Questions tagged [cosmology]

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

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41 views

Are there galaxy clusters or dark matter halos in the void?

The void usually refers to the region of the Universe where the overdensity $\delta(x)$ is less than zero. What I'm curious about is whether there are virialized objects, like galaxy clusters or dark ...
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If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of ...
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1answer
23 views

Calculating comoving volume of 2dF survey

Say I want to calculate the comoving volume in the 2dFGRS survey: I'm using $H_{0} = 70 km s^{-1} Mpc^{-1}$ and the default values for the matter and vacuum density parameters in this calculator (...
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26 views

How can scientists deduce the number of types of neutrinos, or 'effective number', from Planck satellite data?

Is it related to the way they deduce the Hubble constant from Planck data? Would more types of oscillating and mixing neutrinos mean faster or slower expansion of the universe? Would a fourth mass ...
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71 views

Minimum size of closed universe that satisfies ΩK < 0.005

According to this Wikipedia article: https://en.wikipedia.org/wiki/Shape_of_the_universe Results of the Planck mission released in 2015 show the cosmological curvature parameter, ΩK, to be 0.000±0....
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How sensitive are early universe abundance calculations to the neutron half-life?

Phys.org's Physicists announce the world's most precise measurement of neutron lifetime discusses the accepted for publication Phys. Rev. Letter Improved neutron lifetime measurement with UCNτ (links ...
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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....
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1answer
37 views

What's different of power spectrum and correlation function of BAO?

(source) There is the sentence "the harmonic sequence in the power spectrum correspond to a single peak in the correlation function and that the damping envelope corresponds to the broadening of ...
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80 views

Is the sound horizon 500 thousand light years or 500 million light years?

At 06:30 in the nice video Sound Waves from the Beginning of Time about the baryon acoustic oscillations (BAO), the speaker says the sound horizon is about $500$ ...
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1answer
61 views

Observability of proper distance

Hubble's law at current time is as follows. $$v=H(t_0)r$$ But if you look at the explanation on Wikipedia: Strictly speaking, neither $v$ nor $D$ in the formula are directly observable, because they ...
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1answer
51 views

The Lambda-CDM-model: when will the Age of the universe catch up with the Hubble time?

Basic assumptions: A) Within the Lambda-CDM-model we have two separate time measures: the Age of the universe - today roughly 13.8 GY, and the Hubble time - today roughly 14.4 GY B) Very roughly, ...
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380 views

Why would a quantity like the 'Hubble contrast' be squared, then have its square root taken?

From Sabine Hossenfelder's recent video, New Evidence AGAINST Standard Cosmology: And her source.... I don't get why a graph would show a quantity that is squared, then immediately 'square-rooted'......
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What does lowercase 'beta' mean in this graph of radio-emitting galaxies?

Correlation between the mass of the galaxies (X-axis) and the difference of their radio emissions at different radio frequencies (Y-axis). Each symbol represents an individual galaxy. The image of an ...
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What is Euclidean fluence scaling in the context of bounding constraints on FRB all-sky rates? Would non-Euclidean scaling give a different answer?

The recent Chromatic periodic activity down to 120 MHz in a Fast Radio Burst contains two paragraphs in which X's are given "assuming an Euclidean fluence scaling". To me, a non-astronomer, ...
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48 views

Are there ideas about Multiverse in cosmology that are independent of string theory?

It is known that string theory allows for many kinds of universes within it's Landscape which give rise to the concept of the multiverse. But are the concepts/ideas about multiverse in cosmology which ...
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47 views

Why does the 'Big Bang Nucleosynthesis' theory require that neutrinos, or at least sterile neutrinos, have a LARGE mass rather than a tiny one?

From Wikipedia, Sterile neutrino Particles that possess the quantum numbers of sterile neutrinos and masses great enough such that they do not interfere with the current theory of Big Bang ...
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65 views

Do some physicists believe that the universe is a three-dimensional space wrapped on a four-dimension doughnut?

The astrophysicists believe if this is true, the universe would be finite. The entire cosmos may be only three or four times larger than the limits of the observable universe, which is about 45 ...
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112 views

How does the hypothesis of the "inconstant Hubble constant" solve the current crisis in cosmology?

It was published in a paper more or less like two months ago. I'd like to know also if more accurate measurements are necessary to close the gap between the model of the universe and the data reported....
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4answers
281 views

Why do we see a cosmological redshift at all if space is not expanded in our solar system?

It is generally accepted that the large scale redshift of galaxies (as given by the Hubble law) is due to the expansion of the universe. According to this theory, the fabric of space itself stretches ...
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1answer
70 views

How can we know the density of particles in interstellar or intergalactic space and could that density form dark matter?

It is assumed that the density of particles in the empty space between stars or galaxies is extremely low. But how do we know this density? The space surrounding the Sun is contaminated by cosmic ...
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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 ...
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138 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 ...
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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 ...
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3answers
295 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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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 ...
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3answers
2k views

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 ...
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50 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 ...
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1answer
42 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 ...
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1answer
177 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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31 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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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 ...
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1answer
66 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 ...
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1answer
33 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)=\...
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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) ...
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1answer
102 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 ...
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1answer
166 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 ...
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49 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 $H(t)$ ...
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70 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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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
78 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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97 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
193 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
369 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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52 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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90 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 - \...
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1answer
44 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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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 ...
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1answer
87 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 ...
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50 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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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 ...

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