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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21 views

Is the discrepancy in universe expansion in the speed of expansion, the rate that the speed is changing or both?

Is there a simpler way to describe the latest discrepancies or problem with the expansion of the universe. Does the rate of expansion consistently get faster and faster? I was reading this and had ...
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Swampland conjectures and $f(R)$ gravity

Why the swampland conjectures should be satisfied even in backgrounds with the $f(R)$ gravitational model, if the gradient energy (or the kinetic term of the scalar fields) are negligible for the ...
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What is the evidence for the existence of a Hubble sphere beyond which galaxies recede faster than light

The title has been changed from the original title, because that is what I meant, but I misused the terminology, and probably some physics, asking about evidence for the existence of a horizon for the ...
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1answer
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Stress-energy tensor of Jordan-Fierz-Brans-Dicke action

I have a question about the JFBD action, in particular the part that involves a scalar field. It is given by $$ S_\phi = \int d^4x \sqrt{-g} \left( \frac{1}{2} g^{\mu\nu}(\nabla_{ \mu}\phi)(\nabla_{\...
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3answers
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Age of Universe from Friedmann Equation - How to actually solve the integral?

The Friedmann equation for a flat universe can be written as $$ H(t)=\frac{\dot{a}}{a}=H_0\sqrt{\Omega_{m,0}\cdot a^{-3}+\Omega_{\Lambda,0}}=H(a) $$ To calculate the age of the universe, many books ...
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String quantum cosmology

In canonical quantum gravity we have Wheeler–DeWitt equation and we can chose homogeneous and isotropic ansatz (it kills many problems) to describe quantum universe. In string theory, as I understand,...
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2answers
70 views

Various scales of infinite

I was reading a great answer by John Rennie earlier (and took a mental note of the question which I must have mislaid). I wanted clarification by comment so I thought I could look it up amongst John's ...
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2answers
92 views

What caused Big Bang to bang? [duplicate]

What made the big bang start in the first place? I don't see what could have caused it. What made it burst apart?
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Why is Cauchy horizon named after Cauchy?

I think the title says it all; I have done a lot of Googling, but I could not find the answer. Since (I assume) the concept was discovered after Cauchy's death, I speculate that there is a connection ...
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1answer
70 views

The flaws in this explanation of why the universe is so big [closed]

We all know universe is very very big (I’m talking in general meaning of words like “we all know” and “big”) and if we ask why is it so big then it might become a little bit philosophical but what are ...
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Energy is conserved. Dark energy has a fixed value per volume. The universe is expanding. Is non-dark energy decreasing? [duplicate]

I was thinking about this yesterday. Basically, these three premises: The amount of energy in the universe is conserved. Dark energy is a fixed number of joules per cubic meter. The universe is ...
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Why are galaxies so far apart?

With Big Bang, one would expect distribution of mass to be somewhat uniform but we have galaxies that has billions of stars and there are huge distances between galaxies? Why is there is so much ...
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Hubble tension and density parameter

Hi I was reading about the Hubble tension. I know that the Planck Collaboration gives an actual value for the Hubble parameter as $H_{0}=64.3 kms^{-1}Mpc^{-1}$ and the group that measures $H_{0}$ with ...
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Supersymmetry in de Sitter space

Is it possible to construct SUSY theories in de Sitter space? I know statement: dS superalgebras cannot be constructed unless one constructs actions with matter coupled with the wrong signs for ...
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1answer
36 views

Observational evidences of the early universe other than CMB

CMB is a great experimental evidence that the universe was once in an hot themal phase at early times. Supposing all the CMB observations failed for tecnical reason (for example the rocket shipping ...
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1answer
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How do you know what is the fate of the universe in a modified theory of gravity?

Usually when you talk about the final stage of the universe you assume that $\Lambda CDM$ is the correct cosmological model and with that you can analyze what could happen for different values of $\...
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Is standard cosmology $\Lambda$CDM currently in deep trouble? [closed]

The standard cosmology model $\Lambda$CDM can purportedly depict how the universe is evolving with time. But could this confidence in $\Lambda$CDM be misplaced? Is $\Lambda$CDM currently in deep ...
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49 views

How long will last the CBMR?

I am confused about the duration of the cosmic background microwave radiation.Will it eventually be replaced by same radiation from discrete directions?
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What's the difference between Planck mass and bare Planck mass?

I had a quick look at the Wiki definition of bare mass and I get the gist. Could someone explain its significance in the particular case of Planck mass and cosmology? Thanks.
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1answer
49 views

Density of radiation at CMB

I was trying to find the radiation density at the CMB time. For two different methods, the first one is using the Planck Law and the data for the actual measured temperature $T_{0}=2.7255° K$. $$ \...
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2answers
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Validity of Newtonian gravity inside the Hubble radius

When dealing with structure formation in Cosmology, it's often said that well inside the Hubble radius we can use Newtonian gravity and therefore the starting point to obtain equations for the ...
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2answers
525 views

Universe expansion acceleration

[I've read, among others, other questions on this, don't believe this is a duplicate] I (think I) understand the evidence that distant galaxies are moving away from us faster than closer galaxies. ...
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Do redshifted galaxies with $z>1$ look like bigger because red light has a larger diffraction angle?

As a laser beam starts to diverge after a certain distance and has a divergence or diffraction angle which is largest for red light somebody could observe two laser sources distant 1m from each other ...
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2answers
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Acoustic/Sound waves in early universe

Friends, Sound waves, being mechanical in nature cannot travel in empty space. But I came across an article " Magnetic Fields in early Universe" by Dario Grasso and Hector R. Rubinstein in which it ...
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1answer
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If Hubble parameter varies with time how much it differs for low distances (present) and deep space (past)? [duplicate]

If deep space is shown as it was in the past how can we calculate this time lagging regarding the Hubble parameter which is a variable of time?
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1answer
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Is there a “central” cosmological inertial frame?

I know there are no preferred inertial frames from the perspactive of special relativity, and I assume the same is true from the perspective of General Relativity. However, if the same is ...
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Why was helium hydride ion $\text{HeH}^+$, not hydrogen $\text{H}_2$, the first molecule formed in the early universe?

How did NASA scientists confirm it? https://www.nasa.gov/feature/the-universe-s-first-type-of-molecule-is-found-at-last
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Is the photon speed in the reference frame of a distant point in an expanding universe constant?

If we call the photon a worm and the expanding visible universe a apple fruit can we make a next picture of the expanding universe?The apple grows in that way that its surface has a speed of ...
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1answer
127 views

What is the evidence that dark energy exists? (as of 2020)

Astrophysicists believe that dark energy exists and makes up ~70% of the universe's energy content. What is the evidence that dark energy exists? Wikipedia gives five lines of evidence: Supernovae. A ...
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20 views

Models of accelerated expansion

Are there models of accelerated cosmological expansion that involve a changing number of spatial dimensions? I think I've seen a version of string theory that posits a reduction to quasi-3 dimensions ...
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1answer
61 views

How to calculate the radius of curvature of a FLRW universe in terms of energy density $\rho$?

It is easy to calculate the radius of curvature $R_K$ using critical density ${\rho}_{\text{cr}}$ which depends on the Hubble parameter $H$. However, I would like to know how I can calculate the ...
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40 views

Hubble parameter

When we observe a distant galaxy we must see that as it is moving from us its speed regarding Earth increases according hubble constant.But as seems it is not a constant but an increasing function of ...
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2answers
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Why is CMB an evidence of hotter and denser early Universe?

In his book Gravitation and Cosmology, Steven Weinberg says that the Cosmic Microwave Background (CMB) makes it "difficult to doubt that the universe has evolved from a hotter denser early stage". In ...
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Measuring $H(z)$ with the Differential Age method

I was reading about how astronomers measure the Hubble parameter without relying on a cosmological model. They use something called the differential age method. They use galaxies in a similar ...
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Can the metric expansion of space theoretically be exploited to do work?

Other questions tackle why space doesn't expand matter, and I've read their respective top answers, but I'm interested in something a little different. Let's say I set up two masses that are 1 Mpc ...
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Is the universe expanding, or is everything in it getting smaller?

I was recently reading some of the nice answers to the (related) question: If space itself is expanding, how could we ever tell? From many of the answers their appears to be a symmetry between: A ...
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73 views

Expansion of the relation between the cosmological growth factor and scale factor

Hello i'm studying large scale structure of the Universe (in $\Lambda$CDM Universe) using as time the growth factor defined by $$ \ddot{D}+2H\dot{D}-4\pi G\bar{\rho}_{DM}D=0$$ in my equations i have ...
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1answer
58 views

What does the cosmological constant do? [duplicate]

I think just about anyone who has studied Einstein or relativity has heard of the cosmological constant. He added it so that his equation would return an universe that is static. Of course, the ...
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1answer
113 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
52 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
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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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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
75 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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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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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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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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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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