Questions tagged [cosmology]
Questions about the origin, history, evolution and fate of the Universe.
4
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3answers
111 views
cosmic-microwave-background - 2.7 degrees but does the Big Bang care?
The Big Bang is a proposed answer to the beginning of the universe, and the Cosmic Microwave Background or CMB is a central part of the evidence for it.
Now suppose the measurement returned a ...
2
votes
0answers
16 views
How to correct observed flux densities for redshift
Say I have a spectrum of a galaxy at a redshift $z$, in flux density units of erg/s/cm^2/Angstrom. I'd like to recover the spectrum (in the same flux density units) at z=0, i.e. at its rest ...
0
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0answers
32 views
The Size of the Observable Universe [duplicate]
In the history it has been demonstrated that the galaxies have gone away between if there is another thing that has allowed us to calculate the value of the acceleration of the expansion that ...
0
votes
1answer
19 views
Virial Shock Heating
In the review paper on protoclusters, Roderik Overzier (pg 25) mentions the concept of virial shock heating which seems to be a process involved with galaxy cluster formation.
I'm not fully certain ...
0
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0answers
11 views
What is the relative strength of the 21cm compared to the CMB in Spectral Irradiance ($Jy\cdot sr^{-1}$)
I understand that radio astronomers like to work in terms of Brightness Temperature, due to the various conveniences it brings mathematically. The strength of the 21cm appears to be roughly 3 orders ...
3
votes
1answer
29 views
Non-thermal Sunyaev-Zel'dovich effect
Exactly what is the non-thermal Sunyaev-Zel'dovich effect? From what I understand from reading several papers by Mark Birkinshaw and Sergio Colafrancesco, I get the rough idea that the non-thermal SZ ...
0
votes
1answer
35 views
What is the lyman-werner background?
I've seen several papers that discuss the Lyman-Werner Background (LWB) when modeling star and galaxy formation in the early universe. One in particular (https://arxiv.org/abs/1803.04527) even varies ...
-2
votes
5answers
669 views
Vacuum of space [closed]
Firstly I’m genuinely interesting in a working explanation for this question. It is for this reason that I am editing the question to fine tune the question. In essense the question has remained the ...
1
vote
1answer
73 views
Mass of the universe
How do we calculate the mass of the universe taking into consideration dark matter?
Mass of visible matter can be computed by stellar method, but how do we calculate mass of dark matter which we don'...
6
votes
1answer
72 views
Motivation for using $\sigma_{8}$?
Press-Schechter formalism allows one to universally quantify the number of massive dark matter halos in a given volume -- The Halo Mass Function. The density contrast is given by the rms mass variance ...
4
votes
1answer
52 views
Necessary steps to calculate photon's path by using null geodesic equation
Can anyone please give me an explanation on how to calculate photon's path by using the null geodesic equation? N.B. I know all of the non-zero values of Christoffel symbols.
1
vote
1answer
46 views
Ensemble average VS spatial average for a cosmological density field
I'm studying statistical properties of a cosmological density field and I have some doubts; in the following quote and pictures you can see what my problem is:
A fundamental limitation arises in ...
2
votes
0answers
57 views
Correction to inverse square law for cosmology
While reading about the discovery of quasars and the spectroscopic analysis of 3C 273 in this paper by Maarten Schmidt, I came across the following quote:
At these distances, corrections to ...
10
votes
1answer
987 views
How can gravitational lensing makes a quasar appear brighter?
How can gravitational lensing makes a quasar brighter than it would in the absence of a foreground galaxy?
5
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1answer
248 views
How stable is our Universe?
I've been reading and thinking about this subject for some time, but I can't seem to find a clear answer.
After reading Q14:
Aspects of the Higgs boson suggest that our Universe is only
“...
9
votes
6answers
376 views
Are there other proofs of the expanding universe apart from the redshift?
The theory of the expanding universe is so widely accepted, that the redshift is sometimes used as a measure of distance to far away galaxies.
But is it still possible that the redshift is caused by ...
0
votes
1answer
74 views
Time-frame regarding cosmic expansion and the bound state of the Virgo Cluster
Due to accelerating cosmic inflation, other galaxies and galaxy clusters will eventually race toward the Hubble horizon apparently away from us, placing them effectively out of our sphere of influence....
2
votes
0answers
37 views
Any ideas for an astronomy + ML project? [closed]
I wish to do an astronomy related project which incorporates machine learning.Do you have any suggestions?
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0answers
40 views
Deducing distance of closest neighbor galaxy from expression of correlation and mean galaxy density
The original post was on physics exchange but I prefer to move it here :
I am trying to estimate the distance of closest galaxy neighbor knowing the expression of number of neighbors $\text{d}N$ into ...
3
votes
0answers
116 views
Total number density of galaxies and problematic expression in practise
This post comes from physics exchange forum, I have transfered it here, maybe I will be luckier since I have not received answers on the other forum.
Here is the issue :
I am asked to give the ...
0
votes
1answer
71 views
How long would a photon take to reach us if it's emitted from a galaxy receeding at c?
A galaxy at the boundary of the Hubble sphere is receeding from us at the speed of light, right? If it emits a photon now, how long will it take to reach us?
3
votes
1answer
75 views
Searches for dipole form of anisotropies (on luminosity distances) in the universe with type Ia supernovae
There are many articles like this one Testing the isotropy of the Universe by using the JLA compilation of type-Ia supernovae (PDF, arxiv.org) trying to search for a dipole effect in cosmology with ...
5
votes
1answer
136 views
How will nearby galaxies some day expand beyond our horizon? (Neil deGrasse Tyson)
In his appearance on Television contained in the video The Mystery That Keeps Neil deGrasse Tyson Up At Night, Astronomer andeducator Neil deGrasse Tyson says:
I don’t want to be blamed if you can ...
3
votes
3answers
114 views
Why type Ia supernova reaches its peak absolute magnitude(~ -19.3mag) at around 15-20 days after the explosion?
I'm not sure why type Ia supernova reaches its peak magnitude 15-20 days after the explosion. Is this luminosity generated only by the radioactive decay of nickel-56 to cobalt-56?(considering a half-...
2
votes
2answers
126 views
What happens for a “closed” universe without any content?
Let an universe with no content and positive curvature.
Friedmann-Lemaître equation
$$H^2=\frac{8\pi G}{3}\left(\rho_m+\rho_r+\rho_{\Lambda}\right)-\frac{k\, c^2}{a^2},$$
where $a$ corresponds to the ...
2
votes
1answer
80 views
What exactly is the cosmic definition of ‘now’ is?
There is no cosmic definition of now because of special relativity, because of rate at which time flows for a particular object depends upon your point of view and your velocity. For a rocket ...
5
votes
2answers
99 views
Number Density of Dark Matter Halos
Is there a way to calculate the expected number density of Dark Matter Halos above a given mass, in a certain redshift range, and in a certain area?
1
vote
1answer
110 views
Is it possible that the Universe is expanding due to additional Big Bang events?
Is it theoretically possible that the Universe is expanding due to additional Big Bang events that have occurred at the same location that the original Big Bang occurred at?
Perhaps there has been an ...
-1
votes
2answers
55 views
Do we observe Mars Eclipse or Jupiter Eclipse?
We frequently observe lunar eclipse throughout the year. Have we ever observed Mars Eclipse or Jupiter Eclipse or Saturn eclipse?
-1
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1answer
145 views
What exactly is the accretion disk?
Is it really possible for material such as dust, gas and other stellar debris that come near the black hole, doesn't fall into it, instead forms a flattened band of spinning material?
2
votes
2answers
98 views
Can black holes be used as an energy source
I’ve heard that black hole will be rotating continuously which stores a huge amount of energy in its rotation. Will this energy be accessible, since the rotation is imposed on the space outside the ...
3
votes
1answer
97 views
Are black holes really black? [closed]
I don’t think that black holes are really black as they are the only ones who have the capacity to bend light. At the centre of the black hole the laws of physics are totally defied, black holes ...
4
votes
2answers
217 views
Why does earth not collide with the sun? [duplicate]
Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of ...
2
votes
2answers
175 views
What explains the existence of energy/matter if it cannot be created or destroyed?
If the energy/matter that was contained in the singularity before the Big Bang had always existed, and if it’s true that energy/matter cannot be created or destroyed, then what explains its existence?
5
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4answers
213 views
Influences from outside the observable universe explaining dark energy and expansion?
Could some influence from outside the observable universe be causing the expansion?
0
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1answer
65 views
Fundamental axioms in LCDM
What are the axioms (if any) behind the LCDM model of cosmology?
NB: axioms, not postulates (e.g., inflation)
1
vote
0answers
33 views
Efficiently calculating power spectra from galaxy catalogs
I am dealing with a problem which requires calculating either the correlation function or the power spectrum. At times, the 2-point-correlator seems the better route, at others, the PS. Right now I am ...
1
vote
0answers
80 views
Correctly scaling Power Spectrum and Correlation Function of Galaxies from Mock Catalogues
I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
4
votes
1answer
450 views
Angular Diameter Distance
The definition of the angular diameter distance is the ratio of an object's physical transverse size to its angular size. However when I was reading my textbook, Astrophysics in a Nutshell by Dan Maoz ...
2
votes
2answers
111 views
How far away is this galaxy?
Arp 299 is in the news, with most sources reporting it as 140 or 150 million light-years away. But what kind of distance is this?
The paper which is trhe source for the news item has Supplementary ...
5
votes
1answer
192 views
Generating random galaxy catalogs for correlation functions
The galaxy correlation function, when computed directly from a galaxy catalog, relies on computing a randomly generated counterpart catalog.
When dealing with a simulated galaxy mock-catalog, where ...
2
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0answers
52 views
How to derive the equation for the age of a flat universe with a positive cosmological term?
Can anyone show the steps involved in deriving this equation from the friedmann equation with the cosmological constant involved
7
votes
1answer
129 views
Does the CMB pattern evolve in a human lifetime?
I was thinking, as CMB is a snapshot of a slice of the Universe during recombination, how much the cmb is changing with time ?
I mean as we progress in time we look a CMB a bit more faraway each time, ...
1
vote
1answer
66 views
Question on Friedman equation involving cosmological constant
This is the question:
By considering both the Friedmann and acceleration equations, and assuming a pressureless
Universe, demonstrate that in order to have a static Universe we must have
a closed ...
3
votes
1answer
174 views
Question from Introduction to Modern Cosmolgy by Andrew Liddle
The exact question goes like this:
In the real Universe the expansion is not completely uniform. Rather, galaxies exhibit
some random motion relative to the overall Hubble expansion, known as their
...
4
votes
1answer
255 views
Tests for code that computes two-point correlation function of galaxies
I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
1
vote
1answer
60 views
Who discovered the relation between CMB fluctuations and large-scale structure formation?
I haven't been able to find a reference to the first who showed that the small density fluctuations in the cosmic microwave background led to the formation of large scale structure as we know it.
2
votes
2answers
277 views
Approximate conversion of redshift 'z' to a time and/or distance, when reading papers?
I'm looking for some form of "rough and ready" formula to convert between redshift z value, years since BB, and distance, so that when I read an astronomy paper and it discusses an event that occurred ...
6
votes
1answer
89 views
What is difference between horizon and comoving distances?
I wonder if there a difference between horizon and comoving distances in cosmology ?
I mean both correspond to a null geodesic $(ds^2=0)$ right ?
$$d_{horizon} (t) =a(t) \int\limits_{0}^{r} \frac{{\...
4
votes
4answers
534 views
So where are these measurements of galaxies moving faster than light?
https://physics.stackexchange.com/questions/400457/what-does-general-relativity-say-about-the-relative-velocities-of-objects-that-a
"we can actually observe galaxies that are moving away from us at >...
