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.
5,360
questions
0
votes
0
answers
2
views
Determining loaction of Big bang
I am just a high school student. I had already heard that we can't determine the exact location of big bang. So now i have a question can't we approximate it? Like say divide the observable universe ...
1
vote
0
answers
29
views
What mass do neutrinos have in the early universe?
Since neutrinos interact constantly in the early universe, I assume that they are present as flavor eigenstates. However, they are Fermi-Dirac distributed,
\begin{equation}
f(E, T) = \frac{1}{e^{E/T} +...
- 196k
1
vote
0
answers
14
views
Gravitational potential in an expanding universe during the matter era
In the book Cosmology, by Daniel Baumann, there is an exercise to show, using the Poisson equation, that the gravitational potential is a constant during the matter era. As I understand, the Poisson ...
- 11
0
votes
1
answer
15
views
Why only Lyman alpha transition suffer Redshift in the epoch of Recombination in Cosmology?
I was reading Baumann's book on cosmology (topic: Effective three level system). There it is said that in the epoch of recombination to form Hydrogen molecule two type of transition happens.
$$ 2S \to ...
- 128k
0
votes
2
answers
135
views
If the Universe is expanding, how is that galaxies run into each other?
As the title says. If everything is moving outward, thrown by the Big Bang, how could two Galaxies end up with velocity vectors that run them into each other?
0
votes
2
answers
91
views
Time in the Standard Model of Cosmology
Beyond a formal preference for background independence, what is stopping us from setting cosmological time as a de facto universal timeline, analogous to newtonian absolute time? General relativity ...
- 196k
3
votes
3
answers
691
views
Do black holes recapture the CMB?
Basically if I shined a flashlight at a black hole, would I cause it's Hawking radiation temperature increase by more than the temperature of the light I shine at it, at any time during the life of ...
- 30.8k
1
vote
1
answer
33
views
Cosmological horizon and Hawking/Unruh radiation? [closed]
I have two questions about cosmological horizons and their emission of radiation
The first one is:
There are some authors that propose that dark energy or the accelerated expansion of the universe ...
- 16.3k
10
votes
3
answers
2k
views
What are the pros and cons of Einstein-Cartan Theory?
As an alternative to General Relativity, I hear that it can avoid the initial big bang singularity as well as the singularities in black holes, so why does it appear to be talked about so little? If ...
- 8,184
2
votes
1
answer
185
views
What order did the particles of nature appear in?
in the epochs of the very early universe, the different forces separated from each other in succession. if this is true then at one point there was an electroweak force. and before that, there was the ...
CommunityBot
- 1
0
votes
0
answers
19
views
Difference between new inflation and chaotic inflation
I'm trying to understand the difference between new inflation and Linde's chaotic inflation. From what I understand, according to the old inflation, during inflation empty space remains empty, so its ...
- 67
-5
votes
0
answers
56
views
Mystery of cold spot in the CMB [closed]
The cold spot in the CMB is a statistical anomaly there is only a 2% chance there isnt on purpose. Can the cold spot of the CMB be due to an alien Type II or even Type III civilization and not new ...
1
vote
2
answers
107
views
Whether vacuum energy gravitate?
What is the relationship between vacuum energy and gravity, particularly in terms of gravitational effects and its contribution to the overall cosmological constant? Does vacuum energy possess ...
- 1
22
votes
4
answers
4k
views
Why does CMB radiation propagate towards us?
There is something with CMB radiation that does not sit well with me... It seems very counterintuitive that we are able to see it. If CMB radiation formed at the early phases of the universe, would it ...
- 10.9k
1
vote
0
answers
42
views
How big was the surface of the cosmic background radiation?
The cosmic microwave background radiation is the furthest and oldest visible light in the universe. But the universe has expanded considerably since that light was emitted. At the time that that light ...
- 897
1
vote
2
answers
185
views
Assuming that heat death or Big Freeze would be the final outcome for the universe, what would happen with energy?
According to Big Freeze, the universe will expand forever and it will reach a state of thermodynamical equilibrium. It is said that the temperature of the universe will reach an asymptotically ...
CommunityBot
- 1
1
vote
0
answers
55
views
Is the universe closed or flat?
Apparently there is a tension in the measuring of the curvature of the universe (https://arxiv.org/abs/2307.07475) as apparently in 2018 the Planck collaboration got a series of results consistent ...
- 196k
1
vote
0
answers
15
views
Fourier coefficients of fluid velocity in Cosmology
I start with the following equation, that can be obtained by taking the curl of the continuity equation, and in which $\omega=\vec{\nabla}\times\vec{v}$ is the curl of the velocity field of the ...
- 287
2
votes
1
answer
373
views
Calculating energy density in the early Universe
I've been told to calculate the energy density in the early Universe. It states that it is completely dominated by neutrinos (3 species), photons, electrons, and positrons.
Now, I've found an ...
CommunityBot
- 1
2
votes
0
answers
22
views
What is the intuition behind the decrease in comoving Hubble radius during inflation and then its increase later on?
We know that the comoving Hubble radius decreases during inflation when the universe is exponentially expanding and then increases during matter and radius domination. What is the reason behind this? ...
- 73
-4
votes
0
answers
58
views
Visibility of our Universe by an Observer outside [closed]
In my own theory/understanding of our universe I believe that EM radiation does not leave the universe because the method /medium for light travel was created during the expansion thus only existing ...
- 196k
1
vote
1
answer
211
views
Curvature in space time during Big Bang and present scenario
Space time in the presence of masses is curved. But during the time of Big Bang it's presumed that all the matter in this universe was at a single point, so it must have been super dense and had very ...
- 1
7
votes
1
answer
376
views
Are the mass, diameter and age of the Universe frame dependent?
Mass of the observable Universe is known to be $1.50×10^{53}$ kg. Age is approximately known to be 13.7 billion years.The observable Universe is a sphere with diameter of roughly $8.8\times10^{26}$ m.
...
0
votes
0
answers
16
views
Is differential number density the same as flux density?
I'm trying to convert the differential number density of photons to luminosity (using slide 17 of https://www.astro.rug.nl/~sctrager/teaching/OA/Photons.pdf) and ...
- 59
-1
votes
1
answer
48
views
Is there an "escape velocity" for closed dimensions? [closed]
Assuming a closed universe, the shape of the universe is often considered, or at least presented in pop science, to be a glome (4-sphere), and popularly depicted as behaving analogously to a 3-sphere, ...
- 30.8k
2
votes
2
answers
104
views
One-Way Speed of Light and the Big Bang
Variations on this question have been asked a few times (e.g. here, here, and a few YouTube videos here and here). The claim seems to be that because we can only measure the round-trip speed of light, ...
- 94.4k
0
votes
1
answer
30
views
Can a receding body due to the accelerated spacetime expansion be "rescued"?
Once a body "crosses" the limit where dark energy wins over gravitational forces (Is there a distance from a gravitational source where the influence of gravity and dark energy are balanced ...
- 1,972
5
votes
2
answers
657
views
Angular Momentum of the Universe [closed]
Our planet revolves around its axis with a period of roughly 24 hours. The reference are the sun and the stars: Every time we see the Sun rising we conclude that the Earth has completed a full ...
- 83
0
votes
2
answers
653
views
Distance and luminosity distance
In my cosmology lecture notes I read that a way to measure distances in cosmology is to use standard candles and the comparison between "absolute luminosity" of the candle and the apparent luminosity. ...
CommunityBot
- 1
-1
votes
1
answer
60
views
Comoving Space as a Manifold: How Does It Work? [closed]
One of the open questions in cosmology is:
"What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe?&...
- 196k
0
votes
0
answers
45
views
Does inflation theory assume a finite universe?
Inflation theory has it that the early universe was causally connected, and could “mix”, hence explaining relative homogeneity of the CMB. The universe then rapidly expanded and became causally ...
- 196k
1
vote
1
answer
30
views
Is there a Doppler effect on the surface of an expanding balloon?
Imagine a stationary transmitter which vibrates the surface of the balloon and a stationary receiver half way around the balloon that can pick up these waves. Let the balloon expand. Will the ...
- 196k
1
vote
1
answer
89
views
How fast would a hypothetical microscopic quantum black hole evaporate with an effective mass of two protons?
According to the analysis shown in this research here, see link the evaporation time can be calculated in seconds using this equation:
$$
\begin{array}{l}
t_{\text {evap }}=\left(\frac{5120 \pi G^2}{\...
- 351k
0
votes
0
answers
51
views
The covariance matrix $\mathbf{C}$: Why does $\ln \text{det} \mathbf{C}=\text{Tr}\ln \mathbf{C}$ hold? [migrated]
Prof. Max Tegmark first introduced the Fisher information matrix into cosmology in his paper titled Karhunen-Loeve eigenvalue problems in cosmology: How should we tackle large data sets?
As I read the ...
- 196k
2
votes
1
answer
259
views
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 ...
CommunityBot
- 1
2
votes
1
answer
848
views
How come that an Infinite universe will collapse under gravity?
In this PSE post the issue is about the stability of an infinite universe under Newtonian gravity.
Here I'will drop the Newtonian constraint because we know of the finite speed of interactions and I ...
CommunityBot
- 1
1
vote
0
answers
38
views
How to derive deceleration parameter in terms of redshift?
How do I get q in terms of $z$ specifically of this form:
I want to derive the second equation from the first but I'm getting an additional $ \frac{a^2}{H^2}\ $ in the second term
$$ q= -\frac{\ddot{a}...
0
votes
1
answer
395
views
Klein-Gordon equation in FRW spacetime
The metric for frw spacetime is $$ds^2=a(n)^2(dn^2 - dx^2)$$ where $dn$ is the conformal time differential form. The Klein Gordon equation in curved spacetime is $$\left(\frac{1}{g^{1/2}}\partial_{\mu}...
CommunityBot
- 1
1
vote
1
answer
157
views
How can we know if our big bang is a one side of a wormhole?
According to Hawking, he proved that (excluding quantum mechanics) the big bang started at a singularity. Hence showing a connection between the mathematics of the big bang and black holes.
Equally, ...
CommunityBot
- 1
0
votes
0
answers
52
views
Three-Point correlation function in cosmology
I have been studying this review article on Non-Gaussianity from inflation. It was mentioned that $n$-point correlation function can be obtained by the expression
\begin{equation}\label{eq:1}
\langle\...
- 196k
1
vote
0
answers
32
views
Understanding the additional field in shape dynamics calculation of big bang
I read this fascinating paper (RG).
The total collision is a well-known singularity in Newton mechanics: the distances become zero and, therefore, the potential becomes infinite.
In a paper before ...
- 83
2
votes
2
answers
144
views
How does dark energy affect the dynamics of galaxy clusters?
Galaxies interact with each other gravitationally (just as every other celestial object) and in many cases they form groups or clusters.
Does the expansion of the universe (or dark energy) affect the ...
- 196k
0
votes
0
answers
23
views
Is the size/age of the universe dependent on your velocity? [duplicate]
As Photons do not experience time or space, then according to my thought experiment, all photons must occupy some kind of singularity as well as what WE observe from earth.
I was also thinking that ...
- 196k
0
votes
1
answer
102
views
Interaction between energy and spacetime
I hope it's not too stupid but if we say that what we call matter or energy is creating or consuming the spacetime, would not that make the big bang more logical?
- 94.4k
2
votes
1
answer
280
views
How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?
By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
CommunityBot
- 1
8
votes
2
answers
368
views
What does it mean that "relativistic material becomes cosmologically coupled to the expansion rate" in the recent dark-energy black-hole paper?
The recent paper "Observational Evidence for Cosmological Coupling of Black Holes and its Implications for an Astrophysical Source of Dark Energy" has made a splash in the popular press. ...
- 11
5
votes
3
answers
170
views
What is the connection between matter in the universe and the baryon number not being conserved?
Towards the end of "Quarks, the Stuff of Matter", the author discusses the implications of the proton is not stable and ultimately decays. He states, that if the proton decays, then the ...
- 5,718
-4
votes
0
answers
52
views
Are dark energy and vacuum space actually one and the same?
It is my understanding and correct me if I'm wrong, that what we call as vacuum space is the zero-point energy ZPE and its quantum fluctuations. However, because ZPE is constantly increasing with ...
- 3,760
2
votes
4
answers
643
views
Pre Big Bang knowledge
I have read that no one knows what preceded the Big Bang, but is this true?
The Big Bang process (with or without inflation) followed known rules which “existed” prior to time zero. For example, some ...
- 77
1
vote
1
answer
306
views
Prove that the value of the cosmological constant equals the energy density of the vacuum
I know that Einstein introduced his cosmological constant assuming it as an independent parameter, something characteristic of the Universe, in itself, but the term of it in the field equations can be ...
CommunityBot
- 1