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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9
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1answer
147 views

What does QM say about the past rather than the future?

In QM, the wave function (in the Copenhagen interpretation) is not an actual physical wave but a device to derive probabilities about the outcomes of experiments. The wave function encodes all the ...
4
votes
1answer
217 views

Why does the Hubble parameter keep getting smaller if the expansion of the universe accelerates?

Suppose a region of space at a distance D from Earth is escaping from us with the velocity v. Since it seems like the expansion of the universe is accelerating, things at D from Earth should be ...
7
votes
3answers
604 views

Dark Energy / Accelerating universe: naive question

Folks, I have a naive question regarding the subject of dark energy and an accelerating universe: From what I understand/read, it seems that the further we look out into deep space, the faster the ...
2
votes
0answers
17 views

Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? ...
4
votes
1answer
82 views

Infinite number of galaxies?

I understand that the current estimate for the number of galaxies in the observable universe is about 100-200 billion. Is there anything in our understanding of physics and the evolution of the ...
2
votes
4answers
100 views

Is it possible that galaxies' redshift is caused by something else than the expansion of space?

I was thinking that maybe photons loss energy naturally when they travel great distances. Or maybe the mass of all matter is increasing over time and therefore photons emitted in the past are ...
6
votes
2answers
75 views

What is chaotic about Chaotic Inflation?

Chaos is defined as an aperiodic long-termed behavior, that is very sensitive to initial conditions. Now from this definition I can only conclude that the adjective 'chaos' is a mere analogy, since ...
5
votes
1answer
108 views

Hubble time and its derivation? [duplicate]

I know the derivation of Hubble time goes something like this (I am an a-level student so this may not be the actual derivation): Two galaxy that is moving away from each other at speed v are now D ...
5
votes
1answer
47 views

The initial conditions of the CMB spectrum

The CMB spectrum shows the intensity of fluctuation at a certain angular scale: The achievement is the correspondence between the predicted power spectrum and the observed one. My question is as ...
1
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0answers
40 views

Hawking radiation at the mouth of Schwarzschild wormholes

I was researching a question for another post and it occurred to me that you might expect to see Hawking radiation at the mouth of wormholes. Given the mechanism of Hawking radiation at the event ...
27
votes
8answers
3k views

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero?

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero? I just do not see why our model must work the way ...
6
votes
1answer
228 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
31 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
votes
2answers
90 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
votes
1answer
64 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
votes
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
votes
1answer
106 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 ...
1
vote
2answers
50 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
votes
2answers
118 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
votes
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
vote
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
vote
1answer
60 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
323 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
686 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
votes
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
votes
2answers
219 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
votes
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
votes
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
vote
1answer
148 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
votes
0answers
54 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
votes
3answers
115 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
vote
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
votes
1answer
54 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
votes
1answer
57 views

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

does space-time expansion affect on fundamental particle or point particle?
1
vote
1answer
79 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
votes
0answers
74 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
votes
1answer
66 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 ...
1
vote
1answer
136 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
votes
0answers
24 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
115 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
votes
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
votes
5answers
501 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
vote
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
votes
1answer
44 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
votes
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
votes
2answers
163 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
vote
1answer
73 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
votes
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 ...