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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17 views

About estimating the age of the universe [duplicate]

So the age of the universe can be estimated by doing 1 over Hubble's constant, but why is this?
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Reason for negligible chemical potentials of different particles in early universe

For the early Universe at high temperatures, the chemical potential is assumed to be zero for all types of particles is negligible. Why is this true?
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How did the Universe behave just at the end of inflation?

Edit When the inflaton field $\phi$ rolls down the potential $V(\phi)$ to the value $\phi=\phi_f$ where the slow-roll parameter $\epsilon(\phi_f)\to 1$, inflation ends. By calculating the behaviour of ...
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2answers
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Does the Cosmic Background Radiation (CMB) indicate that we may be any kind of privileged observer?

To first order, the Cosmic Background Radiation (CMB) looks the same in every direction. Apparently, however, at the level of parts per million, there are small distinctive variations in the ...
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What is the maximum amount of computation that can be performed in the future lifespan of the universe?

The ultimate question I have been trying to answer is the maximum universal population allowed by physical limits. Making some transhumanist assumptions I am perfectly happy with, I am equating this ...
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Intuition for the freeze-in scenario of the dark matter

Let the coupling of the dark matter (DM) particles to the Standard Model (SM) particles is tiny. In that case, the interaction rate of the DM particles with the primordial cosmic plasma could be so ...
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Comoving coordinate system

In Ta Pei Cheng's GR book, the comoving coordinate system is defined as $$t=\text{the proper time of each fluid element}$$ $$x^i=\text{the spatial coordinates carried by each fluid element}$$ The ...
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Integrals in cosmology, Friedmann equation (Gorbunov and Rubakov book problems)

I'm currently taking a course in cosmology which uses Gorbunov and Rubakov's Introduction to the Theory of the Early Universe: Hot Big Bang Theory as the main literature, and am working on some ...
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1answer
75 views

Why does gravitational attraction result in a decelerating universe

I read the following statement: There is matter and energy in the universe, and their mutual gravitational attraction will slow down the expansion of the universe, leading to a monotonically ...
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Redshift time relation

I was reading that the relation $dt=\frac{dz}{H(z)(1+z)} $ between time and redshift (H is the Hubble constant) holds. I don't understand this. I thought the relation between time and redshift is $z=H(...
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How much of the universe will get flatter in how long?

So I learned recently that star systems are relatively flat due to the conservation of angular momentum. On how large a scale does this degree of flattening apply. i.e will the universe eventually ...
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Why is hubble time the age of the universe? [duplicate]

For a universe with a constant expansion rate, i.e. the Hubble constant $H_0$ is a constant, the Hubble law is $$v=H_0d$$ where $v$ is relative velocity between two galaxies and $d$ is the separation ...
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Is there a standard distance/z data set for cosmology given new measurements?

I've seen this data set in a few places. Found here: https://lco.global/spacebook/light/redshift/ Do the measurements from SH0ES and Planck that cause the Hubble tension alter this table? I can find ...
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Is it realistic for an unstable vacuum to have an infinite past, but a finite future?

Is it realistic for an unstable vacuum to have an infinite past, but a finite future? In other words, can there be a plausible scenario in which an unstable vacuum did not have a beginning, but ...
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Do cosmological event horizon evaporates like black-hole horizon?

While the inflation phase of the universe, expansion was exponential and so the universe was de Sitter-like. So, for a point-like observer, there was a cosmological event horizon. Do this horizon ...
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Shape of spacetime [closed]

If you could snap a picture of the entire universe, this would show you the entire spacetime structure. We will call this spacetime A. Every point in this structure has the ability to say it's NOW. ...
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Can coldness be converted to heat energy?

We know that the heat can be converted into heat energy with the help of thermoelectric generators, but why can't we generate energy from coldness? Like the temperature of the universe in 1 K, can ...
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Hydrogen-One (HI) Image and R-band

In the paper below, on page 3, it says "In the lack of HI image, we use the r-band b/a ratio to correct for the inclination". Am I correct in thinking that what this means is that if we ...
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What's a minisuperspace in quantum cosmology?

I read this paper on Lorentzian Quantum Cosmology. But I couldn't understand the term minisuperspace which is used to define the path integral $$\int\mathcal{D}N \mathcal{D}\pi\mathcal{D}a\mathcal{D}...
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Is energy of universe constant? [closed]

First of all please do not flag this question as duplicate(I have read all the others like this one, this one and this one is just talking about observable universe. none of them talked in this ...
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What is “nothing”? [duplicate]

In some cosmological models, the possibility of the emergence of the Universe "out of nothing" is considered. What do scientists mean when they say "nothing"?
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What law prohibits the existence of universes that are completely unrelated to our universe?

Could there be such a law of nature that prohibits the existence of universes that are absolutely not connected with our universe and do not affect it in any way? If so, how?
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How to create a cosmic string?

I know about cosmic strings. They are 1-D topological defects in space-time which may had been created during the symmetry breaking phase transition during the big bang. But my question is : Are ...
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By definition, does the term “cosmological redshift” comprehend the gravitational redshift as well or are both terms mutually exclusive? [closed]

In what way has it been - if at all - considered, when determining and measuring the expansion of the universe, that the redshift is or might not be caused by Doppler's effect but by the ...
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Continuity equation for the stress-energy tensor in the FLRW metric

I'm trying to compute the continuity equation for the stress-energy tensor $\nabla^\mu T_{\mu\nu}$ in the FLRW metric $$ds^2=-dt^2+a^2(t)ds^2_3$$ where $ds^2_3=g^3_{ij}dx^idx^j$ is the metric for the ...
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Is the temperature of the universe rising?

I know the temperature of the universe is decreasing due to it's expansion after the big bang but after I came up with this article in AOP(please note I don't have the access of the journal,so I have ...
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A rod is attached to a star in a galaxy at the edge of the observable universe and it reaches earth. Will that rod start moving faster than light? [closed]

It is known that the galaxies at the edge of the observable universe are accelerating away from us and eventually they will start moving faster than light with respect to our frame. However, suppose ...
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Are there theories about the end of the universe where time ends?

We always tend to think that future will come no matter how far in the future that is. But according to some theories, time started at the big bang and the term "before the big bang" doesn't ...
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What factors affect what happens in the far, far future of the universe, after we undergo a heat death?

I've been reading about Boltzmann brains, especially Sean Carroll's stuff, and am interested in the scenario he describes in which, after the heat death, the universe is populated with objects ...
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4answers
125 views

Block universe theory [closed]

How to make sense of change in static block universe if every event already exist in block? if you say it's an illusion created by brain even that requires forward evolution of patterns of information ...
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1answer
109 views

Could there be a region of the universe that is dominated by antimatter?

I was reading some texts explaining that since our region of the world is overwhelmingly dominated by matter, and not by anti-particles, this suggests that in some region of the universe, there will ...
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Derive density fluctuation dependence on scale factor

I want to show that in the radiation-dominated era, $\delta_H^2 \propto a^{1-n}$. I know that the wavenumber corresponding to the Hubble scale goes like $a^{-1}$ and that $\delta_H^2 \propto k^3 P(k)$ ...
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What are the predicted and measured times it takes light to reach distances in an expanding universe?

Imagine you shoot a laser beam into space, and there is a target placed every 500 million light years from you. In a static universe, the laser beam will reach the first target in 500 million years, ...
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The block universe theory

i watched a video on YouTube https://www.youtube.com/watch?v=_A7wk40AeiI regarding the block universe where brian greene seem to be saying we live "forever" (i am probably misinterpreting ...
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Is the equivalence of axial vectors with skew symmetric tensors in 3 dimensions a coincidence?

We happen to live in the only number of (large space-like) dimensions that permits an equivalence between skew symmetric tensors, like the magnetic field, and vectors. Similarly (equivalently? not ...
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Why is the universe so stable and consistent? [closed]

Why is it we can discover patterns? Like math, chemistry, physics, biology, etc. Why does the universe never fail us? For example in one second I'm standing next to you, the next I'm still standing ...
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1answer
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Is Loop quantum cosmology replacing the Big Bang singularity with a big bounce?

Am I correct in understanding that in the cosmological version of Loop quantum gravity, namely Loop quantum cosmology, the universe in all models starts with a big bounce? Are there other models, for ...
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Contributions of light relics to the effective number of neutrino species

These slides show that contribution $\Delta N_{\rm eff}$ to the "effective number of neutrino species" $N_{\rm eff}$ by light relics can be computed by first noting that \begin{equation} g_*(...
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How to calculate cosmological parameters such as scalar spectrum index and the tensor-to-scalar ratio in constant-roll method?

As we know, the scalar spectrum index and the ratio of tensor-to-scalar in the slow-roll method are obtained with a certain relation according to the parameters of the slow-roll. Now I want to ...
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104 views

Can the universe be considered a thermodynamic system?

With matter a type of energy, can/should the universe be considered a thermodynamic system? At what scale would it be relevant?
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Exact solution of scalar field cosmology with exponential potential [closed]

Considering a homogenious and isotropic universe described by the Robertson-Walker metric: $$ ds^2= dt^2-a^2(t)\left[\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2(\theta)d \phi ^2)\right]$$ and also ...
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What is the meaning of “current” CMB rest frame or the CMB rest frame at redshift $z$?

As I know that the definition of the CMB rest frame is the comoving frame of the plasma emitting CMB. And the anisotropy of CMB is due to the velocity difference between the earth frame and the CMB ...
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What is brane inflation, and can it be eternal?

What is Brane Inflation and how does it describe inflation process? Can it be eternal?
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Does cosmology provide evidence against a non-uniform one-way speed of light?

In a recent popular video by Veritasium it is claimed that no one has devised an experiment that measures the one-way speed of light, and that in theory a directional difference in the speed of light ...
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Thermal equilbirium and intensity of interactions

It is stated in several books and papers on Cosmology, that the condition for system to be in thermal equilibrium is that the interactions are sufficiently strong. For example, neutrinos in universe ...
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Did the dominant cosmic structure forming force change over time?

Hypothesis During an earlier epoch the universe was filled with hot dense plasma. During this time the dominant structure forming force was electromagnetic. As plasma is prone to self organizing the ...
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They say the universe is expanding. Is that the same as saying that time is contracting?

Apparently the distance between galaxies used to be smaller in the distant past. That means it used to take fewer units of time to get from one galaxy to the next. So either the distance really was ...
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Research in next decade on dark matter: sky surveys vs particle detectors

The next decade is very promising for cosmology as new surveys such as SPHEREx, Roman, Euclid, DESI which will shed light on inflation, dark matter/energy and more. At the same time there are particle ...
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What is the equation that tells us that the universe is expanding?

Well I know that the universe is expanding, but is there any equation or letter or simply just an expression that tells us that the universe is expanding...
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Cosmological time in the Milne region of Misner space

I refer to this paper. In the Milne regions of Misner space, the coordinates $x^{\pm}$ are defined as \begin{equation*}x^{\pm} = \frac{T\mathbb{e}^{\pm \beta \theta}}{\sqrt{2}}, x^{+}x^{-} > 0. \...

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