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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151 views

Origin of Universe

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
560 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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1answer
74 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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1answer
31 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
149 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
73 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
19 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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0answers
22 views

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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0answers
32 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 ...
2
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1answer
51 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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0answers
36 views

Astronomy - Hubble Law and Universe age [on hold]

We know that the approximated age of the Universe is calculated as 1/H0 where H0 is expressed like (km/s/Mpc). If I try to calculate the dimensional equation I obtain: (10^3 m/(s* 10^6*10^16 m))^-1, ...
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0answers
48 views

Applicability of Occam's Razor [closed]

Is there a solid mathematical or physical argument on the applicability of Occam's Razor at cosmological scales, or with regard to physical laws? I've recently become aware of mathematical ...
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0answers
29 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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0answers
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
103 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
75 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
62 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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0answers
56 views

How can we measure the speed of light in spacetime, and the rate of universe expansion, and come up with conflicting answers? [closed]

In spacetime, there should be no way to measure anything at the speed of light or passed it. The conclusion I make from here, is that we can't observe anything moving faster than the speed of light. ...
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1answer
37 views
+50

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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0answers
38 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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0answers
29 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
27 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
129 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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0answers
53 views

End of the universe [duplicate]

Is there hope that the universe does not die? How sure do the physicist know that the universe will die? What do you think: will the universe die?
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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
77 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
38 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
533 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
41 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
129 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
41 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
33 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 $R_{0}$.
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2answers
196 views

How can the universe be flat?

Okay, so I just want to clarify a few things. According to what I have read, we have measured the universe to be flat, and the shape of the universe is directly related to the mass-energy density. ...
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1answer
42 views

General questions related to inflation of the universe

(i) We know that the universe is expanding. As it expands, something has to "fill in", even if it is vacuum. Isn't energy required for this? For creating new "dark matter"? But we can't get energy ...
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0answers
35 views

Does the total particle energy increase in FRW Universe?

If a particle travels on a geodesic with 4-momentum $P^\mu$ in a spacetime with a Killing vector $K_\mu$ then we have a constant of motion, $K$, given by: $$K=K_\mu P^\mu$$ Using the relationships: ...
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0answers
26 views

what are the relativistic/cosmological implications on a long rigid body? [duplicate]

This is my first question. Hope it's in respect of quality standards. Let's suppose to have a "long" rigid almost unbreakable bar of fixed negligible diameter . (we can imagine it as a rigid wire) ...
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0answers
36 views

Using a mask with a (2D) Fourier Transform

For a project I'm working on, I (1) take a (gaussian) random field of 256x256 pixels, (2) apply numpy's fast fourier transform (numpy.fft package), (3) filter the k-space image (using a Tukey ...
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0answers
28 views

CMB and Gibbs free energy

I was doing my homework and I have a doubt. I proved that if I have the radiation energy density $$u = \frac{U}{V} $$ and $$P = \frac{1}{3}u $$ Then the Gibbs free energy is 0. But, how can I to ...
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2answers
68 views

What do we mean by the size of the universe? [duplicate]

Isn't it true that the concept of length exists only within the universe? Are we simply measuring the observable universe, that is to say, the spatial limits of our observation?
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0answers
22 views

What's the difference between cosmic time and conformal time? [duplicate]

When they say that the age of the Universe is about 13.7 billion years, they are giving the age in cosmic time, is that right?. There's this other thing called conformal time which I don't understand. ...
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0answers
54 views

Is the Big Bang notion compatible with the flat space? (A problem with the **moment** of Big Bang, not with the place of it) [duplicate]

A passage from a paper: "If one imagines running the clock backward in time, any given region of the universe shrinks and all galaxies in it get closer and closer until they smash together in ...
2
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2answers
135 views

Why doesn't a global frame of reference exist for GR?

I only have at best a layperson's familiarity with GR, so forgive me if I am asking a basic question, but I have heard that in GR, we cannot have a global frame of reference, that is a frame of ...
3
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1answer
91 views

Could the universe's antimatter be hiding in black holes?

As far as we know almost all the mass in the universe is matter, not antimatter. There are three scenarios: The universe started off with more matter than antimatter and the small amount of ...
-6
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0answers
50 views

Does time exist in the universe? [duplicate]

Does time start with the start of our universe, or doesn't it even exist? It is either fundamental in our universe or it is just invented by us for our ease.
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0answers
46 views

Is there any explanation for these objects apparently traveling faster than light? [duplicate]

This is from a documentary on the science channel: https://www.youtube.com/watch?v=RaELad94KZs
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0answers
43 views

Meaning of $R(t)$ in FLRW metric

In the FLRW metric what is the meaning of $R(t)$ from a geometric point of view? And from a physical point of view? $$ds^2 = dt^2 - R^2(t) \left( \frac{d\bar r ^2}{1-\kappa \bar r^2} + \bar r ^2 d ...
5
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2answers
113 views

By what process did the era of inflation end, or has it just slowed down (a lot)?

Could someone please explain the current model regarding: a) how the era of inflation was turned off (the graceful exit idea or alternatives)? b) what the timeline is between the era of inflation ...
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1answer
55 views

Where is the universe expanding from? [duplicate]

It seems to be quite a common question to ask "what is the universe expanding into?" However I want to ask "where is the universe expanding from?" Is there a "centre of expansion" of our universe ...
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0answers
52 views

Implications of dark matter imprints on Cosmic Microwave Background Radiation?

Looking at this link, CMB Anisotropy, I have two questions regarding the possible creation and properties of dark matter: If dark matter has left it's imprint on the CMB, that to me, would imply ...