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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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FRW metric spacelike slice

In Steven Winberg's Cosmology book (p. 6) he says that the spatial part (co-moving part) of the FRW metric can be written as $$ \tilde g_{ij} = \delta_{ij}+K \frac{x^i x^j}{1- K \mathbf x ^2} $$ ...
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Conventions in the FRW metric

So the FRW metric is $$ds^2=-c^2dt^2+a(t)^2\left(\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2\theta d\phi^2)\right). $$ I have seen books mention that $k=1,0,-1$ - depending one of the values listed ...
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Can anyone please provide paper by Peebles. P.J.E 1980 and 1993 [on hold]

Can anyone please provide me the paper by Peebles, P.J.E in 1980 and 1993 where linear perturbation theory work is done.... I am unable to find it.....thanks Or if possible the link to the books by ...
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1answer
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Is there a relationship between Black Holes and the Big Bang?

As the question mentions, is there a relationship between the Big Bang and Black Holes? It appears the Big Bang is a ‘tacked on’ theory that doesn’t align with current theories (since we don’t know or ...
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18 views

What is the current most accurate measurement of the cosmological transition redshift

Does anyone know what the current most accurate measurement of the redshift for when the Universe's expansion changed from decelerating to accelerating (with a reference)?
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1answer
51 views

Could six copies of the universe exist only interacting by gravity? [on hold]

The ratio of dark matter to matter is roughly in proportions $1:5.4$ This suggests that if there were about 6 Universes placed in the same space that were completely invisible to each other, except ...
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2answers
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Modern arguments against Geocentrism?

In ancient times, the arguments for geocentrism were primarily from observation. It looks like the heavens revolve around the Earth, so "heavens revolve around Earth" is a reasonable first model. ...
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Traveling to another Multiverse doesn't seem to solve the heat death problem [on hold]

Travelling to another universe (of the multi-verse) seems to be one of the proposed solution to escape the heat death of the universe. But this doesn't really seem to be a solution because the other ...
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0answers
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Scale factor at surface of last scattering

Is there an exact theoretical scale factor associated with the CMB? More specifically, since the CMB is associated with a temperature of around 3000K (as I understand), is there an exact thermodynamic ...
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4answers
3k views

Does the universe have a fixed centre of mass?

Does the universe have a fixed centre of mass? If it does, doesn't it necessarily mean that every action of ours has to be balanced by a counteraction somewhere in the universe so as to neutralize the ...
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1answer
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De Sitter spacetime affine parameter

I am reading Chapter 8 in Carroll's "Spacetime and geometry " textbook and I was lead to exercise 8.2, given as: Consider de Sitter space in coordinates where the metric takes the form $$ds^{2} =...
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1answer
25 views

Could the Universe have a finite number of states?

I think most people would say the Universe is infinite, and the number of ways of arranging infinite particles in infinite space is infinite. But we know the "visible" Universe is finite, there is a ...
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1answer
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Was Helium hydride really the first molecule?

This press release by NASA: https://www.nasa.gov/feature/the-universe-s-first-type-of-molecule-is-found-at-last/ When the universe was still very young, only a few kinds of atoms existed. ...
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1answer
48 views

Olber's paradox with Rational Numbers [on hold]

Does Olber's paradox claim that in an infinite universe (both in space and in time) light should be able to reach you from any direction? Consider the thought experiment of placing an infinite number ...
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0answers
39 views

Is it there any physics-basis for Tegmark's hypothesis? [closed]

Tegmark's mathematical universe hypothesis is very interesting (https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis) but it has virtually no support among physicists because it's too ...
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0answers
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General covariance and a running cosmological constant

A running cosmological constant $\Lambda(t)$ can always be included in the perfect fluid source tensor. By the transformation properties of tensors Einstein's field equation is still independent of ...
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1answer
63 views

Are repulsive and attractive gravity connected?

My question is simple: Can attractive and repulsive gravity (the expansion of space between masses, caused by the still hypothetical dark energy, so it seems they are repelled while in fact, they ...
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2answers
37 views

Are pressureless, collisionless and self-interacting dark matter all synonymous?

Is there any distinction between pressureless, collisionless and self-interacting dark matter or does being one imply the other?
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0answers
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Why do some claim that the universe is infinite if it has the cosmological boundary at 5 Gpc from us with finite de Sitter temperature? [closed]

Why do some claim that the universe is infinite if it has the cosmological event horizon (boundary) at about 5 Gpc from us with finite de Sitter temperature?
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No dark matter, weight of the universe measured by luminosity is unaccounted/distorted relections? [closed]

If calculating the universe's mass by luminosity, how do we account for all the combined reflections from our planets or planets from other solar systems, and throughout all of time and space? And can ...
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1answer
32 views

Particle decoupling, universe expansion and QFT

A particle (say neutrino) decouples from the plasma in the early universe when its interaction rate $\Gamma$, with the plasma, is slower than the expansion rate $=H = \frac{\dot{a}}{a}$, i.e. $\Gamma &...
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0answers
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Relationship between time derivatives at fixed physical r and at fixed comoving x coordinates

I am currently studying Newtonian Perturbation Theory in cosmology. We have introduced the relation between the physical coordinates r and the comoving coordinates x in an expanding universe: $\bf r$...
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2answers
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Do we need dark matter and dark energy, if the behaviour of the universe in its initial stages was similar to that of the Sun? [closed]

According to CMBR the universe was a cloud of plasma and was a perfect black body, $380,\!000$ years after big bang. But the Sun in our solar system also is in the state of plasma, thus making it a ...
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0answers
46 views

How accurate is the De Sitter solution of the Einstein field equations of general relativity?

The De sitter solution models the universe as spatially flat, neglecting ordinary matter, such that it is dominated by the cosmological constant. The cosmological constant notion arguably suggests the ...
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1answer
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How do you calculate conformal time?

I've got several references that indicate that conformal time is the integral of the scale factor: $$\eta=\int_0^t \frac{1}{a(t)}\mathrm dt.$$ So I tried to calculating this over a range of {0, ...
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0answers
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If the universe is expanding, then the distance between the atoms must increase, which should result in decrease in density of everything [duplicate]

In the expansion of universe theory, it is already written that the scale of universe is itself expanding, which means the consequence for expansion of universe is not that atoms are growing in size, ...
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1answer
46 views

Are objects repelled by the cosmological horizon in Lambda-CDM cosmology?

When an observer watches a distant object, is there a gravitational influence of the horizon on that object? ("Distant" here is meant to mean: at a distance that is a sizeable fraction of the Hubble ...
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0answers
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Proving that universe is a blackbody

How can you theoretically explain why cosmic microwave background seems to be coming from a perfect blackbody given that the universe is no longer at thermal equilibrium?
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2answers
39 views

Physically acceptable energy-momentum tensor

From a problem the line element is: $$ds^2 = -c^2e^{-2ax}dt^2 + dx^2+ dy^2+ dz^2$$ I found energy-momentum tensor ($T_{\mu\nu}$) from Einstein field equation by using the above line element. Only T$...
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1answer
102 views

The largest black hole [duplicate]

The Schwarzschild radius involves an expression in terms of Newton's constant $G$, the mass $M$ inside a radius $r$, and the speed of light squared $c^2$. Current estimates of the universe's matter ...
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0answers
25 views

Galactic or Extra-galactic Cosmic Rays?

If we want to show whether or not a specific Cosmic Ray particle is confined to our galaxy we must use the Larmor radius (relativistic version), $$r = \gamma \, \frac{ m c}{q B}$$ Considering a $10^...
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0answers
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Vanishing integral in deriving stress-energy tensor from action

In the derivation of the energy-stress tensor for a scalar field (context: Inflation Theory for Cosmology) by varying the action with respect to the metric, the integral over spacetime that is ...
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0answers
19 views

Inside a shell full of strings, what are the components of the network, and what the possible defects?

Imagine that we have a (very) large shell full of holes, and flexible strings go (randomly) from one hole to another. Inside the shell, there will be a network of strings. Can one split this network ...
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2answers
1k views

Schwarzschild Radius of the Universe

According to the Wiki on the Rs, the Rs of the observable universe is 13.7BLY. https://en.wikipedia.org/wiki/Schwarzschild_radius (The observable universe's mass has a Schwarzschild radius of ...
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1answer
51 views

Which decoupled first? The dark matter or the CMB photons?

According theories that work best for the structure formation which happened first in the history of the evolution of the Universe, the decoupling of dark matter or the decoupling of photons? How sure ...
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0answers
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In what ways can gravitational wave research help particle physicists? [closed]

Old content (put on hold as too broad) I was massively ill-informed about the experiments that shape the modern particle physics research. For example, I believed that most of the information that ...
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0answers
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How can CMB anisotropies be due only to inhomogeneities?

I'm studying from these lecture notes (Author: Wayne Hu, title: Lecture Notes on CMB Theory: From Nucleosynthesis to Recombination). At the very end of page 10 and beginning of page 11 he says: ...
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1answer
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Why doesn't the centrifugal force cause rotating dark halos to flatten?

Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below Assuming that the halo is purely made up of dark matter which has only gravitational ...
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1answer
68 views

Dark matter should be nonrelativistic at $1$ KeV or earlier. Why?

If the dark matter (DM) is non-relativistic at freeze-out and if the kinetic decoupling occurs much later than freeze-out, it is called cold dark matter. More precisely, it is stated that if the DM ...
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2answers
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Where is the work done of adiabatically expanding universe going into?

According to this Wikipedia article, the universe is expanding adiabatically. However, any system that is expanding adiabatically must be losing energy $\left(\mathrm{d}E = -p\,\mathrm{d}V\right) .$ ...
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1answer
28 views

How does cosmological principle imply an expanding space?

My understanding of cosmological principle is this: Space looks same in all directions and is homogeneous. But this principle was under heading of Observational Evidence for expansion of space in this ...
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4answers
75 views

Can we measure dark matter's effect on spacetime?

I'm not a physicist or mathematician at all, just very curious, so sorry if this is a dumb question. Now I understand we can make astronomical observations and say, "Oh, 85% of gravity in the ...
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0answers
38 views

At what point is a shrinking black hole unable to feed itself due to gravity?

Let's suppose that Hawking Radiation does occur. According to my limited knowledge, as the radius of the event horizon decreases, the emitted Hawking Radiation can (must?) assume ever-smaller ...
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1answer
41 views

Friedmann vacuum solution linear rather than exponential?

Does the Friedmann vacuum equation have a linear solution rather than an exponential one? Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray} \left(\frac{\dot ...
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0answers
25 views

Hubble parameter in radiation dominated era and dimensional analysis

Starting from dimensional arguments, is it possible to give the dependence of Hubble parameter on temperature $T$ in the radiation dominated (RD) era? In the RD era, the only relevant mass/energy ...
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1answer
32 views

Does the scale factor evolution in a matter dominated universe allow for acceleration?

One probably very trivial issue which is bothering me. Standard cosmology says that in a FRW metric, in a matter dominated flat universe (Pressure $P = 0$ and $K = 0$), the scale factor, $a \propto t^{...
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1answer
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Guessing the temperature dependence of a decay rate $\Gamma(A\to B+B)$

For a two-body decay of the form $$A\to B+B$$ if the interaction strength controlling the decay is $\lambda$, the Feynman amplitude $\mathcal{M}$ will contain a factor of $\lambda$ from the vertex ...
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4answers
147 views

Why hasn't anybody done a two-telescope test on the speed of starlight?

It seems that even though it wouldn't be too hard, nobody has taken the trouble to use a pair of telescopes to confirm that light originating from red-shifted / distant stars is indeed traveling at c. ...
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1answer
81 views

Evidence for zero curvature of universe (besides CMB)

There is a beautiful argument, based on the spherical harmonics of the cosmic microwave background, which calculates the curvature of the universe. Are there any other methods of computing the ...
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1answer
59 views

How much did the universe expand since recombination time? [closed]

Assuming adiabatic expansion, and a temperature at recombination time of $2700^{\circ} C$, how much has the universe expanded if the temperature today is $2.7K$? My idea is to use $\frac{dQ}{dt}=A\...