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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16 views

Perturbation of the energy-momentum tensor: mistake in my computations or in the book?

In the book Cosmology - S. Weinberg eqauation $(5.1.28)$ is $$ \delta T^{\mu}_{\;\;\nu} = \bar{g}^{\mu\lambda} [\delta T_{\lambda\nu} - h_{\lambda\kappa} \bar{T}^{\kappa}_{\;\;\nu} ] \tag{5.1.28} $$ ...
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Calculating the lifetime of the universe

I am working on the following exercise for my class on general relativity: Suppose that the spatial volume of a closed, matter dominated FLRW universe with spherical space sections and vanishing ...
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Why is life impossible in the supersymmetric Minkowski vacuum?

I read in several sources that the existence of life is impossible in the supersymmetric Minkowski vacuum (also called the terminal vacuum, the true vacuum or the global minimum). Why? After all, ...
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Scalar field Hamiltonian $H = 0$ from parameterization independence

This question is related (but not similar) to this old one of mine: How to derive the two Friedmann-Lemaître equations from a Lagrangian? Consider the Lagrangian of an isotropic-homogeneous ...
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Computation of error on $C_\ell$ with fraction of sky $f_{\text{sky}}$ and $\Delta\ell$

I try to demonstrate the expression of error on a $C_{\ell}$. Below the expression of standard deviation on a $C_{\ell}$ that I would like to get: $$\sigma_{C_{\ell}}=\sqrt{\dfrac{2}{(2 \ell+1) f_{\...
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Knowable and Unknowable Hidden Variable theories

Following a recent interesting question about the collapse of the wave function (link at the bottom). It seems that the wave function is just a mathematical way to give predictions of various outcomes ...
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Mixture of two fluids: Is that EoS possible and how to interpret it?

In the context of cosmology, a perfect fluid is generally described by an equation of state (EoS) of the following form: $$\tag{1} p = w \rho, $$ where $p$ is the fluid's pressure and $\rho$ is its ...
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What is the reason only one axis measure is given for the Universe?

We are given the diameter of the Universe to be approximately 880 Ym. Is this the the distance observed at any place when looking into space, or is this value more of an average?
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Time evolution of scale factor

A de Sitter universe is a cosmological solution to the Einstein field equations of general relativity, named after Willem de Sitter. It models the universe as spatially flat and neglects ordinary ...
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Application of Kasner metric

Kasner metric is given by: $$ds^2=-dt^2+\frac{t}{t_0}e^{2K_1}dx^2+\frac{t}{t_0}e^{2K_2}dy^2++\frac{t}{t_0}e^{2K_3}dz^2$$ where $K_i$ are constants How is this metric useful in exploring manifolds with ...
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Killing Vectors from Killing Equations

I need to find the killing vectors of the FLRW metric. However, it seems that they are complicated. Is there a simple/general equation that gives the killing vectors for a given metric? Or do I have ...
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Is it valid to add energy densities of *interacting* perfect fluids?

In several papers on interacting perfect fluids in cosmology, the authors assume that we still can add the energy densities and pressures of the individual fluids, as if there wasn't any interaction: \...
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Scalar field displacement from the minimum of the potential gives rise to particles/dark matter, why?

In This paper (Kobayashi et al -- Lyman-alpha Constraints on Ultralight Scalar Dark Matter: Implications for the Early and Late Universe) it says, at the beginning of Section 3.1: A light scalar ...
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If the universe is 13,7 billion years old how it could be infinitely large? [duplicate]

If the universe is 13,7 billion years old how it could be infinitely large? Maybe it is curved to account for being finite? But then there should be more spatial dimensions...is it right?
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What would the wavelength of the Cosmic Background Gravitational Wave radiation be?

Considering electromagnetic CMB can only see light as old as 380,000 years after the Big Bang, whilst theoretically those being gravitational should be formed from the beginning, what would their ...
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Galaxies: Negative pressure, positive feedback

As part of research trying to develop a cosmological model that can be static (scale-factor), but in dynamic equilibrium - a way is being considered whereby dense collapsing regions of matter 'bounce' ...
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Relation between Matter Power spectrum $P(k)$ and Matter Angular power spectrum $C_{\ell}$

Summary: $\quad$ I would like to go deeper in the relationship between Matter power spectrum and Angular power spectrum. From a previous post about the Relationship between the angular and 3D power ...
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How should I calculate the redshift of an object if I know its light travel distance?

If an object placed at hubble horizon i.e. 14.4 billion light years approx sends a light ray today and it reaches on earth sometimes in the future so how can I calculate its redshift when it will ...
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Interacting multi-fluids in FLRW cosmology?

In the following, I only consider fully isotropic and homogeneous cosmology. Because of local isotropy and homogeneity, the Robertson-Walker metric (and its two FLRW equations) imply a diagonal energy-...
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Am I correct in understanding that in the many-worlds interpretation the Universe is considered as a single quantum object?

Among the tags there is also a topic of interpretation, so I hope that the question will not be closed. In the many-worlds interpretation, the wave function acquires an onotological meaning, that is, ...
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What is time till false vacuum decay? (Which source is in error?)

https://arxiv.org/abs/1308.4686 says that the time till false vacuum collapse is just about the age of the universe: ΛCDM ... can be achieved if the top quark pole mass is approximately 178 GeV That ...
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Energy density of a neutrino species

My question refers to the derivation of the energy density $p_{\nu,i}$ of a neutrino species given in the footnote on page 16 of Lecture Notes on Cosmology by Komatsu. I have reproduced this below: $$...
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Conformal Flatness in FLRW cosmology and interpretation of Stress Energy Tensor

The FLRW metric is used to model our Universe on large cosmological scale. It is a conformally flat metric and the form of stress energy tensor that we get from Einstein's equations are often equated ...
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Gravity in a universe where all energy has been turned to mass [closed]

If all energy in the universe was turned to mass would gravity work?
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Distances in cosmology (relativistic) [closed]

so, my problem is that distance measures in cosmology seem to ignore General Relativity GR (or even STR). There, the proper distance is the distance w.r.t. a specific rest frame! This means that, in ...
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The distance to last scattering surface incerease?

The cosmic background radiation was released 38 million years after the Big Bang and is observed as the last scanning surface. How does the distance to the last scattering surface differ when the ...
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Version of Olber's Paradox for gravity [closed]

Olber's Paradox is a famous problem in cosmology. In astrophysics and physical cosmology, Olbers' paradox, named after the German astronomer Heinrich Wilhelm Olbers (1758–1840), also known as the &...
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Compute the variance of spectroscopic Shot Noise in cosmology context

I would like to know the right expression for the expression of variance of Shot noise in spectroscopic probe. Sometimes, I saw $$\sigma_{SN,sp}^{2} = 1/n_{sp}$$ with $n_{sp}$ the average density of ...
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Distances in cosmology

I want to make sure that I understand the different distance measures is cosmology. To do that I consider the FLRW metric: $$ ds^2=dt^2-R(t)^2\left(\frac{dr^2}{1-kr^2}+r^2d\theta^2+r^2\sin^2\theta d\...
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How is energy in the universe constant? [duplicate]

We have been taught that the universe has a constant amount of energy but if energy can be converted into matter, how is it constant?
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Four Momentum in an Isotropic and Homogeneous Universe

We define the four-momentum of a particle by $p^\mu = (E, \mathbf{p})$, where $E$ is energy and $\mathbf{p}$ the three momentum of the particle. In cosmology, we conventionally use the Friedmann-...
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Cosmological models of the expansion of the universe for different curvatures

My question refers to page 38 of Tong's Notes on Cosmology. Here we consider a matter dominated universe and derive the equation $$2h^\prime + h^2 + k = 0$$ for $h = a^\prime/a$, where $^\prime$ ...
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Where is the warped throat in Klebanov-Strassler geometry?

In string theory, it is common to work in the Klebanov-Strassler geometry to find AdS and dS vacua. Applications are that anti-branes can be placed at the tip of this deformed/warped conifold to ...
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Is the cosmological Doppler effect more effective at long wavelengths?

When observing the Doppler effect of a particular galaxy, is the wavelength change greater for long wavelengths? According to the simple Doppler effect formula, the wavelength change is proportional ...
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How can we be sure that gravity is not the reason for the perceived expansion of the universe? [closed]

The question has been partially discussed here but I believe the following formulation is a bit different. What experience could disprove the notion that the universe is not expanding despite the pull ...
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Why should there be a Big Crunch?

Everyone knows it all started with the Big Bang. And then on, all objects have been moving away from each other very fast. And this rate itself is accelerated, according to Hubble's Theory. However, ...
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Are we inside the event horizon of a gigantic black hole? [duplicate]

Is it possible that the Milky Way and the local cluster or maybe the entire visible universe is inside the event horizon of a truly gigantic black hole that is so far away that we haven't noticed it ...
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Gondolo-Gelmini Change of Variables

In the article Cosmic abundances of stable particles: Improved analysis, P. Gondolo and G. Gelmini, Nucl. Phys. B 360 (1991), p. 145-179, they convert $\rm{d}^3p_1\rm{d}^3p_2=2\pi^2p_1p_2\rm{d}E_1\rm{...
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Photon temperature at nucleosynthesis

I've been asked about photon temperature at nucleosynthesis (3 minutes from Big Bang). So I guessed this is the moment when Matter and Radiation where in equilibrium: $$ \rho_M(T) = \rho_R(T)$$ Taking ...
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Does the history of the universe change when the initial conditions change in superdeterministic theories?

In quantum mechanics, superdeterminism is a loophole in Bell's theorem, that allows one to evade it by postulating that all systems being measured are causally correlated with the choices of which ...
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Why do hot and cold spots in CMB maps have an average angular size of 1°?

CMB all-sky maps issued by the PLANCK and WMAP satellites show that CMB hot and cold spots have a physical average angular diameter of about 1°. Why is this size statistically preferred, and what is ...
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Present value of the cosmological scale factor, important or arbitrary?

I'm now confused about a subject that I thought was very clear to me, until recently. So I need to sort this clearly once and for all. Consider the standard FLRW cosmology in classical general ...
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Histories of the Universe under Different Initial Conditions

The history of the evolution of the Universe (we are talking about the observable part) on ultra-large scales (larger than the scale of galactic superclusters) under any initial conditions would be ...
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How does space change as the universe expands?

Consider a metre ruler. Despite the universe – and space itself – constantly expanding, the ruler maintains its size. If this ruler was alone in empty-ish space, other distant objects would appear to ...
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Conservation of phase space density during decoupling of semi-relativistic particles

In "The Early Universe" by Kolb and Turner it is stated that for particles in thermal equilibrium that decouple semi-relativistically ($m\simeq T$) the phase space distribution is not ...
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What inflation theories can be considered viable?

Observations made by the Planck consortium excluded many potentials relevant to inflation theories. What inflation theories are still considered viable at the present date?
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Spectrum of particles in primordial cosmology

I am reading Primordial cosmology by P.Peter and J. Uzan. In chapter 4 (pag 191 in my edition) the authors study the distributions of particles in the primordial plasma. They write that the number ...
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Cosmology: Is there any experimental evidence for the redshift scale-factor relation?

Modern cosmology relies heavily on the redshift scale-factor relation $$a=\frac{1}{1+z}$$ But what is the experimental evidence for it? It's derived in textbooks from General Relativity, but for all ...
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What would happen if universe wouldn’t accelerate faster than speed of light? [closed]

Its quite stunning to assume that not only space-time is expanding but also the rate of expansion is greather that speed of light. But after the initial surprise, I ve been wondering if that fenomena ...
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Correspondence between average size of spots in CMB map and location of first peak in CMB spectrum

The size of an average hot or cold spot in the CMB all-sky map is about 1° in diameter. Can you explain why this scale also corresponds approximately to the location of the first peak in the CMB ...

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