Questions tagged [cosmology]

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

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301 views

Will the Universe end by time stopping?

I know there are several theories as to how the universe might end, if it does. Is it possible for it to end by time stopping? A friend of mine told me about this theory, but her explanation that it ...
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1answer
174 views

Age of the universe and age of stars

The age of the universe is 13.798±0.037 billion years, yet the age of HD 140283 is 14.46±0.8 billion years, how this can be the case?
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152 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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1answer
143 views

How does inflation justify the nonexistence of magnetic monopoles?

It is said that inflation justifies that magnetic monopoles don't exist. Can anyone explain how inflation theory explains the non existence of magnetic monopoles?
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How wary should I be of old cosmology books?

Recently I bought a couple of books on cosmology from a second-hand bookshop. One was published in 1993 and the other is from the 1970. I know, from a recent book, that the inflaton theory is ...
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1answer
51 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
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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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
79 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)}$...
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1answer
138 views

Difference between 3D real space and 3D redshift space?

With regards to cosmological simulations (e.g., N-body dark matter only simulations), I've seen people talk about the positions of objects (halos, galaxies) in the simulation in terms of 3D real space ...
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169 views

What would the CMB look like somewhere else in the universe?

An answer to a question on Worldbuilding.SE opined that if you were suddenly transported an unknown distance and direction away by magic you could use the Cosmic Microwave Background to determine your ...
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830 views

What is the cosmic mean density?

I know cosmic mean density for baryons is around $(2\ \mathrm{to}\ 5)\times10^{-31}\ \mathrm{g/cm^3}$, assuming that Hubble's Constant is $100\ \mathrm{km/s/Mpc}$. But what is the current cosmic mean ...
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48 views

Does Big Bang red shift affect the frequency of both light waves and individual photons?

With regard to the wave/particle duality of light, does Big Bang red shift affect both the frequency of light waves, and, the frequency of individual photons? I assume that the e=hf formula relates to ...
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417 views

Future redshifting and effect on the 'pitch' of CMB radiation

After discovering this question exploring the sound of a blackbody, I started wondering about the sound of the Cosmic Microwave Background radiation from the Big Bang, specifically what the current ...
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1answer
488 views

Physical laws in other universes

In the theory of multi-verses, we can imagine countless other universes where the constants of nature, including the amount of dark energy in a volume of space and possibly even shapes of hidden ...
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1answer
161 views

Are galaxies moving away with constant acceleration?

Suppose that we could measure the current acceleration with respect to us of a distant galaxy chosen at random in our observable universe (Ignore the peculiar motion of the galaxy). Is the ...
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1answer
136 views

Difference in redshift between 2 interacting galaxies

I have a galaxy 'A', say at redshift 1. Let's suppose this galaxy has no peculiar velocity. What would be the redshift of another galaxy 'B', that has a radial velocity of 500 km/s, relative to A? ...
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1answer
229 views

Beta profile fit of Virgo cluster gas?

I'm looking for the parameters $r_c$, $\rho_0$ and $\beta$ in a $\beta$-profile fit of the Virgo cluster's ICM density. I just can't find a reference for it. Unlike for Coma (Abell 1656) where it's in ...
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137 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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131 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' ...
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92 views

Could different views of the universe simply be different points in time?

I'm not sure if this is the correct stackexchange site for this question. I've been reading this article on Vice Motherboard There’s Growing Evidence That the Universe Is Connected by Giant ...
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88 views

Why do we expect Type Ia Supernovae to be brighter in a decelerating Universe than what has been observed?

This seminal review paper in the PNAS by Robert P. Kirshner (https://www.pnas.org/content/96/8/4224) says the following: If the universe had been decelerating—–in the way it would if it contained ...
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70 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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144 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
55 views

galaxies in a specific radius in different shapes of universe

Let the distribution of galaxies be a number like $n$. we want to find the total number of galaxies within the radius $r$ from a specified point. in which shape of the universe (flat, open, closed) ...
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534 views

How can a naked singularity be possible?

How can we have a singularity, without a black hole around it? How would one form? What does it do?
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1answer
109 views

The largest discrepancy in the history of science

I watched a cosmology lecture given by Leonard Susskind that convinced me to put my money on dark energy being a pressure caused by energy that is evenly distributed throughout the "vacuum" of space. ...
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77 views

Cosmic microwave background radiation

If the big bang is true, after the emission of light from the hydrogen plasma, the universe was still expanding. Why would we expect to see uniform radiation if earth very well could have formed ...
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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_{\...
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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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87 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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90 views

Neutrino Modelling in Friedmann Equation

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 ...
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77 views

Redshift and Velocity relation

Let us say that we have an object so its total velocity is defined as \begin{equation} v_{tot} = v_{pec} + V_{rec} \end{equation} Where \begin{equation} V_{rec} = H_0r \end{equation} and \...
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56 views

Redshift, velocity, distance

How are redshift, (recessional) velocity and distance of a galaxy related? I understand what v=HD states. So I assume that Hubble measured a direct relation between the distance and recessional ...
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1answer
62 views

How might WFIRST help us better understand dark energy?

In the video linked in the Space.com article What Would It Mean for Astronomers If the WFIRST Space Telescope Is Killed?, (available in YouTube as WFIRST: The Best of Both Worlds) after ...
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1answer
107 views

Ionization Fraction of Helium in alternate, He dominated early universe

I'm asked to verify an expression for the ionization fraction of helium in a universe in which helium dominates baryonic matter. I'm given that the ionization fraction $X = \frac{n_{He^+}}{n_{He^+}+...
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1answer
523 views

Will Hubble's law always be accurate?

We have Hubble's law: $$v=Hd$$ where H is the Hubble parameter, which is decreasing in value, but it will be constant in the distant future. So, assuming the Standard Model of Cosmology holds true, ...
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44 views

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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79 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 ...
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102 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 ...
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53 views

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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56 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 ...
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71 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 ...
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51 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 ...
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95 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 ...
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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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128 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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68 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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75 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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106 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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