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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2answers
271 views

How can the universe be flat?

Okay, so I just want to clarify a few things. According to what I have read, we have measured the universe to be flat, and the shape of the universe is directly related to the mass-energy density. ...
15
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3answers
1k views

Where's the missing helium in the Universe?

I'm confused: Big Bang nucleosynthesis is adamant about the 1 neutron to 7 proton ratio which yields 75% hydrogen to 25% helium (with a nominal amount of partially-reacted deuterium and heavier ...
9
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3answers
498 views

What is the exact meaning of homogeneity in cosmology?

I understand that, in general, homogeneity is the physical attribute of being uniform in composition (" of the same form at every point"), but I'm slightly confused when it is used in cosmology as ...
9
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1answer
336 views

evidence on the equation of state for dark energy?

If dark energy contributes mass-energy density $\rho$ and pressure $p$ to the stress-energy tensor, then you can define $w=p/\rho$, where $w=-1$ gives a cosmological constant, $w<-1$ gives a big ...
8
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2answers
144 views

In a random direction, am I more likely to find a dwarf or giant galaxy?

First a couple of disclaimers: My title explains the idea of my question, but I will pose it slightly differently to make it less subjective. This ends up being in the style of a homework exercise ...
8
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1answer
182 views

Different age of universe

According to general relativity time runs slowly near a massive body. So, I have a question bothering me from some time, that if time for every body/object in universe runs at a different rate, then ...
8
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3answers
341 views

Why doesn't dark energy vary with time?

Trying to teach myself the basics of cosmology. I can see why, in an expanding universe, radiation and matter density, and radiation and matter pressure vary with time. But why is it assumed that dark ...
7
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4answers
948 views

de sitter cosmologic limit

It has been said that our universe is going to eventually become a de sitter universe. Expansion will accelerate until their relative speed become higher than the speed of light. So i want to ...
7
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3answers
692 views

The Big Bang in an infinite universe

If the universe is spatially infinite (and assuming, if it makes a difference, that we don't have eternal inflation), what actually happened 13.7 billion years ago? Was the energy density infinite (or ...
6
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4answers
775 views

Is space unending?

Is the space unending, i.e it has no boundaries? If yes, how can a thing exist which is non-ending? Its impossible for me to imagine something like that. Secondly, if its not and has boundaries then ...
6
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1answer
1k views

Can/has string theory solved cosmological constant problem?

Can someone explain to me what the KKLT paper says, and what has and hasn't it achieved regarding the ability to construct solutions with a small positive or negative cosmological constant in string ...
5
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2answers
666 views

Abundances of the light element of the Big Bang Nucleosynthesis

This question is related to the Big Bang Nucleosynthesis of light elements, more concretely I do not understand some features of the graph Why do the $^3$He and D abundances go down with ...
5
votes
3answers
432 views

Is observable universe an explanation against Olbers' paradox?

First of all, let me tell you that I'm not a physicist but rather a computer scientist with a mere interest in physics at nowhere near a professional level so feel free to close this question if it ...
5
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6answers
2k views

Does gravity slow the expansion of the universe?

Does gravity slow the expansion of the universe? I read through the thread http://www.physicsforums.com/showthread.php?t=322633 and I have the same question. I know that the universe is not being ...
5
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5answers
353 views

Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)?

Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a ...
4
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2answers
350 views

Number density of CMB photons?

When we talk about the present-day number density of photons ($n_\gamma\approx 10^8 $ $m^{-3}$) in the universe do we mean the number density of CMB photons? I mean there are other sources of ...
4
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1answer
300 views

Is dark energy around a black hole locally curved?

The repartition of dark energy in the vacuum is homogeneous all over the universe. The diagram below represents space with a black hole in the middle. The square is divided in small unit squares. If ...
4
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2answers
212 views

Could Dust Equation of State have some negative pressure?

Traditionally the cosmological equation of state of cold matter (so-called dust) is simply: $$p = 0.$$ But, in Newtonian terms, each particle is gravitationally attracting every other particle. ...
4
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1answer
54 views

Qualtitative explanation for the link between low magnitude of high z supernova and accelerated expansion

Is there any explanation on a qualitative level why we can see in the observed magnitude vs. redshift z plot that the universe is expanding accelerated? See for example here: ...
4
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1answer
132 views

How can contracting dimensions lead to cosmological inflation?

Using the Kasner metric, given by $$ ds^2 = -dt^2 + \sum_{j=1}^D t^{2p_j}(dx^j) $$ it is possible to not only describe the cosmological expansion of some space directions (the ones with positive ...
4
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5answers
516 views

What is the significance of Planck force?

I have been curious to find what could be the significance of Planck force? It is calculated by the formula $c^4/G = 1.21031359\times 10^{44} \, \mathrm{N}$, where $c$ is the speed of light and $G$ is ...
4
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3answers
450 views

Question on inflation

I have two particular questions regarding the inflationary scenario. They are: 1.) What is the physical origin of the inflaton field? 2.) Why has the potential of the inflation field its particular ...
3
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0answers
58 views

Would quantum fluctuations cause problems for scalar-field inflation?

Wheeler once said that spacetime would be highly curved at very small scales because of the uncertainty principle for energy-momentum. In which case the spacetime becomes very bumpy and not smooth ...
3
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1answer
127 views

A true singularity at $t=0$, coordinate independent Big Bang

Consider a flat Robertson-Walker metric. When we say that there is a singularity at $t=0$, clearly it is a coordinate dependent statement. So it is a "candidate" singularity. In principle there is ...
3
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1answer
158 views

What is the difference between matter & spacetime? [duplicate]

If the universe is expanding why doesn't the matter in it expand proportionally making it seem as if the universe is static? Alternatively, as spacetime expands why does it not just slide past matter ...
3
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1answer
180 views

The first $10^{-35}$ seconds [closed]

I am a rank amateur, so please forgive me if the answer to this is well-known. The following quote can in a weekly update for an EdX course I am following in astrophysics: "And what a week it's ...
3
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1answer
578 views

How Does Cosmology Constrain the Number of Neutrino Species?

I know that based upon theories of structure formation cosmologists can constrain the sum of the masses of neutrinos - if neutrinos were too light or too heavy it would simply change the power ...
3
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0answers
107 views

Were fermions ever massless?

In a discussion of the Standard model and Higgs mechanism it was claimed that accordingly: "During an early phase of the cosmos all fermions were massless." I wonder whether this claim can be ...
3
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3answers
340 views

How much of the universes mass is bound up in photons traveling between stars?

I was watching a Scott Manley video on youtube and he mentioned that the Sun was loosing 4 million tons of mass a second as it converts to energy. With a few trillion trillion stars also converting ...
3
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0answers
55 views

squeezed radiation astronomy

Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier. Are there ...
3
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1answer
240 views

If Fine Structure Constant in not uniform in space what would that imply for cosmic background?

Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
2
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3answers
314 views

Is the accelerated expansion of the universe consistent with conservation of energy? [duplicate]

If energy can neither be created nor destroyed, how come the universe is expanding at an accelerating rate? Where does this energy come from?
2
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0answers
50 views

If the universe is expanding then why does the distance between Sun and the Earth is not expanding? [duplicate]

It has been proved that universe is expanding in the accelerating way. If that is true than the space between the Sun and the Earth must also be expanding. But it is not so, why?
2
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1answer
298 views

How come that an Infinite universe will collapse under gravity?

In this PSE post the issue is about the stability of an infinite universe under Newtonian gravity. Here I'will drop the Newtonian constraint because we know of the finite speed of interactions and I ...
2
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1answer
762 views

Photon on null geodesic

If given an FRW metric $ds^2 = -dt^2 + a^2(t)[dx^2+dy^2+dz^2]$ and for the trajectory followed by a photon (null geodesic; $ds^2=0$) with affine parameter $\lambda$, know that ...
2
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1answer
211 views

Varying an action (cosmological perturbation theory)

I am stuck varying an action, trying to get an equation of motion. (Going from eq. 91 to eq. 92 in the image.) This is the action $$S~=~\int d^{4}x \frac{a^{2}(t)}{2}(\dot{h}^{2}-(\nabla h)^2).$$ ...
2
votes
1answer
269 views

acceleration of the universe

Moments after the Big Bang, the universe was expanding at an incredible rate, (I've heard) faster than the speed of light. Due to dark energy, scientists predict the rate of expansion will pick up ...
2
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0answers
138 views

Can decompactification explain the inflation of the early universe?

I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
2
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1answer
191 views

Can someone explain probability flux in the tunneling boundary condition of Vilenkin?

This is what's leading to the notion of a quantum universe tunneling from nothing into existence, right? The idea is that probability flux flows out of superspace (configuration space) at ...
2
votes
5answers
499 views

Can computers survive bubble nucleations?

According to string landscape theory, our vacua with a cosmological constant of $10^{-123}$ is a metastable vacua which can decay to a supersymmetric vacua with either a zero or negative cosmological ...
2
votes
1answer
322 views

How does inflation drive Ω close to 1?

I'll keep it simple. How does inflation drive Ω close to 1?
2
votes
2answers
426 views

Have CMB photons “cooled” or been “stretched”?

Introductory texts and popular accounts of why we see the "once hot" CMB as microwaves nearly always say something about the photons "cooling" since the Big Bang. But isn't that misleading? Don't ...
1
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2answers
62 views

Is the expanding of the universe also a cause which can induce gravitational waves from mass? [duplicate]

Gravitational waves arises when mass is rotating in another mass'orbital, in explosions and of course in case of colliding black holes. But are they also created when mass is moving and speeding ...
1
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1answer
75 views

Does time expand along with space? [duplicate]

The flat FRW metric is given by: $$ds^2=-c^2dt^2+a(t)^2dr^2$$ If we take $dt=0$ then we get: $$ds=a(t)\ dr$$ Thus we find that space expands. If we take $ds=0$ to find the null geodesic followed ...
1
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1answer
103 views

How fast am I moving? [duplicate]

Given the speed of light is 299,792,458 meters per second and that it is constant through out the universe, (i.e a person who measures the speed of light while standing still will get the same result ...
1
vote
1answer
142 views

What is the identity of Dark Matter?

What is the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an ...
1
vote
2answers
153 views

Could these hypothetical very long distance properties of light offer alternatives to an expanding universe or a finite number of galaxies?

Assume that there could be a property of light that it gets very gradually steadily red-shifted by traveling large astronomical distances, in exactly the same way that Doppler shift from receding ...
1
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1answer
84 views

Increasing matter density in the Friedman equations?

From what I understand, the Friedman equation is $H^2=\frac{8\pi G}{3} \rho$ and the $H$, the Hubble constant, is a measure of the rate of expansion of the universe. What I am confused about is that ...
1
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1answer
189 views

Infinite space or finite space?

I have been reading about the nature of space (Brian Greene's and Max Tegmark's recent books) and have a question on the size of space. Both references acknowledge that general relativity equations ...
1
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1answer
330 views

Reconciling “The Big Crunch” with the 2nd Law of Thermodynamics

Assume "The Big Crunch" scenario (the universe will collapse to a singularity). In this case, I think of the entire universe as an isolated system; in the "Big Crunch" scenario, it seems to me gravity ...