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.
5,565
questions
0
votes
0
answers
29
views
Is the tensor product involved in the metric a symmetric product?
The expression of the FRW metric in Cosmology in usually written as:
$$ds^2=-dt^2+a^2(t)d\vec{x}^2$$
where $c=1$. However, $dt^2$ is a shortening of $dt\otimes dt$, that is, of the tensor product of $...
- 293
-2
votes
0
answers
29
views
Can Poincaré recurrences happen eventually in a universe with zero vacuum energy?
I am interested in the topic of possible vacuum up-tunneling and down-tunneling events in cosmology. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "...
- 2,462
0
votes
1
answer
36
views
Question coming from Cosmological Perturbation
We consider the following scalar perturbation on the FRW metric:
$$ ds^2 = -(1 + 2\phi)dt^2 +2a\partial_i B dx^i dt + a^2 \left( (1 - 2\psi)\delta_{ij} + 2\partial_{ij}E\right) dx^i dx^j $$
where $\...
-5
votes
0
answers
50
views
Can self-indication assumption solve the fermi paradox? [closed]
Consider $P$ the probability that our civilization appears from beginning to the end. $P$ should also be the probability that at least 1 extraterestrial civilization exists. If we asses that $P$ is 50%...
0
votes
1
answer
62
views
Clarification of Weinberg's cosmology book eqns 5.1.44-5.1.47 for scalar perturbation
Has anyone clarified the equations in Weinberg's cosmology book for scalar perturbation for nonzero $F$ and $B$, eqns 5.1.44-5.1.47. I am not sure why there are terms with $\nabla^2 \dot{B}$ and $\...
- 1
0
votes
1
answer
40
views
Spatial Curvature of Universe at recombination vs now
From my understanding, we use the CMB data to measure the spatial curvature of the universe today. Why is it the value for today if the CMB data reflects the universe at recombination (380K years ...
- 25
0
votes
0
answers
86
views
Is there a metric, a solution to Einstein's field equations, for a single body in a space of uniform non-zero density?
The Swarzschild metric describes a single body in an empty space with zero density, while the FLRW metric is presumably for a space with uniform non-zero density but no single body. But is there a ...
- 131
0
votes
4
answers
146
views
Are there closed simply connected 2D manifolds that do not require a third dimension?
In the context of cosmology, space is commonly described as potentially having a global curvature that can be positive, zero, or negative. A common way that textbooks describe positive curvature is by ...
- 11
0
votes
0
answers
68
views
Are vacuum "up-tunneling" phase transition in AdS and Minkowski spaces impossible?
I am interested in the topic of vacuum phase transitions in models of the universe. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "true" one (in ...
- 2,462
-2
votes
3
answers
100
views
What is the gravitational field of a hole in an infinite perfect crystal?
Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
2
votes
1
answer
86
views
As per Friedmann Equations, is big-bang singularity necessary?
The Friedmann Equations do not directly require that the scale factor $a(t)$ was zero in the beginning. Since Einstein's static universe is still a valid solution, is it possible that before the Plank ...
- 1,161
3
votes
1
answer
180
views
Will expanding space rupture an empty box floating in outer space
Under the theory that space itself is expanding, but the space inside of atoms and molecules doesn't expand because nuclear and electromagnetic bonding forces exceed the forces that expand space, ...
- 85
1
vote
2
answers
48
views
Viable values for the $K$ parameter in the FLRW metric
The FLWR metric is sometimes given as $$c^2 d\tau^2 = c^2 dt^2 - \frac{a(t)^2}{(1-KX^2)} dX^2. $$
I am not interested in the tangential motion so I set $d \Omega = 0$ although it is of interest in ...
- 6,102
2
votes
0
answers
53
views
Cosmological numerical computations
I am unsure where to ask this question, whether here or in the Mathematica stackexchange, but either way, I was wondering what are some recommendations for cosmological computations specifically using ...
2
votes
3
answers
132
views
Can a light signal from Earth reach a galaxy outside the Hubble Horizon?
Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
- 6,102
1
vote
2
answers
133
views
Is the FRW metric, based on spatial homogeneity and isotropy, rotationally and translationally invariant? If so, how?
The spatial part of the Minkowski metric, written in the Cartesian coordinates, $$d\vec{ x}^2=dx^2+dy^2+dz^2,$$ is invariant under spatial translations: $\vec{x}\to \vec{x}+\vec{a}$, where $\vec{a}$ ...
- 11.8k
2
votes
0
answers
90
views
Gravitational halos made of neutrinos...?
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
- 2,462
0
votes
1
answer
67
views
Why can we use Bose-Einstein statistics in this expression for number density
In a system with $N$ particles in some volume $V$ in contact with a reservoir of temperature $T$, we find that
$$\bar{n_i}=\frac{g_i}{e^{\frac{{\epsilon}_i -\mu}{kT}} \pm 1}$$
depending on whether the ...
- 125
0
votes
0
answers
51
views
The data file of the LISA Interferometer results
How to extract the data of the strain versus frequency plot of the Laser Interferometer Space Antenna (LISA): Figure (2) in this paper:
https://arxiv.org/abs/1702.00786
The paper doesn't contain any ...
- 395
0
votes
0
answers
30
views
Magnetic monopole in CPT universe model
I've recently read this paper CPT universe, and a thought came into my mind.
Is it possible to discuss magnetic monopole based on this CPT universe model?
This paper points out that some mysterious ...
- 529
2
votes
1
answer
110
views
Saddle Shaped Universe
The universe, as described by FLRW metric, if $k = -1$ is clearly a 2 sheet 3-hyperboloid described by $x^2+y^2+z^2-w^2=-R^2$. So where does the more common saddle shaped picture of the open universe ...
- 1,161
0
votes
1
answer
64
views
Beginning of a dark energy (cosmological constant) dominated universe
Consider a flat universe with only dark energy (cosmological constant)
In such universe, the Friedmann Equation can be written as:
$$H(t)^2 = \left(\frac{\dot a}{a}\right)^2 = \frac{8\pi G \...
- 411
0
votes
1
answer
42
views
Friedmann Equation and a contracting universe
Consider a universe with a nonzero curvature and matter.
One can write the Friedmann Equation in this universe as such:
$$\frac{H(t)^2}{H_0^2} = \frac{\Omega_0}{a^3}+\frac{1-\Omega_0}{a^2}$$
Where $H(...
- 411
-4
votes
0
answers
46
views
Simple question about finite Universe [duplicate]
If, by Big Bang, Universe was created from initial singularity, with finite "speed" of expansion of matter, shouldnt it be finite as well?
3
votes
2
answers
108
views
Free Neutron Decay
I've read that free neutrons can decay into hydrogen, but it's rare because the energy from the decay usually sends the electrons away, unable to bind with the protons. But if trillions of free ...
- 85
2
votes
1
answer
75
views
Doppler Effect and the concept of relative velocity in GR
While reading Sean Carroll's book on General Relativity, I understood that the concept of velocity is ill-defined over large distances in arbitrarily curved manifolds, like the one used to describe ...
- 442
0
votes
0
answers
26
views
Is there an estimate for how many monopoles would be produced in the very early universe?
The question really hits it with this one. I haven't really found some good numbers other than the whole “It'd be so many (per Hubble vol) to recollapse the universe”. I don't know if that comes of as ...
- 33
2
votes
1
answer
95
views
What causes the 41k obliquity cycle?
The obliquity cycle refers to the oscillation of the Earth's axial tilt, which has a period of about $41,000$ years.
When the Moon is the main cause of the tilt, and it is receding linearly, how can ...
- 21
0
votes
0
answers
27
views
How to plot the curve of the gravitational wave energy density giving the data of the strain versus frequency
I want to plot the curve of energy density ($\Omega$) of the gravitational waves versus frequency that are predicted by the Einstein telescope. But in the ET pages:
https://moscow.sci-hub.se/4444/...
- 395
3
votes
1
answer
146
views
Cyclic Universe Problems
In Penroses's hypothesis, at the end of each iteration the universe undergoes a conformal transformation, meaning distances are rescaled. If I am right, it implies that a planet from the previous ...
- 1,161
0
votes
2
answers
68
views
Homogeneous and Isotropic But not Maximally Symmetric Space
Is this statement correct: "In a homogeneous and Isotropic space the sectional curvature is constant, while in a maximally symmetric space the Riemann Curvature Tensor is covariantly constant in ...
- 1,161
9
votes
4
answers
651
views
Understanding expansion of the Universe as things flying apart
Say that we have a Universe uniformly filled just with matter (let's not bring dark energy into this). And say that we fill it with very light particles (so that the gravitational interaction between ...
0
votes
0
answers
21
views
About redshift and distant objects observation
I did a few Time&Length dilation factor calculations. This is for a distant clock away from any field and a clock on Earth on the equator with the moon passing over clock coordinates, the factors ...
0
votes
1
answer
78
views
Could cosmic rays induce a vacuum decay in the future?
I've been told that very energetic cosmic rays could cause a vacuum phase transition or vacuum decay (and even could cause a true vacuum level to go "uphill" to a false vacuum) due to their ...
- 2,462
0
votes
3
answers
73
views
Are there universes where "symmetry breaking" went differently? [closed]
What have happened with other possible variants of asymmetry?
Are there other universes being run in parallel to our universe where the ball is not at C, but at B?
Stephen Wolfram told
I have found ...
- 129
0
votes
2
answers
111
views
Is it true that $\dot{H}(t)\sim H(t)$, and if so, why?
In the context of working with the FRW metric in Cosmology, I'm trying to reproduce the results of a paper where an expansion of the metric in terms of perturbations is performed. The author gives a ...
- 293
0
votes
1
answer
76
views
Black Hole Formation -- How Can an Event Horizon be Observed to Grow? [duplicate]
This is a question about black hole formation. To be clear, I’m not suggesting that black holes don’t form. It’s that I’m having trouble with the accepted explanation so there’s a flaw in my logic ...
- 21
-1
votes
1
answer
55
views
Relation between Earth's and Sun's magnetic axis relative to their paths of travel and relative to each other?
Pretext
As non-educated citizen with no academic background, and based on watched PBS Space Time's latest video I've got curious, what's the relation between Earth's and Sun's magnetic axis relative ...
- 530
0
votes
0
answers
47
views
Inflation in background free models of the universe
There are many authors who are attempting to construct a model of physics that doesn't rely on the objective existence of spacetime. This is part of the work in quantum gravity. This leads to things ...
- 1,430
3
votes
2
answers
143
views
How can we model the primordial Universe while the interior of a neutron star and comparable states of matter are still mostly unknown?
There is something I never quite understood about the physics of the primordial Universe. There are states of matter at high temperature/high pressure that are still, today, poorly understood. The ...
- 1,109
11
votes
2
answers
513
views
Why is nonzero net charge density incompatible with the cosmological principle?
In an answer to a question about the overall charge-neutrality of the universe, benrg writes,
A nonzero net charge density is incompatible with the cosmological
principle. Unlike the gravitational ...
rob♦
- 91.5k
1
vote
2
answers
69
views
Can Bose-Einstein condensates and Fermionic condensates survive for long periods of time in space?
Imagine we have a cold region of the universe, almost devoid of matter and radiation. Or perhaps in a future universe where the CMB has "cooled" down to sufficiently low "temperatures&...
- 2,462
5
votes
3
answers
414
views
Why is the universe charge-neutral?
The positive charges (such as from the protons) of the universe are almost neutralized by the negative charges (such as from the electrons).
Is there an explanation for this neutrality? Does it ...
- 4,452
1
vote
2
answers
530
views
Please help me with this paradox [closed]
Physicists believe that some galaxies are moving away from us at faster than the speed of light. A galaxy that is moving away from us at faster than the speed of light would be moving backwards in ...
- 53
10
votes
1
answer
526
views
Age of a dark energy dominated universe
In a flat universe that is dominated by dark energy (or cosmological constant), the Friedmann equation can be written as:
$H^2 = (\frac{\dot a}{a})^2 = \frac{8\pi G\varepsilon_{\Lambda}}{3c^2}$
Where $...
- 411
2
votes
1
answer
281
views
Negative Horizon distance
Consider a flat universe, here, proper distance can be given by R-W Metric:
$$d_p (t_0) = c\int_{t_e}^{t_0}\frac{dt}{a(t)},$$ $t_e$ is the time when a photon is emitted from a distant galaxy, $t_0$ is ...
- 411
0
votes
0
answers
49
views
Dark energy and conservation of energy in General relativity [duplicate]
i know that conservation of energy in general relativity has been discussed multiple times here at PE, a popular explanation on the topic is Sean Carroll's blog "Energy is not conserved" ...
- 117
1
vote
1
answer
71
views
Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
- 411
0
votes
1
answer
55
views
How to find critical density?
In Cosmology critical density is defined as the minimum density for a flat universe to keep expanding, by Friedmann Equation:
${\left({\frac {\dot {a}}{a}}\right)^{2}={\frac {8\pi G}{3}}\rho -{\frac {...
- 411
1
vote
1
answer
61
views
How to understand critical density?
In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
- 411