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
179 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 ...
5
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1answer
78 views

What fraction of baryonic matter is in stars?

We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of ...
5
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1answer
568 views

Additional merits to Wetterich's “Universe without expansion” compared to standard cosmological redshift interpretation?

A recent news item in Nature promotes Wetterich's preprint "A Universe without expansion". All sounds very exciting but hard to judge for non-experts. As I understand from the Nature's article, the ...
5
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2answers
669 views

Big bang and time

I heard Carl Sagan talking about the Universe 15 Billion years ago, and the Big Bang. He made the statement that it was the biggest explosion of all time (at first I thought this a subtle pun). This ...
4
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3answers
422 views

Do photons lose energy due to gravitational redshift? If so, where does the lost energy go? [duplicate]

In the gravitational redshift, the frequency of photons radiated from some source is reduced. As the energy of a photon is given by $\hbar\omega$, if the frequency is reduced where is the lost energy? ...
4
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2answers
174 views

How can a black hole zap a galaxy into existence?

I am referring to this picture published here. Apparently super massive black holes emit radiation and matter in astrophysical jets. And these jets can form galaxies. I have some questions: Isn't ...
3
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3answers
145 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 ...
3
votes
1answer
208 views

Difference between horizon and flatness problems & how inflation solves flatness (w/out math)

Layman here so I'm hoping for an answer for my query that doesn't involve math. I'm reading about inflation and how it solves the flatness and horizon problems. I get that the horizon problem deals ...
3
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1answer
308 views

Fulvio Melia's linear Universe

I would be interested in what people think of Fulvio Melia's argument for a linearly expanding Universe. I realize that the experimental evidence seems to be pointing to an accelerating Universe ...
3
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0answers
109 views

At what expansion rate (H) would virtual particles be ripped apart into real particles, and what might be the density (temperature) of such an event?

Is it even possible for a virtual particle-antiparticle pair to be torn apart by cosmological expansion? Virtual particle-antiparticle pairs are ripped apart near black hole event horizons, creating ...
3
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3answers
227 views

What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe's expansion?

Brian Greene in this TED talk about possible multiverse, claims tomwards the end (At around 18:00 mark) this statement. 'Because the expansion is speeding up, in the very far future, those galaxies ...
3
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1answer
163 views

extracting energy from cosmological expansion

This question is a more concrete reincarnation of an old question about energy conservation in GR. Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
3
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1answer
166 views

The range of light

It occurs to me that the empirical evidence shows that there is a point out in space where light stops coming from. Putting aside the expansion of the universe for a second, and focusing strictly on ...
2
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2answers
2k views

Differences between astronomy, astrophysics and cosmology? [closed]

What is the main difference between Astronomy, Astrophysics, and Cosmology? I have the impression that astronomy is a subject that runs parallel to physics but it is outside the physics field. This ...
2
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0answers
89 views

Does cosmological time expand like space does?

The FRW metric is given by: $$ds^2=-dt^2+a^2(t)\ dr^2$$ where $ds$ is an interval of proper length, $dt$ is an interval of cosmic time, $dr$ is an interval of co-moving co-ordinate distance and $a(t)$ ...
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1answer
135 views

Calculating Hubble's constant at earlier times [duplicate]

I want to calculate Hubble's constant at some redshift $z$. I have found the following formula: $$H^2=H_0^2\left(\Omega_m\left(1+z\right)^3+\Omega_{\Lambda}\right)$$ Now it's obvious that at higher ...
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1answer
1k views

Is it possible that the Big Bang was caused by virtual particle creation?

As far as I understand, it is understood that throughout the universe there exists, what is known as, a quantum field from which, due to its fluctuations, temporary (pairs of) virtual particles ...
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2answers
329 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
1k views

How is it possible the universe expanded faster than the speed of light during inflation?

In a documentary written in collaboration with Stephen Hawking, the narrator (supposedly Stephen Hawkings) says that by the time the cosmos was 10 minutes old, it had already expanded thousands of ...
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3answers
135 views

What's dark matter and who discovered it?

I have heard about dark matter that's called the Master Of The Universe. What's this and is the dark matter the reason galaxies exist?
8
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3answers
390 views

Does MOND make good predictions?

Well, it does according to this preprint for certain scales. What would be a simple way to explain MOND to a layman? Does it ignore mainstream physics? How much?
7
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2answers
549 views

What experiments compete with BICEP 2, and when are their results expected?

The recent results of the BICEP 2 experiment published on March 17th 2014, has generated a lot of media attention, with the general consensus being that "this is a major discovery" perhaps leading to ...
6
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3answers
738 views

Baryon asymmetry

Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of antimatter or observe gamma rays that would be ...
5
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3answers
274 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 ...
5
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3answers
506 views

Could the acceleration of universe expansion be caused by gravity itself?

Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...
4
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1answer
165 views

How does the Cosmic Microwave Background give us information about the Big Bang?

I was reading about CMB after this new breakthrough last week and I could not figure this out. The CMB did not exist before the epoch of Last Scattering. They were just photons which were formed at ...
4
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1answer
211 views

What happens at the interface between two universes with opposite thermodynamic arrows of time? [closed]

I was trying to think but cannot figure it out. For instance, if the interaction is small, for instance limited to a windows, the observers in each universe will see that the other goes in reverse. ...
3
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1answer
53 views

Spacial curvature and expanding space

If we take the analogy that in an empty space the space is just a flat sheet then if there is a single planet or a star then the flat sheet will curve below the planet leaving a curvature shaped like ...
3
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3answers
288 views

How do we know for certain that space is expanding?

How do we know for certain that space is expanding? Let's say that in the year 1950, we observe that galaxy 1 is 5 billion light years away from us and galaxy 2 is 10 billion light years away from ...
3
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2answers
117 views

Cosmic radiation cutoff at LOW energies?

The energy spectrum of the cosmic radiation (not CMB) is limited to both sides. I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
2
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2answers
279 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 ...
2
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4answers
221 views

Expansion of the universe and strain

From cosmological models that involve expansion of the universe, can we not say that there are ever increasing tidal forces felt by solid bodies? If so, the material in solid bodies like metal ...
2
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1answer
368 views

How is it possible that we see light from shortly after the big bang?

How can astronomers see light from shortly after the big bang? How did we get "here" before the light that emanated from our "creation"?
2
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0answers
89 views

Topics in particle cosmology [closed]

I am interested in learning more about this interdisciplinary approach. 1) What are some of the top questions in particle cosmology (e.g nature of dark matter, inflationary structure, topological ...
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1answer
108 views

Are we comoving observers of space expansion?

In cosmology: A comoving observer is the only observer that will perceive the universe, including the cosmic microwave background radiation, to be isotropic. (Wikipedia) According to this ...
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1answer
146 views

Redshift and conformal time

What is the relationship between redshift and conformal time ? For example in a paper i found: taking $z_e = 3234 $ at the time of radiaton-matter equality yields the conformal time ...
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3answers
207 views

Is the universe flat?

There are more than one way to view the description of the universe as flat. There is the description of an open, flat or closed universe in terms of it's fate, expansion forever away from gravity, or ...
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2answers
172 views

Expanding universe and the peculiar velocity

Hubble's law states that the universe is expanding with a velocity equals Hubble's constant*distance from earth. But, recent findings show that the Andromeda galaxy is actually blueshifting towards us ...
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3answers
128 views

If everything in existence were increasing in size at some rate, would we be able to detect it?

Would our eyes observe any changes? What about electronic measurement devices?
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0answers
163 views

Why is “dark matter” a more parsimonious explanation than “more normal matter than we thought”? [duplicate]

Possible Duplicate: Why isn't dark matter just matter ? The evidence usually presented in support of dark matter is larger-than-expected masses in galaxy clusters: the mass required for ...
0
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2answers
310 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 ...
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2answers
151 views

Size of Universe after inflation

I read in some website that during the period of inflation, the expansion of the universe underwent incredibly fast, and its size increased by a factor of $10 ^{50}$, see this link In this field, I ...
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0answers
91 views

Calculating the age of the Universe now and 30 billion years from now [duplicate]

I am confused about the age of the Universe. If you calculate it by $1 / H_0$, won't the answer be roughly the same today as it will be 30 billion years from now? I know $H_0$ is a parameter, not a ...
0
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3answers
268 views

Cosmological redshift interpretation

Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a ...
0
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1answer
511 views

Does the Big Bang need a cause? [duplicate]

Possible Duplicate: on causality and The Big Bang Theory Asking here in layman's terms.. When theoretical physicsists discuss the origin of our Universe, the wider consensus appears to be ...
11
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2answers
417 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 ...
13
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5answers
2k views

Does the amount of gravitational potential energy in the universe increase as it expands?

It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
10
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10answers
2k views

What is the most efficient way to destroy the universe? [closed]

Don't worry... I won't actually do it, just would like to know how.
27
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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 ...
13
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2answers
1k views

What exactly is meant by the “Gaussianity” of CMBR?

What does it mean when we say that the CMBR is mostly gaussian? What are non-gaussianities in CMBR? How does evaluation of 3-point correlation functions of the inflaton field tells us that there is ...