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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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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 ...
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0 answers
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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 ...
2 votes
1 answer
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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 ...
3 votes
2 answers
155 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, ...
-2 votes
3 answers
91 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 ...
0 votes
0 answers
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Questions on vacuum phase transitions in the universe...? [closed]

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 votes
3 answers
122 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 ...
2 votes
1 answer
351 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 ...
1 vote
2 answers
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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 ...
2 votes
1 answer
542 views

Dilaton field causes apparent universe expansion?

In string theory low-energy $n$-dimensional gravity is described by an action of the following form: $$S^{(n)}=\frac{1}{2\kappa^{(n)}}\int d^nx\sqrt{-G}e^{-2\Phi}\Big(\mathcal{R}+4\partial_\mu\Phi\...
2 votes
0 answers
84 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....
1 vote
2 answers
130 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 $\vec{a}$ ...
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 ...
9 votes
4 answers
638 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 ...
2 votes
1 answer
468 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 ...
1 vote
1 answer
206 views

How do I interpret the unit of [1/dex]?

I am trying to reproduce the mass function graph from a paper. I have calculated the mass function $\frac{\rm{d}f}{\rm{d}\log M}$, where $f$ is the mass fraction with respect to the total mass in the ...
10 votes
7 answers
5k views

Why is FTL travel impossible if the universe expands FTL?

If the universe is expanding spacetime faster than light (FTL), is FTL travel no longer completely impossible? Do not care about energy requirements or needing new tech, just if it is NOT physically ...
-2 votes
0 answers
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What is the energy density of gravitational fields around neutron stars? [closed]

The field strength of gravitational fields around neutron stars is extremely high. This would lead to an extreme negative value for the energy density. But if an absolute negative energy density is ...
0 votes
1 answer
60 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 ...
2 votes
1 answer
93 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 ...
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0 answers
50 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 ...
2 votes
0 answers
264 views

Singularities in Bianchi models in general relativity (physical science)

what are the conditions to check point type singularity in a bianchi type model? bianchi type model are of Type I,II,III,IX,IV or u can say we use different Bianchi type models having some specific ...
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 ...
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 votes
1 answer
108 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 ...
0 votes
1 answer
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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 \...
3 votes
2 answers
142 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 ...
0 votes
1 answer
41 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(...
-4 votes
0 answers
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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?
60 votes
4 answers
8k views

Does the universe have a center? [duplicate]

If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center? Where was the ...
10 votes
1 answer
2k views

Lagrangian for Perfect fluid Stress-Energy tensor

The wiki article on the Einstein-Hilbert action for General Relativity says that the stress-energy tensor $T_{\mu\nu}$ is related to the Lagrangian of matter, $\mathcal{L}_M$, by $$T_{\mu\nu}=-2\frac{\...
2 votes
1 answer
73 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 ...
3 votes
2 answers
107 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 ...
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 ...
21 votes
4 answers
21k views

How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?

The actual number: How far apart are galaxies on average? An attempt to visualize such a thing: If galaxies were the size of peas, how many would be in a cubic meter?
3 votes
1 answer
143 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 ...
6 votes
3 answers
2k views

Schwarzschild Radius of the Universe

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

Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
0 votes
0 answers
50 views

In theory, shouldn't there be multiple BAO peaks and even troughs?

Shouldn't acoustic oscillations created by primordial matter anisotropies create multiple peaks and troughs when those oscillations are frozen by decoupling ? It's not unthinkable to imagine that the ...
0 votes
2 answers
66 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 ...
0 votes
0 answers
25 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/...
2 votes
6 answers
973 views

Can there be a theoretical synchronised ‘now’ moment at all points across the universe?

Einstein’s relativity rejects the notion of a universal ‘now’ moment. It underlines how the concept of ‘now’ is compromised due to time passing at differing rates in differing frames of reference, ...
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
76 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 ...
0 votes
3 answers
72 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 ...
0 votes
2 answers
110 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 ...
0 votes
1 answer
72 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 ...
4 votes
2 answers
267 views

How did the big bang's low entropy (which comes from gravity) get converted to sunlight?

In many places you will read that just after the big bang, the universe was filled with very hot gas which appeared to be in thermal equilibrium. You will then read that this is actually quite a low ...
6 votes
2 answers
1k views

Inverse Laplacian

I have seen the following operator somewhere in a paper on cosmology $$ \frac{\partial_i \partial_j}{\nabla^2} - \frac{1}{3} \delta_{ij}. $$ What is the definition of the inverse Laplacian? What is ...
-1 votes
1 answer
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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 ...

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