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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116 views

What is the meaning of Hubble tension?

The people say that the value measured for the Hubble constant for the CMB is 4.4 $\sigma$ away from the value measured by Type Ia supernovae. I don't get it The value for CMB is $H_{0} = 67.4 \pm 0....
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1answer
54 views

Jeans Instability in an Expanding Universe, Understanding the perturbation

I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding universe. There are 4 equations but I think exercising on one of them would be helpful for me. So ...
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1answer
31 views

What is cosmological critical density?

I'm taking a first course in cosmology and I'm struggling to understand exactly what critical density is. I've been told "Critical density is the energy density of the universe that corresponds to the ...
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29 views

Principle of homogeneity and isotropy of space [duplicate]

How is principle of homogeneity of space different from principle of isotropy of space?
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6answers
5k views

If spacetime itself is expanding, how could we ever tell?

If space is the measured distance between 2 objects, then saying the space expanded is nonsensical unless we have a measuring stick outside of the space fabric to measure the expansion. 2 objects ...
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1answer
78 views

What is the largest gravitationally bound structure in the universe?

I understand the galaxy superclusters are not bound due to dark energy. I wonder what matter scale will remain gravitationally bound - galaxy clusters, galaxies, stars - if anything at all, as dark ...
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1answer
60 views

Did Big-Bang dimensional-expansion initially occur stepwise, i.e 1 dimension at a time?

I’ve been investigating the topological Casimir effect from compactified dimensions as a mechanism to explain dark energy, and this raised a question I was hoping someone could shed some light on. I ...
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59 views

Are photons with a wavelength of the size of the universe possible - and what is their equation of state?

Could the universe be filled with photons with a wavelength of the size of the universe? Do such photons exist? If so, what would be their equation of state?
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32 views

Could the big bang singularity initial condition include more than radial expansion?

The Big Bang Theory states that at t=0 the big bang singularity occurs with an initial condition of ~luminal radial expansion. The Big Bang Theory also states that the energy of the big bang ...
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52 views

Does anyone recognize this function for the Hubble value as a function of density?

I'm trying to reverse engineer a C++ package I found for computing the power spectrum of CMB (EasyCMB). There's a formula that appears to be calculating the Hubble value at a given scale factor, a, ...
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0answers
22 views

Can cosmological parameters be quoted more precisely at other epochs?

Inspired by Value of density parameters in CMB time it seems to me that when cosmological parameters are quoted (e.g. from the CMB or type Ia supernovae), they are almost exclusively quoted in terms ...
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2answers
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Value of density parameters in CMB time

I was looking for the value of the density parameters ($\Omega_{i}$) in cosmology at the time of decoupling (CMB Time). I know that the Planck Colaboration aproximatelly gives $\Omega_{m}=0.315$, $\...
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2answers
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Does an alternative explanation exists for the observed structure of matter in the universe instead of the dark energy explanation? [closed]

I asked in this question if type Ia supernovae could be different in the past. According to me, this would imply that the existence of dark energy would not be so certain. After reading the answer ...
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14 views

The distance sound waves could have traveled in the time before recombination

I was reading this page and find a concept that I dind't know, I was searching for the specific value of $r_{s}$ (The distance sound waves could have traveled in the time before recombination) and $\...
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1answer
55 views

Effects of the cosmological expansion

If the universe is expanding, and perhaps even in an accelerated way, it would seem natural that as space expanded some of the distant matter inside the observable universe would leak out to be ...
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1answer
129 views

Does visible universe have shape of a 3-sphere?

Here's my logic: If you look out in the visible universe you see further back in time. Look enough back and you get to the big bang singularity. This means whichever way you look in the visible ...
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37 views

Flat inflationary spectrum

As far as I know, any successful model of inflation must predict an almost flat spectrum of primordial perturbations ($n_s$ close to one). However, according to some reviews: http://pdg.lbl.gov/2019/...
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47 views

Null chemical potential and expansion of the Universe

In the post Chemical potential of particles with zero mass it's explained that for any species such that in equilibrium satisfies entropy and volume constant, the chemical potential of said species ...
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1answer
58 views

Age of universe

It's said that at the moment of the Big Bang, space expanded so that to an objective observer matter moved at thousands of times the speed of light. If so, how can we say the universe is 10-30 billion ...
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28 views

Was the singularity at the big bang a black hole? [duplicate]

Was the singularity at the big bang a black hole? If not, how does the singularity compare to a black hole?
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49 views

The Wheeler-de-Witt metric as a probability measure?

According to the Wheeler-de-Witt equation arranged like this: $$\left(G^{ij,kl}(x,y)\frac{\delta}{\delta g^{ij}(x)}\frac{\delta}{\delta g^{jk}(y)}-1\right)\Psi[g]=0$$ (Summing over repeated indices ...
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2answers
39 views

Particle decoupling during Universe's evolution

Studying cosmology, I've seen that when you want to calculate the number of degrees of freedom of the thermal bath before and after the decoupling of some species, it is used the mass of that particle ...
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1answer
105 views

Time between Big Bang and 'now'

To point at first my question has a part of trying to make a concept and is not about was there a Big Bang or not. We accept expansion so it is right to think about a starting point of expansion and I ...
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0answers
121 views

Misconeption about age of universe [duplicate]

If we take the most distant quasars and galaxies from as in all directions we get a sphere of radius 13,7 billion light years. As we know they were there 13,7 billion years ago.This suggests the ...
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49 views

Introduction article/book to Lambda cdm Model

After reading Spacetime and Geometry of Caroll, I would like to understand better cosmology: in particular the $\Lambda$CDM model which is referred to as "the standard model of Big Bang cosmology ...
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14 views

full width half maximum (fwhm) optically thin case,

I have been reading The Physics of Star Formation and Early Stellar Evolution edited by Charles J. Lada, N.D. Kylafis where I found about the line profiles How can i derive eq(36)?
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1answer
103 views

Einstein Field equations for the whole universe

Can someone please show the EFE solved for the entire universe. I've heard people say that 'Einstein solved the Einstein equations for the universe and found this'. If I am missing something, what ...
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1answer
57 views

Vacuum energy and cosmic graviton background

What is the best way to show that vacuum energy (the cosmological constant) cannot be the same as a cosmic graviton background (or cosmic gravitational radiation background)?
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105 views

Instead of space expanding maybe atoms are shrinking? [duplicate]

Naturally we would not be able to detect this effect directly by measuring the size of objects using rulers because any such ruler would be shrinking too. Also cosmological redshift can be explained ...
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1answer
62 views

Curved spacetime and geodesics

Is it right to take in account that the Universe might be similar to the Earth regarding curvature so when we look at two galaxies equidistant to Earth but at a right angle observed from Earth their ...
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64 views

What is “The Universe in a helium droplet” about?

The book by E. Volovik seems very complete but I was unable to read it since it requires a very technical understanding of various areas of physics. Can anyone explain what this theory is about and ...
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0answers
26 views

The passing of time [duplicate]

Did the passing of time come before everything else? As in, how long was there "nothing"? And if we can put a time on that, wouldn't time itself be something? And if so, is time the thing that "always ...
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1answer
28 views

Continuity equation in cosmology in terms of derivatives of redshift

I was trying to write this equation $$\dot{\rho}+3\frac{\dot{a}}{a}\rho_{m}=-\frac{\dot{\Lambda (t)}}{8\pi G}$$ as $$(1+z)^{3}\frac{d}{dz}(\frac{\rho_{m}(z)} {(1+z)^{3}})=-\frac{1}{8\pi G}\frac{d\...
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45 views

Understanding quantum cosmology

As I understand it there have been different attempts to use Loop Quantum Gravity to make a theory of cosmology. The first one being done by Martin Bojowald and then later one being performed by ...
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1answer
55 views

Does the moon formation theory consider the necessary precision in the angle of ejection of material caused by the collision?

When Newton considered that a projectile fired horizontally at 8km/h from a very high mountain would enter orbit, I suppose that he also understood the importance of the firing angle being strictly ...
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36 views

Sum of all contributions to energy density in the universe

Assume that the universe contains pressureless matter and a cosmological constant. Assume further that we normalise the scale factor $a$ to $1$. Why is the sum of all contributions to energy density $\...
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31 views

Demonstration of comoving volume between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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1answer
75 views

Can everything be accelerating away with a nearly constant Hubble Parameter?

Simple Physics shows everything accelerating away over time with a nearly constant Hubble parameter. The velocity of recession is $V=DH_o$: The change in distance over time is $∆D=V∆T=DH_o∆T$: The ...
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2answers
69 views

What is the mathematical expression of the Lambda-CDM model?

The conceptual meaning of this model is clear to me. But where can I find the actual mathematical model? The Wikipedia page says "The simple ΛCDM model is based on six parameters". Six parameter ...
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0answers
28 views

What are the key pieces of cosmological evidence if you want to compare the success of dark matter versus quantised inertia?

Quantised Inertia is a controversial fringe theory that is claimed to be an alternative explanation of, for example, galactic motion. The consensus explanation for why the outer stars in galaxies ...
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2answers
121 views

Light's redshift in a vacuum tube due to expansion

If we have two bodies separated by 1 Mpc, but still connected with a tube then it's evident that these two bodies distance won't change because they are connected (If they're not connected their ...
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1answer
79 views

Book suggestion about Neutrino effect on Cosmic Structure

I am trying to find some nice explanatory books about neutrino effects on the cosmic structure. I did not take GR so I prefer sources that contain not much GR. I prefer lecture note series or books ...
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1answer
43 views

Entropy increase after the Big Bang?

I'm trying to understand the increase of entropy just after the big bang. The way i see it going from a "soup" of particles to organized and ordered sytems like solar system, galaxies and clouster of ...
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1answer
65 views

Early Universe Flatness Problem

My question is about the curvature of spacetime in the early universe (Plank era) https://en.wikipedia.org/wiki/Flatness_problem Based on data from Wilkinson Microwave Anisotropy Probe, in order to ...
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0answers
13 views

Newton constant in scalar-tensor Brans-Dicke theories

I'm reading Valerio Faraoni's Cosmology in scalar tensor theories where he says From the action (1.25) or from the field equations (1.29), one reads the gravitational coupling of Brans-Dicke theory ...
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2answers
118 views

If time is relative, then how is the age of the universe calculated? [duplicate]

I am very new to the world of relativity, so the answer may be trivial, but to me measuring the age of the universe seems an impossibility due to there being no fixed frame of reference from which to ...
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0answers
37 views

Density parameters in cosmology and flatness of the universe

I was reading about the density parameter $\Omega=\frac{\rho}{\rho_{c}}$ in cosmology. I find that when cosmologists calculate the value of the density parameters for $\Omega_{m}$, $\Omega_{r}$ and $...
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0answers
14 views

Observers frame of reference for history / chronolgy of very early universe [duplicate]

What is the observers frame of reference when discussing t=0 the instant of the big bang, or the Plank epoch, or the Grand Unification Epoch, or the Inflation, or the Electroweak Epoch?
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1answer
45 views

Does the shape of the Higgs mexican hat depend on the temperature of the universe?

I am reading this paper and i wonder if it is mainstream. Does the VeV of the Higgs field evolve with time? What about the mass given to the fermions?
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48 views

Might local gaps in spacetime suffice to leave its infinity and eternality incomplete?

At https://arxiv.org/pdf/1403.1599.pdf, Vilenkin, in 2014, used the Borde-Guth-Vilenkin theorem's premise that the "bouncing" local universes of an expanding or contracting multiverse would ...

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