Questions tagged [cosmology]
Questions about the origin, history, evolution and fate of the Universe.
317
questions
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1answer
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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. ...
8
votes
2answers
462 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 𝐺 measured? cites the November 2017 arXiv ...
2
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1answer
139 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
vote
0answers
14 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
18 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
votes
1answer
57 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
votes
1answer
51 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
votes
1answer
43 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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0answers
24 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
votes
1answer
54 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
38 views
Which redshift is used to determine the Hubbleconstant?
I think they measure cosmological redshift to use in the Law of Hubble-Lemaître together with the distance to calculate $H_0$. Is this correct, or do they use Doppler shift (too)?
$H_0$ indicates how ...
1
vote
1answer
61 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 '...
4
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0answers
71 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
votes
3answers
1k views
Does a merging massive binary black hole ‘emits’ 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 ‘see’ it simultaneously ...
3
votes
1answer
339 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 ...
0
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0answers
61 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
29 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 ...
2
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0answers
66 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 ...
2
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0answers
50 views
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
votes
1answer
79 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
votes
1answer
64 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
702 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
77 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 ...
3
votes
1answer
75 views
'Little h' usage in cosmological simulations
I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
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0answers
24 views
How to derive the formula of magnitude error caused by peculiar velocity?
I want to understand how to derive the formula of magnitude error ($\sigma_m$) which is caused by peculiar velocity of a galaxy, like this
$$
\sigma_m = \frac{5\ \sigma_V}{cz\ \rm{ln(10)}}
$$
,where $\...
1
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0answers
42 views
Astronomy A Physical Perspective [closed]
Is "Astronomy: A Physical Perspective" by M. Kutner written in 2003 a good introductory textbook? What other textbooks could you recommend? I want to study astronomy on my own but with scientific ...
1
vote
1answer
90 views
Why dimmer high-redshift supernovae means the expansion is accelerating, if the dilated region pertains the distant past?
I must be seeing this wrong, because it seems to me the data indicates faster expansion in the past, contrary to Adam Reiss' study that led to the birth of the notion of 'dark energy'.
In Reiss' ...
2
votes
1answer
61 views
Neutrino Modelling in Friedmann Equation
so I'm trying to model neutrinos in the Friedmann Equation. I've covered the case of the Benchmark Model where we have matter, radiation, curvature, and the cosmological constant, Lambda. I know my ...
2
votes
0answers
33 views
Paper recommendations for career guidance
I'm an undergraduate student in astronomy and I'm one year from graduating, but I feel I still can't choose what career path I want to pursue. To date I've only had two courses of proper astrophysics ...
5
votes
2answers
155 views
What good is the Einstein de-Sitter model?
Why is the EdS model still taught and used to predict the age of the universe if ours is not an EdS Universe. Does this model have any pros?
1
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1answer
78 views
What exactly is a “conclusive association” in Astronomy?
Phys.org's Astronomers use slime mold model to reveal dark threads of the cosmic web says
A computational approach inspired by the growth patterns of a bright yellow slime mold has enabled a team of ...
2
votes
2answers
142 views
The Matter Anti Matter Asymmetry
The matter anti matter asymmetry is still a mystery that still can't be explained. But what if there wasn't an asymmetry? Isn't it possible that there was exactly equal amounts of matter and anti ...
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2answers
98 views
Create an inhabited black hole
Hello I was watching one of Isaac Arthur's videos about the end of time and i just got a crazy idea
Is it theoretically possible to "create" a "solar system" that would look like a supermasive black ...
2
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0answers
85 views
Is the Big Bounce Theory valid? If so does that mean all this have happened before?
IF the universe is just an infinite loop of events from expanding to contracting does that mean all this has happened before? The initial conditions for the next big bang would (or rather could) be ...
1
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0answers
32 views
Absorption line in quasar spectrum
I have quasar spectrum which includes emission and absorption lines. I need to find absorption lines in between 2 wavelength. How can I do it with coding?
2
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0answers
71 views
Force to fix $\sigma_{8}$ or $A_{s}$ for Forecasts
In the context of Forecasts with Fisher's formalism, I make vary cosmological parameters to compute the elements of the Fisher matrix.
First, I generate with CAMB code a linear power spectrum. Then, ...
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0answers
78 views
Could the Sun's planetary nebula be dangerous for the planets?
Since the Sun's mass isn't high enough it won't go supernova being a red giant but instead release a planetary nebula. So that seems to be kind of a substitute for a supernova. Would ejecting the ...
2
votes
1answer
107 views
Did Einstein supported Big Bang Theory cosmological model?
Einstein made many predictions, including gravitational waves and the possibility of black holes.
Relativity is taken into consideration for the Big Bang model, so did Einstein agree with it or did ...
2
votes
3answers
78 views
light emission of quasars
I'm studying Astrophysics and I have some video's from a past course. I'm currently studying the reionization of the universe. At the current point in the class, the instructor is talking about ...
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0answers
30 views
Nebula and its Colours [duplicate]
A Nebula boasts of Too Bright Awesome Colors that includes red, blue, green, orange, white, etc?
They're result of excitation of electrons.
Is there a clear explanation as in which Color is ...
4
votes
1answer
62 views
How is the Dating of Samples of Cosmic Objects Performed, Including Pre-Solar Dust?
In the new Space.com article 7 billion-year-old stardust is oldest material found on Earth I had read about some pre-solar grains dating back to 7 billion years. What techniques are used in ...
1
vote
1answer
50 views
Metal lines in Lyman-alpha forest
I need to find metal lines in the Lyman-alpha forest of quasar spectra, without the red side of lyman alpha emission line.
Can you guide me how to do this?
0
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1answer
74 views
Why is constant velocity drift not an accepted explanation of Hubble expansion?
Edward A Milne at the Oxford University proposed that what Hubble was
observing was simply a natural sorting of random galactic motions. If
a group of galaxies formed together moving at various ...
3
votes
2answers
76 views
Are there predictions of the distribution of matter at the end of time
Are there any theoretical predictions or "educated speculation" by cosmologists on how matter will be distributed at very, very distant times in the future?
For example, will matter be:
agglutinated ...
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0answers
23 views
Formula to Compute sigma8 for correction in non-linear regime
I try to explain my issue on this forum since it seems to be appropriate.
I need to apply a correction on $\sigma_{8}$ between linear and non-linear regime to keep it fixed (I make change the values ...
0
votes
1answer
44 views
What reference frame do age-of-the-universe calculations assume?
I'm thinking, in particular, about general relativity. When we speak, for example, of neutrino decoupling, what do we mean when we say this happened in the first second after the Big Bang? Do we mean ...
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0answers
125 views
Scale factor 0 for expanding universe
Does the FLRW model have a scale factor $0$ at the singularity, meaning that even infinite distances become $0$ there according to the mapping $x \mapsto a(t)x$, for $a(t)=0$?
I am asking this ...
2
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2answers
105 views
Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant?
The Supernova 2006cm has a redshift of 0.0153 which translates into a recession speed of 4600 km/s.
It has a distance modulus of 34.71 which translates into a luminosity distance of 87 Mpc.
This ...
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0answers
91 views
Aligning of 7 unidentified objects in the sky, what are they? What did I see? [duplicate]
So this morning (4th December 2019, 0518hrs) I was out turning my horses out in the fields and noticed a strange arrangement of what I perceive to be stars, only I’ve never seen it before and wondered ...
15
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1answer
296 views
A Sky Map of Gravity - what would it look like?
PREAMBLE
Sky Maps exist which show the distribution and intensity of electromagnetic radiation received on Earth at various wavelengths. An obvious form is a map of the stars (Star Chart).
Maps ...
