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Questions tagged [cosmology]

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

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Would we know if the universe was rotating?

I was wondering what we would observe if the universe had a small rotation. My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than ...
KDP's user avatar
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8 votes
1 answer
559 views

References about neutrinos getting clumped in "structures" in the future?

I was wondering whether (standard model) neutrinos could form clumped structures (like halos perhaps). The problem with this is that neutrinos have a very light mass, have a great speed and do not ...
vengaq's user avatar
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-2 votes
0 answers
70 views

Does Mach's Principle imply the Newtonian constant of Gravitation or the speed of light (or both) change with time as the Universe expands?

In his 1999 book Relational Mechanics Professor Andre K. T. Assis gives an equation on page 180 (Equation 8.37) for the Newtonian constant of gravitation, $G$, that he has arrived at heuristically, ...
James Arathoon's user avatar
1 vote
1 answer
103 views

Does the lower value of the Hubble Constant predicted by the Planck Satellite and now confirmed by Wendy Freedman invalidate the idea of dark energy?

Is the dark energy part of the Lambda-CDM model (the 'lambda') still necessary if the value of the Hubble 'constant' is lower than that predicted by Adam Reiss and collaborators when they examined ...
Kurt Hikes's user avatar
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20 views

Is the hypothetical '(Don) Page curve' of evaporating black holes equal in time, so to speak, on both sides of the curve?

I have heard that Hawking radiation is slow at first, but rapidly increases when the black hole becomes much smaller... (I forget the formula...) Does that mean that the second half of Page's curve, ...
Kurt Hikes's user avatar
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7 votes
1 answer
123 views

What exactly is a 'gravastar'? How is it different from a 'regular' black hole?

The Wikipedia article on gravastar says that it has a 'regular' black hole metric (Schwarzschild, I presume?) on the outside, but a de Sitter metric on the inside.... What exactly does that mean? Don'...
Kurt Hikes's user avatar
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-1 votes
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68 views

Spinning black holes and ring singularities

2 days ago, I asked this question on the physics stack exchange: https://physics.stackexchange.com/questions/815217/what-happens-to-the-ring-singularities-when-two-kerr-black-holes-merge?. However, ...
Flamethrower's user avatar
0 votes
2 answers
77 views

How can $\Omega_m<1$ indicate an infinitely expanding universe?

I was reading up about the cosmological density parameters $\Omega_m. \Omega_{rel}, \Omega_{\Lambda}$ corresponding to total matter(baryonic+dark), relativistic particle matter and dark energy. I ...
Ambica Govind's user avatar
5 votes
2 answers
490 views

White dwarf supernova luminosity

I understand that the mass limit for a white dwarf is 1.4 solar masses, and therefore approaching it would cause a white dwarf (type Ia) supernova, and thus it is a standard candle. I understand this ...
Cosmo's user avatar
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What is Helix Nebula made up of?

The Helix is a planetary nebula, the glowing gaseous envelope expelled by a dying, sun-like star. Helix nebula consists of two gaseous disks, how they are formed or which material is present in it.
Helix Nebula's user avatar
4 votes
1 answer
537 views

Can an astronomer fully work remotely? [closed]

My question today is addressed to those astronomers who have quite a lot of experience working remotely. In general terms, the answer to the question includes the following elements: the range of ...
ayr's user avatar
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0 answers
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How can I convert luminosity from [erg/s/Hz] to [erg/s]

I have the dust-corrected luminosity in the Ks band in the unit erg/s/Hz. I want to convert it to erg/s. I would like to know which method is correct: $L[erg/s]=L[erg/s/Hz]\times(C/\lambda)$ $L[erg/s]...
Fatemeh Abedini's user avatar
3 votes
1 answer
162 views

A couple of questions on Hawking radiation

Black holes progressively evaporate due to the emission of Hawking radiation just outside their event horizon primarily in the form of photons. However, there is a small probability that they emit ...
vengaq's user avatar
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2 votes
1 answer
106 views

Earth-Moon receding-approaching cycle?

I made the following question in an another more informal forum of discussion on physics: My question comes after reading this article: https://www.coastreporter.net/in-the-community/the-moon-is-...
vengaq's user avatar
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2 votes
0 answers
151 views

What happens at the edge of a finite and "bounded" universe?

So far, we know only about two types of universes: Infinite and finite universes (Unbounded edition). In an infinite universe, there is no edge. Because well, an infinite space doesn't have an edge. ...
Alastor's user avatar
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3 votes
0 answers
125 views

What are the loopholes in recent claims that dark energy is not necessary?

I read an article on spacechatter about recent work that eliminates the need for dark energy in explaining observations. The pertinent quote from that article is “The study’s findings confirm that ...
tnknepp's user avatar
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16 votes
9 answers
11k views

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

It strikes me as contradictory that the scientific community will say that we don't know what dark matter is, but be happy to state things like "dark matter makes up about 85% of the cosmos" ...
Zinn's user avatar
  • 405
1 vote
0 answers
55 views

Fourier transform of the density fluctuation

There is a Fourier transform that I don't really understand in my textbook (p.218). I have the following equation: $$\ddot{\delta} + 2H\dot{\delta} -\frac{3}{2} \Omega_m H^2 \delta = 0 $$ Then using ...
merlinbluepickle's user avatar
2 votes
0 answers
34 views

Local anisotropies in the expansion of the universe and tidal forces...?

In the context of the large-scale structure of the universe, there can be local anisotropies (https://www.aanda.org/articles/aa/full_html/2021/05/aa40296-21/aa40296-21.html). I understand from this ...
vengaq's user avatar
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4 votes
1 answer
175 views

Energy density of the inflaton field

In the textbook introduction to cosmology by Barbara Ryden, the author say that the energy density of the inflaton field is: $$\epsilon_{\phi} = \frac{\dot{\phi^2}}{2 \hbar c^3} + V(\phi)$$ However, ...
merlinbluepickle's user avatar
2 votes
1 answer
70 views

Expanding voids ending being collapsed?

According to some papers I've found 1, 2 expanding voids can be foun inside clouds of denser materials that can cause them to eventually collapse. I have two questions about this: Overdensities ...
vengaq's user avatar
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0 votes
0 answers
23 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
0 votes
1 answer
74 views

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
  • 405
2 votes
2 answers
253 views

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 ...
ayr's user avatar
  • 793
-2 votes
1 answer
206 views

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
  • 21
2 votes
3 answers
321 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
0 votes
2 answers
308 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
99 views

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
0 votes
0 answers
33 views

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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1 vote
0 answers
70 views

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
  • 1,017
0 votes
0 answers
46 views

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
1 vote
0 answers
36 views

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
  • 1,017
3 votes
1 answer
2k views

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
  • 47
8 votes
1 answer
2k views

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
  • 30.6k
7 votes
2 answers
2k views

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
  • 5,177
3 votes
2 answers
3k views

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 ...
Alastor's user avatar
  • 2,668
2 votes
1 answer
81 views

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
89 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
  • 1,017
1 vote
1 answer
109 views

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
0 answers
94 views

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
  • 111
2 votes
0 answers
42 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
  • 21
1 vote
0 answers
83 views

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
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
101 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
5 votes
2 answers
841 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
0 votes
0 answers
42 views

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
71 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
0 votes
0 answers
97 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 ...
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