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Questions tagged [cosmology]

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

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56 views

Black holes in galactic rotation [on hold]

I'm wondering whether the total mass of a galaxy's hypothetical black holes, that may have each been formed within the gravitational horizons of larger black holes (that are indirectly apparent, ...
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2answers
71 views

The Expansion Of The Universe [on hold]

I am informed that a volume of nothingness can expand many times over for no good reason,becoming filled as it does so by colossal numbers of virtual particles which appear ultra-briefly out of ...
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1answer
52 views

Discrepancy Between Results of Cosmological Observations & the Assertions of Quantum Mechanics [closed]

Quantum mechanics requires a vacuum energy 40 orders of magnitude higher than that indicated by cosmological observations. Which side is most likely to be right,& when the truth is finally ...
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0answers
36 views

How are constraints on dark energy improved the most?

Today we have various data available from independent probes to constrain dark energy, i.e. its equation of state $w$. These include Type 1a supernovae, BAO, CMB, large-scale galaxy surveys, etc. Are ...
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1answer
144 views

Where do Neutrinos Go if They Don't Interact?

It is said that neutrinos can travel for billons of years at almost the speed of light without interacting with anything. That being the case,what do they do - is it possible they could orbit the ...
4
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1answer
148 views

Has the Big Crunch been ruled out?

If the dark energy equation of state $\omega$ evolves with time, and becomes larger, this would result in decelerated expansion and eventually a Big Crunch. If $\omega$ decreases with time, becoming ...
2
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1answer
40 views

How is the expansion rate of the universe obtained from Baryon Acoustic Oscillations?

I understand that we can measure the sound horizon from the anisotropies in the CMB. Large galaxy surveys today can construct two-point correlation functions, which reveal peaks at distances equal to ...
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0answers
43 views

Expansion of space or constriction of length scales?

According to wikipedia, "Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. Although ...
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0answers
19 views

How is “surface brightness dimming” derived from redshift in the NED extragalactic database

I have several questions regarding “Surface Brightness Dimming” in the “Quantities Derived from Redshift” section of the Nasa/ipac Extragalactic Database. As an example, please use quasar IRAS F08279+...
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0answers
51 views

Understanding the statistical mechanics of Recombination Epoch (“Cosmology” Weinberg)

In Steven Weinberg's 'Cosmology' Chapter 2.3 (pg 113), he begins with a number density for some particle via the Maxwell-Boltzmann Formula: While I have some familiarity with statistical mechanics, I ...
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1answer
96 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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2answers
239 views

What covers the outside of the Universe? [closed]

Is the Universe a sphere with water or ice surrounding its' confinement? Are there any theories that imply the Universe being confined within a sphere of water/ice material?
2
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1answer
177 views

What exactly is wrong with this plaque in Neil deGrasse Tyson's “back yard”? Has it been fixed?

The Wikipedia subsection Observable Universe; Misconceptions on its size shows this image of a plaque, with the caption: An example of the misconception that the radius of the observable universe ...
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2answers
183 views

Are there ways to estimate size of the “whole universe”?

Words escape me, but by "whole universe" (I think) I mean everything that's spatially connected to the observable universe in a conventional sense. If there is a better term for it, please let me know!...
22
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1answer
2k views

When will the number of stars be a maximum?

There are very roughly a "mole" of stars in the universe. Wikipedia quotes an estimate of $3 \times 10^{23}$ though the number is associated with some debate and uncertainly. I'd like to know if ...
4
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3answers
135 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
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0answers
25 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 ...
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1answer
24 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 ...
3
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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 ...
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1answer
52 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 ...
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5answers
685 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
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1answer
78 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
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1answer
81 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
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1answer
55 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.
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1answer
47 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
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0answers
59 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 ...
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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?
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1answer
322 views

Is our universe stable or metastable?

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 “...
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6answers
435 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 ...
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1answer
77 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
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0answers
40 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
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0answers
123 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 ...
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1answer
75 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
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1answer
76 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 ...
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1answer
145 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 ...
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3answers
120 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
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2answers
128 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 ...
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1answer
84 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 ...
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2answers
118 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?
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1answer
120 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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2answers
59 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
157 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
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2answers
105 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
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1answer
101 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
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2answers
238 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 ...
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2answers
188 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?
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4answers
223 views

Influences from outside the observable universe explaining dark energy and expansion?

Could some influence from outside the observable universe be causing the expansion?
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1answer
67 views

Fundamental axioms in LCDM

What are the axioms (if any) behind the LCDM model of cosmology? NB: axioms, not postulates (e.g., inflation)
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0answers
37 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 ...