Questions tagged [cosmology]

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

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Evolution of the Hubble parameter

In the lambda-CDM model describing an accelerating Universe, the Hubble parameter is currently decreasing with time. Will it continue to decrease forever?
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How to disentangle a very distant star's relative velocity vs. redshift distance

We measure a star's relative velocity towards or away from us via its Doppler-shifted spectrum. This is also how we measure the distance of very distant stars: measuring the shifts in the spectrum ...
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What does Stephen Hawking mean by 'an infinite universe'?

In the recent $100m search for extra terrestrial life project project, Stephen Hawking is quoted in the following way: "We believe that life arose spontaneously on Earth," Hawking said at Monday's ...
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166 views

Assumptions necessary for the strong form of Olber's Paradox

From Wikipedia, here's Lord Kelvin's statement of Olbers' Paradox: Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by ...
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113 views

How do we measure the mass of a star?

I recently came across the book intro to cosmology by j.v. Narlikar, where it mentions about quarks and other star and inadvertently uses mass of sun as reference. I want to know how me measure mass ...
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161 views

Critical density of the universe (no, not that one)?

I seem to recall reading something that said if the density of the universe was off by as little as one atom per cubic meter, the universe, as we know it, would not exist; one less and there wouldn't ...
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How can cosmic inflation make an infinite universe homogeneous?

As it is explained in this video, one of cosmic inflation's observable effects is the homogeneity of our universe. Inflation allows two points on the different sides of the observable universe to be ...
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224 views

Is mass+energy conserved when a new universe forms inside a black hole?

My understanding is that there are credible theories out there in which the formation of a singularity in a black hole also represents the beginning of a new universe via a big bang. We can't see the ...
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62 views

Galactic revolutions relative to the age of the universe [duplicate]

If the universe is only 40 to 50 galactic revolutions old, how has it acquired its present state so "quickly" relative to this time scale?
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Formation of Cosmic Microwave Background

It is said that the cosmic microwave radiation (CMB) was formed when the universe was 379,000 years old. How is this calculated?
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How did the reionization epoch make the universe “transparent”?

I've been reading about the reionization epoch of the universe, and it isn't entirely clear to me. As I understand it, very shortly after the big bang the universe was ionized, with the electrons ...
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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 ...
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245 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 ...
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How are the element abundances calculated for a meteorite in the Hydrogen log10 scale?

In astronomy, solar abundances are often calculated or tabulated per $10^{12}$ H atoms. I understand that in case of the Sun this can be done because H atoms are in majority or are present for a scale ...
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what is the current explanation for the formation of cosmic voids?

Why aren't galaxies distributed evenly in space, but instead form those sponge-like structures, with huge voids between filaments?
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175 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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597 views

How can the observable universe shrink in a Big Rip?

As far as I know, the Big Rip occurs when the scale factor reaches infinity in a finite time. This will only happen in a universe dominated by phantom energy (i.e. a universe with an equation of state ...
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445 views

Distinction between metric expansion and objects just moving apart from each other?

I see explanations likes this: Galaxies and other sources, then, are not strictly expanding away from each other but rather are attached to the fixed grid on the expanding fabric of spacetime. Thus, ...
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275 views

Are galaxies moving away from us faster than before?

Are all galaxies moving away from us at constant speed even those that may be moving in our direction as the space is being formed? How does nothingness appear to push matter? A black hole sucks as ...
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87 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 $r_0=\int_{t_{em}}^{to} c/a_{(t)} dt$ ...
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133 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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2k views

Angular Diameter Distance

The definition of the angular diameter distance is the ratio of an object's physical transverse size to its angular size. However when I was reading my textbook, Astrophysics in a Nutshell by Dan Maoz ...
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827 views

Tests for code that computes two-point correlation function of galaxies

I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
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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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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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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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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 ...
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Why didn't we have inflation when the theory of everything and GUTS broke symmetry? [closed]

Inflation seems to have occurred when the symmetry breaking of the electroweak occurred. But do we know of any reason why we did not have inflation when gravity separated from the other forces or ...
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Missing Terms in Weinberg's treatment of perturbations on Newtonian Cosmology

I was reading Appendix F of Steven Weingberg's book "Cosmology". In this Appendix he works out the perturbations to a cosmological fluid described by non-relativistic hydrodynamics and Newtonian ...
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What is the effect of this expanding universe on the energy density of it?

We know that our universe is expanding i.e. volume of the universe is getting larger with time with total energy being conserved. My question is whether the energy density is getting smaller with ...
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753 views

Latest cosmological parameters

I'm looking for the latest values (with uncertainties) of the four main cosmological density parameters $\Omega_i$ : \begin{align}\tag{1} \Omega_{\text{mat}} &={} ?, &\Omega_{\text{rad}} &=...
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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 ...
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171 views

Where are heavier elements than iron produced?

Specifically in the milky way, if that changes anything. I feel like it should be in the disk, since stars there are richer in heavier elements, but I'm not sure.
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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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186 views

Time dilation due to the expansion of the universe

This is not a homework question. I want some help in being able to clearly perceive the expansion of the universe and the consequences thereof. We know that the universe is expanding in an ...
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1answer
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What will the universe be like in a googolplex years time?

It will be cold and dark in 10^(10^100) years. What else can be said of the state of things in the far distant future? Will everything have fallen into supermassive black holes and then radiated ...
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355 views

Star versus Black Hole

I'm making a simplified animation for college project on how a star reacts with a black hole. I had known that a black hole sucks in everything and thus should be doing the same for stars. But upon my ...
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81 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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1answer
115 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 ...
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79 views

Does wavelength affect redshift caused by the metric expansion of space?

Does wavelength play a role in cosmological redshift? Do we see certain wavelengths affected less by expansion or notice any delay in arrival of certain wavelengths? How accurate are any observations?...
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323 views

Can the Universe be spatially closed and expand forever?

In the lambda-CDM model, the Universe is spatially flat, and will expand forever. Is possible that a spatially closed Universe expands forever?
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145 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 ...
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1answer
37 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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2answers
114 views

Question about $3p/c^2$ in Friedmann equation

The Friedmann Equation: $$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}$$ I want to know, where does the $3p/c^2$ come from. I know it is something ...
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360 views

What is left of a universe where no matter and energy exist and is there a term for this?

In the final energy state of the universe, there exists a hypothesis that all matter will have fallen into black holes and neutron stars. If, through Hawking radiation, after all matter is consumed ...
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1answer
237 views

How could we tell if the Universe is infinite? [duplicate]

I've heard from a number of people that the Universe is infinite, and from my perspective, I can't see how this can actually be known, especially given that the Universe started out a finite size and ...
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409 views

Looking for help in understanding how black holes can move

I'm looking for help in understanding how black holes can move through space. This is probably a trivial question for anyone who knows astronomy, but probably due to my lack of education on the topic, ...
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1answer
164 views

Original Hubble diagram units incorrect?

Figure 1 from Hubble 1929 (original paper with open access pdf) has velocity with units of km. Is this just a typo or was there a reason the plot is printed this way?
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What is assembly bias in context of cosmology?

I see the term "assembly bias" being used quite a bit in cosmology literature? I have not been able to find a good primer on what it means, and why it is important.

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