Questions tagged [cosmology]
Questions about the origin, history, evolution and fate of the Universe.
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Why do gravitational waves travel “only” at the speed of light but the gravitational scalar potential is instantaneous?
To my old Space SE question Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch? @DavidHammen's excellent answer replies that one should not add a ...
1
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1answer
109 views
Is it “nonsense to even talk about” objects outside the observable universe not having gravitational influence on us? (finite speed of gravity)
In this supplemental answer to Is the zero gravity experienced in ISS the “artificial” kind? in Space Exploration SE I said:
Gravity moves at the speed of light so nothing outside out observable ...
2
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0answers
36 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 ...
2
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3answers
268 views
Could it be that Universe is rotating while expanding? [duplicate]
I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales.
Could one assume then that the Universe is acceleratingly rotating ...
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0answers
22 views
How to locate helpful and instructive books and resources about numerical methods in cosmology?
I am interested in the hydrodynamical and N-body simulations, and I want to use simulation results to examine the theoretical models.
Hence I am searching for some helpful and instructive books and ...
5
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3answers
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How can a black hole have a charge, or be charged?
So-called 'hairless' black holes (no-hair theory, or theorem?) , which is what real black holes are, can be described by just three characteristics: Mass, spin, and charge.
It is easy enough to ...
2
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0answers
48 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 ...
3
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1answer
39 views
Dark Matter Density Parameter Variation
The definition of the dark energy density parameter is $$\Omega_{\Lambda} = \frac{\epsilon_{\Lambda}}{\epsilon_c}$$
where $\epsilon$ is the energy density, $\Lambda$ subscripts represents dark energy ...
5
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1answer
141 views
If we watched extremely red-shifted galaxies near the edge of the observable universe for a very long time, how would they change? Would more appear?
I have understanding sphere eversion as #1 on my bucket list (if I ever get a round tuit) but understanding metric expansion seems to be a rapidly receding possibility :-)
Question: Suppose it takes ...
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0answers
30 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 ...
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0answers
28 views
Questions re galaxies clusters and galaxies mergers within them
An introduction (containing the info I gathered from several Wikipedia articles).
A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of ...
3
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1answer
48 views
Scalar field displacement from the minimum of the potential gives rise to particles/dark matter, why?
The paper Lyman-alpha Constraints on Ultralight Scalar Dark Matter by Kobayashi et al. says, at the beginning of Section 3.1:
A light scalar field stays frozen at its initial field value in the early ...
2
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1answer
29 views
What do “overdense” and “underdense” mean in cosmology?
“Overdense" and "underdense" often appear in various documents to refer to high density and low density respectively. And I know that "overdensity" is defined as
$$
\delta(x)=\...
2
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0answers
26 views
Explain as simply as possible how the Sunyaev-Zeldovich (SZ) effect is used to estimate the Hubble constant
The Sunyaev–Zeldovich effect (SZ effect) is useful in determining the Hubble constant because it is independent of the cosmic distance ladder. This effect occurs when CMB (cosmic microwave background) ...
1
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1answer
90 views
Energy conservation of the universe
I have tried to derive an equation for the total energy of the universe.
I have found that,
$$E(t)= \delta\dot a(t)^2a(t)\Omega(t)$$
Where $\delta$ is just a positive constant, a(t) is the scale ...
1
vote
1answer
52 views
Comoving Volume Calculation
Suppose I have data from an astronomical survey at redshifts in the range $z = [2,3]$. Suppose that, on average in this range, the data covers an area on the sky of $A=1$ $\mathrm{Mpc}^2$. How would I ...
5
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0answers
45 views
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)$ ...
0
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1answer
69 views
Would it be a good idea to dockerize softwares in order to conduct hands-on tutorial sessions? [closed]
I am going to organize a tutorial in which I intend to train students on semi numerical simulations for reionization. Is it a good idea to supply use Docker to supply the codes?
What do you think ...
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0answers
146 views
Relationship between 3 quantities : Density Matter power spectrum, Density Matter angular power spectrum and Temperature angular power spectrum
I take over another post that is going to be deleted since I have none answer.
Summary: I would like to go deeper in the relationship between Matter power spectrum and Angular power spectrum.
From a ...
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0answers
17 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 ...
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2answers
66 views
Does black hole singularity really exists with respective to relative frame of reference? [duplicate]
How can one describe this phenomena?
What are the factors that we should consider for solving such a problem?
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0answers
96 views
Where does the energy of the decaying CMB go?
I would expect that energy of photons in the Cosmic ray Microwave Background gets less and less because their wavelengths are stretched due to the expansion of space.
How can this be possible? Does it ...
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3answers
176 views
How is it possible that a universe with mass does not have curvature?
I may know the answer to my question. Mass must curve the universe but the pressure due to energy density may curve the universe in the opposite direction, leading to a flat universe, if they exactly ...
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4answers
251 views
What are the differences between astronomy and cosmology?
Are astronomy and cosmology same? If not, what are the differences? And which one is greater in terms of subject area?
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1answer
51 views
In any particular space or universe, is the shape of its metric the same as the shape of its geodesic (perhaps the sum of its geodesics)?
Perhaps I mean the curvature tensor, not the metric.
I suspect it is not.
I think the metric is a 4-dimensional object whereas the sum of the geodesics is 3-dimensional (probably 3+1 but not 4).
...
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0answers
47 views
Shot noise in cosmology : Expectation and Variance expression
I am trying to express the Shot noise in cosmology context, mostly the Shot noise expression into error on a $C_\ell$
If this would be only for myself, I would take $$\text{Variance}(Shot\,noise) = 1/...
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1answer
40 views
Why is the FLRW universe (general relativity solution(s)) sometimes called the 'FRW universe'?
Why is the letter L for Georges LeMaîtres often, or even usually, left out?
Does he, or does he not, deserve some credit for this cosmological solution to Einstein's general relativity?
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0answers
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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 ...
2
votes
1answer
62 views
Can the Hubble Sphere ever extend over the cosmic event horizon?
I am struggling to reconcile the Hubble volume with the idea of a cosmic event horizon.
As I understand, the Hubble volume is increasing over time because Hubble's constant is decreasing. This should ...
6
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0answers
68 views
Reproduce and plot myself from a paper with appropriate scripts and Fisher/Covariance matrixes
I would like to reproduce and plot myself the figure below. This plot, even it begins to date, is full of informations for me. The paper is located here : paper Suzuki et al. 2011
I have contacted ...
2
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0answers
48 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 ...
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3answers
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Why did astronomers believe most or all stellar black holes had masses no greater than 15 solar masses?
The so-called 'mass gaps' for black holes, according to theoretical models, are between 2-5 solar masses and 50 to 150 solar masses. (Actually, I have read that there is no good theoretical reason ...
5
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1answer
94 views
Understanding better the factors on $C_l$ in Angular power spectrum and Relation with Matter power spectrum
I am looking for an explanation on the angular power spectrum. I found this extract that is interesting but not fully understood for me (I will cite the step that I didn't understand)
"what is ...
6
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0answers
56 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 ...
2
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0answers
51 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, ...
2
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0answers
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I have a scientific paper on gravity, how do I publish it, and will my research benefit me? [closed]
I read a lot about gravity and discovered a hole in it, and I want to publish that hole, but I am not good at mathematics. To show that gap, what should I do? Should I use Newton's equations which are ...
2
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1answer
44 views
Cosmology calculator where curvature density $\Omega_k$ can be independently set?
I am looking for a cosmology calculator that does not have the default that
$$\Omega_k = 1 - (\Omega_r + \Omega_m + \Omega_{\Lambda}).$$
I particularly want to run
$$\Omega_r = \Omega_m = \Omega_{\...
3
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0answers
47 views
Why are CMB peak heights sensitive to the physical densities
A very simple question: why should it be that the CMB power spectrum allows constraints to be placed on the combination of parameters
$$\omega_c = \Omega_c h^2$$
$$\omega_b = \Omega_b h^2$$
as opposed ...
0
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0answers
66 views
Objects beyond 13.3 billion light-years away are (apparently) moving away from us at faster-than-light speeds? [duplicate]
That is, if you use the cosmic distance ladder method, and a value of about 73.5 for the Hubble constant....
But, if you plug in the Planck CMB value of about 67.5, you get a distance of about 14.5 ...
6
votes
1answer
96 views
Medium sized Black Holes
For many years the existence of medium sized Black Holes (IMBH$^1$) have eluded scientists. BH of several times the mass of our sun have been found, as well as SMBH with millions of sun masses. SMBH's ...
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1answer
58 views
Shouldn't the estimate of the universe's age be higher, not lower, after the attractive strength of gravity is taken into account?
From 'Fundamentals: Ten Keys to Reality', by Frank Wilczek :
"Running the movie of cosmic history backward in our minds, we found the galaxies all coming together to meet at a definite time. ...
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0answers
62 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 ...
4
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0answers
44 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 ...
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0answers
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Calculating the equation of state parameter for matter
am a grad student in a cosmology class right now and I've got something of a head scratcher.
"Assuming the velocity of the atomic particles can be described by a Maxwellian distribution, ...
2
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1answer
58 views
Demo to get Matter Power Spectrum in cosmology
I would like please to demonstrate the expression of Power spectrum in Cosmology :
First, I have the relative contrast:
$$\delta_{i}(\vec{x}, z) \equiv \rho_{i}(\vec{x}, z) / \bar{\rho}_{i}(z)-1\quad(...
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1answer
61 views
Evidence for homogeneity of the universe
Cosmologists assume that the universe is homogeneous on large scales and, in particular, that the unobservable parts are similar to the observable universe. As I understand it, one important reason ...
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1answer
63 views
How do you calculate comoving distance and light's travel distance? According to the formulae below?
According to Wikipedia, Distance measures (cosmology),
Comoving distance:
$${\displaystyle d_{C}(z)=d_{H}\int _{0}^{z}{\frac {dz'}{E(z')}}}$$
Light-travel distance:
$${\displaystyle d_{T}(z)=d_{H}\int ...
2
votes
1answer
58 views
Why is the boundary of friends-of-friends (FOF) halo corresponding to iso-density contour?
The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. (For more information, please refer to here and here)
I found that some literature argues ...
11
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2answers
2k views
Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?
The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. And FOF depends on only one parameter, linking length, $l_\mathrm{link}=b \left(\frac{V_u}{...
3
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1answer
64 views
Historical estimates of the density parameter
From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the ...
