Questions tagged [cosmology]

Questions about the origin, history, evolution and fate of the Universe.

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How do I estimate uncertainties in mass function?

I constructed a galaxy mass function in Python reading the stellar mass of galaxy in a numpy array, and the weights of each galaxy in another numpy array. Now, I want to propagate errors assuming a ...
Matías Liempi's user avatar
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19 views

Deriving Faber-Jackson Relation from Milgrom's Law

I am reading Famaey & McGaugh 2011, a review paper on Modified Newtonian Dynamics. My question concerns bullet 3 in Sec 5.2, where the authors try to explain how Faber-Jackson Relation can be ...
Ambica Govind's user avatar
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HUBBLE EXPANSION: scale factor change

If the FRLW scale factor is 1 now, was it smaller or larger at earlier times? Does it asymptotically approach zero or infinity as you look back closer to the big bang? Or what? ty!
Miss_Understands's user avatar
10 votes
1 answer
2k views

Is the amount of dark matter the same as you look back through time (further away from earth)?

In the hope that it may provide information on the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large ...
Zinn's user avatar
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Fractal structure of the universe

I decided to return to the question first written here after some time, since a short and succinct phrase had formed in my head that described the phenomenon that I was trying to understand. What are ...
dtn's user avatar
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Shouldn't the Friedmann equation take into account the massive extra distance traveled during the time for gravity to travel across the universe? [closed]

I’ve got a question that has been bothering me for the past few years that of the few sources that are only somewhat related to this, none of them say anything about this should be wrong so I can’t ...
Gref's user avatar
  • 11
2 votes
3 answers
224 views

Is there evidence against a Universe Black Hole?

As far as I know, the critical density of our observable universe coincides with the density of a huge black hole of the same radius. It could be a BH expanding at the speed of light and growing in ...
Juan Casado's user avatar
-1 votes
2 answers
278 views

Big bang cosmology [closed]

What is the main evidence that the Big Bang is real. Why can we know that the big bang exists. In a short summary, the evidence? In your owns words?
Brendan Darrer's user avatar
2 votes
2 answers
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Where can I find the $\mu$ value in galaxy clusters for ideal gas law?

I am studying hydrostatic equilibrium in galaxy clusters and encountered the following expression: $P=(kT/\mu m_p)\rho$ The interpretation of this formula is obvious. It is just the ideal gas law ...
Youngsub Yoon's user avatar
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Is the difference of redshift parameter z able to prove expansion is accelerating in flat FLRW universe model?

Sorry for my bad english, this is not my mother tongue. Recently, I've encountered a problem in cosmology class that is about FLRW universe. Assuming that the universe we inhabit is composed of non-...
ALLin's user avatar
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Does dark energy increase eccentricity in orbits of galaxies?

I came across this thesis dissertation which indicates that if a pair of orbiting galaxies are sufficiently far apart (with a sufficiently large orbital radius) then their orbits would change from ...
vengaq's user avatar
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Consistent values for density of galaxies between degree squared and steradian

I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, $\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{...
guizmo133's user avatar
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Dark energy affecting the ejection and infall of material in galaxies?

Some galaxies have active outflows of material (mostly gas) that would eventually stop at some distance and then fall again into the galaxy due to gravitational attraction. However, can dark energy ...
vengaq's user avatar
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3 votes
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Why is WR124 not brighter?

Wikipedia says the Wolf-Rayet star WR124 is 44,700 degrees, and is 562,000 times brighter than our sun. But the Stefan-Boltzmann Law says luminosity per sq meter is proportional to the temperature $T^...
Jack's user avatar
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8 votes
1 answer
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What would evidence of stars and galaxies significantly older than 13.8 billion years old look like? In what parts of space has it not been found?

The September 19, 2023 podcast with transcript Why the earliest galaxies are sparking drama and controversy among astronomers includes the following: An article published earlier this year in the ...
uhoh's user avatar
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7 votes
2 answers
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How long has it been since dark energy started to reverse our slowdown and accelerate the expansion of the universe? 4 billion years ago? 4.5? 5?

Or has it been as long as six billion years? Over the last few years I keep seeing longer and longer numbers... Have researchers noticed something different recently? Or found a mistake in their ...
Kurt Hikes's user avatar
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Why can't dark energy be considered a 5th fundamental force?

So I have recently been researching for multiple articles about fundamental forces, currently there are just 4 forces: Gravity Electromagnetism Strong nuclear force Weak nuclear force So far there ...
Furious Arcturus's user avatar
2 votes
1 answer
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Gravitational-wave Angle?

As a train of gravitational waves generate from its source and passes through, is it stretch and compression of space-time with same angle all through the distance it is passing and damping over time, ...
Hamidreza Abdollahi's user avatar
45 votes
1 answer
50k views

Shouldn't very very distant objects appear magnified?

My understanding of the expansion of space itself is very shaky, but if space itself is expanding, then shouldn't very very distant objects appear magnified? For example these James Webb Space ...
orion elenzil's user avatar
2 votes
0 answers
83 views

Could entropy be reversed eventually in the far future?

In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe and after the last black hole evaporates there could be no more structures and no ...
vengaq's user avatar
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Finding an expression for the rate of change of Comoving volume with respect to redshift

I am trying to find the expression for the rate of change of comoving volume with respect to redshift, that is $\frac{\mathrm{d}V_c}{\mathrm{d}z}$. In this paper (Hogg, David W.), the comoving volume ...
noob anomaly's user avatar
1 vote
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Is there such a thing as a map that shows the current (not observed) positions of celestial objects?

As I understand it, maps of the sky and three-dimensional maps of the universe all show the observed positions of celestial objects. Are there any maps that show our best guess at their current ...
John Wu's user avatar
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2 votes
0 answers
37 views

Colossus sigma function not reading the changes in the code

am currently using the colossus package on Python, which helps calculating functions that are relevant in Cosmology. I am interested in calculating a Matter power spectrum, and use it as a base to ...
Lorenzo's user avatar
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Are we certain that there is a cosmic event horizon in our Universe?

There is a cosmic event horizon in our Universe iff $\int_{t_0}^{\infty} \frac{dt}{a(t)}$ converges, where $t_0$ is the cosmic time today and $a(t)$ is the scale factor of the metric. $\dot{a}$ and $\...
Rond point's user avatar
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27 views

Can the cosmological constant be coupled to redshift of radiation

Can space-time be more embedded with the things inside it than we actually think?Can the redshift of radiation be a property of spacetime and that dark energy exists only to conserve the overall ...
appliedSciences's user avatar
8 votes
1 answer
2k views

Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The (Davis and Lineweaver (2003)) "Expanding Confusion" paper states that "the expected change in redshift due to cosmological acceleration or deceleration is only ∆z ∼ 10^(−8) over 100 ...
Glycoversi's user avatar
2 votes
0 answers
82 views

How to plot the freeze out equation for massive particle species in Mathematica? [closed]

I want to recreate the figure . This comes from the Cosmological Boltzmann equation which can be modified as the number density equation as $$\frac{dY}{dx}=-\frac{s(m)<{\sigma}|v|>[y^2-Y_{eq}^2]}...
Trinayan Saikia's user avatar
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0 answers
43 views

What courses should one take for working towards Astronomy from an AI / Data Analysis perspective?

Note:- This is a suggestions seeking question As a prospective student about to embark on pursuing the MEngg. in Applied AI program, I am eager to align my education with my passion for Astronomy. I ...
Hacker1's user avatar
5 votes
2 answers
831 views

Hubble expansion rate and reaction rates

I've come across sentences like ...the reaction rates (the number of reactions per particle per unit time) must be larger than the cosmic expansion rate $H(t)$ in order for the particles to maintain ...
Arnab Chowdhury's user avatar
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Trying to understand how to properly implement this equation to a healpix map

So i am working on reproducing a paper (Secrest, N. J., von Hausegger, S., Rameez, M., Mohayaee, R., Sarkar, S., & Colin, J. (2021). A test of the cosmological principle with quasars. The ...
qasidaleem's user avatar
4 votes
1 answer
67 views

Combining Systems Engineering and Cosmology/Astrophysics

in a nutshell, my undergrad was in Astrophysics, my master's is in Systems Engineering (SE), and I am currently completing my Ph.D. in SE focusing on deep learning applications towards engineering ...
Tony Sirico's user avatar
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0 answers
95 views

Why is it assumed that the universe started out with a size equal to the Planck length?

What if the Planck length is the distance between particles when the universe was very small . For a spherical ball of 10^ 78 protons that would give the universe a radius of 10^26 X 10^-35 = 10^-9 ...
user avatar
17 votes
1 answer
3k views

Why do cosmologists assume that inflation began shortly after the Big Bang, rather than at the exact start?

Somehow, I have never come across an explanation of why cosmologists claim that the alleged inflation of the very early universe occurred not at the Big Bang, but very shortly afterwards (~10^-36 to ...
Kurt Hikes's user avatar
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4 votes
0 answers
64 views

What does it exactly mean for spacetime to have no global symmetries?

Are there spacetimes or metrics with no global symmetries? Spacetimes/metrics with no global Poincaré, Lorentz, diffeomorphism, CPT, translational and gauge invariances? And if there are, what does it ...
vengaq's user avatar
  • 827
1 vote
1 answer
190 views

Critical density of universe with intrinsic spatial curvature

$H^2 = \frac{8 \pi G}{3}\rho-\frac{kc^2}{a^2}$ Shown above is the first Friedmann equation. I understand that the curvature parameter, k, represents the spatial curvature of the universe which is ...
Scott's user avatar
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1 vote
0 answers
42 views

In CMB data analysis, when calculating the signal-to-noise ratio for a binned power spectrum, do we use the bin centers in the SNR calculation?

The signal-to-noise ratio for angular power spectrum signal $C_l$ under theoretical noise $N_l$, where $C_l$ and $N_l$ are functions of multipole $l$, is given as $$(S/N)^2= \sum (2l+1) (C_l/N_l)^2$$ ...
Yami's user avatar
  • 89
0 votes
0 answers
170 views

where are we in the universe as compared to the observable universe? [duplicate]

For distant galaxies to be accelerating away at equal speed in every direction, Earth would have to be at the centre of the universe. Since it is not, some region of the distant galaxies should appear ...
Clifford Lingwood's user avatar
1 vote
1 answer
76 views

Could inhomogeneities or topological defects break the fundamental symmetries of the Universe?

I have heard that some types of inhomogeneities and topological defects in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, Lorentz, ...
vengaq's user avatar
  • 827
3 votes
1 answer
247 views

Conditions in the Early Universe

I am interested in how the large-scale properties of the Universe change over time. Does anyone know a trusted website which gives the temperature, density and radius (distance to the particle horizon)...
John Davies's user avatar
21 votes
2 answers
4k views

What equation tells you how far in space you can go from a point and return?

We know that hubble expansion increases the distance between points in space, and that the cosmological event horizon represents the distance from the observer at which objects are receding faster ...
user52978's user avatar
  • 313
5 votes
2 answers
304 views

Size of the whole universe if it were spherical

Suppose the universe is spherical and its density ratio is $\Omega \leq 1.00125$ $\Omega = 1.00125$ is approximately the maximum possible value of the density ratio according to the Planck Mission ...
Albert's user avatar
  • 1,910
1 vote
1 answer
70 views

Other Hubble spheres with no Lorentz symmetry?

Perhaps this may be a stupid question but anyways here it goes... If the Lorentz symmetry is not global but rather local, wouldn't that mean that is possible that other Hubble spheres outside our ...
vengaq's user avatar
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4 votes
0 answers
93 views

Why is the latest Webb discovery of early-age universe considered so important?

The past week, news were flooding of an unexpected discovery of James Webb Space Telescope. In many articles it is mentioned that cosmologists cannot explain the formation of galaxies of this ...
Demis's user avatar
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7 votes
4 answers
253 views

What does Dark Energy explain other than the accelerated expansion of the universe?

Dark energy is a vital part of standard cosmology and allows for an explanation of the accelerated expansion of the universe. Does it explain anything else? Or put into other words: If a different ...
Liberty's user avatar
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0 votes
1 answer
132 views

Can the curvature of a closed universe be measured?

Suppose we were in a closed universe likened with a spherical surface such as that of the Earth's surface. Assume also that the limits, represented by the limits of the observable universe, ...
Mikael Jensen's user avatar
2 votes
0 answers
97 views

Empirical Constraints on the Time Evolution of Cosmological Redshifts

The standard model of Cosmology predicts that the redshift of a single source at fixed comoving distance varies with observation time according to $$ \dot z = \left( 1+z - \frac{H(z)}{H_0} \right) H_0....
jawheele's user avatar
  • 121
1 vote
1 answer
104 views

Coasting universe Hubble parameter behaviour

I’ve recently been trying to wrap my head around the potential behaviours of the Hubble parameter over time for various cosmological models but I’ve run into a little snag when it comes to a coasting ...
Scott's user avatar
  • 23
7 votes
1 answer
4k views

In 2023, is space still thought to be "filled with a network of wormholes"?

The September 2009 YouTube music video Carl Sagan - 'A Glorious Dawn' ft Stephen Hawking (Symphony of Science) at about 00:30 depicts a clip from (what I think is) ...
uhoh's user avatar
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1 vote
0 answers
32 views

Gas heating due to high viscosity?

Has there been any observation where a gas or a fluid expands following the Hubble flow (the expansion of the universe) and gets heated because it has a large viscosity (transforming its kinetic ...
vengaq's user avatar
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1 vote
0 answers
41 views

Early Dark Energy [closed]

Early dark energy is one of the possible solutions for the Hubble tension. If it is correct then wouldn't the universe be somewhat larger than our current best estimates? early dark energy:https://...
Kontrola Faktů's user avatar

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