Questions tagged [cosmology]
Questions about the origin, history, evolution and fate of the Universe.
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How can the CMB have a “monopole anisotropy”?
Wikipedia's Cosmic Microwave Background (CMBR) radiation monopole anisotropy (ℓ = 0) says
When ℓ = 0, the ${\displaystyle Y(\theta ,\varphi )}{\displaystyle Y(\theta ,\varphi )}$ term reduced to 1, ...
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77 views
I have a scientific paper on gravity, how do I publish it, and will my research benefit me? [closed]
I read a lot about gravity and discovered a hole in it, and I want to publish that hole, but I am not good at mathematics. To show that gap, what should I do? Should I use Newton's equations which are ...
2
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1answer
39 views
Cosmology calculator where curvature density $\Omega_k$ can be independently set?
I am looking for a cosmology calculator that does not have the default that
$$\Omega_k = 1 - (\Omega_r + \Omega_m + \Omega_{\Lambda}).$$
I particularly want to run
$$\Omega_r = \Omega_m = \Omega_{\...
3
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0answers
46 views
Why are CMB peak heights sensitive to the physical densities
A very simple question: why should it be that the CMB power spectrum allows constraints to be placed on the combination of parameters
$$\omega_c = \Omega_c h^2$$
$$\omega_b = \Omega_b h^2$$
as opposed ...
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62 views
Objects beyond 13.3 billion light-years away are (apparently) moving away from us at faster-than-light speeds? [duplicate]
That is, if you use the cosmic distance ladder method, and a value of about 73.5 for the Hubble constant....
But, if you plug in the Planck CMB value of about 67.5, you get a distance of about 14.5 ...
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Medium sized Black Holes
For many years the existence of medium sized Black Holes (IMBH) have eluded scientists. BH of several times the mass of our sun have been found, as well as SMBH with millions of sun masses. SMBH's and ...
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1answer
50 views
Shouldn't the estimate of the universe's age be higher, not lower, after the attractive strength of gravity is taken into account?
From 'Fundamentals: Ten Keys to Reality', by Frank Wilczek :
"Running the movie of cosmic history backward in our minds, we found the galaxies all coming together to meet at a definite time. ...
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Significantly blueshifted Lyman-$\alpha$
Lyman-$\alpha$ line in galaxies is notably known to show a double peaked profile, mainly due to its scattering in a moving medium, see e.g., the very recent Matthee et al. 2021, The X-SHOOTER Lyman-α ...
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How to place an observer in a cosmological simulation
Suppose I have a cosmological simulation with points in xyz space at a certain redshift in a cube like this:
And I have these for many cubes of different redshifts say from z=0 to z=2. Now I want to ...
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Calculating the equation of state parameter for matter
am a grad student in a cosmology class right now and I've got something of a head scratcher.
"Assuming the velocity of the atomic particles can be described by a Maxwellian distribution, ...
2
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1answer
49 views
Demo to get Matter Power Spectrum in cosmology
I would like please to demonstrate the expression of Power spectrum in Cosmology :
First, I have the relative contrast:
$$\delta_{i}(\vec{x}, z) \equiv \rho_{i}(\vec{x}, z) / \bar{\rho}_{i}(z)-1\quad(...
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1answer
54 views
Evidence for homogeneity of the universe
Cosmologists assume that the universe is homogeneous on large scales and, in particular, that the unobservable parts are similar to the observable universe. As I understand it, one important reason ...
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1answer
49 views
How do you calculate comoving distance and light's travel distance? According to the formulae below?
According to Wikipedia, Distance measures (cosmology),
Comoving distance:
$${\displaystyle d_{C}(z)=d_{H}\int _{0}^{z}{\frac {dz'}{E(z')}}}$$
Light-travel distance:
$${\displaystyle d_{T}(z)=d_{H}\int ...
2
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1answer
57 views
Why is the boundary of friends-of-friends (FOF) halo corresponding to iso-density contour?
The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. (For more information, please refer to here and here)
I found that some literature argues ...
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2answers
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Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?
The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. And FOF depends on only one parameter, linking length, $l_\mathrm{link}=b \left(\frac{V_u}{...
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1answer
63 views
Historical estimates of the density parameter
From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the ...
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1answer
79 views
I need to study space, planets and black holes, how do I study them on my own [closed]
I need to study space, planets and black holes, how do I study them on my own, knowing that I am an Arab and I do not know English. I use translation applications. I speak with you. Can you help me ...
2
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1answer
41 views
Optical emission line data archives
I am trying to do exhaustive optical emission line analysis of some active galaxies using archival data.
What are some of the publically available data archives from where one could obtain optical ...
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1answer
62 views
Researching mechanics without “fixed stars”
In the history of humanity, easily observable extra-(Solar System) objects greatly helped understanding certain phenomena inside the Solar System. Importantly, the “precession of the equinox”, and ...
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1answer
51 views
Computing the over-density of Lyman-alpha emitters
Let's assume that I have observations of $N$ galaxies in a projected area $A$, within a redshift range $\Delta z = z_{max} - z_{min}$. What is the correct way of computing their number volume density? ...
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2answers
95 views
Do objects lose momentum as space expands
As far as I know, photons' wavelengths can be considered increasing as space expands, making them lose energy and momentum. Does the same apply to physical objects? I understand a photon's speed is ...
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1answer
90 views
If rotation has nothing to do with gravity, why do astronomers always link swirling dust and gas to stellar evolution?
All my life I was taught in school that we have gravity because the earth spins. Now I know that is not true. Online academics were baffled why anyone thought so. I can only say that's what they ...
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How, exactly, does the precise measurement of the CMB's polarization modes and temperature fluctuations tell us the value of Hubble's 'constant'?
Amidst all the talk a year and change ago about the value of the Hubble parameter reached by the Planck satellite team, and how it's value differed from the value reached by the 'distance-ladder' team(...
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What are the pros and cons of angular cross-correlation in real space and harmonic space?
In observational cosmology people often measure and model cross-correlations between different tracer maps. There are generally two ways to measure cross-correlations:
in real space (two-point ...
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Cosmological density fluctuation dependence on scale factor
I want to show that in the radiation-dominated era, $\delta_H^2 \propto a^{1-n}$. I know that the wavenumber corresponding to the Hubble scale goes like $a^{-1}$ and that $\delta_H^2 \propto k^3 P(k)$ ...
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From what distances do the atoms in you come from?
If the atoms of a human come from stars, comets, nebulas and magnetrons, then what is the greatest distance that two atoms of a human can possibly have been away from each other previously? Perhaps ...
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1answer
851 views
Could there really be a preferential direction to the speed of light?
This question is inspired by a recent video by Veritasium Why no one has measured the speed of light linked below.
To summarize the video, Derek points out that it is impossible to measure the one-way ...
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1answer
77 views
Transverse Comoving distance
I don't understand the link between the comoving distance and transverse comoving distance : how can they be equal ?
Here an example of definition that I have found :
Angular Diameter Distance :
The ...
2
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0answers
100 views
Clarifications about distances in cosmology
I would like to get clarifications about some usual notions of distances in cosmoogy.
First, the comoving distance is the current distance of objets whose light ...
2
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1answer
92 views
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$$
Then there must exist a Big Bang ...
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1answer
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Why does the Hubble flow does not carry photons with it?
I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is ...
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Cross-correlations between Lagrange and Fourier synthesis
In the context of forecast for large surveys, I have to make cross-correlations between 2D (with angular coordinates of Lagrange transformation for GC photometric and Weak Lensing) and 3D (Fourier ...
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1answer
85 views
What is the 'scale factor' equation for a dark-matter dominated universe?
The Friedmann equations can be solved exactly in presence of a perfect fluid with equation of state
$${\displaystyle p=w\rho c^{2}} \qquad p=w\rho c^2$$
where ${\displaystyle p}$ is the pressure, ${\...
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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&...
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108 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 ...
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1answer
38 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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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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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
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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
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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, ...
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1answer
342 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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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 ...
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1answer
126 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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Is the Universe infinite? [duplicate]
https://en.wikipedia.org/wiki/Universe
Is the Universe infinite or finite ?
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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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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
135 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
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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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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 ...
