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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14 views

Energy density and pressure in thermal quantum field theory

In QFT, energy density and pressure can be defined from Noether current due to Poincare translation invariance. What if we are considering a system at finite temperature? For a scalar field, we have ...
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0answers
29 views

When was the temperature of the CMB between 0°C? When was it 100°C? [duplicate]

The current temperature of the CMB is something like 3°K. When was the CMB temperature between 0° and 100° Celsius?
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0answers
28 views

How to explain the following concepts regarding the behavior of the universe at early stages. [on hold]

Hello fellow physicists I'm working on a long question and having a little bit of trouble. "Discuss each of the following concepts in the context of theories of the early Universe and its ...
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3answers
297 views

What does the accelerating expansion of the universe mean about its end?

So we know that the expansion of the universe is accelerating.When we thought that the expansion of the universe was slowing down,we predicted that it would reach a plateaux and maybe reverse the ...
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0answers
15 views

Validity of Press-Schechter formalism at the end of recombination

I was wondering if Press-Schechter formalism on Spherical collapse is applicable sometime at the end of recombination, i.e. the photons have started to free stream but the free streaming length is ...
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2answers
61 views

We stretch as the universe does [duplicate]

Here's an idea I thought about; more of a thought experiment than anything: The universe is essentially the same as the universe 14 billion years ago, just stretched. However, all matter stretches ...
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1answer
30 views

Why is a negatively curved universe older than a flat one?

And a positively curved universe younger than a flat one? I get that negatively curved means an open universe, and positively curved is a closed one. But how can this affect their age? Is it ...
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1answer
109 views

Age of the universe [closed]

Does the fact that the edge of our observable Universe contains information from the beginning of the Universe give us an ability to determine its age? The edge of our observable universe surely has ...
4
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0answers
173 views

Dark matter clumping [duplicate]

How can dark matter particles (WIMP's) get ever more densely clumped during the evolution of the universe, as we see for example in the Millennium simulation, if they can't loose kinetic energy by ...
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1answer
60 views

How to design an experiment to see if we are living in a brane or not?

Theories in string theory and cosmology lead to an idea that our universe is a single "membrane" (or brane, as it is commonly called) suspended within a higher dimensional "bulk". See Wikipedia. Is ...
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28 views

Are there any grounds for thinking that the distribution of matter in the universe is unbounded?

Even though we can make no observations beyond the cosmological horizon, I think people tend to picture a universe that is either homogeneous to infinity, or possibly having spacelike slices that are ...
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0answers
54 views

Are neutrinos weekly interacting particles? [closed]

Occasionally, I read about weekly interacting neutrinos, and although I know this is a typo, I wonder if it happens to be true. Do average neutrinos, as they fly through the universe, really interact ...
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0answers
20 views

Relation between black hole mass and size applied to observable universe [duplicate]

If I apply the relations given in http://xaonon.dyndns.org/hawking/hrcalc.js and http://www0.arch.cuhk.edu.hk/~hall/ag/sw/SpinCalc/SpinCalc.htm to the values for the mass of the observable universe ...
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1answer
53 views

Test to determine if universe is infinite or not

Currently it is not known whether the universe is finite or infinite. Is there any test that can be performed (theoretically) to know whether the universe is infinite or not? I'm still in high ...
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2answers
95 views

Where did energy come from?

So it is my understanding that before the big bang there was a very small point that held all of the matter in the universe. My question is what form did energy of the universe take on when it was in ...
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2answers
215 views

When will physics understand it is the 'dark mass' not the 'dark matter'? [closed]

There is new thinking occurring in physics where 'dark matter' is considered to be incorrect. The new thinking is that mass fills 'empty' space. The mass which fill's 'empty' space is being referred ...
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2answers
58 views

Could the missing antimatter lie outside the observable universe?

While I was reading a similar question asking if other galaxy could be made of antimatter, to which the answer was: if they were, we should detect the radiation from matter interacting with antimatter ...
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1answer
2k views

Why haven't we seen the big bang?

The Andromeda galaxy is 2,538,000 light years away, so if we view Andromeda from a telescope, we see Andromeda how it was 2,538,000 years ago. Now the diameter of the visible universe is 92 billion ...
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1answer
40 views

Cosmological Boltzmann equation [closed]

Consider the Boltzmann equation: $$\frac{d \ln{n^c(T)}}{d \ln{T}} = \frac{\Gamma}{H}(1 - \frac{n^c_{eq}(T)}{n^c(T)})$$ We know that the ratio $\Gamma/H$ can be considered constant, let us put it ...
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1answer
72 views

When they say the universe was the size of a baseball about a billion billion billion billionth of a second

after the big bang. Does that means the observable universe was the size of a baseball, or does it mean the entire universe? I'm guessing it means the observable universe - as we really don't ...
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1answer
41 views

Flat universe a torus?

As shown by cosmologists space is presumably a flat expanse. In his book the Fabric of the Cosmos, Brian Greene shows that one of the possible shapes of this flat universe is a Torus. My question is, ...
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0answers
21 views

Luminosity distance as a function of redshift

We know that the luminosity distance is: $d_L = a_0 R (1+z)$ with $z$ the redshift. How can we mathematically make explicit the dependence of $d_L$ from the redshift? I guess we need to make it ...
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4answers
58 views

Does our universe have an even distribution of matter in every direction

It occurred to me that it does since gravity in the space I exist appears to be uniform Ie there is no net movement of objects I observe in a particular direction. I maybe speaking from classical ...
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1answer
31 views

Null geodesics in FRW metric: why angular coordinates are constant?

Consider a ray passing through $r=0$ in the FRW metric $ds^2 = -dt^2 +a(t)^2(\frac{dr^2}{1-kr^2} + r^2(d\theta^2 + \sin{\theta}^2d\phi^2))$ The geodesic curve is parametrized by the affine parameter ...
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2answers
58 views

Why should I believe that matter does not expand along with the space between it [duplicate]

This question has been asked before under other guises. I am not a scientific profesional however i have some schooling in pyhsics and mathematics and have a keen interest in these subjects. It seems ...
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2answers
142 views

Does the universe have a different age to different observers?

Although I understand that the universe is approximately 12 billion light years wide, and that may mean that it took the light photons 12 billion years at 186,000 miles per second to reach the ...
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1answer
37 views

Discrepancy problem in lithium?

Why is there a discrepancy between the amount of lithium-7 predicted to be produced in Big Bang nucleosynthesis and the amount observed in very old stars?
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2answers
63 views

Expanding universe - does the Hubble red shift come from gravity? [duplicate]

Measurement of the expansion of the universe is based, in part, on the red shift of light. The speed of light is a constant, although some people argue that it may be changing over time, I am going to ...
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4answers
2k views

Why is the singularity of the Big Bang not considered to be the center of the Universe? [duplicate]

If the universe is expanding, then at some time in the past, it must have started from a single point but why this point is not the center of the Universe. Just like the singularity of black holes is ...
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0answers
22 views

FRW Metric maximally symmetric, derivation, $R=3K$ or $R=6K$ confusion, two different texts

I'm looking at Tod and Hughston Introduction to GR and writing the metric in the two forms; [1]$$ ds^{2}=dt^{2}-R^{2}(t)(\frac{dr^{2}}{1-kr^{2}}+r^{2}(d\theta^{2}+sin^{2}\theta d\phi^{2})) $$ [2] $$ ...
4
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1answer
83 views

Light clocks measure conformal time - detailed argument

Let us assume that an observer is stationary at the origin in expanding space. We assume the FRW metric near the origin is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Let us assume that the observer ...
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0answers
16 views

Deriving General Expression for Total Density Parameter

I'm trying to derive an expression for the total density parameter of the universe, where $\Omega_0=1.001$, as a function of the scalefactor $a$.The expression should be for a universe with curvature, ...
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0answers
24 views

Are there any in depth superfluid mechanic analyses of spacetime?

Has there been much work done that treats particles as vortexes in a fluid, or dark matter as bubbles in this fluid (bending space in the same way massive particles (vortexes) are observed to do, but ...
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0answers
25 views

Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
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1answer
51 views

Is our locally measured time actually conformal time?

The FRW metric at the origin $r=0$, with $c=1$, is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Now one can change variables so that near the origin the FRW metric is approximated by the Minkowski metric ...
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0answers
40 views

Are ultracold atoms only created by intelligent life?

Nature has particle accelerators that are far beyond our capacity, but occasionally I hear atomic physicists claim that they are able to make something that has never been formed in any natural ...
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1answer
47 views

Flat universe and accelerating expansion

As current data suggests that the universe is flat, a flat universe would imply a slowing expansion rather than an accelerating one which we observe. Neither in an open nor a closed universe there is ...
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0answers
152 views

Can the universe be round but still infinite?

Can the universe still be infinite in space if its curvature is > 1? Is a manifold of positive curvature necessarily compact? Does the Tarski paradox have any bearing on the finite or infinite ...
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1answer
69 views

Are we really sure that the whole universe is expanding?

Haven't we thought about the possibility of only a region of the universe is expanding? What about space is expanding in this region and space is compressed in another region, that satisfies the ...
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1answer
52 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 ...
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1answer
93 views

I wonder what kind of proof we have that dark matter is really made of not-yet-discovered WIMP particles and not WISPy? [closed]

There is quite reasonable argument by Joerg Jaeckel that dark matter might be actually very light, very weakly interacting particles, so-called WISPs (for weakly interacting slim particles), as ...
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0answers
20 views

Do cosmic strings or global monopoles interact with magnetic field?

Does anyone know any phenomenon that shows the interaction between cosmic strings or global monopoles with magnetic field? I looked for that in Vilenkin and Shellard's book but, as I'm not a ...
3
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1answer
64 views

The actual space curvature

What is the curvature of our physical space, according to the latest experimental data? I've found it somewhat difficult to find a definitive answer to the question, because the spacetime curvature ...
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0answers
50 views

Is the Weyl Postulate correct?

The Weyl postulate in cosmology states that worldlines do not intersect but it can be shown in GR that using Raychaudhuri equation that geodesics can intersect if there is curvature so I'm really ...
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0answers
53 views

Can the Milky Way's halo be considered to be curved spacetime? [duplicate]

Scientists are beginning to understand dark matter fills 'empty' space. Dark matter which fills 'empty' space would be displaced by the matter. The Milky Way's halo would be the state of displacement ...
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0answers
22 views

Are atomic energies increasing as the Universe expands? [duplicate]

Starting from the FRW metric (for simplicity flat space, radial direction only): $$ds^2=-c^2dt^2+a(t)^2dr^2$$ If we take $dt=0$ then the proper distance $ds(t)$ between two spatially separated ...
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1answer
61 views

How many electrons are there in the universe?

Someone on io9 estimated there were about 10^80 electrons in the universe, but I want to ask the Stack Exchange physics community.
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1answer
27 views

Does the readoption of the cosmological constant apply across all models of GR?

I'm currently reading about cosmology, and the reintroduction of the cosmological constant, Lambda, into the field equations, to give a more accurate model for cosmology. My question is this: If it ...
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0answers
81 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
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0answers
33 views

Linear Perturbation theory in General Relativity, what to do with products of derivatives?

I'm trying to do a problem in which I am given the Einstein tensor for the following metric: $$ ds^2 = -e^{2\Phi}(d{x^0})^2 + e^{2\Psi}\delta_{ij}dx^idx^j, $$ And then asked to find the Einstein ...