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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135 views

Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?

IIs it valid to apply Einstein's Relativity to scenarios involving expansion of space? For a practical example of this: Is it legitimate to speak of distant red-shift galaxies as experiencing time ...
2
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1answer
163 views

Difference between Gunn Peterson trough and the Lyman Alpha Forest? Cosmological implications?

I'm having difficulty understanding the full implications of the Lyman alpha forest and its use in cosmology. My understanding is this: we detect features in the Intergalactic Medium (IGM) by very ...
1
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1answer
91 views

How can the universe by infinite when it has known start and finite time since? [duplicate]

Just watched the 'How Big is the Universe?' TV program and it indicated that the universe is believed to be infinite in size. They used an approach involving measuring the internal angles of ...
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1answer
65 views

Is the Big Bang instant considered a beginning of all the coordinates of our universe? [duplicate]

Is the Big Bang instant considered a beginning of all the coordinates of our universe? If we consider the BB to be the Beginning of of our universe, can we call this the Origin of all of the 3 ...
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3answers
61 views

Can we tell the difference types of recession? [duplicate]

Because of the the Hubble red shift pattern, Cosmologist generally assume that space is expanding. They assume this is a fact because of the observed redshifts and GR says it can be so. But how can we ...
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1answer
34 views

Power spectrum calculation in Weinberg's cosmology

In Weinberg's Cosmology, on page 412 eq. 8.1.42 he provides the power spectrum as; $$P(k)=\frac{4(2\pi)^3 N^2 C^2(\Omega_\Lambda / \Omega_M)}{25 \Omega_M^2H_0^4 ...
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2answers
52 views

How tall is the universe?

Let's suppose that I am on a spaceship in low-earth orbit, if I rise my spaceship higher and higher and hover it upwards forever, will there ever come a point where I just can't get up any further or ...
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1answer
69 views

What lies at the very edge of the expanding universe? [duplicate]

We all know that the universe is expanding at an accelerated rate and it might appear much like a soap bubble. That is where the phrase dark energy whose essence is unknown and which is thought to ...
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0answers
61 views

Big Freeze or Heat Death?

I can't seem to find this on the Internet.... Which is most likely to occur to "end" the universe, the big freeze scenario? Or the heat death scenario? P.s. When, if it does at all, will the ...
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0answers
28 views

About a *center of the universe* [duplicate]

I understand that the history of our universe began with an explosion - the Big Bang. Now, I saw a comment in some old post, that "the universe has no center". Is that true? If there was initially a ...
4
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1answer
139 views

Is inflation theory really dead?

I know the title is little bit challenging but maybe most of you heard about the last BICEP2 paper on February. As I have read about it here and here. My understanding is, BICEP2 results released on ...
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0answers
28 views

On the probability of the existence of a similar observable universe

Let's assume a standard ($\Lambda$CDM + some simple inflation model) cosmology in an infinite universe. Really it doesn't matter much what cosmology we take, just that we're considering an infinite ...
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0answers
52 views

What does string theory and loop quantum gravity say about the Planck Epoch and anything before the Big Bang?

General Relativity is known to commit suicide exuberantly at the planck epoch, and is unable to say anything further, whereas string theory and loop quantum gravity breathe well under these ...
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2answers
93 views

Harvesting hydrogen from a star

I've been thinking about what highly technologically advanced civilisations would do once their energy requirements become comparable to the total output of a star, and how such activity could be ...
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1answer
42 views

Can accelerating universe avoid Big Bang?

Expansion of our universe is accelerating. This means that it was expanding less rapidly in the past. Is in accordance with the laws of physics a model of accelerating universe in which the rate of ...
3
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1answer
273 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 ...
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0answers
27 views

Upper Limit of the Hubble Parameter right after Inflation

When I take the FLRW equation $$H_{(t)}=H_0\cdot\sqrt{\Omega_R\cdot a_{(t)}^{-4}+\Omega_M\cdot a_{(t)}^{-3}+\Omega_K\cdot a_{(t)}^{-2}+\Omega_{\Lambda}}$$ and calculate the value of the ...
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0answers
34 views

Did spacetime curve infinitely about 13.7 billion years ago? [duplicate]

GR/Big Bang Model implies that there was a singularity about 13 billion years ago, in which all the matter and energy along with the observable universe (or perhaps, the entire universe) was ...
0
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1answer
56 views

Expansion of the universe in “everyday live” [duplicate]

According to space.com the universe is expanding with 75km/1Mpc/1s (megaparsec), which comes down to $\beta=2.43\times10^{-18}/s$, a relative rate per second. To put this in perspective: The average ...
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2answers
21 views

Is it better to define the observable universe as the universe?

This is somewhat triggered by this question: It's established that universal energy is not constant. But is the net change positive or negative? When I say entire universe, is that equivalent to ...
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4answers
183 views

The fainter the star, the further away it is?

If a star appears to be very faint, would it imply that it's really far away or that it's luminosity is minimum?
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1answer
118 views

Could there be a time dilation of 2,000,000x between Earth and other parts of the universe?

Don't ask me why, but you may be able to guess why I would ask such a question. The question is, what kind of conditions would have to be present in a universe in order for billions of years pass in ...
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2answers
99 views

How could the “Big Bang” singularity have actually expanded? [duplicate]

We have mountains of evidence about singularities and how they work, and we have mountains of evidence that the "Big Bang" was the origin of the universe as we know it. But if compressing enough ...
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0answers
22 views

Cosmological and astrophysical bounds on physics beyond SM [closed]

There are many cases when cosmological and astrophysical observations make restrictions on particle physics models. For example, some axion models are rejected by white dwarf luminocity functions, ...
86
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3answers
14k views

How come some people are claiming that the Big Bang never happened?

A news story is going viral on social media networks claiming that two physicists have found a way to eliminate the Big Bang singularity, or in layman's terms (as claimed by many sensationalist news ...
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2answers
119 views

Cosmology: proper evolution of energy density ratios with time?

Solving Einsteins equations for the FLRW metric with ideal quantum gases for matter and radiation, assuming that pressure is proportional to the energy density $p=w\rho$ and deriving $w$ from ...
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0answers
24 views

Recommendations of books on Newtonian cosmology (without reference to general relativity)

I only know basic special relativity, but am interested in learning Newtonian cosmology. I have heard that on introductory level, most of the material can be generated purely from Newton's work. I am ...
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2answers
39 views

Cosmology: what is a quantity that is called “$h$” in regard to angular size of a galaxy?

I am trying to solve a Cosmology problem, but a certain quantity $h$ appears in it, of which I do not know the definition (I have never seen it mentioned anywhere before). So I thought maybe someone ...
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0answers
28 views

Luminosity, brightness and magnitdes in cosmology

I just started learning Cosmology and am confused with these terms. What I know so far: Luminosity $(L)$ is the energy output (inc. visible light, radiation etc.) per unit time of a star. It is an ...
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3answers
237 views

It's established that universal energy is not constant. But is the net change positive or negative?

Dark energy is density is constant and that's something like 75% of the universe, so I am pretty sure that the net change must be positive. But photons redshift and so loose energy. I assume other ...
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3answers
253 views

Is there an exact formal definition of the Universe?

I've read several articles about observable Universe, Universe and Hubble volume, including Wikipedia article and references on it, and I wondered: Is there a formal, rigorous definition in physics of ...
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0answers
46 views

Does the curvature of spacetime by gravity affect homogeneity and isotropy of the space of the universe?

The FLRW metric starts with the assumption of homogeneity and isotropy of space.(Wikipedia) FLRW metrics of the universe have no or only very weak curvature - It is curved space. In contrast, ...
3
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1answer
106 views

What is meant with the statement: “the universe in infinite”? [duplicate]

I'd like to know what is thought of when saying the universe is infinite. I've read about what is meant when astronomers are speaking of a "flat" universe and I have read that its size is most likely ...
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2answers
63 views

Did the Earth (amongst other things) travel faster than light?

It's often said that when you look far across space with a telescope you are looking back in time, as the light has only just reached our position in space. However, given that the Earth and many ...
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0answers
29 views

Will any massless particle/object in the vacuum of space always travel at the speed of light? [duplicate]

The question as stated above, is "Will any massless particle/object in the vacuum of space always travel at the speed of light?"
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2answers
87 views

Could Dark Energy just be particles with negative mass?

Title speaks for itself. Dark matter: We see extra attractive force, and we posit that there are particles which create such a force, and use the measure of that force to guess their locations. Dark ...
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1answer
69 views

Did light already experience death of the universe immediately after the Big Bang?

Let's suppose that I am on Point A with a light-beam launcher. My goal is to launch a beam of light to Point B, which is about 8,000 quadrillion light years away. My friend named Jack is at Point B ...
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3answers
158 views

Is simulating the entire universe possible?

Is it concievable that we may one day simulate the entire universe with every single particle, field and law of physics factored in? Can n number of particles (say the number of particles that make up ...
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2answers
63 views

Expansion of the universe a thermodynamic process or not?

Can the expansion of the universe be thought of as a thermodynamic process? If so, is it a closed system? Is it a reversible system?
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1answer
68 views

absorption of cosmic microwave background radiation

My understanding of the CMB radiation is that it comes from everywhere, and goes in every direction. But how can this be the case, when there is a lot of matter in space which could absorb, or for ...
2
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1answer
101 views

Question about universe expansion

In general relativity, we cannot determine the global structure of the universe (since it is not flat), therefore all measurements and observations are only meaningful locally. In particular, we can ...
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0answers
44 views

Convert angular power spectrum to spatial power spectrum

If we have a signal projected on a sphere, one routinely decomposes this in spherical harmonics, in analogy to a Fourier decomposition in flat space. One can then make the decomposition: ...
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0answers
24 views

What does velocity relative to CMB mean? [duplicate]

I've see some velocities such as that of the Sun quoted as "relative to the cosmic microwave background"? How is this different to measuring velocities relative to that old, discredited idea, the ...
0
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1answer
39 views

Big bang theory

How much energy and heat were produced by the big bang? I'm reading Bill Bryson's Short history.... He mentioned a point that to forage light elements like hydrogen, helium and lithium into elements ...
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1answer
46 views

Why do scientists use a different method from redshift to find the distance of distant galaxies?

The method I am referring to is measuring the brightness of a super nova. Does the redshift method even work at that distance and if it doesn't why not? If it does work why do they prefer to use the ...
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0answers
14 views

Intensity of spectral lines

I am trying to calculate the scattering of stellar light from electrons and atoms in space. To complete the calculation I need the intensity of a spectral line from a star. Does anyone know the ...
4
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3answers
171 views

Coordinates for FLRW metric

In GR, coordinate are just a tool for us to describe the physics, they should be equivalent. However, in standard form of FLRW metric, it can be inferred that the universe is expanding, but we can do ...
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1answer
48 views

How the expanding universe model takes into account the fact that information from far objects is not what happens today?

Different models on the universe are based on data obtained from far celestial objects, e.g. galaxies. But the light coming to us from them, comprises information on their state millions or billions ...
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1answer
35 views

isotropy of 3-space and spacetime metric

The most general spacetime metric is given by $$ds^2=g_{\mu\nu}dx^\mu dx^\nu=c^2dt^2+g_{0i}dtdx^i-g_{ij}dx^i dx^j$$ Why does the second term said to violate isotropy of 3-space? It is true that, ...
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3answers
98 views

Average temperature of the universe

Is it possible to define some average temperature of the universe? If yes, what fixes this temperature and how t estimate today's temperature? Is it different from the temperature of the black-body ...