Questions tagged [cosmology]

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

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86 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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1answer
129 views

The energy density of DM (to be fluid,state parameter=-2)increases with time. Does it violate the concept of accelerating universe?

We know that dark energy with value of state parameter $\omega<-1$ is called phantom dark energy model. In this model if we assume dark energy to be a fluid with value of state parameter $\omega=-2$...
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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 ...
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78 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 ...
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111 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
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1answer
217 views

Flatness of celestial systems like galaxies and Solar System [duplicate]

If we look around the cosmos, we will see most of the systems are just flat disks. Either it is solar system, galaxies, or the rings of Saturn. What is the matter, does it depend on how they were ...
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50 views

How the observed deuterium abundance constrains big bang nucleosynthesis?

I know deuterium can be used as a good tracer to constrain the big bang nucleosynthesis. But can anyone tell me the in details how this is actually done?
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152 views

How to to compute the expected number of collisions between stars when Milkyway and Andromeda merged? [duplicate]

Can some one tell me how you can roughly estimate the number of collisions when Milkyway and Andromeda merged with each other. I know it is somehow related to the mean free path of the star. But ...
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105 views

Galactic Filaments and their connection to Galaxy centre

If alliptical galaxies seem to a line with their associated galactic filament and that orientation is related to connections between the mass of said galaxy and the dominating central object, which ...
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54 views

How many times has the sun orbited the galaxy? [duplicate]

Even if our solar system is not in a stable orbit around the center of the galaxy (is it?), since the birth of our sun, how far has it traveled relative to the center of the galaxy?
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1answer
264 views

Is the Drake equation based on a logical fallacy? [closed]

I often wonder why the Drake equation gets taken so seriously and why popular scientists in general say "it's probable' life exists elsewhere in the cosmos. My problem is that the mechanism for ...
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2answers
142 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?
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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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2answers
81 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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1answer
85 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 ...
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1answer
459 views

Power spectrum of matter in the Universe

I have a picture of a power spectrum of matter in the Universe. What is the connection to Lyman alpha? Please describe it for me (horizontal and vertical and shape).
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1answer
81 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 ...
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1answer
57 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?
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1answer
51 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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1answer
57 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
50 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
67 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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2answers
962 views

Are objects in the universe moving away from each other at the same acceleration?

This is a follow up question to Does the universe expand at the same rate everywhere in the universe?. If the universe is expanding, then it would seem necessary for each object (ie galaxy) to be ...
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1answer
604 views

Hubble law, cosmological redshift and distance

I have got confused with some cosmology formulas which calculate distance. The most famous formula is indeed the Hubble law which says (nowadays) velocity is proportional to Hubble's parameter at ...
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1answer
245 views

Mass resolution

Can someone define mass resolution in terms of cosmological simulations such as the millennium simulation? Is it just the smallest mass that a dark matter particle can have and still be detected by ...
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1answer
93 views

Does cosmological horizon grow or decrease as it radiates?

Black holes decrease as they evaporate and their radius decreases as well. So what is with a cosmological horizon? If cosmological horizon is just a black hole centered at the opposite side of the ...
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1answer
83 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 ...
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1answer
64 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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1answer
152 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 ...
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1answer
128 views

What's the oldest possible age of the universe? (LCDM parameter range)

What would happen if you took today's six-parameter LCDM model down to the 5% confidence level? Understanding that the number of permutations is high, I would be particularly interested just in the ...
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1answer
92 views

Measuring the size and the geometry of Universe [closed]

When we would be able to tell with surety that universe is finite or infinite? How can we exactly determine the density of the universe so, that we compare it with critical density, in order to ...
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2answers
130 views

Why aren't other kinds of energies considered in the mass-energy of the universe?

Mass-Energy of the Universe: 5% ordinary matter, 27% dark matter, 68% dark energy What about other energies such as thermal energy, potential energy, kinetic energy, chemical energy, and radiant ...
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1answer
674 views

Finding equation for energy density of matter

I'm trying to get an equation for the energy density of matter of the universe $\rho(t)$, assuming the number of particles is conserved and rest mass energy is much greater than kinetic energy. $\rho(...
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1answer
114 views

How can space expand in accelerating speed when dark energy is constant?

I came to know that dark energy is constant.As mentioned in the Friedmann equations How can a constant energy cause an ...
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1answer
69 views

Does the universe prefer a velocity?

As I understand it, the distribution of matter is roughly homogeneous across space -- anywhere I stand in the universe, there will about the same amount of matter in every direction around me. Is the ...
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1answer
39 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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1answer
22 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. ...
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1answer
445 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 ...
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1answer
211 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 ...
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1answer
75 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.
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1answer
100 views

Universe and black holes

I know that the radius of the observable universe is about 46.6 billion light years and is expanding at every instant of time due to dark energy . So is it possible that we our entire universe could ...
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1answer
74 views

A good place to start? [duplicate]

I am a high school student (going into 11th grade next year), who will be taking an introduction to astronomy and astrophysics as part of my core curriculum. As such, I would like to get a head start ...
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1answer
1k views

Three-torus model of the universe

I want to ask a question about the Three-torus model of the universe, as described in wikipedia: https://en.wikipedia.org/wiki/Three-torus_model_of_the_universe "is a proposed model describing the ...
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1answer
66 views

Light that travels eternally because of inflation?

What happens with electromagentic waves that are emitted into intergalactic space, but reach not any object because of inflation? Lets assume that photon A and photon B are emitted. Photon B hits ...
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1answer
90 views

Were magnetic monopoles created before, during or after inflation?

In the case that magnetic monopoles exist, when were they created? I thought that all particles were created after inflation, but I've read some comments recently that hint at a pre-inflationary ...
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1answer
116 views

What do cosmologists mean when they talk about “the running of the spectral index”?

All I know is that the spectral index is related to the CMB, and that it has to be slightly less than 1 to favor inflation.
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8 views

Cosmic variance - Fourier Transform - a few precisions

I have posted initially on physics exchange but none answer, so I try my chance here (I am going to delete the physics exchange one) : I cite an original report of a colleague : If we are interested ...
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15 views

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, ...
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30 views

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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21 views

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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