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.

learn more… | top users | synonyms (1)

1
vote
0answers
74 views

World's Largest Radio Telesope : How accurately does it need to be figured?

In Guizhou Province, China, the worlds largest radio telescope is almost complete, measuring 500 metres across. I am aware that todays largest optical telescope require very accurate figuring and ...
2
votes
1answer
119 views

How much noise is in the Cosmic Background Radiation, especially from Cosmic Rays

Do we have an estimate of how much noise, if any, say caused by cosmic rays in particular, is present in the CMB datasets and the maps based upon them? Can we extrapolate a figure from the cosmic ray ...
5
votes
2answers
71 views

On the expansion of space on small distances

After reading this post :Why does space expansion not expand matter? and after posting some comments to Peter Diehr, I was invited to make my argument to a question. The main things one needs from the ...
4
votes
1answer
52 views

Which 'annihilation' processes dump energy to the cosmological plasma?

I have been considering a generalisation of the cosmological process involved in computing the temperature of the cosmic neutrino background. It is well-known that (simplistically) this temperature $...
0
votes
0answers
9 views

What is the estimator that used to de-rotation of CMB polarization? [duplicate]

For de-rotation of CMB polarization we need to use of estimator for rotation angle. and used many estimator for this process.
0
votes
0answers
32 views

Where is the origin of cosmic expansion? [duplicate]

Perhaps, a rudimentary or dumb question, but nevertheless am curious. There is the idea that space is expanding at an exponential rate. From where? If everywhere, why doesn't this rapid creation of ...
2
votes
0answers
27 views

How many times does baryogenesis happen during the history of the universe?

Does the baryogenesis happen both in GUT epoch and electroweak Epoch during the history of the universe? If yes, are there any differences during this two times?
5
votes
1answer
186 views

Will the CMB rest frame always coincide with co-moving coordinates?

I understand that the CMB rest frame for a typical FLRW universe should coincide with co-moving coordinates, but under what conditions won't the two coincide? For example if the universe had some net ...
2
votes
1answer
80 views

Is a nearby parallel universe still in use as an explanation for dark matter?

Is the theory which explains dark matter as the effect of a nearby parallel universe still used as an explanation for dark matter, or does this theory already lie on the pile of forgotten theories?
3
votes
1answer
70 views

Hubble's constant measured by observer approaching light speed

I recently read about expansion of the universe and I can't figure out if Hubble's law (and other models of expansion of the universe) is compatible with the theory of relativity. My question is: In ...
22
votes
4answers
3k views

How did the universe shift from “dark matter dominated” to “dark energy dominated”?

In order to get dark energy to dominate, wouldn't you first need another form of energy to push the expansion until dark energy could dominate? Otherwise I don't understand how the universe could ...
0
votes
1answer
50 views

In an infinite universe, would there be an infinite amount of matter as well?

Or would you eventually reach a point past which there is no matter? How would an infinite amount of matter work? Would it sort of "loop back" to the beginning, or would it be entirely unique the ...
5
votes
1answer
119 views

Gravitational wave and string theory

I'm new to physics and have been reading about fundamental and textbook physics text, which is the Young & Freedman University Physics (good book). I'm little skeptical towards string theory as ...
3
votes
1answer
59 views

Friedmann equation in a matter dominated universe? [closed]

Prove that the Friedmann equation in a matter dominated universe is: $$\dot{R}^2=\frac{8\pi G\rho}{3}R^2-k$$ Where $R$ is a scale factor, $\rho$ is the matter density and $k$ is a positive constant. ...
2
votes
0answers
28 views

How to understand that Inflation generate almost scale-invariant perturbations?

Since during the inflation, the Hubble rate is approximately constant, the perturbations generated by inflation are almost scale-invariant. How to understand that?
6
votes
1answer
297 views

What is meant by the term “value” of a scalar quantum field?

During the slow roll of a scalar field, the scalar field is changing its value over time. But what is meant by the term "value" of a scalar field? Since the scalar field is quantized, I don't ...
1
vote
1answer
42 views

Slow-Roll: What is the energy of a scalar field transformed to due to Hubble friction?

So, I've lately been reading about the Slow-Roll approximation of scalar fields. However - since I don't really know that much about cosmology - I have some trouble regarding the understanding of the ...
1
vote
0answers
29 views

Going to the Einstein frame in f(R) theories

First of all thank you for your time! I have a question that I can't solve. In every review that I read, I find that when you want to go to the Einstein frame in a $f(R)$ theory what you have to do ...
7
votes
3answers
150 views

Considering dark energy and other factors, what is the most distant object light could reach?

If we send a photon from Earth at this moment, what is the most distant object that it could reach? This question is partly inspired by this video which states we cannot ever travel beyond our local ...
1
vote
1answer
48 views

Baryon / lepton number violation equivalence in Sakharov conditions

In the Sakharov conditions for matter-antimatter asymmetry the requirement for the baryon number violation is often interchanged with lepton number violation. In the context of the Sakharov conditions,...
0
votes
0answers
17 views

About the use of Newtonian Relations for the movement of stars in the Galaxy [duplicate]

From a General Relativity point of view Gravity is given as the result of spacetime curvature interacting with energy-mass density. To get to the Newtonian limit one needs to take a) Non-...
0
votes
0answers
40 views

Can we see the past view of our galaxy while observing in the sky because of the homogeneous structure of our universe?

As a sense, we think that simple speed addition should work ($V_1+V_2$) for all references but we know today that universe does not match our this sense. We cannot exceed the speed of light in any ...
2
votes
1answer
53 views

Why does the flatness problem (of the universe) present a fine tuning problem?

The Friedmann equation can be written $$|\Omega-1|=\frac{|k|}{a^2H^2}$$ In a period where the universe is decelerated $a^2H^2$ becomes bigger as we extrapolate into the past. We know $|\Omega-1|$ ...
1
vote
1answer
69 views

How inflation solves the horizon problem

This is a little embarrassing but I don't understand how the inflation model solves the horizon problem. My doubt is the following: What does the occurrence of an exponential expansion of the early ...
0
votes
1answer
68 views

Poincare recurrence time of the Universe

I've read around a bit, and it seems to be universal that the notion of a Poincare recurrence time for the universe exists. And it seems to be debated that the universe can be given an entropy, as it ...
1
vote
1answer
79 views

At a center of Gödel's universe

A few quick questions clarifying a picture about Gödel's universe, they bug me badly! Taken from here. So Gödel's universe is made out of dust particles. All of them have angular velocity. Do this ...
3
votes
0answers
34 views

Basic question on Cosmic Strings

What is the typical size of Cosmic String Loops? If a loop is formed today, what are the restrictions on its size? I would like to know if there is a distribution of cosmic strings depending on its ...
4
votes
1answer
66 views

How are the particles produced at the very early universe?

We know that according the standard model of cosmology $\Lambda {\rm CDM}$, there is a primordial plasma after the inflation. And then we study the non-equilibrium evolution of this plasma such as ...
4
votes
1answer
68 views

Hubble time and age of the universe

I'm having trouble with the following derivation of the 'age of the universe': http://imgur.com/gRvLWX8 The parts I'm struggling to conceptualize is what a 'universe expanding' means, and also why ...
-1
votes
2answers
74 views

Universe Expanding

Quick question that I was just hoping to get some clarification on. Current theories state that the universe is expanding; with those planets farther away moving away from us at a faster rate. Is it ...
0
votes
0answers
29 views

Space expanding within atoms [duplicate]

I'm hoping that "all questions are good questions." I'm a layman enthusiastically trying to comprehend cosmology and physics, and the more I learn the more questions I have. My understanding is that ...
6
votes
1answer
83 views

Intuition about the cosmological significance of the chemical potential

A definition of the chemical potential that has always served me well is $$\mu_i = \Big(\frac{\partial U}{\partial N_i}\Big)_{S,V},$$ that is, the amount of energy one would have to add to a system ...
0
votes
0answers
20 views

The density parameter consistency relations in cosmology?

Hopefully this is an easy one. I've been asked to derive something using something referred to as the density parameter consistency relations. I'm not to sure what these are referring to exactly? ...
-1
votes
1answer
53 views

What is the difference and/or similarity between multiple universes in inflation and in quantumphysics? [duplicate]

Some theories propose that there are more universes. In one of them it is said that there are parallel universes, due to inflations after the big bang that created them. But in the two-slit ...
-2
votes
2answers
89 views

Entropy always increases in a closed system - what if the universe is open?

An interesting question I was asked: Entropy always increases in a closed system - what if the universe is open? Does that mean that entropy can decrease in such a system? Of course, I think there is ...
1
vote
1answer
17 views

Baryon Acoustic oscillations - simple confusion about length scales

I am confused about length scales and baryon acoustic oscillations. I am clearly missing something very simple. The length scale found in the $SDSS$ and $2dF$ data is quoted as being $150Mpc$. ...
2
votes
1answer
35 views

Measuring the growth of the Hubble sphere

I have a question which I believe will greatly help my understanding of the Hubble sphere and what it means physically. Say we have a flat ($k=0$) Universe with no dark energy ($\Lambda=0$) during ...
5
votes
2answers
156 views

How does the inclusion of vacuum energy modify Newtonian gravity equation of motion?

The equation of motion (in the center of mass frame) due only to gravitational forces between two point masses is: $$\frac{d^2r}{dt^2} = -\frac{GM}{r^2}$$ How does the equation get modified when a ...
1
vote
0answers
53 views

The reason for the fading of galaxies?

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years the future ...
1
vote
0answers
37 views

Are there characteristics that distinguish our universe?

Are there any defining characteristics of our universe that could be different for other universes (or "instances of our universe") that operate under the same laws of physics? And which, if any, are ...
2
votes
0answers
36 views

Any method to determine the gravitational constant $G$ from observations in cosmological scale?

By the observations in cosmological scale, I mean observations by astrophysicists like motions of quasars in far distance from us, relative motions of our galaxy to others, etc. I know the famous ...
0
votes
0answers
37 views

Gravitational entropy [duplicate]

In the beginning (big bang), mass was evenly distributed (if you consider an elementary particle as an extended object, like a circle on a very thin cylinder, with a radius near the Planck length, but ...
6
votes
3answers
122 views

Two Robertson-Walker observers, at what time will a light signal be received?

Here is a question I have that is inspired by this question here. The spacetime metric of a radiation-filled, spatially flat ($k = 0$) Robertson-Walker universe is given by$$ds^2 = - dT^2 + T[dx^2 + ...
2
votes
1answer
64 views

Cosmic Rest Frame

Cosmic Rest Frame is defined in the paper "Adventures in Friedmann cosmology :A detailed expansion of cosmological Friedmann equations" by R Nemiroff and B Patla as the frame at a point where average ...
1
vote
0answers
38 views

Is it possible for dark energy density be decreasing? [closed]

I have an equation relating dark energy density and temperature, actually they have a linear relationship, my question is how to choose the temperature? If I choose the temperature of universe, the ...
4
votes
1answer
256 views

What is the difference between the Big Bang Model and the Λ-CDM Model?

If I'm going to write about "The Consensus Model of Cosmology" should I include Big Bang Model or should I go with just the $\Lambda$-CDM Model?
1
vote
0answers
47 views

Could dark energy and Hawking radiation be the same thing? [closed]

Specifically, could black holes effectively emit some kind of negative mass or other form of dark energy, so that the mass of the black hole itself would actually increase rather than decrease (...
0
votes
2answers
65 views

Cosmic Scale Factor $R$ and redshift

I'm slightly confused by an idea of redshift: If we assume the distance between two points is given by: $$d = R Δx$$ we can assume that $$λ_0 = R_0 Δx$$ And when we receive the emission it will ...
2
votes
1answer
62 views

Cosmic inflation and space flatness

I am new here, an avid Physics reader, but have lots of questions and doubts on which I'd like to get the help of a pro physicist. Here goes some about the theory of Inflation: a) after the first ...