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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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From the expansion to temperature and recovering equilibrium number densities from the scale factor

For a particle species of mass $m$, when the temperature $T$ of the Universe is $T>m$, the equilibrium number density $n$ falls as $$n\propto T^3\tag{1}$$ and for $T<m$, it falls as $$n\propto ...
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2answers
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How does calculating Hubble time work?

For calculating we take $t=\frac dV$, where $V=Hd$ . But velocity isn't uniform for an object as distance also increases with time. And if velocity isn't constant the equation $t=\frac H d$ also isn't ...
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1answer
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Spatial variation in the fine structure constant

An article in Time (I saw it reprinted in Medium), and questions on this site discuss observations that suggest that the fine structure constant might vary spatially in the universe. If true, wouldn't ...
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1answer
44 views

How do we know what happened during the Planck epoch era of the big bang?

The Cosmic Microwave Background provides the left over radiation of the big bang. Cosmologists have theorised what happened at the Planck epoch era of the big bang. Where is the evidence of what ...
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2answers
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Could photons decay into Dark Matter? [on hold]

So, we can "see" 13.7 billion (intentionally not including expansion) light years in all directions and we "see" a red shift. What if, the reason we can't observe photons beyond this limit is that ...
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1answer
23 views

Relationship between Energy density and Curvature

I don't know GR so while answering the question so keep in mind that. In the Friedmann Equations, is energy density has an effect on curvature or vice versa? Or they are separate things and they don'...
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1answer
28 views

Physical explanation of Dirac-Born-Infeld (DBI) inflation

I am studying the Dirac-Born-Infeld (DBI) inflation model and came across this question in a past exam paper from Cambridge that considered the following Lagrangian: $$\mathcal{L}=\sqrt{-g}A(X,\phi)$$...
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23 views

Why Empty universe have to obey the Negative Curvature? [duplicate]

For empty universe it seems to me that we can have two solutions. $$H^2=\frac {8\pi G\epsilon} {3c^2}-\frac {\kappa c^2} {R^2a^2(t)}$$ For an empty universe when we set $\epsilon=0$ we get $$H^2=\...
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Did inflationary expansion “abruptly” turn into non-inflationary, rather than gradually decelerate to non-inflationary?

From the $\Lambda$CDM model, the universe was in an expanding state and then inflation occurred which turned non-inflationary expansion into inflationary expansion. After inflation ended, did ...
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1answer
63 views

Why is the heat death, rather than a Big Crunch, the most accepted theory of the ultimate fate of the Universe?

The Heat Death is accepted by most as the end of the Universe, but how can that be? Wouldn't the Big Crunch make a lot more sense? I mean, even if everything in the Universe is spread out uniformly ...
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1answer
52 views

Age of empty universe vs actual universe [on hold]

Can anyone please explain the following question for me? I know how to get to the formula to find age from Friedmann equation but I am not sure how to answer this question.
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Redshift of decoupled photons

How many years was passed after the photon decoupling when the wavelength of the caused by decoupling visible light redshifted out of visible one?
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1answer
56 views

What is the present day photon density, $\rho_{\gamma, 0}$?

I'm attempting to perform the integration that will yield the sound horizon at recombination: $$ c_s^2 = \frac{c^2}{3}\left[\frac{3}{4}\frac{\rho_{b,0}(1+z)^3}{\rho_{\gamma,0}(1+z)^4} + 1\right]^{-1}...
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2answers
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What if humans doubled size… and everything else… could we notice? [duplicate]

After the big bang, everything expanded from a small mass. That expansion is said to be still happening. Imagine if everything observable constantly grew in size. EG. Everything slowly doubled in ...
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Must the whole universe have the same entropic arrow of time?

Theoretically, could it be possible for one galaxy to have low entropy in the past, but another nearby to have low entropy in the future? I understand that, cosmologically, there almost certainly are ...
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1answer
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How is the energy density of radiation relatedd to the energy density of particles and the critical density?

In a solution set I was reading for cosmology I saw that the energy density of radiation is defined as $\epsilon_r=\epsilon_c-\epsilon_p$. Where $\epsilon_p$ is the energy density of particles at ...
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1answer
68 views

Why is $\Lambda=0$ for a flat matter dominated universe?

I was looking over some of my cosmology notes and was considering the Einstein de-sitter model of a matter dominated universe. I know that in this case that k=0 and also furthermore that $\rho(t)$ is ...
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Is there any use of a quadratic equation of state in FLRW cosmology?

Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear: \begin{equation}\tag{1} p = w \, \rho, \end{...
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1answer
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How do we measure the age of the universe? [duplicate]

How do we measure the age of the universe if time is relative to the observer? What is the reference frame we use to measure the age of the universe?
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0answers
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Can a no boundary proposal work without the big bang?

In Hawking's no boundary proposal he assumed that the the amplitude for a state A (such as a given 3-manifold) is the sum of all amplitudes for histories starting at a special big-bang-state (like the ...
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1answer
51 views

number density of photons

My lecturer put up as a solution to the number density of photons to be $n_{o\gamma}=\tfrac{8\pi}{c^3}\int^{\infty}_0(\tfrac{kT}{h})^3\tfrac{x^2dx}{e^x-1}$. The integral on the right hand side being ...
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1answer
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number density of photons and the total energy of the universe

I was reading over some assignment solutions that my lecturer put up and one of them said that instead of calculating the number of photons in the universe with the familiar method of $\int^{\infty}_0(...
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What is the correct value of the matter density of the universe?

I'm studying for an exam right now and I came across a solution my lecturer gave that is really confusing me. So in finding the density of matter in the universe we can use the density parameter $\...
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1answer
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How is the co-moving co-ordinate defined mathematically in cosmology?

The relationship between proper distance and co-moving distance in cosmology is given by: $$R(t)=a(t)r$$ Where $R(t)=$ the proper distance which corresponds to where a distant object would be at a ...
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2answers
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How can the universe be dodecahedron-shaped?

Physicsworld references "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background" by J.-P. Luminet et al., published in Nature (...
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1answer
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How is Inflation able to create an infinite amount of energy?

Follow up to this question here: If the universe is flat, does that imply that the Big Bang produced an infinite amount of energy? As I understand Inflation theory, some time after the Big Bang, the ...
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How big is the universe 1 second after the Big Bang? [duplicate]

In his book Astrophysics for People in a Hurry, Neil de Grass Tyson starts of with the Big Bang and narrates on what (probably) happened within the first moments of our universe. He further states (p. ...
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1answer
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Continuity equation from the Friedmann equation

I know that one can derive the continuity equation: $$\dot{\rho}=3H(\rho+P)$$ from the Friedmann equations. $H^2=\frac{8\pi{G}\rho}{3}$ and $\frac{\ddot{a}}{a}=\frac{4\pi{G}\rho}{3}(\rho+3P)$ But ...
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28 views

Why do we assume that there had to be a baryon-antibaryon-symmetry in the first place?

Maybe this is obvious to some, however I have not found a satisfying answer yet. Detailed answers appreciated!
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2answers
72 views

If the universe is flat, does that imply that the Big Bang produced an infinite amount of energy?

Too much density and the universe is closed, analogous to a sphere in four dimensions: you travel in a straight line and you end up where you started. Too little and you have a saddle: not sure about ...
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1answer
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Modification of Gravitational force's law and related problem

I'm facing a problem which states the following: Suppose that it is discovered that Newton's law of gravitation is incorrect and that the force $F$ on test particle of mass $m$ due to a body of ...
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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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Looking for a geometric cosmology model where time behaves like an orientation

This question is literally inspired from seeing the above scene unfold. Let the merging and splitting light spots you saw in the above gif be pairs of particles and anti particles, let the shape of ...
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1answer
66 views

Is the total information in the (visible) universe compatible with the one on the surface of a black hole with the same radius?

Is the total information available in the universe compatible with the entropy we would find on the surface of an hypothetical black hole whose radius is the visible universe one ($\sim14$ Gpc)? We ...
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2answers
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Could the cosmic fluid be better described as a kind of van der Waals gas, or a kind of foam, instead of a dust gas?

In cosmology, it is usually assumed that the cosmological fluid made of galaxies could be described as a gas of "particles" without any pressure (the dust gas), of density $\rho_{\text{matter}} \...
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How would gray dust work, on a microscopic level?

I came across an article on something called gray dust, a hypothetical substance explored theoretically during the early 2000s. It attempted to explain cosmological supernova observations by ...
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3answers
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formation of atomic nuclei from nucleons

What holds the nucleus together? In a nucleus there are several protons, all of which are positive. Why don't they push themselves apart? It turns out that in nuclei there are, in addition to ...
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1answer
66 views

Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
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Would a cellular-automata universe allow singularities to exist?

It is usually assumed (almost always) that cosmological singularities (like black hole singularities) are impossible and do not exist in the universe. If the universe was like a cellular automata (...
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1answer
60 views

Why does dark energy make universe accelerate not slow down?

If mass=energy, then why doesn't dark energy not make the expansion slow down rather than accelerate seeing as gravity pulls masses together?
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3D positively curved space [migrated]

If we consider 2D euclidean surface consists of infinite concentric circles and 3D euclidean surface consists of infinite concentric spheres. If 2D surface is positively curved the radius of the ...
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2answers
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Proper distance in cosmology

If considering two galaxies $\mathcal{A}$ and $\mathcal{B}$ being separated by $d$ ($d$ being the comoving distance) and light is emmited from $\mathcal{A}$ at time $t_1$ and that light is received to ...
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Does quantum cosmology work for open spacetime manifolds?

Hawkings no boundary proposal assumes a closed spacetime. But why would it not work with a infinite spacetime? With instead if periodic boundary conditions you might have asymptotic boundary ...
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0answers
37 views

Can quantum cosmology have a starting state?

In quantum cosmology, Hawking's no boundary proposal hypothesised that the wave function of the Universe can be found by summing over closed Universes, which have no beginning in time. One of the ...
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Alternative values for the constant in the cosmological equation of state? (Mislaid list)

A couple of days ago I came across - but now cannot locate - an online list of about eight or nine alternative theoretical values for the constant in the cosmological equation of state (W = 1, -1/3, 0,...
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1answer
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Tired light hypothesis question deriving the distance redshift relation

I need to find that distance(r) and redhshift(z) gives a linear relationship for $z<<1$ in tired light hypothesis and find the K value for the Hubble constant of $H=70 km s^{-1} Mpc^{-1}$ I ...
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1answer
37 views

Do cosmic voids have to exist to prevent gravitational collapse of the Universe?

Since a black hole radius is 2 times the mass, it seems like any infinite distribution of matter has to have voids of increasing size as the scale changes in order for the whole Universe not to ...
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2answers
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What is the biggest structure in the Universe?

The CMB cold spot (if it exists) may imply a supervoid of approximately 1 billion light years across. If the diameter of the Observable Universe is 46 billion light years this is about 2% the ...
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1answer
120 views

How did the Planck Study calculate the angular size of the sound horizon?

The Planck Collaboration has quoted the angular size of the Sound Horizon:$$\Theta_*= 0.0104$$I decided I'd try a rough, back-of-the-envelope calculation to understand how they arrived at this value. ...
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Natural classification of fundamental constants of Nature

I was wondering about the fundamental constants of Nature since several years, and still pondering on them. Of course, I have read a lot of papers on them, but never found any satisfying ...