**2.917 cosmology questions.**

I'm trying to solve the equation as followings for the WIMP miracle as on my lecture notes, but when I tried to arrive at the same result, I get different units, can someone help me with that?
I get ...

Perhaps this is a naive question, but I have been reading about conformal invariance and conformally related metrics and I would like to know if someone can clarify me some concepts on this.
Anti de ...

It is established by different observations a 13.4 Gyr and 93 Glyr diameter Universe. However, how can be sure about the size of the Universe if there are portions of it we can not see? Is due to ...

With regard to the density parameter derived from Friedmann Equations which is:
$$
Age = D_H\int_{z}^{\infty}\frac{1}{(1+z)\sqrt{\Omega_R(1+z)^4 + \Omega_M(1+z)^3 + \Omega_K(1+z)^2 + \Omega_L(1+z)^{(...

In inflation theory, the inflaton field produce negative pressure to drive inflation.
But now I’m going to ask about scalar fields in general, not the inflaton field.
Can a scalar field also has ...

First of all, I have never studied quantum field theory.
Since Higgs field is a scalar field, does Higgs field also exert positive/negative pressure which contributes to the stress-energy tensor ...

I’ve read that there are inflationary models which use vector field instead of scalar field.
What are the equations used to define energy density and pressure in inflation driven by vector field?
I ...

Suppose the black hole interior (non-kerr, kerr-like,...) is made of some kind of subsubatomic stuff (e.g., folded strings or superstrings, folded branes, buches of preonic pregeometry or whatever). ...

Has any of these hypothetical theories a "higher" number of possible universes or has string theory landscape the same type of possible universes than brane cosmology?

I read/heard that the expansion of the universe would lead to any observer not being able to see other galaxies as time goes on, due to their light not reaching them anymore.
But there's something I ...

From this wikipedia link relation about BAO, I can't get this relation :
$$c\Delta z = H(z)\Delta \chi\quad\quad(1)$$
with $z$ redshift, $H(z)$ Hubble constant at redshift = $z$ and $\chi$ radial ...

I know that adiabatic modes in cosmology has close relationship with large gauge transformations mostly discussed in asymptotically flat gravity or in gauge theory.
However, there are differences and ...

How has CMBR temperature dropped as function of time? A graph would be nice, but I'd be happy with times (age of universe) when it cooled enough to not be visible to human eye, became room temperature ...

I have a question about inflation theory.
Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion?
Could it be that the energy density does dilute with ...

I've found standard derivations showing how one can integrate the Friedmann equation to obtain $a(t)$ given $P_{tot}$ and $\rho_{tot}$. I'd like to know how $a(t)$ depends on the inflation potential.

Could matter in the universe come from some process other than the decay of inflaton field?
Do we have any theoretical reasons that support the idea that matter was created from the decay of ...

Due to the space expansion, the universe should act as a giant telescope. The farther objects are from us (beyond the redshift of $z\approx 1.5$), the larger (not smaller!) they should appear in the ...

From a certain inflationary model I have obtained the theoretical values for the spectral indexes $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...

From this Ethan Siegel’s article
The temperature fluctuations in the CMB are only 1-part-in-30,000, thousands of times smaller than a singular Big Bang predicts.
Does big bang predict the size ...

While exploring variations on Shroedinger's Cat, I came up with the following puzzle:
Define a Schroedinger's Black Box (SBB) as a volume whose contents have no causal connection to an observer A. ...

From the Friedmann equation
$$
H^2=\frac{8\pi G}{3}\rho_m - \frac{kc^2}{a^2} + \frac{\Lambda c^2}{3}.
$$
The dimension of Hubble parameter $H$ is $\frac{1}{\text{time}}$ or $\frac{1}{[T]}$. Therefore, ...

I know the implication :
$\text{Space isotropic everywhere (i.e in each point)} \Rightarrow \text{Space homogeneous}\quad\quad(1)$
but what can we conclude about the example of a torus ?
A torus is ...

I would like to know if the moon where to shine like the sun would the Earth get the same energy and light as it is getting now from the Sun?

I am approaching this from a layman’s perspective and curious about what those more expert in the field have to say about my understanding of Superfluid vacuum theory and the holographic principle and ...

Was the universe already expanding before inflation occured or did inflation cause the universe to start off expanding?
By “cause it to start off expanding” , I mean the cause of the initial ...

Unless the earth is located at the exact center of the universe can we not aim a telescope in various directions, finally finding the edge of the big bang. I mean, we are about 200 million years after ...

What caused the universe to start off expanding in the first place.
Was it negative pressure that drove the initial expansion or was it just something else?

I presume plasma is #1, while superfluid, BEC, QGP, etc, are pretty rare in nature, so the question is "how do solid, liquid, gas, and degenerate matter compare, by mass, in the known universe?"

"Angular diameter" and "apparent size" seem to be used interchangeably in cosmology. But if an object is outside of the observable universe, then isn't it no longer "apparent", yet it would still have ...

In Scott Dodleson's book, he considers that baryons are all tightly coupled. I know this can be proved by calculating the interaction time scales and comparing with Hubble's Expansion time scale.
...

I'm currently trying to self study inflationary cosmology and am finding it difficult to find good resources which explain the motivation behind such theories while providing all the mathematical ...

not just the universe at the point of the big bang
in order for something to exists there must be a t =0.
However, if nothing exists, that means nothing has ever existed since t = negative infinity.
...

The relaxion is a new model-building gadget that solves the hierarchy problem by allowing the Higgs mass term to dynamically relax to zero.
To describe it as simply as possible, leaving a lot out: we ...

Recombination is the time in the universe when free electrons and protons combined into neutral hydrogen. One of the most important features of this transition is that after recombination the photons "...

I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure:
For this, I don't know what considering as abcissa (comoving distance ?, i.e
$$D_{comoving} = ...

Wikipedia says that in new inflation, the slow-roll conditions must be satisfied for inflation to occur.
What is fast-roll or non-slow-roll inflation and why is slow-roll is prefered?

I’ve read somewhere that quantum fluctuations in the inflaton field are usually very rare and very small.
If the effect of quantum fluctuations pushing the potential uphill is too rare and too small ...

I found an article written by physicist George Ellis that confused me a little.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.498.4569&rep=rep1&type=pdf
At some part, he says:
3....

If we observe two galaxies that are diametrically opposed from the Earth, and each $1000\,{\rm ly}$ away from the Earth, is the separation distance between the galaxies $2000\,{\rm ly}$? Really the ...

The Friedmann equation in question is:$$
\dot\rho + 3H \left(ρ + P\right) = 0
\,.$$
I’m a layperson so I don’t know why this equation is sometimes called fluid equation, energy conservation equation, ...

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of ...

On large scales, matter seems to spread uniformly in our observable universe so we think the Big Bang happened everywhere in the observable universe.
Is there any way to tell if the Big Bang happened ...

In many research papers about inhomogeneous cosmology, one often considers spherically symmetric (LTB) spacetimes where in the co-ordinate frame $(t,r,\theta,\varphi)$ wherein the metric assumes the ...

I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...

I'm from engineering background and not firmly familiar with cosmological equations and concepts. but i know what Redshift is and how it is related to the Doppler effect.
My question is how ...

In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...

If spacetime is curved by object with mass, just like the sun curves space time and so makes the planets orbit around it and will eventually in time make them crash towards it then shouldn't the ...

Heat death is the predicted fate of the universe.
Eternal inflation theory states that new hydrogen atoms are created in each pocket universe so that means new hydrogen atoms are still being created ...

I am having trouble understanding expansion of the universe.
Say my coordinate system uses atomic clock cycles for time and the number of clock cycles for light to bounce back to me for distance.
Say ...

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