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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2answers
84 views

Where did the universe get its initial momentum?

If, according to Newton's third law, forces come in pairs then what about the big bang? where did the universe get that initial push/momentum?
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1answer
67 views

Is it reasonable to visualise an additional dimension of time as a part of 4th dimensional spacetime

[EDIT] In the spirit of asking a "good question", this question is considerably more refined that the one I decided to blurt out earlier thanks to a little additional research and nudges from ...
2
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2answers
122 views

Time-like Killing vector in FRW metric?

The spatially flat FRW metric in cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it there are Killing vectors in the $x$, $y$, $z$ directions implying ...
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0answers
34 views

Infinite Redshift [closed]

My text says that the signals emitted at a physical size of horizon $l_{H}(t)$ come to an observer at time $t$ with an infinite redshift. How do I show this mathematically? My approach is as follows: ...
5
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2answers
186 views

Proving one field equation leads to the other [closed]

Assume that the universe is homogenous and isotropic, and the following equation holds: \begin{equation}R_{00}-\frac{1}{2}g_{00}R=8\pi GT_{00}; \space \space \nabla_{\mu}T^{\mu 0}=0.\end{equation} ...
2
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1answer
27 views

What is the current rate of change of the volume of the Universe?

Is it possible to know the current rate of change of the volume of the Universe? If we can't see past the cosmic horizon, can we still get a value for this rate of change, and if so, how would it be ...
9
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3answers
208 views

Is the edge of our Hubble Sphere within our Cosmic Event Horizon and why?

I was recently shown a pretty cool video about common cosmological misconceptions. It got me reviewing the different between event horizon (current distance within which we will see/interact), ...
4
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2answers
84 views

Can the cosmological constant change with time?

This post is a specialization of the post: Can the proportion of dark energy change? Can the cosmological constant change with time? If so, is there a measurement of this evolution up to now, ...
2
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1answer
46 views

Can the proportion of dark energy change?

This post generalizes the posts: - Proportion of dark matter/energy to other matters/energy at the beginning of the universe? - How do people calculate proportions of dark matter, dark energy ...
7
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1answer
118 views

What is the concept of cosmic strings?

What is the concept of cosmic strings? Is it related to the strings in the string theory, and if it is, then how?
6
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1answer
76 views

What is the cosmological footprint of humanity?

What is the the cosmological footprint of humanity? By this I mean, how might a cosmological observer detect humanity or its products on earth? How far away can humanity or it's products (not mere ...
1
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1answer
13 views

Non-zero net charge on cosmological scale

It seems very clear, that because atoms and ion compounds are electrically neutral, our world seems to consist of an equal amount of positive and negative charge resulting in a zero net charge. What ...
2
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1answer
380 views

Surely proper time expands like proper space?

The FRW metric at the origin $r=0$ is simply given by: $$ds^2 = -dt^2 + a^2(t)\ dr^2$$ Setting $dt=0$ gives us an element of proper distance $ds$ given by: $$ds = a(t)\ dr$$ Thus we get the well known ...
8
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7answers
1k views

Mathematically possible vs physically probable outcomes

A good buddy of mine and I have had a friendly debate about the origins of the current state of our universe (namely; Earth and life on Earth) and have fundamentally disagreed in our stances with ...
1
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2answers
43 views

Redshift 1+z - CMB Temperature lower?

I know that $\frac{\lambda_2}{\lambda_1} = 1 + z$ Suppose a galaxy had redshfit $z=3$. Does this mean that the wavelength becomes $4\lambda$? Then by wien's law where $\lambda \propto \frac{1}{T}$, ...
1
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1answer
83 views

Reconciling “The Big Crunch” with the 2nd Law of Thermodynamics

Assume "The Big Crunch" scenario (the universe will collapse to a singularity). In this case, I think of the entire universe as an isolated system; in the "Big Crunch" scenario, it seems to me gravity ...
3
votes
3answers
102 views

If we say the universe is expanding, shouldn't it be expanding relative to something?

I don't understand, if everything in this world is relative to something else, then cannot we essentially say that nothing exists independently? We say that the universe is considered to be the ...
4
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1answer
127 views

Non-stationary spacetime

What is an example for a spacetime that is non-stationary that is considered as a description of something in nature? So far all the spacetimes I encounted have always been stationary ...
0
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1answer
95 views

Aside from Noether's theorem, what other concepts would explain energy conservation?

Energy is defined more in the mathematical sense, and tends to show true with observations in the physical world. But why is energy conserved aside from "Noether's theorem"? In a closed system that ...
0
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0answers
26 views

Question about Hubble's Law - expansion vs receding velocity [duplicate]

The distance to the galaxy NGC3198 is found to be $15.9 MPc$ and recession velocity is $680km s^{-1}$. What value of Hubble Constant is implied? If $H_0$ is in fact $72 km s^{-1} Mpc^{-1}$, what ...
2
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1answer
23 views

The Helium mass fraction from the Big Bang Nucleosynthesis

In Perkin's book Particle Astrophysics (page 144): I do not understand how one comes to the following expression (the second equality with $r$) for the Helium mass fraction due to the Big Bang ...
5
votes
2answers
93 views

Abundances of the light element of the Big Bang Nucleosynthesis

This question is related to the Big Bang Nucleosynthesis of light elements, more concretely I do not understand some features of the graph Why do the $^3$He and D abundances go down with ...
2
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2answers
81 views

Entropy was created after inflation?

I'm puzzeled by a statement in Big Bang Cosmology-review about the reheating phase subsequent to the exponential expansion during inflation: In this reheating process, entropy has been created ...
7
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1answer
71 views

Homogeneity of space

How do I verify homogeneity and isotropy of space, for example for hyperbolic space? My idea is to verify that Lorentz transformations in 4-vectors can move any desired point on the hyperboloid into ...
11
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4answers
525 views

With redshift, energy is lost. Where does it go? [duplicate]

A photon emitted by a distant source billions of light years away arrives here with a lower frequency hence less energy than it started with. What happened to the energy?
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1answer
59 views

Origin of Space-time fabric

I have seen on this site some questions regarding the nature of gravitational-force, and the ways in which it could be interpreted. Most of them mention that the space-time fabric came into existence ...
2
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1answer
53 views

What is intrinsic gravitational entropy?

What is intrinsic gravitational entropy? Does it have to do with dark matter or coarse graining in the universe? Is it unique to general relativity, or there are predictions from quantum mechanics as ...
2
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3answers
143 views

How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
4
votes
2answers
64 views

What is the error in the measured value of the Hubble Constant?

I've found on the internet that the Hubble constant has been measured to be about $2.3\times 10^{-18} \, \mathrm{s}^{-1}$ Does anyone know what the current error bounds are on this value?
4
votes
5answers
193 views

Is the universe expanding at a speed of almost $2c$?

I've been told nothing can travel faster than the speed of light. Therefore, from my vantage point the diameter of the universe is increasing at a rate of $2c$. Are there any flaws in my thinking?
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0answers
55 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 $a(t)$ ...
1
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1answer
31 views

Factors in predictions for Tolman's Surface Brightness Test?

http://en.wikipedia.org/wiki/Tolman_surface_brightness_test It says: In a simple (static and flat) universe, the light received from an object drops inversely with the square of its distance, but ...
3
votes
1answer
129 views

Time slowing down vs. universe expanding

Einstein said that it is impossible to distinguish between the effect of gravity and acceleration (so if you stand in an accelerating elevator in space it would not feel any different than if you were ...
9
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1answer
145 views

What does QM say about the past rather than the future?

In QM, the wave function (in the Copenhagen interpretation) is not an actual physical wave but a device to derive probabilities about the outcomes of experiments. The wave function encodes all the ...
4
votes
1answer
209 views

Why does the Hubble parameter keep getting smaller if the expansion of the universe accelerates?

Suppose a region of space at a distance D from Earth is escaping from us with the velocity v. Since it seems like the expansion of the universe is accelerating, things at D from Earth should be ...
7
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3answers
596 views

Dark Energy / Accelerating universe: naive question

Folks, I have a naive question regarding the subject of dark energy and an accelerating universe: From what I understand/read, it seems that the further we look out into deep space, the faster the ...
2
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0answers
17 views

Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? ...
4
votes
1answer
80 views

Infinite number of galaxies?

I understand that the current estimate for the number of galaxies in the observable universe is about 100-200 billion. Is there anything in our understanding of physics and the evolution of the ...
2
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4answers
94 views

Is it possible that galaxies' redshift is caused by something else than the expansion of space?

I was thinking that maybe photons loss energy naturally when they travel great distances. Or maybe the mass of all matter is increasing over time and therefore photons emitted in the past are ...
6
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2answers
73 views

What is chaotic about Chaotic Inflation?

Chaos is defined as an aperiodic long-termed behavior, that is very sensitive to initial conditions. Now from this definition I can only conclude that the adjective 'chaos' is a mere analogy, since ...
5
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1answer
107 views

Hubble time and its derivation? [duplicate]

I know the derivation of Hubble time goes something like this (I am an a-level student so this may not be the actual derivation): Two galaxy that is moving away from each other at speed v are now D ...
5
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1answer
47 views

The initial conditions of the CMB spectrum

The CMB spectrum shows the intensity of fluctuation at a certain angular scale: The achievement is the correspondence between the predicted power spectrum and the observed one. My question is as ...
1
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0answers
40 views

Hawking radiation at the mouth of Schwarzschild wormholes

I was researching a question for another post and it occurred to me that you might expect to see Hawking radiation at the mouth of wormholes. Given the mechanism of Hawking radiation at the event ...
27
votes
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 ...
6
votes
1answer
227 views

Frequency of cosmic microwave background

Some people refer to cosmic microwave background's (CMB) frame of reference as an absolute one. If I understand correctly, we can measure 'absolute' velocity in this frame by using the Doppler effect. ...
1
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0answers
29 views

Dimension of galaxy filaments

Galaxy filaments are amongst the largest known cosmic structures in the universe. They [...] form the boundaries between large voids in the universe. As their name suggests filaments are ...
3
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2answers
85 views

Smolin on Cosmological selection and neutron stars

Regarding the cosmological selection hypothesis and testable predictions, Lee Smolin asserted the following: "Smolin: I did make two predictions which were eminently checkable by astrophysical ...
0
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1answer
62 views

Is the universe expanding faster than we could catch up with it? [closed]

Like the title says, could we ever catch up with the expanding universe?
-1
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1answer
20 views

Cosmic Microwave Background and heat flow

Can the fact that energy is distributed homogeneously in the universe be explained through heat flow and not the Cosmic Microwave Background(or in other words, can we say CMB is a result of heat flow ...
5
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1answer
101 views

Differentiating the gravitational redshift and the cosmological redshift?

If general relativity accounts for a redshift, independent of inflation, how can we still know that inflation is viable? Moreover, how do we differentiate the the gravitational redshift and the ...