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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Do gamma ray bursts play a role in cosmic evolution?

If gamma ray bursts were to interact with gas clouds in the early universe, might it be a relevant factor in star production?
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45 views

Does Only Flat universe have zero energy? [on hold]

Hello I am very new to cosmology and quantum physics. I need some basic understanding (in LAYMANs term )of the following: I read that only closed universe has zero energy. But I heard Astrophysicist ...
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50 views

What $f(R)$ models pass most of the known constraints?

In most papers and talks about $f(R)$ gravity authors repeatedly state that the model proposed by Starobinsky 2007 $$ f(R)=R+\lambda\,R_{0} \bigg[\bigg(1+\frac{R^{2}}{R_{0}^{2}}\bigg)^{-n}-1\bigg] ...
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3answers
88 views

Can a Dyson Sphere around a Black Hole be built so that it would not radiate significant IR?

This is related to this question If the sphere surrounded a BH and used it as a heat dump (as well as extracting energy from it by dropping in mass) could its exterior be engineered to match the ...
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1answer
36 views

Current constraints on lightest neutrino mass?

This paper from 2005 claims that the mass of the lightest neutrino is unconstrained. (see p9) Oscillations are only able to constrain the differences in squares as far as I know, but perhaps ...
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42 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, ...
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1answer
119 views

Speed of light and current dimensions of the universe [duplicate]

I've seen several documentaries explaining that the diameter of the universe is currently estimated at over 90 billion light-years. And which that - in the face of the age of the universe being about ...
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1answer
387 views

Creation of a miniature universe?

I am not referring to the experiment being conducted at Lancaster University involving liquid helium and a magnetic field to build a marble sized representation of the early cosmos. My question is: ...
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2answers
74 views

Special and general relativity and space time

I have a question. I'm reading The Elegant Universe and it's talking about the special and general theory of relativity. One of the things it mentions is that time and the three dimensions of space ...
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Correlation length during phase transitions in early Universe

During phase transitions of the second kind topological defects may form on the bounds of two areas separated by correlation length. In early Universe during phase transitions correlation length ...
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1answer
29 views

How was hydrogen gas (H) obtained by spectroscopists? Why is there more H than H2 in space?

Introductory quantum mechanics lessons talk about emission and absorption spectra for the hydrogen gas, and then give you an explanation as if this gas were pure $H$ atoms, and not the $H_2$ molecule ...
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33 views

Is the scale factor Lorentz invariant?

Given that the Minkowski metric does not change under a Lorentz transformation, the scale factor does not change in the special case when it is equal to 1. Is this result true in general? i.e. is the ...
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2answers
71 views

Where does the energy go when light is redshifted? [duplicate]

Imagine a galaxy millions of lightyears away and, obeying Hubble's law, moving very quickly away from us. Now imagine the same galaxy emitted a green photon in our direction (a photon with a ...
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28 views

Is there any evidence from observational cosmology to say Dark Energy dominated era begins 5 billion years ago

People say that Dark Energy Dominated era begin 5 billion years ago. Do we have evidence for that from observations
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117 views

Can anyone explain the unit for rate of expansion of universe?

If you google for 'what is rate of expansion of universe' you get Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 ...
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1answer
54 views

Where would dark matter be produced?

There are a zoo of dark matter candidates. Are there any candidates which could be produced in extreme conditions such as black holes/active galactic nuclei/pulsars? After reading an article on WIMPs ...
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A start point to learn cosmology [duplicate]

My major is not physics in university. So, My knowledge about physics is limited to the general physics I , II courses which we had to pass. On the other hand I heartfully like to learn cosmology. I ...
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3answers
119 views

Does general relativity entail singularities if there's a positive cosmological constant?

I've heard that Hawking and Penrose proved that general relativity entails singularities. But it says in the abstract of what seems to be the paper in which they proved it (The Singularities of ...
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1answer
64 views

Stephen Hawking's theory of multiple universes [closed]

Is it correct to think that there are multiple universes? Does my thinking of having more than one big bang?
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82 views

Master's Thesis in General Relativity [closed]

just throwing a query out to the Physics community. I'm about to embark on a master's in Gravitation, Cosmology and General Relativity and was looking for possible subjects to start researching. My ...
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2answers
100 views

If the universe is expanding will gravitational attraction eventually go to zero?

Let's assume that we prove that dark matter exists (after all, only about 4 percent of the entire universal mass is atoms, and 22% dark matter, 74% dark energy (I think I got the numbers right)). ...
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194 views

Origin of Universe [duplicate]

The universe is believed to have originated from absolutely nothing, and we know it is still expanding. Physics says "something can arise from nothing". I understand how mass and energy are related ...
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1answer
672 views

Do quasars exist today?

Are there likely to be any quasars right now, or are all the ones we're seeing old galaxies today, like the Milky Way?
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2answers
100 views

Why is universe expanding?

Okay, this question may sound silly: base on the observation Besides an expanding universe, would there be other possibilities? Would it be possible, say, there exists a fundamental repelling ...
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38 views

Difference between Enzo & Gadget astronomy simulation codes

Enzo and Gadget are simulation codes used in astronomy. What are the largest differences between them both in terms of physics they simulate and in their implementations?
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1answer
194 views

Is there any evidence against the Cosmological Principle?

I have been slowly getting more into math, but haven't gone into differential geometry or anything like that yet, so this question might be basic. Trying to get a deeper understanding of the ...
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2answers
81 views

Cosmological constants

I've heard that the cosmological constant is 0.[123 more zeros] and then a 1 [in some units]. Does that means that it used to be exactly zero? Is the value of this constant changing or is it fixed at ...
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0answers
24 views

How redshift-space distortions affects baryonic acoustic oscillations

I have a question with BAO (baryonic acoustic oscillations) and RSD (redshift-space distortions). On large scales, Kaiser effect makes the observed radial separation of galaxies smaller then ...
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cosmology - Hubble parameter equation [duplicate]

I have a question. We know that the Hubble constant in the space is a constant, is the same everywhere, but it changes in time, so we don't talk of H0 but H(t). Is there a way, a formula to compute ...
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39 views

Cosmological Bulk Viscosity - is the universe a fluid?

I was trying to make heads of tails of both this paper and the press release about it. The scientists have used bulk viscosity (as opposed to shear viscosity) and popped it into a model and somehow ...
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1answer
59 views

Raychauduri equation for Milne universe

I am trying to solve this kind of problem: Consider the Milne model, i.e., the empty $ \kappa = −1 $ Friedmann model. Verify by a direct calculation that the expansion $ \Theta $ of the unit ...
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30 views

Global cosmic strings evolution

Recently I've read about axion string. It can be shown that the energy per unit length of the string located along $z$ axis is $$ \mu = 2 \pi f_{a}^{2}\ln\left( \frac{L}{\delta}\right), $$ where $L$ ...
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50 views

What fills the space between the galaxies, stars and planets? [duplicate]

What fills the space between the galaxies, stars and planets with the exception of gas clouds, dust, atoms, elementary particles and electromagnetic radiation? What the authoritative opinions exist on ...
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4answers
111 views

Can a photon have little to no energy and/or speed?

Can a photon move more slowly than the speed of light and behave 'non-relativistically,' so to speak. Perhaps another way to express my thought is: could we stop a photon from moving?
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1answer
81 views

The universe ringing like a bell? What would that mean for the cosmological constant?

The claim that the rate of expansion accelerates and decelerates periodically over time is here and that it has already gone through seven such cycles, each being damped further with each oscillation. ...
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1answer
70 views

Does isotropy imply homogeneity?

This question comes from exercise 27.1 in Gravitation by Misner, Thorne and Wheeler. They required the following: Use elementary thought experiments to show that isotropy of the universe implies ...
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1answer
48 views

Single field vs single clock inflation?

My question is very simple and short, but I couldn't find any explicit answer in papers and/or lecture notes. What is the difference between single field and single clock inflation? Or, maybe, what ...
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40 views

Do The Diameters of Distant Objects Seem to Get Larger?

So I'm curious about the dynamics of an expanding universe, My question may stem from misunderstanding so i'll outline what I know. When we view distant objects in the observable universe we are not ...
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0answers
29 views

Best evidence for Big Bang? [duplicate]

Why is the existence of cosmic microwave background regarded as the best evidence of Big Bang (or Big Bang model?)?
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33 views

Significance of light travel time in cosmology

When in astronomical news they say something like "It took $T$ billion years for the light from that galaxy to reach the Earth" or "The light traveled $L$ light years" what is the physical ...
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1answer
28 views

Galaxy bias and baryon acoustic oscillations (BAO)

I have a doubt with the concept of galaxy bias and how it affects baryon acoustic oscillations, it is supposed to mean that by measuring the distribution of galaxies we are not measuring the actual ...
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3answers
135 views

Is the Universe being dragged into a singularity rather than expanding?

Recently I've read that the expansion of the Universe as observed is not exactly uniform as proposed by Hubble. Therefore is it possible that instead of space expanding in all directions all matter is ...
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1answer
40 views

Boltzmann equation collisional operator in thermal equilibrium

Edited after Thomas' answer http://jila.colorado.edu/~ajsh/astr5770_14/grbook.pdf#section.30.5 Question 30.6. "Detailed balance": System is in thermal equilibrium, and the physics of the system is ...
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2answers
83 views

What's the point of looking at distances beyond $13,7$ billion light years?

Question: Provided that the age of the universe is $13.7$ billion years, but the actual radius is $48$ billion ly, what are we really going to see if we built a telescope powerful enough to reach ...
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2answers
41 views

How the mass is conserved during continuous expansion?

If universe is continuously expanding doesn't it contradict the fact that mass is conserved? Because if mass is conserved so how is universe expanding?
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1answer
537 views

What is the future of gravitional lensing? [closed]

What do physicists expect to find or accomplish with gravitational lensing in the next 15 years? Is there a specific type of object, or a concept that they are looking for?
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1answer
44 views

Distance and luminosity distance

In my cosmology lecture notes I read that a way to measure distances in cosmology is to use standard candles and the comparison between "absolute luminosity" of the candle and the apparent luminosity. ...
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2answers
133 views

The Big Bang theory hypothesis

Is there a simple way to state the hypotheses of the Big Bang theory? I have the impression that the Big Bang singularity is merely a consequence of Freedman equations. Could somebody clarify what ...
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1answer
48 views

Why is the Baryon Acoustic Oscillations (BAO) calculated like this?

In the Wikipedia, it says that, when calculating the Baryon Acoustic Oscillations (sound horizon), we measure $150\text{ Mpc}$, saying that the sound horizon is the "Physical Length of sound horizon ...
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1answer
35 views

modified gravitational model

In the modified gravitational model $ f(R)=R+\lambda{R_{0}}\left(\left(1+\frac{R^{2}}{R_{0}^{2}}\right)^{-n}-1\right) $ what are the units of $\lambda$ and $R_{0}$.