Questions tagged [cosmology]
Questions about the origin, history, evolution and fate of the Universe.
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Question on the singularity theorem
I have just started studying Cosmology and we have been asked to prove that in an expanding FRW Universe which obeys the strong energy condition: $$\rho + 3P >0$$
Then there must exist a Big Bang ...
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1answer
57 views
Why does the Hubble flow does not carry photons with it?
I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is ...
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Cross-correlations between Lagrange and Fourier synthesis
In the context of forecast for large surveys, I have to make cross-correlations between 2D (with angular coordinates of Lagrange transformation for GC photometric and Weak Lensing) and 3D (Fourier ...
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0answers
30 views
What is the 'scale factor' equation for a dark-matter dominated universe?
The Friedmann equations can be solved exactly in presence of a perfect fluid with equation of state
$${\displaystyle p=w\rho c^{2},}p=w\rho c^2$$,
where ${\displaystyle p}$ is the pressure, ${\...
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0answers
14 views
Do apparent magnitude (V) and photometric magnitude (specifically in optical V band between 500-600 nm) have a similar scale to stellar magnitude?
I'm graphing a table and I used the magnitude values from this database
https://vizier.u-strasbg.fr/viz-bin/VizieR-4
(It's the V/19/data catalog for "Masses and Ages of stars in 68 open clusters&...
6
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1answer
73 views
If an object 1 billion light years away emits light, does it take more than 1 billion years to reach us because of the expansion of the universe?
From page 7 of the recent (September 26, 2020) edition of Science News Magazine:
Detected May 21, 2019, the gravitational waves came from a source about 17 billion light-years from Earth, making this ...
2
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1answer
31 views
What is a UBV source for stars or stellar objects?
I was using the V19(VizieR) catalog and the main identifier of the stars was in terms of an 'ID' number which they mentioned as being 'Star number in UBV source' I have no idea what that means. How ...
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0answers
21 views
Particle horizon distance as $t \to \infty$
So I know that the particle horizon distance in our universe is about 46 billion light years. I also know that as $t \to \infty$, the particle horizon distance approaches a finite value. What is this ...
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20 views
What is this 3D graph on velocity dispersions of galaxies showing, exactly?
I don't fully understand these 3-dimensional graphs sometimes.... Why do the x-axis and the y show the same thing? Why do the horizontal axes run into the negative numbers?
I've seen this on other ...
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2answers
51 views
How far away are objects whose light will never reach us, because of the expansion of the universe?
I thought I had read this question on Stack Exchange before, but now I cannot find it... In fact, I thought I had posted this question before, somewhere, on Stack Exchange...
I believe the answer ...
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2answers
125 views
Why can't we point the centre of the universe from inflation graph we see?
This question is not about whether it makes sense to have a centre of the universe or not instead, it's about the inflation graph we used to see while describing the Big Bang. From the inflation graph ...
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Units of angle in comoving transverse distance calculation
For two objects at a redshift $z$, I know that the proper distance between them is given by $\delta \theta d_M(z)$, where $d_M(z)$ is the transverse comoving distance. What are the units of the angle ...
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1answer
40 views
Are the most distant known objects in the universe more than 14 billion light years away? [duplicate]
When I hear about the most distant objects in the universe, such as the recently discovered galaxy GN-z11, their distances are usually stated to be a little under 14 billion light years away....
But, ...
9
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1answer
305 views
Are black holes considered part of baryonic content of universe?
Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?
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0answers
11 views
Radio loud AGN restarting as Radio Quiet AGN?
Is it possible for a Radio-loud object (eg., an FRI galaxy) to evolve into a Radio-quiet (eg., a Seyfert) source?
Also, can this happen via AGN restart? That is, have there been instances where the ...
5
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1answer
107 views
Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?
I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored ...
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53 views
Is the Universe infinite? [duplicate]
https://en.wikipedia.org/wiki/Universe
Is the Universe infinite or finite ?
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25 views
What features are different in old/young galaxies, what can be seen?
As you all know, the further the galaxy is from us, measured by redshift, the younger it is supposed to be, but what other features can be seen in those younger galaxies to confirm their age?
Hints: ...
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42 views
Relation between photometric redshift in a given bin and conditional probability
I take over an old question that is not yet clear for me. Indeed, I would like to formulate the equation (1) below under the form of "conditional probability" but I can't manage to do that. ...
2
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1answer
112 views
What was the size of the universe around 400 million years after the Big Bang?
I was trying to find out how large the universe was around 400 million years after the Big Bang as this is when some of the first galaxies were formed but I am having trouble finding out the answer or ...
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2answers
76 views
How are younger objects outside our observable universe when the older CMB is an observer's temporal “edge”?
We know the observable universe is limited to objects whose light has had time to reach us and that an observer very far away will see a slightly different (assuming cosmological principle) observable ...
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0answers
28 views
spectral index of primordial power spectrum
According to eq (7.62) here, the dimensionless primordial power spectrum takes a power law form: $$\mathcal P(k)=A_s\left(\frac{k}{k_0}\right)^{n_s-1},$$ with $n_s\approx 1$, i.e. $\mathcal P(k)$ is ...
5
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1answer
191 views
What are GEODEs?
Phys.org's Researchers predict location of novel candidate for mysterious dark energy says that GEODEs may explain the dark energy problem. I'm not an astronomer but I've never heard of GEODEs. I know ...
2
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1answer
50 views
What does “located in the Hubble flow” mean?
I do not understand that much about the term Hubble flow.
For cosmological studies with type Ia supernova, I often see the sentence like "We used 100 SNe Ia in the Hubble flow". What does ...
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1answer
38 views
Does the critical density of the Universe also include Dark Energy?
It is often said that it seems that the Universe is quite close to critical density (for example https://astronomy.swin.edu.au/cosmos/C/Critical+Density).
Does this include Dark Energy as well (apart ...
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1answer
130 views
What is dark energy made of? [duplicate]
What are the fundamental constituents of Dark Energy?
Can we quantize dark energy in the first place, if it is a kind of Energy?
3
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1answer
127 views
What happened before, Big Bang or inflation?
I always thought that the cosmic inflation happened after the big bang (10-36 sec) and lasted for a very tiny fraction of time).
Now, I recently came across a couple of articles (links below) claiming ...
1
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1answer
21 views
Approximate formula to find velocity from cosmological redshift
From IOAA 2013 (Greece) Theory question no. 15, they stated that the approximate formula to find velocity from cosmological redshift is $$v = c*log_e (1+z)$$ and that it is often used by cosmologists. ...
9
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2answers
496 views
Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?
This newly updated answer to How precise are the observational measurements for the speed of gravity? and this answer to How is the most accurate value of šŗ measured? cites the November 2017 arXiv ...
2
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1answer
150 views
Is cosmology an accurate field? [closed]
I'm new here this is my first question. I'm an engineering student and I really like mathematics, I've recently watched a movie about Stephen Hawking (The Theory of Everything) In which he makes ...
1
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0answers
17 views
Demonstration of scales for the two components (transverse and perpendicular) of Baryonic Acoustic Oscillations (BAO)
I transfer here a post that has not had any answer pn physics exchange, so I am going to delete this latter. Maybe I will have more luckiness here.
In an astrophysics context about BAO (Baryonic ...
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0answers
32 views
Get the expression of probed volume 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 ...
3
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1answer
60 views
Basic explanation of baryon acoustic oscillations; what if anything is actually oscillating?
Sci-News' Sloan Digital Sky Survey Collaboration Releases New 3D Map of Universe links to the two videos
The eBOSS 3D map of the Universe
SDSS releases largest 3D map of the universe ever created
...
3
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1answer
63 views
Calculate object apparent diameter by distance and radius [duplicate]
I'm trying to create a dynamic starfield for game using the HYG database. Because there are no data about the apparent size of the stars from earth, I will need to get it myself. I found a formula ...
3
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1answer
44 views
Would DCBH form supermassive stars first before BH formation
In reading some literature it seems ambiguous (intentionally?) as to the path that Direct Collapse Black Holes (DCBH) take. Some papers do talk about pristine Halos collapsing to form supermassive ...
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27 views
Transition of Domination in the Universe
While deriving the epoch of matter-radiation equality, we assume the transition from radiation dominated universe to matter dominated universe as instantaneous. But how instantaneous is this ...
2
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1answer
62 views
Expresion of comoving distance
I have a simple question :
How to prove the following relation :
The comoving distance to an object at redshift $z$ can be computed as
$$r(z)=\dfrac{c}{H_{0}} \int_{0}^{z} \dfrac{\mathrm{d} z}{E(z)}$...
0
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1answer
40 views
Which redshift is used to determine the Hubbleconstant?
I think they measure cosmological redshift to use in the Law of Hubble-LemaƮtre together with the distance to calculate $H_0$. Is this correct, or do they use Doppler shift (too)?
$H_0$ indicates how ...
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1answer
63 views
Can the electromagnetic counterpart of a gravitational be seen by 'small' telescopes?
As follow-up question for my previous question I'm thinking about the search for the electromagnetic (EM) counterpart of a gravitational wave (GW). Is it possible to search for those events with '...
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0answers
81 views
Does the cosmological principle apply to the entire universe?
Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If ...
5
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3answers
1k views
Does a merging massive binary black hole āemitsā more than one gravitational wave?
If we want to use gravitational waves (GW) to determine the Hubble constant, we need to find the source in the electromagnetic spectrum (EMS). However, we need to be lucky to āseeā it simultaneously ...
3
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1answer
341 views
Why are quasars so far away that they couldn't be optically resolved in the 1950's? [duplicate]
Wikipedia's Quasar includes the following in its introduction:
The term quasar originated as a contraction of quasi-stellar (star-like) radio source, because quasars were first identified during the ...
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0answers
62 views
Does time flow in a Minkowski spacetime?
In a spacetime where the stress-energy tensor is null (no energy, no matter, no entropy), I wonder whether any motion of the variable time in the phase-space is well defined.
The arrow of time, meant ...
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0answers
31 views
Pivot scale in cosmology, CMB observatiion
What is a pivot scale, pivot frequency in general, and particularly in the cosmology? I don't get the idea.
Almost everywhere I found the same the same sentence "we take the standard value of a ...
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0answers
67 views
If time is not absolute and depends on the gravitational field, then why we keep using our own (particular) time in cosmology?
Sorry if my question is not really clear, but indeed it is not clear in my mind neither.
But maybe some of you can help.
Time is not absolute and can be affected by the gravitational potential.
Now ...
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0answers
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Is cosmological redshift caused due to expansion of space or distance traveled by light?
If the universe wasn't expanding, would light still be redshifted after traveling some distance?
Is there a way of being sure that it's the expansion that causes light to redshift and not just a ...
4
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1answer
82 views
Puzzle about proper distance
So I tried deriving the radial co-moving coordinate formula to an observable galaxy for which the light observed at time $t_o$ was emitted at time $t_{em}$. i.e $r_0=\int_{t_{em}}^{to} c/a_{(t)} dt$ ...
2
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1answer
65 views
Bounded Observable Universe and General Relativity
The book General Relativity for Mathematicians by Sachs and Wu defines a "spacetime" as a connected 4-dimensional, oriented and time oriented Lorentzian manifold. I emphasize the word "connected." ...
14
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1answer
715 views
How is the Hubble constant determined from gravitational waves?
We know there is a discrepancy between measurements of the Hubble constant, $H_0$. On one side there is the method of the Planck mission, where they use the CMB and the $\Lambda$CDM model to determine ...
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1answer
82 views
Is our universe a singularity?
According to the Big Bang theory our universe used to be a lot smaller in size.
It actually used to be so small that in the beginning it used to be a singularity.
And the universe started to expand ...
