# cosmology's questions - English 1answer

2.917 cosmology questions.

### WIMP Miracle result

0 answers, 18 views cosmology dark-matter wimps
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 ...

### Anti de Sitter and FLRW metrics

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 ...

### Limits of the Universe: how can be sure about the size of unobservable Universe?

0 answers, 36 views cosmology universe
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 ...

### 1 Density parameter in matter-dominated universe

1 answers, 48 views cosmology space-expansion

### 1 Why is slow-roll is prefered over non-slow-roll inflation?

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?

### Would eternal inflation be really eternal since quantum fluctuations are usually very rare and very small?

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 ...

### -2 Is there any scientific multiverse theory that contains impossible worlds? (even if it is highly hypothetical)

0 answers, 76 views cosmology multiverse laws-of-physics
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....

1 answers, 196 views cosmology galaxies distance
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 ...

### 2 Does Friedmann fluid equation prove that energy of the fluid is not conserved?

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, ...

### 55 Is the total energy of the universe zero?

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 ...

### 1 Is there any way to tell if the Big Bang happened everywhere or just in some regions of the entire universe?

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 ...

### 3 Why should LTB dust be comoving?

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 ...

### 2 How could the universe's expansion “remove the mean gravitational potential”?

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 ...

### 3 How scientists know that the Redshift is because of moving objects and not due to local phenomena

1 answers, 67 views cosmology doppler-effect redshift
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 ...

### 2 Curiosity episode with Stephen Hawking. The Big-Bang

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 ...

### 22 Do the laws of physics evolve?

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 ...

### Is the universe expanding?

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 ...

### 1 Heat death in eternal inflation

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 ...

### Expansion of universe, distance confusion

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 ...