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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1answer
227 views

Frequency of cosmic microwave background

Some people refer to cosmic microwave background's (CMB) frame of reference as an absolute one. If I understand correctly, we can measure 'absolute' velocity in this frame by using the Doppler effect. ...
1
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0answers
29 views

Dimension of galaxy filaments

Galaxy filaments are amongst the largest known cosmic structures in the universe. They [...] form the boundaries between large voids in the universe. As their name suggests filaments are ...
3
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2answers
85 views

Smolin on Cosmological selection and neutron stars

Regarding the cosmological selection hypothesis and testable predictions, Lee Smolin asserted the following: "Smolin: I did make two predictions which were eminently checkable by astrophysical ...
0
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1answer
62 views

Is the universe expanding faster than we could catch up with it? [closed]

Like the title says, could we ever catch up with the expanding universe?
-1
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1answer
20 views

Cosmic Microwave Background and heat flow

Can the fact that energy is distributed homogeneously in the universe be explained through heat flow and not the Cosmic Microwave Background(or in other words, can we say CMB is a result of heat flow ...
5
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1answer
101 views

Differentiating the gravitational redshift and the cosmological redshift?

If general relativity accounts for a redshift, independent of inflation, how can we still know that inflation is viable? Moreover, how do we differentiate the the gravitational redshift and the ...
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2answers
49 views

The relationship between the Hubble Constant and cosmological time [duplicate]

I am a first year student at varsity and I am new to Astronomy. A question came up in my tut asking about the Hubble constant and cosmological time. I couldn't answer it because I have not understood ...
-1
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2answers
116 views

I understand the Big Bang Theory (BBT), but how was the matter in the BBT created?

I understand the Big Bang Theory to consist of all of the matter being pulled into one great gravitational pull. such a great force that it expelled the matter out causing the idea of Red-Shift and ...
0
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0answers
20 views

Linear size of everything in the Universe is doubled overnight [duplicate]

Suppose the linear size of everything is doubled overnight. Can you test the statement by measuring sizes by a meter stick? Can you test it by using the fact that the speed of light is a universal ...
1
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1answer
28 views

Luminosity distance from angular diameter distance

Consider that I know the cosmological angular diameter distance at a given redshift : $$D_{A}\left(z\right)=\frac{x_{object}}{\theta_{observer}}$$ Is there a general formula to compute the ...
1
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1answer
58 views

Black holes: Behaviour of light near an event horizon

Electromagnetic waves travel mostly in vacuum medium, in outer space, but sometimes in gaseous media, such as in gaseous atmospheres involved in nebulae. If electromagnetic-waves come anywhere in the ...
5
votes
1answer
321 views

Amateur's question on Black Holes [duplicate]

Black holes are caused by massive curvature of the fabric of space-time. Is it right in believing theoretically that forces of electromagnetic origin could also lead to distortion of the fabric of ...
8
votes
2answers
680 views

Can the coordinate of the big bang point be calculated via observed universe or it is impossible? [duplicate]

We know all galaxies spread out after Big Bang theory.The key idea is that the universe is expanding after that theory. Can we play back the scenes via observable universe (galaxies) and can we ...
0
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0answers
19 views

Angular and luminosity distance in general?

Consider a non-Friedmannian Universe in which we know the trajectories of photons, ie in which we know null geodesics $\left(\eta, x^{1}, x^{2}, x^{3}, a, z\right)$ where : $\eta$ is the conformal ...
3
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0answers
56 views

Dark Matter and Modified Gravity

Could someone please explain briefly or refer me to an article or manuscript that shows how f(R) modified gravity theories can be used to explain the problem of Dark Matter, particularly Galaxy ...
-12
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2answers
212 views

if the universe is flat does it mean it exists only in our mind as math? [closed]

My dad, who is a retired mathematician, has this attitude, which I think we all have as kids, that ultimately reality is made of stuff. End of story. If you look around yourself in the world, ...
0
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0answers
25 views

If the universe has a shape, what is the surface? [duplicate]

Since the idea of the universe was that it was expanding, is there a way to know the surface area of the universe like the growing 3D object it is illustrated? The idea was that the universe was ...
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 ...
1
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1answer
141 views

If nothing can travel faster than speed of light then how the Universe is only 13.7 billion years old? [duplicate]

The light would take 93 billion years to reach the edge of universe but nothing can travel faster than the speed of light not even the big bang?
4
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0answers
52 views

What does it mean to have infinite negative conformal time?

In the context of Inflationary Cosmology, it is postulated that there was a period of shrinking Hubble Sphere radius $(aH)^{-1}$. $$ \frac{d}{dt} (aH)^{-1} < 0 $$ Then the regions of the ...
2
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3answers
97 views

Expansion of the Universe regarding Time and lightspeed

According to our current understanding, the universe is expanding... it is often said that every galaxy is moving away from us (I'm assuming that isn't true for galaxies in our local group), and it's ...
1
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1answer
54 views

Isotropy of Space

Weinberg writes in his Cosmology text "Likewise,isotropy requires the mean value of any three-tensor $t_{ij}$ at $x=0$ to be proportional to $\delta_{ij}$ and hence to $g_{ij}$, which equals ...
-1
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1answer
53 views

Reference frames and speed of light

If the speed of light is a constant, and the universe is expanding, wouldn't shining a light in the direction of the center of the universe have a different color (frequency and wave length) than ...
0
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1answer
57 views

does space-time expansion affect on fundamental particle? [duplicate]

does space-time expansion affect on fundamental particle or point particle?
1
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1answer
77 views

Why is scale factor inversely proprotional to temperature?

In all lectures, books and papers I have read about, the scale factor of the universe is inversely proportional to temperature $$a \propto \frac{1}{T}$$ What is the reasoning behind this relation? ...
0
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0answers
20 views

How did WMAP measure the flatness of space? [duplicate]

I read that WMAP constrained $\Omega_0$ to be $1$ within $1%$. I realize that the baryon acoustic oscillations will produce a preferred scale on the CMB. Knowing $H_0$ one could measure the angle of ...
2
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0answers
71 views

Probability of spontaneous Boltzmann brain formation

I was reading through: http://www.scottaaronson.com/papers/giqtm3.pdf But I can't make sense of page 61 discussing Boltzmann brains. Specifically the fact that it says: But the problem is worse. ...
2
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1answer
65 views

Friedmann Equations with varying G?

If Newton's constant $G$ actually varies with cosmological time $t$ would a suitably modified form of the Einstein field equations: $$G_{\mu \nu} + \Lambda g_{\mu \nu} = \frac{8 \pi G(t)}{c^4} T_{\mu ...
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1answer
130 views

Is the cosmological redshift caused by the Planck mass increasing?

The standard explanation for the cosmological redshift is that photons emitted from far away galaxies have their wavelengths lengthened as they travel through the expanding Universe. But perhaps the ...
0
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0answers
23 views

Expanding Universe and more [duplicate]

We all are pretty familiar with the Friedmann models of the expanding universe, but at present I would like to concentrate on the model where the universe goes on expanding with the lapse of time. We ...
12
votes
1answer
114 views

Spectrum of CMB vs. duration of last scattering

The epoch of last scattering took over 100,000 years. The Visibility Function has maximum at about 370,000 years after the Big Bang when the temperature was about 5,600 K. 50,000 years earlier the ...
0
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0answers
32 views

Open-source up-to-date cosmological datasets [duplicate]

Where could I find open-source up-to-date cosmological datasets? Are there any available as a data stream with an API? I already know about these: http://cas.sdss.org ...
4
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5answers
496 views

Is there no radioactive decay between nuclear fusion and solid material formation?

I'm aware my question might be considered a duplicate of this one: Radio-dating and the age of the earth I read that one and I looked everywhere and I still can't find my piece of mind. I would ...
1
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1answer
59 views

In a universe that is expanding at a constant rate, do objects that are attracted to each other feel a force opposite to their attraction?

In this article, the authors make the claim (pg 44) that "Expansion by itself—that is, a coasting expansion neither accelerating nor decelerating—produces no force." I'm having a hard time convincing ...
0
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1answer
39 views

Virgo SuperCluster and space time

Is the Virgo Supercluster affected at all by the expansion of space? What about the larger filament that Virgo resides in?I don't quite understand how gravity affects space time locally in groups of ...
2
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1answer
59 views

How does cosmic horizon work?

This is about the horizon which divides us from stuff that is too far to see because it's moving away from us faster than the speed of light. If point $A$ and $B$ are so far away that $B$ is a bit ...
3
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2answers
162 views

Is energy conserved in general relativity? Does $\nabla_aT^{ab}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is propotional to $a^{-4}$ and the volume is propotional to $a^3$, where $a$ is the scale factor. So the total energy ...
1
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1answer
67 views

Deriving the slow-roll parameter $\eta$

In inflationary theory, many papers start off by making the slow-roll approximation, on which many things depend. This approximation is usually presented by requiring that two 'slow-roll parameters' ...
2
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1answer
73 views

Universe inflation [duplicate]

Read in several publications that the Universe during a very short time (inflation), increased its size by a factor of $10^{50}$. What does it mean to increase the size of $10^{50}$ if you do not ...
2
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2answers
235 views

Why are we blind for the era before the recombination?

At recombination, Universe became transparent to electromagnetic radiation after universe expanded enough to cool down to form neutral atoms. Before that, the matter plasma was effectively opaque to ...
0
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0answers
130 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 ...
1
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1answer
67 views

Is cosmic background radiation absolute [duplicate]

Now that we don't have permeating Ether anymore, why don't we consider the omnipresent Cosmic background radiation in place of it? and measure Speeds with respect to the CMBR? In that way an object ...
0
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2answers
79 views

Does all information in the universe come from the observer?

In absence of the observer any system undergoes unitary evolution, that is reversible evolution without entropy change. It is believed that the initial state of the universe had very low entropy, ...
3
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2answers
107 views

Quantum entanglement on cosmological scales

This may be a foolish question given my limited understanding of QM but here it is. As I understand quantum entanglement basically means that two particles evolve as a single "unit", i.e., are ...
0
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2answers
133 views

How does measuring redshift make us conclude that rate universe expansion is accelerating?

The universe is expanding in all directions. We can detect this by the redshift of electromagnetic radiation from other galaxies. We detect a larger redshift in galaxies which are further away, and ...
0
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1answer
56 views

Speed=mass=time? [closed]

I was wondering why objects gain mass with speed? How can we measure speed if there is no truly stationary point to measure from? And if an object comes to a stand still will it loose its mass? ...
2
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0answers
25 views

Question on Equipartition at Reheating?

Apologies in advance if this is a silly question. I am fimiliar with the idea of Reheating in cosmology, as discussed briefly here. I have been reading several papers which make the assumption of ...
10
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2answers
263 views

The difference between The Dilaton and The Radion?

I have read this question on the Dilaton, but I am a little confused with the distinction between the Dilaton and the Radion. I definitely have the feeling that these two scalar fields are different ...
2
votes
1answer
93 views

Why can't we detect redshift of Galaxies/Clusters moving parallel to us?

If a moving Galaxy/Cluster doesn't have radially outward motion component from us, we are unable to detect its redshift. The answer may look obvious: There's no wavelength squeezing/expanding ju-ju. ...
2
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1answer
40 views

Experimental evidence for the relic neutrinos

What are the experimental (indirect) evidence for the cosmic neutrino background? Where can I read more about this? The discussion on the wikipedia page about the C$\nu$B seems to me to be more ...