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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Can we really see new objects in future? [duplicate]

From Wikipedia: Some parts of the Universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth, so these portions of the Universe lie outside the ...
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66 views

Does Expanding Universe conflict with the Thermodynamics 3rd Law? [duplicate]

I know that thermodynamics 3rd law states that if a system in equilibrium is at zero temperature, its entropy is minimum. Expansion of the universe made a transition from radiation dominated era ...
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2answers
71 views

What is the process by which the density fluctuations that were detected in the CMB evolved into the first stars?

We can detect very slight fluctuations in the temperature (mass distribution) when the CMB was released (decoupling). It is believed that these fluctuations led to the first stars. However, these ...
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201 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?
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54 views

Assuming the universe is finite, at some point, CMB should go away, right?

I'm not talking about gradual red-shifting, that happens slowly anyway, but with a universe of finite size, we should actually see Cosmic Background Radiation turn off in a sense, first at a point, ...
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70 views

Negative energy of universe

According to a little I have understood whatever we see in the universe is its postive energy and there is so an equal negative energy. Is this Negative energy the Dark energy? which is responsible ...
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45 views

The “Big-Freeze”, laws of physics and empty space

We currently have data, that suggests the universe will end up in a Big-Freeze. My questions are: Will the laws of physics break down, when the last black hole disintegrates? If so, why? And how long, ...
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1answer
107 views

Is there any paper analyzing the validity of Relativity in expanding space? [duplicate]

Big Bang models result from applying Relativity equations to expanding space; however, as Einstein noted, Relativity was not established in expanding space. A basic question arises: is Relativity ...
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1answer
56 views

Long rod in an expanding universe

Suppose you have a metal rod, of vast length, floating in space. From it's center-of-mass reference frame it is assumed to be at rest with respect to the CMB. The rod will have some degree of ...
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2answers
240 views

Cosmic event horizon changes over time

The cosmic event horizon is the comoving distance beyond which the light signal from distant galaxy can never reach us. With dark matter, the event horizon converges to a finite value so does it mean ...
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1answer
36 views

Is the age of the universe different for different people located at different points in the Universe? [duplicate]

From Earth we say that the age of the universe is 14.5 billion years ago because that is how far we are able to see. But since we are probably not the center of the universe, wouldn't some one many ...
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52 views

CMB parameter, what's the meaning of the matter power spectrum normalization sigma_8?

Most CMB experiments like WMAP and Planck include a certain cosmological parameter called $\sigma_8$. My understanding is that normalization of the matter power spectrum is not a theoretical ...
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52 views

Even the universe is infinite, it could keep expanding? [closed]

the study showed Cantor, that there are infinite types, the universe would be expanding as well as the possibility of Stephen Hawking. http://phys.org/news/2015-02-big-quantum-equation-universe.html
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75 views

How do we know the expansion of the universe is “of space” and not “in space” or “into space” or another less intuitive arrangement?

How do we know the expansion of the universe is "of space" and not "in space" or "into space" or another less intuitive arrangement? For example, what implicit and explicit assumptions underlie the ...
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3answers
137 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 ...
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1answer
188 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 ...
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1answer
93 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
66 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
80 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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64 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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29 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 ...
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1answer
144 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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29 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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56 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
95 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 ...
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1answer
276 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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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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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 ...
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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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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
187 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
119 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
100 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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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, ...
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3answers
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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
120 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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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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29 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
239 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
258 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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47 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, ...
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1answer
107 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
66 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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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
89 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 ...