Questions tagged [cosmology]

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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33 views

Work and energy: law of conservation of energy? [duplicate]

According to the law of conservation of energy: Energy can neither be created nor be destroyed Then how did energy come into existence?
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1answer
136 views

Why is the flatness problem a problem?

Friedmann equation states $$ H^2 \equiv \left( \frac{\dot{a}}{a} \right) = \frac{8\pi G \rho}{3} - \frac{k}{a^2} $$ which describes the evolution of the expansion of the universe. Writing $\Omega(t)...
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A question about conformal time

I have a function to calculate the Hubble parameter at a given redshift: $$H(z)=\sqrt{\Omega_R(1+z)^4+\Omega_m(1+z)^3+\Omega_k(1+z)^2+\Omega_{\Lambda}}$$ And I have another function to calculate the ...
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1answer
50 views

What does a rotating universe look like?

If everything in the universe were revolving around an axis, how would that affect space-time? In my mind it would be no different from a normal universe since it would look normal in a comoving frame,...
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1answer
232 views

Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
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5answers
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If string theory is inconsistent with observations, why hasn't it been rejected yet?

I'm no expert on string theory, but I've been reading about it. I've been quite surprised because of how it appears to be inconsistent with observations, but hasn't been rejected yet. Examples: On the ...
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2answers
462 views

Curvature of the Universe imaginary?

If the curvature of the universe is zero, then $$Ω = 1$$ and the Pythagorean Theorem is correct. If instead $$Ω> 1$$ there will be a positive curvature, and if $$Ω <1$$ there will be a negative ...
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1answer
2k views

Is the Universe Past-Eternal?

Does the Borde-Guth-Vilenkin theorem definitively demonstrate that the Universe cannot be past-eternal, whatsoever? Does it not assume a classical space-time while the real world requires Quantum ...
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1answer
204 views

What were the dimensions of the universe during Planck epoch?

Wikipedia says: It is believed that, due to the extraordinarily small scale of the universe at the time, quantum effects of gravity dominated physical interactions. But I wonder whether there ...
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1answer
54 views

Age versus size of the universe [duplicate]

QUESTION CONTEXT: The universe is roughly 14B years while its diameter 93B light years, making radius 46.5B ly. If I understand correctly if we froze everything, go on the one side, then go to the ...
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1answer
62 views

Lagrangian in reduced Horndeski Theory for i=2

I am trying to understand the calculations of the latest Charles Dalang's paper "Scalar and Tensor Gravitational Waves", arXiv:2009.11827. Since I just learned basic general relativity, I ...
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1answer
36 views

Radially-dependent stress-energy-tensor(?)

When we solve the Einstein Field Equations $G_{\mu\nu} = 8\pi T_{\mu\nu}$ one way of doing it is by specifying a symmetry (and thus a general form of the metric) and then specifying the stress-energy-...
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1answer
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Light ray in Friedmann–Lemaître–Robertson–Walker metric

What are the main differences in the path of light ray for different values of $k$ in the FLRW metric? Is it possible to build an experiment (even a mental one) with a single light ray to distinguish ...
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1answer
76 views

Why did recombination make the universe transparent?

It is commonly said that after the universe cooled enough for ionized Hydrogen to settle down into neutral Hydrogen, i.e. recombination, the universe became transparent. A reason I have heard for this ...
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1answer
129 views

Calculating the age of the empty universe from the Hubble's constant

This is from p.120 of Ta-Pei Cheng's Relativity, Gravitation and Cosmology. I cannot understand the way the book calculated the age of the empty universe. The velocity $v$ of expansion is proportional ...
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0answers
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Is the universe a thermodynamically isolated system?

Heat death of the universe can be understood as attainment of thermodynamic equilibrium. But the concept of thermodynamic equilibrium is only valid for isolated systems. What evidence do we have which ...
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2answers
429 views

How could the universe be hyperbolic if hyperbolic space isn't symmetrical?

In the 2-D projections of the shape of the universe shown here, we see that the flat universe and the spherical universe are perfectly symmetrical, so any triangle drawn anywhere on them will be the ...
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1answer
319 views

Scale factor negative

Can the scale factor in a Robertson-Walker metric with $k=0$ (flat universe, with only matter, no cosmological constant or quintessence) depend on a polynomial function? For example: $$a(t)=-t^3+4t^2-...
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1answer
45 views

How would quarks behave in the event of 'The Big Rip'?

I have always heard that you can't get a quark by itself because "the energy required to split them apart is enough to create another." But, in the case of The Big Rip, the idea is that ...
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1answer
87 views

Are there any grounds for thinking that the distribution of matter in the universe is unbounded?

Even though we can make no observations beyond the cosmological horizon, I think people tend to picture a universe that is either homogeneous to infinity, or possibly having spacelike slices that are ...
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2answers
7k views

What's the avarage temperature of the Universe?

I'm sorry if this was asked before but with all the stars and dark matter and all the other stuff, curently in the Universe, what's the avarage temperature of the Universe? Is it like extremely high ...
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1answer
70 views

Prove that the value of the cosmological constant equals the energy density of the vacuum

I know that Einstein introduced his cosmological constant assuming it as an independent parameter, something characteristic of the Universe, in itself, but the term of it in the field equations can be ...
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1answer
78 views

Can one quantify the total luminosity of the cosmological horizon?

If I integrate all the power (radiation, matter particles, neutrinos) radiated from the cosmological horizon into the universe, what number do I get? Is it true that the integral power/luminosity is ...
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0answers
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Demonstration of comoving volume probed between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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1answer
22 views

Density Parameter instability (Guth 1980)

I'm reading through Guth's 1980 paper "The Inflationary Universe" and I have one simple question (so far)... He gives this definition of the density parameter: \begin{equation} .01 < \...
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1answer
41 views

What is Conformal cyclic cosmology?

The concept, as explained in wikipedia is more or less clear to me. But the page is not very detailed. How does CCC works? Also in another question, someone answered Keep in mind that CCC is a dead ...
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2answers
128 views

Event Horizon and Particle Horizon

The furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the event horizon is nearly 16 Gly. Is this means the galaxies that further ...
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1answer
51 views

How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?

By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
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1answer
118 views

Equations of motion from Lagrangian

I have the action $$S=\int d^4x\sqrt{-g} \Big[\frac{1}{8}\phi^2R- \frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi - \frac{1}{2}m^2\phi^2\Big]$$ where $\phi$ is a scalar field and $R$ is the ...
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1answer
21 views

Deriving vacuum FRW equations directly from action

Using the Einstein-Hilbert action for a Universe with just the cosmological constant $\Lambda$: $$S=\int\Big[\frac{R}{2}-\Lambda\Big]\sqrt{-g}\ d^4x$$ I would like to derive the equations of motion: $$...
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2answers
242 views

Is the expansion of the universe really accelerating? Or does it just look like it?

I pulled this quote from an article on the Hubble Constant: "...for example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a ...
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6answers
83 views

In an infinite Universe, would we necessarily see multiple “Earths”?

With my memory issues I can't be certain it was Sean Carroll, though I believe it was a video of him that explained that in an infinite Universe, we would inevitably see large scale patterns repeated, ...
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1answer
42 views

How to calculate $ |\Omega_{tot} - 1| $ at inflation time?

Suppose that we live in a universe that has the following properties: $$ |\Omega_{tot,0} - 1| = 0.001 $$ How can I calculate the value of $$|\Omega_{tot} - 1|$$ at some other time as, for example, the ...
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1answer
304 views

Is a universe without massive particles scale-invariant?

In a popular talk by Roger Penrose about spacetime geometry, when introducing his conformal cyclic cosmology starting at 17:15 I think he says that as soon as there are no massive particles left in ...
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2answers
48 views

Is there any way to prove that contrarly to a flat 3D space, a curved 3D space can only be constructed in a 4D manifold?

This question is a result of me trying to understand how this universe can be possibly infinite if it isn't infinitely old. So to compare with an area that is flat it can be constructed both in 2D and ...
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1answer
46 views

How did a large amount of concentrated energy come in the first place? [closed]

If the universe wants to reach a state of entropy, how did it come to have such a large amount of concentrated energy in the first place?
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0answers
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Question regarding different mode functions in FRW spacetime

I am currently reading Mukhanov's and Winitzki's book with title: "Introduction to Quantum Fields and Classical Backgrounds". At chapter 6, they argue that in a FRW (Freedman - Robertson - ...
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0answers
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Is there a convenient way to find questions which I have previously answered? [migrated]

I started my participation yesterday, and I was very gratefully and pleasantly surprised to receive a perfect answer. I also looked through 10 pages of unanswered questions, and I answered a few. Now ...
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1answer
37 views

Is the value of the scalar components of the Higgs field known?

As I understand the relevant popular sources (example), since the end of the Electroweak epoch, the Higgs field became fixed in the whole known Universe. Thus, we are somewhere on the deep blue part (...
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1answer
33 views

Angular size of the largest region in causal contact at the moment of recombination

I want to calculate the angular size of the largest region in causal contact at the moment of recombination (z = 1100) to our universe, so we have: $H_0 = 68$ (km/s)/Mpc, $\Omega_{m,0} = 0.31$, $\...
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1answer
63 views

Could our universe be expanding due to loss of mass from light waves, gravitational waves and other radiation that left our universe? [closed]

so I know that current evidence points to the universe expanding faster and faster, i.e. red-shift in the stars getting more and more the further they are. I know the current theory is that "dark ...
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2answers
55 views

Can the universe ever contract?

I am currently going through this answer related to the Big Bang and from there a question arose in my mind: Can the universe ever contract? Can it ever come back to singularity? (Although I am aware ...
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0answers
21 views

Fluctuations in the cosmological equation of state during the radiation dominated era

The radiation dominated era is described as a universe full of photons and ultra-relativistic particles with little slow moving matter. Ideally it has w = P/ρ = 1/3. But didn't the actual value of w ...
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2answers
48 views

If you can simulate logic with math, and simulate math with logic, what came first? [closed]

If you can simulate logic with math and simulate math with logic, which came first? if 2 + 2 = 4 it is made up of logical rules. but an if-else can also be simulated by math. In other words, is the ...
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4answers
120 views

The nature of the space in which we exist

I'm looking for pointers to give me a better understanding as to what 'space' is. I do not mean the descriptions of what's in 'space' i.e. not interested in descriptions of outer space filled with ...
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1answer
284 views

Was Sun a part of a binary system?

I've read that many stars and other celestial bodies are found to constitute binary systems where the two bodies spin around each other. But our Sun is one of the exceptions. Could it be possible that ...
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2answers
293 views

Could dark energy just be another characteristic of the gravitational force?

Recent observations of the accelerating expansion of the universe have been quantified and for the time being given a name as to the cause: Dark Energy. And from what I've read from other, similar ...
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1answer
76 views

Reason for matter rather than antimatter? [closed]

If the vacuum had a high enough energy density in the early universe (perhaps relative to changing local energy scale) then different species of particles and antiparticles would have been ...
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1answer
44 views

Will the entropy of the universe be every real number (greater than whatever it currently is)?

Since the universe's entropy is going to increase indefinitely and it is going to be a continuous, does this mean that it will be equal to some power of pi or e or any other transcendental number?
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Does Hubble's constant apply to galaxies that are blue-shifted/ moving towards us? Another question

Does Hubble's constant apply to galaxies that are moving towards Earth with a velocity and does Hubble's constant also able to different systems such as binary stars, stars in another galaxies or ...

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