aa/full_html/2021/05/aa40296-21/aa40296-21.html). I understand from this ...
vengaq's user avatar
  • 1,017
2 votes
0 answers
89 views

Could entropy be reversed eventually in the far future?

In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe and after the last black hole evaporates there could be no more structures and no ...
vengaq's user avatar
  • 1,017
2 votes
0 answers
42 views

Colossus sigma function not reading the changes in the code

am currently using the colossus package on Python, which helps calculating functions that are relevant in Cosmology. I am interested in calculating a Matter power spectrum, and use it as a base to ...
Lorenzo's user avatar
  • 21
2 votes
0 answers
98 views

Empirical Constraints on the Time Evolution of Cosmological Redshifts

The standard model of Cosmology predicts that the redshift of a single source at fixed comoving distance varies with observation time according to $$ \dot z = \left( 1+z - \frac{H(z)}{H_0} \right) H_0....
jawheele's user avatar
  • 121
2 votes
0 answers
73 views

Limits of the universe

I am considering the limit in the expanding universe where the expansion matches the speed of light. If we consider that the expansion is accelerating, how will the situation be after the next 14 ...
Mikael Jensen's user avatar
2 votes
0 answers
37 views

How do the abundances of heavy elements near the Solar System compare with the abundances at other places and times in the Universe?

We have measured the relative abundances of heavy elements (e.g. gold, lead, uranium) near the solar system and in cosmic rays. I'm interested in whether those abundances are particularly high (or low)...
Alex319's user avatar
  • 181
2 votes
0 answers
31 views

How much does foreground clumping affect the estimated sizes of high-redshift objects?

I've always converted angular to linear size by using the angular-diameter distance without really thinking about it. But in light of recent stories about high-$z$ galaxies observed by JWST being ...
benrg's user avatar
  • 3,860
2 votes
0 answers
191 views

Plotting galaxies in 3-D

Suppose I have a catalogue of galaxies in a .dat file with each row representing information about a galaxy: right ascension of the galaxy, declination of the ...
peakcipher's user avatar
2 votes
0 answers
158 views

By dint of some 'reverse engineering' I have produced the equation below. Does anyone recognise it? Does it come from general relativity?

The equation is $$a = \frac{\sqrt{2} \cdot c^2}{k}$$ where a is the acceleration due to gravity, c is the speed of light and k is the radius of curvature of the geodesic. The equation is dimensionally ...
John Hobson's user avatar
2 votes
0 answers
337 views

Cosmology context plotting : Covariance matrix - Script to plot Omega_m vs Omega_Lambda diagram in Python

I have covariance matrices and Markov chains coming from the SCP (Supernova Cosmology Project) and I would like to plot all these data in the particular diagram Omega_Lambda vs Omega_m or w vs Omega_m....
user avatar
2 votes
0 answers
91 views

Formula to calculate round-trip travel time based on current distance to far-away galaxy

Cross-posted from Physics Stack Exchange - original The question was closed as HW-like there. The Premise You travel at a constant velocity towards a far-away galaxy. You then travel back at the same ...
trinalbadger587's user avatar
2 votes
0 answers
54 views

What are the one-dimensional observables that can reveal the matter clustering properties of the Universe?

As we all know, the distribution of galaxies, which depend on the spatial coordinates, can help us understand the large-scale structure of the Universe. We can measure the distribution of galaxies ...
Wang Yun's user avatar
  • 431
2 votes
0 answers
60 views

How to find the scale factor on given time in the future if initial distance between source and destination is known

Assume the received redshift $z$ of a photon is known. We then can calculate the initial distance (when the received photon was emitted), the light Travel Distance, the current Distance (proper ...
Vishwas Prabhakar's user avatar
2 votes
0 answers
43 views

Looking for analytical expression of Cosmic Variance - Poisson distribution?

I have an expression of Matter Angular power spectrum which can be computed numerically by a simple rectangular integration method (see below). I make appear in this expression the spectroscopic ...
user avatar
2 votes
0 answers
140 views

Poisson noise for a squared contrast of density - expression of variance equal to $1/n^2$

I manage to compute the Poisson noise of a density field like this : If I take $N$ the density of galaxies and compute the Shot noise with a Poisson distribution, I get : $\langle N^2\rangle - \...
user avatar
2 votes
0 answers
48 views

Cosmic variance - Fourier Transform - a few precisions

I have posted initially on physics exchange but none answer, so I try my chance here (I am going to delete the physics exchange one) : I cite an original report of a colleague : If we are interested ...
user avatar
2 votes
0 answers
77 views

How do we know or predict which particles were present before Big Bang Nucleosynthesis occurred?

I'm reading Carroll and Ostlie's "An Introduction to Modern Astrophysics". In the BBN section, they describe that the universe contains a mixture of photons, electron-positron pairs, and ...
Astroturf's user avatar
  • 1,111
2 votes
0 answers
161 views

How can the CMB have a "monopole anisotropy"?

Wikipedia's Cosmic Microwave Background (CMBR) radiation monopole anisotropy (ℓ = 0) says When ℓ = 0, the ${\displaystyle Y(\theta ,\varphi )}{\displaystyle Y(\theta ,\varphi )}$ term reduced to 1, ...
uhoh's user avatar
  • 30.6k
2 votes
0 answers
185 views

Clarifications about distances in cosmology

I would like to get clarifications about some usual notions of distances in cosmology. First, is the comoving distance the current distance of objects whose light has been reached by us now, i.e. ...
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