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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29 views

Is there a formula for volume in a open universe?

Recently while reading a bit about cosmology, I have stumbled upon two different formulas that describe the full volume of a universe with its spatial section $dt=0$. For instance, for a flat universe ...
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Considering below metric, find out $V_{r;θ;φ}$−$V_{r;φ;θ}$ for any arbitrary 4-vector $V^v$ , and how to interpret the result.? [closed]

Considering this spacetime metric, $$ds^2 = g_{\mu\nu} dx^\mu dx^\nu = \left(1 − \frac{r_g}{r}\right)c^2 dt^2 − \left(1 + \frac{r_g}{r}\right) dr^2 − r^2 dθ^2 − r^2 \mathrm{sin} ^2θ dφ^2$$ . Where $r{...
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How to compute volume of universe? then how to show that $k= 0$, $1$, and $-1$ corresponds flat , closed and open universe

Using the matric $$ds^2 = g_{\mu\nu} dx^\mu dx^\nu = c^2 dt^2 − a^2(t)(\left( \frac{1}{1-kr^2}\right) dr^2 − r^2 dθ^2 − r^2 \mathrm{sin} ^2θ dφ^2)$$ . where a(t) is scale fector . This is one of my ...
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How can we show that the very early universe can be considered to be flat?

Let me confess that it's a homework problem. I was working on a heuristic proof of the following problem. How can we prove that the very early universe can be considered to be flat. It was asked in ...
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1answer
41 views

What is the relationship b/w mass’s effect on the curvature of space vs the expansion of space?

As is well known, General relativity explains that mass and energy bend the curvature of spacetime. Mass energy of different amounts lead to different space time curvatures. As is also well known, the ...
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A finer tuned universe

I was reading about the idea of the fine-tuned universe and the anthropic principle. It is stated how small changes in some of the physical constants would render life as we know it impossible. My ...
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Why entropy of the universe is increasing even when universe is expanding with time?

Why entropy of the universe is increasing even when universe is expanding with time? As we know the observable universe is expanding that means new space creating new unknown information with time and ...
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What are $δ$ and $ε$ in the list of Dirac's five 'fundamental constants', concerning his 'Large number hypothesis'?

From Jean-Philippe Uzan's Varying Constants, Gravitation and Cosmology: Dirac formed five dimensionless ratios among which1 δ ≡ H0ħ/mpc2 ∼ 2h × 10−42 and equation M1 and asked the question of which of ...
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Photon geodesic at first order [closed]

I would like to calculate photon geodesic $k^{\alpha} \nabla _{\alpha} k^\mu=0$ at first order in perturbed FLRW metric (in particular the expression of $k^0$). I'm so confused I do not have any ...
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1answer
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Fokker-Planck equation from Langevin equation in stochastic inflation

I'm reading this paper by Starobinsky and Yokoyama where they give the coarse-grained equation of motion, $$ \dot{\bar{\phi}}({\bf x},t ) = -\frac{1}{3H}V'(\bar{\phi}) + f({\bf x},t) $$ where $f({\bf ...
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Big Bang and relativity [duplicate]

Wasn't the big bang's explosion itself faster than speed of light? how does this not violate relativity, I had read an explanation earlier but it wasn't sufficient, can someone explain to me how the ...
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Where did the CMB come from? What is due to the matter/antimatter annihilation? Or perhaps the radiation released when the electrons decoupled?

Where did the CMB originate from? I get that at the beginning of the universe, by the Big Bang theory temperatures and pressures were too high for matter to exist, and even if it did, it would just ...
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Making point transformations of the coordinates of the whole universe [closed]

In theoretical mechanics, we make a transformation from regular coordinates to general coordinates, and then we make a coordinate space, using these independent general coordinates on the ...
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Define absolutely NOTHING prior to the initial singularity before the inflation of space-time [duplicate]

British physicist Stephen Hawking stated "There was absolutely nothing before the Big Bang". Not only him Einstein and most of reputable scientists said that there was nothing before the ...
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The Difference between angular and anisotropic BAO

I was reading an article and saw these 2 different BAO definitions. anisotropic BAO; $$\alpha_{\perp} \equiv \frac{d_A(z)}{r_d}\frac{r^{fid}_d}{d_A(z)^{fid}}$$ angular BAO; $$\theta(z) = \frac{r_d}{...
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About the acquisition of mass by particles

In particle accelerators, ephemeral particles are produced by reactions between particles, as sufficiently high energies are reached. In the early life stages of the universe there were very high ...
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How is it possible to have a quantum wave function used by quantum cosmology describing early stages of our Universe without time being considered?

In quantum cosmology a quantum wave function is used to describe the early stages of our Universe rather classical spacetime. However, the probability distribution does not depend on time. How is such ...
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1answer
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A universe with $\rho_{\text{vac}}>0$ but $p_{\text{vac}}=0$: how would it be like? Would it be stable?

The vacuum energy and vacuum pressure is $\rho_{\text{vac}} = -p_{\text{vac}}$ due to the cosmological constant in our universe. I try to think about another universe, where $\rho_{\text{vac}}$ does ...
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Is Dark Matter in Motion?

What is known about the motion of dark matter, especially in galaxies? It seems as though a particular distribution of dark matter might be required to cause the very flat galactic rotation curves ...
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441 views

Local effects of cosmic expansion

Suppose we place two galaxies 100 Mpc apart, with zero initial velocity with respect to each other. In other words, they are static with respect to each other initially, with a negligible ...
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Can you explain this line from "A brief history of time"?

Newton realized that, according to his theory of gravity the stars should attract each other, so it seemed they could not remain essentially motionless. Would they not all fall together at some point? ...
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Spacetime of a quantum measurement

In a classical(or wave) picture, when we measure a doppler effect from a receding galaxy, we are working on two wave crests essentially. Therefore, there are two events in spacetime for such detection....
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Did the universe need the presence of matter and radiation to start expanding?

I have read this question: Hence it is not possible that photons generated by stars is contributing to dark energy. Could photons generated from the many trillions of stars be some how contributing ...
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1answer
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Demonstration of the variance on a $C_{\ell}$ : can't make appear into demonstration a term "$-1$"

Regardings the definition of $C_{\ell}$ on a survey, we measure all the $2 \ell+1$ coefficients. We are thus led to define an estimator of the observed power spectrum $$ \hat{C}_{\ell}=\frac{1}{2 \ell+...
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Is there any "Cosmological Principle"?

The term "Cosmological Principle" is widely invoked in cosmology discussions. I think the basic idea here is that one might guess that the Milky Way is not at a special location in the ...
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Calculating $Φ(t)$ during inflation

I am trying to solve for $\Phi(t)$ during inflation for a given metric, $$ ds^2 = dt^2 - a^2dx^2 - b^2dy^2 - c^2dz^2. $$ I just wanted to know how should I proceed while taking Lagrangian $L$ as $ L =...
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Do more distant galaxies move faster with respect to the cosmic background radiation rest frame?

Aparently our galaxy is moving approximately 1/500th lightspeed with respect to cosmic background radiation. While this is certainly fast, it stands to reason that it could had been a lot faster, so ...
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Is cosmic background radiation consistent with the Cosmological Principle? [duplicate]

Is the observation of cosmic background radiation really consistent with the cosmological principle? It implies that there is a "special" rest frame of motion with respect to the big bang. ...
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What is the motion of the Earth compared with cosmic background radiation? [duplicate]

Does cosmic background radiation have a frame of reference where it is symmetrical in all directions? If so, what is the motion of this planet, solar system, or galaxy with respect to this?
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Is the age and expansion of the universe the same from all perspectives? [duplicate]

If I understand the Cosmological Principle correctly the age and rate of expansion of the universe must be the same from all frames of reference, yet is this consistent with relativity that states ...
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Quantum fluctuations in the early universe

A Quanta Magazine article on the relationship between inflation and the large-scale structure of the universe mentions the relationship between the spatial distribution of galaxies and quantum ...
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Integrate the expression of error for photometric galaxy clustering and after compute the variance of shot noise

The error on photometric galaxy clustering under the form of covariance which is actually a standard deviation expression for a fixed multipole $\ell$ : $$ \sigma_{C, i j}^{A B}(\ell)=\Delta C_{i j}^{...
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1answer
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Surface of Last Scattering Spacelike or Timelike from us

As far I understand, the whole universe, at a redshift of 1100, became transparent to light, and the phenomenon occurred throughout the whole universe. It is in our past, not in our left or right ...
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1answer
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What are the conditions needed for baryogenesis? [duplicate]

Physicists have created antimatter in the laboratory. But when they do, they create an equal amount of matter. That suggests that the Big Bang must have created matter and antimatter in equal ...
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Big Bang as a Shotgun Blast, not Raisin Bread

I was reviewing various questions regarding the big bang, including: The answer to the question here Did the Big Bang happen at a point? by @JohnRennie and the community that describes how the Big ...
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About the spatial distribution of vacuum energy around strongly gravitating objects in the galaxy

We know that the distribution of vacuum energy is spatially uniform. But we also know that it couples to gravity. Anything with energy, such as a beam of light is affected by the gravitational field ...
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Is it experimentally verifiable if we are a inside a white hole that never fully formed?

There are a lot of questions on this site about white holes, but none of them specifically answer my question. Most of these questions talk about observing white holes from outside, but do not talk ...
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Is it possible something in the universe is not orbiting anything?

I thought I heard that astronomical bodies could be ejected from a galaxy. If a body was ejected from a galaxy, could it have been ejected faster than the escape velocity of the galaxy? If that is ...
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Base and structure of wormhole formation

the symbols $r$ and $r_ 0$ (minimum value of $r$) in wormholes structure are given as non dimensional quantities. What reference radius are these quantities defined relative to?
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How can I understand these averages? (CMB context)

I am currently reading Prof. Mukhanov's book on "Foundations of Cosmology". In chapter 9 he is very briefly explaining free streaming and all of a sudden averages labeled with a direction ...
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1answer
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What is "bulk" in brane cosmology?

The central idea of brane cosmology is that the visible, three-dimensional universe is restricted to a brane inside a higher-dimensional space, called the "bulk". What is "bulk"? ...
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Could the rotating superfluid halo around a galaxy form a regular Abrikosov vortex lattice and serve as a naturally generated Quantum Matrix?

Spherical halo (shown in blue) surrounding galaxies could be a superfluid [Bose-Einstein condensate] according to Prof. Justin Khoury's hypothesis and may help to understand dark matter and galactic ...
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An explanation about "why is the Universe charge neutral" [duplicate]

I am new at physics. I have a question: What evidence is there that support the statement that the Universe is charge neutral? Is there anybody explain to me?
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Is $10^{10}$ Planck Times sufficient to create all the mass-energy in the universe

I understand that there are $10^{T_P}$ (Planck Times that I think of as Planck Seconds) during the interval beginning with the interval $10^{-43}$ seconds and the end of the inflationary epoch at $10^{...
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Infalling Observer and Black Hole Information Loss?

I was thinking about the following scenario today, and I was hoping to find a more detailed explanation. Suppose you have an observer $A$ falling into a black hole and an observer $B$ at infinity. In ...
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1answer
184 views

Dilaton field causes universe expansion?

In string theory low-energy $n$-dimensional gravity is described by an action of the following form: $$S^{(n)}=\frac{1}{2\kappa^{(n)}}\int d^nx\sqrt{-G}e^{-2\Phi}\Big(\mathcal{R}+4\partial_\mu\Phi\...
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Has the luminosity of the universe ever been larger than $\frac{c^5}{4G}$?

$\frac{c^5}{4G}$ is about $10^{52}\,\rm W$. The value is sometimes called the maximum power, e.g. by Gibbons. Has the luminosity of the universe ever been that high or higher? Is there a large sphere ...
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2answers
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Why do matter and radiation densities scale differently in expanding universe, when all matter is energy and all energy is matter? [duplicate]

I sometimes see the first Friedmann equation expressed with different terms for the density of "matter" and the density of "radiation". It is said that with an increase in scale ...
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3answers
706 views

Does GR explain cosmological time dilation?

There is plenty of evidence for a cosmological time dilation effect. For instance a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift $z=1$ (see Ned ...
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Stephen Wolfram Computational Model and the hypergraphs structure

I just discovered Stephen Wolfram`s project and model and I am fascinated especially since i had a similar ideea a few years ago without ever reading about it. I have a question regarding the ...

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