The Stack Overflow podcast is back! Listen to an interview with our new CEO.

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.

Filter by
Sorted by
Tagged with
5
votes
1answer
261 views

Is space itself expanding or is it just momentum from the Big Bang carrying things apart? [duplicate]

So, we observe that galaxies are moving apart (on average) and the further away from us they are the faster they are moving away from us. If space was expanding that would explain this. However, I ...
2
votes
1answer
53 views

Can I measure the expansion of the universe with absolute rest frames?

Let's say I go out into deep space and set up two observation stations a long ways apart. I want to find an absolute rest frame for each one. I decide that if the microwave background radiation is ...
1
vote
2answers
108 views

When did the radiation domination end and matter domination start?

After the inflationary era the Universe became radiation dominated. The era of radiation domination is defined as the phase during which the temperature of the Universe was so high that the kinetic ...
8
votes
2answers
419 views

What wavelength was the CMB originally?

What wavelength was the light that became the CMB after factoring out redshift? Is it safe to assume that what we're seeing in the CMB was emitted at a distance of approx: ...
1
vote
0answers
21 views

Star formation after big bang doesn't violate entropy? [duplicate]

Ok I know this has been asked a lot but I'm still a bit confused. After big bang, there are a lot of equally distributed elementary particles around with evenly distributed heat(or photons). So why ...
2
votes
0answers
17 views

How to show no boundary wave function satisifies Wheeler-de-Witt equation?

I've seen various places it says the no boundary wave function (Hartle Hawking state) satisfies the Wheeler-de-Witt equation. But I don't think I've seen a proof. Is there a proof of this somewhere? ...
3
votes
1answer
179 views

What is the temperature of the black night sky, outside the atmosphere?

The sky at night is rather black. If it were completely black, it would correspond to a black body at absolute zero. But the sky is not completely black. Is there a way to assign a temperature value ...
0
votes
1answer
37 views

Cosmological densities when universe was/will be a different size

Supposing a spatially flat, matter-dominated universe, the expression for the Hubble parameter is: $$ H^{2}=H_{0}^{2}\left(\frac{\Omega_{m}}{a^{3}}+\Omega_{\Lambda}\right) $$ The previous equation ...
1
vote
2answers
59 views

What is the radiation density in a spherical oven? Does this pertain to the universe? [closed]

Imagine a spherical oven with radius R. The inside of the spherical wall is at temperature T (everywhere) and is assumed emit perfect black body radiation (everywhere). (1) What is the energy ...
0
votes
0answers
49 views

Tests of inflation theory

So I'm following a course in Early Universe Cosmology right now. For the next lecture we have to solve a few questions about inflation, just as preparation. I read some closely related lecture notes ...
0
votes
0answers
32 views

A book about extragalactic astrophysics?

I'm an undergraduate student, writing my thesis. And I look for an advanced book of extragalactic astrophysics that contains information about Line Emission Galaxies. I already read "Extragalactic ...
2
votes
1answer
74 views

Why is Cosmic Microwave Background Radiation attributed to the Big Bang? [duplicate]

When searching for CMBR all articles start by stating as a fact that it is a remnant of the Big Bang. Why? Why is it well understood and accepted? Why sure that it's not an effect of something else ...
0
votes
0answers
9 views

Regarding the initial curvature conditions in simple single scalar field slow roll inflation models

So while learning about modelling inflation using a single scalar field slow roll, most authors seem to use the Friedmann equation with the spatial curvature taken to be 0. Now one of the precursors ...
0
votes
1answer
38 views

Could the redshift interpreted as space expansion also be a time dilatation (change in tick speeds)?

This might be a very naive first thought but I was not able to find any further information if it can be ruled out by some observations or some theories immediately. Can the increasing mean redshift ...
7
votes
2answers
149 views

What actually is the flatness problem?

One of the Friedman equations is given by $$ H^2\equiv\Big(\frac{\dot{a}}{a}\Big)^2=\frac{8\pi G}{3}\rho-\frac{k}{a^2},\tag{1} $$ which fixes the evolution of $a$ (with given $\rho(t)$) if values for $...
1
vote
1answer
67 views

Unit conversion in cosmology

In cosmology (Dodelson) , the natural unit is $\hbar=1, c=1, k_B=1 $. There the author writes the critical density of the universe $\rho_{cr}$ as \begin{align*} \rho_{cr}&=1.879 \ h^2\ \times 10^{-...
1
vote
1answer
37 views

Momentum of a moving object in FRW metric according to an observer comoving with cosmic expansion

I would like to show that in an FRW metric the momentum of a freely falling object decays as the inverse of the scale factor. I know there are many proofs and arguments for this but I am trying to get ...
1
vote
0answers
56 views

Could the source of dark energy be the universe rotating in a higher dimension?

As I understand it, the centrifugal force in a merry-go-round increases as we move further away from the center. Could the universe be spinning in a higher dimension (or even just spinning in a ...
1
vote
2answers
80 views

Geodesics for FRW metric using variational principle

I am trying to find geodesics for the FRW metric, $$ d\tau^2 = dt^2 - a(t)^2 \left(d\mathbf{x}^2 + K \frac{(\mathbf{x}\cdot d\mathbf{x})^2}{1-K\mathbf{x}^2} \right), $$ where $\mathbf{x}$ is 3-...
1
vote
1answer
33 views

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 $$ \tilde g_{ij} = \delta_{ij}+K \frac{x^i x^j}{1- K \mathbf x ^2} $$ ...
0
votes
1answer
55 views

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 ...
0
votes
1answer
78 views

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 ...
0
votes
0answers
22 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)?
0
votes
1answer
68 views

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

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 ...
2
votes
2answers
114 views

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. ...
2
votes
0answers
26 views

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 ...
24
votes
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 ...
1
vote
1answer
101 views

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} =...
0
votes
1answer
39 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 ...
6
votes
1answer
104 views

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. ...
0
votes
1answer
62 views

Olber's paradox with Rational Numbers [closed]

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 ...
1
vote
0answers
56 views

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 ...
0
votes
1answer
66 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 ...
0
votes
1answer
50 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?
3
votes
1answer
64 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 &...
0
votes
1answer
121 views

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: $\bf r$...
3
votes
2answers
3k views

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 ...
0
votes
0answers
56 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 ...
1
vote
1answer
49 views

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, ...
0
votes
0answers
15 views

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, ...
1
vote
1answer
62 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 ...
0
votes
0answers
38 views

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?
0
votes
2answers
46 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$...
1
vote
1answer
110 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 ...
1
vote
0answers
37 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^...
0
votes
0answers
28 views

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 ...
0
votes
0answers
29 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 ...
6
votes
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 ...
1
vote
1answer
65 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 ...
0
votes
0answers
32 views

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: ...