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Questions tagged [cosmology]

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

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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!...
uhoh's user avatar
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6 votes
1 answer
345 views

Collision with Andromeda Galaxy

According to Hubble's law, the universe is expanding and this expansion is proportional to distance between the two objects and thus objects are moving away at a very fast rate, then why is it ...
Holy Answerer's user avatar
10 votes
3 answers
3k views

Cosmological redshift vs doppler redshift

I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift. He ...
user120112's user avatar
7 votes
1 answer
2k views

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?
set5's user avatar
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6 votes
2 answers
9k views

what is a Friedmann model?

Can you explain what is a Friedmann model to a layman? And also give some examples of Friedmann models, specially I would like to know if the lambda-CDM model is considered a Friedmann model.
set5's user avatar
  • 559
13 votes
2 answers
7k views

Observable universe equals its Schwarzschild radius (event horizon)?

The estimated age of the universe is 14 billion years. The estimated Schwarzschild radius (event horizon) of the observable universe is 14 billion light-years. What are the ramifications?
Dirk Helgemo's user avatar
4 votes
2 answers
395 views

Is it "nonsense to even talk about" objects outside the observable universe not having gravitational influence on us? (finite speed of gravity)

In this supplemental answer to Is the zero gravity experienced in ISS the “artificial” kind? in Space Exploration SE I said: Gravity moves at the speed of light so nothing outside out observable ...
uhoh's user avatar
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9 votes
6 answers
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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 ...
cuckoo's user avatar
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29 votes
1 answer
3k 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 ...
uhoh's user avatar
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22 votes
1 answer
3k views

Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?

Nature: Universe has ten times more galaxies than researchers thought NASA feature: Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought Headlines sometimes ...
uhoh's user avatar
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16 votes
5 answers
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Why are all quasars so far away?

Why are all quasars so far away? If the universe is homogeneous, we should expect to have a homogeneous distribution of quasars, but all of then seem to be far away from Earth. Why is that?
Carlos's user avatar
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11 votes
3 answers
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On the cosmological principle

Just more of a conceptual question on the mutual inclusivity of the cosmological principle. That is to say, I was wondering if it were possible to have a Universe that were isotropic but NOT ...
MichaelJRoberts's user avatar
10 votes
1 answer
7k views

When will the Milky Way "arrive" at the Great Attractor, and what all happen then?

The Great Attractor is described as a location to towards which the Milky Way, along with all other galaxies in the Laniakea Supercluster are moving. How long will it take for the Milky Way to "arrive"...
orome's user avatar
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9 votes
4 answers
728 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, ...
eyenstine's user avatar
5 votes
4 answers
2k views

So where are these measurements of galaxies moving faster than light?

https://physics.stackexchange.com/questions/400457/what-does-general-relativity-say-about-the-relative-velocities-of-objects-that-a "we can actually observe galaxies that are moving away from us at >...
John Joe's user avatar
  • 261
4 votes
1 answer
1k 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}} &=...
Cham's user avatar
  • 273
2 votes
2 answers
2k 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?
user avatar
1 vote
1 answer
127 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 ...
Munj Patel's user avatar
18 votes
2 answers
8k views

Understanding The Turnover Point of Angular Diameter Distance

I am trying to get a better understanding of cosmological distances, in particular the angular diameter distance which I have also seen referred to as angular size distance. What I am looking for is ...
Michael's user avatar
  • 183
18 votes
1 answer
482 views

Which came first: black holes or galaxies?

In other terms, did galaxies grow around black holes at their center?
Kyriakos Kyritsis's user avatar
16 votes
2 answers
920 views

Boötes Void numbers, 2016

As a general science (hence Wikipedia!) reader, the latest information I have is that 60 galaxies "have been found and counted" inside the Boötes void, this of 1997. (1) What is the latest count of ...
Fattie's user avatar
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13 votes
4 answers
6k views

How can a black hole have a charge, or be charged?

So-called 'hairless' black holes (no-hair theory, or theorem?) , which is what real black holes are, can be described by just three characteristics: Mass, spin, and charge. It is easy enough to ...
Kurt Hikes's user avatar
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12 votes
3 answers
2k views

Why did astronomers believe most or all stellar black holes had masses no greater than 15 solar masses?

The so-called 'mass gaps' for black holes, according to theoretical models, are between 2-5 solar masses and 50 to 150 solar masses. (Actually, I have read that there is no good theoretical reason ...
Kurt Hikes's user avatar
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9 votes
2 answers
640 views

Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?

This newly updated answer to How precise are the observational measurements for the speed of gravity? and this answer to How is the most accurate value of 𝐺 measured? cites the November 2017 arXiv ...
uhoh's user avatar
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9 votes
1 answer
222 views

Does the CMB pattern evolve in a human lifetime?

I was thinking, as CMB is a snapshot of a slice of the Universe during recombination, how much the cmb is changing with time ? I mean as we progress in time we look a CMB a bit more faraway each time, ...
AlbertBranson's user avatar
9 votes
2 answers
7k views

Which came first: Galaxies <=> Stars <=> Planets?

If a galaxy is defined as a collection of planetary systems (and all matter in between), and a planetary system is defined as a collection of planets circling a star (and all matter in between), and a ...
EveryBitHelps's user avatar
8 votes
1 answer
2k views

Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The (Davis and Lineweaver (2003)) "Expanding Confusion" paper states that "the expected change in redshift due to cosmological acceleration or deceleration is only ∆z ∼ 10^(−8) over 100 ...
Glycoversi's user avatar
8 votes
1 answer
2k views

How likely and severe is the threat of a gamma ray burst to earth?

In the National Geographic article: http://news.nationalgeographic.com/news/2009/04/090403-gamma-ray-extinction_2.html it is suggested that a gamma ray burst likely caused a mass extinction in earth's ...
Jonathan's user avatar
  • 4,381
7 votes
3 answers
11k views

Approximate conversion of redshift 'z' to a time and/or distance, when reading papers?

I'm looking for some form of "rough and ready" formula to convert between redshift z value, years since BB, and distance, so that when I read an astronomy paper and it discusses an event that occurred ...
Stilez's user avatar
  • 1,022
5 votes
1 answer
350 views

If we watched extremely red-shifted galaxies near the edge of the observable universe for a very long time, how would they change? Would more appear?

I have understanding sphere eversion as #1 on my bucket list (if I ever get a round tuit) but understanding metric expansion seems to be a rapidly receding possibility :-) Question: Suppose it takes ...
uhoh's user avatar
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5 votes
2 answers
900 views

Comoving distance between two points [(RA1, Dec1, z1) and (RA2, Dec2, z2)]

I have Redshift (z), RA and Dec positions for two points in a galaxy catalogue (say P1 = [<...
Ling Guo's user avatar
  • 151
4 votes
1 answer
1k views

Is the local group bound to the Virgo cluster?

The Virgo Cluster is currently red-shifted by 0.003 according to Wikipedia; this indicates that the cluster is expanding. Will the cluster's enormous gravity eventually "win" and pull the ...
Kevin Kostlan's user avatar
4 votes
2 answers
289 views

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 ...
jvriesem's user avatar
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3 votes
1 answer
159 views

Intergalactic Lyman-alpha absorption for high redshift quasars (Gunn-Peterson effect)

This is a follow up to a recent question on SE asking about the apparent suppression of radiation shortward of the (red-shifted) Ly-$\alpha$ line of a quasar at redshift $z=6.53$. The general ...
Thomas's user avatar
  • 3,504
3 votes
1 answer
304 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 ...
Pulchritude's user avatar
2 votes
0 answers
565 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?
Pranav Satheesh's user avatar
2 votes
1 answer
472 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 ...
user's user avatar
  • 21
21 votes
2 answers
4k views

What equation tells you how far in space you can go from a point and return?

We know that hubble expansion increases the distance between points in space, and that the cosmological event horizon represents the distance from the observer at which objects are receding faster ...
user52978's user avatar
  • 313
21 votes
1 answer
493 views

Dark Matter Particle Candidates

Dark matter appears to dominate the matter component of the universe as compared to luminous, or baryonic, matter. Though it does not interact electromagnetically (it doesn't absorb, scatter, or emit ...
astromax's user avatar
  • 5,953
20 votes
3 answers
2k views

Existence of gravitons?

For a lot of my uninformed life, I have doubted the existence of gravitons or even that gravity is an actual "force" (like electromagnetism). This is because my vision of general relativity was that ...
Jack R. Woods's user avatar
16 votes
3 answers
2k views

How do we know dark matter/dark energy exists?

I've never quite understood the theory behind why dark matter and dark energy exist. I know it has something to do with gravitational pull being stronger than what we calculate it SHOULD be, could ...
Rich's user avatar
  • 609
14 votes
1 answer
1k views

How is the Hubble constant determined from gravitational waves?

We know there is a discrepancy between measurements of the Hubble constant, $H_0$. On one side there is the method of the Planck mission, where they use the CMB and the $\Lambda$CDM model to determine ...
PrincepsMaximus's user avatar
13 votes
1 answer
700 views

What is the mass of hotspots in the Cosmic Microwave Background radiation?

A lot of pop science articles (for example, this Space.com article) discuss the cold spots on the WMAP data of Cosmic Microwave Background (CMB) radiation, but have any studies been done on the large ...
called2voyage's user avatar
  • 6,300
11 votes
2 answers
583 views

Recommendation for introductory cosmology text

I am looking for recommendations for an introductory text (or texts) on cosmology on the advanced undergrad or beginning grad level. I am coming from primarily a physics undergrad background (doing ...
NeutronStar's user avatar
  • 2,673
11 votes
2 answers
2k views

Are we moving ever closer to the center of our Galaxy due to a super massive black hole?

I've seen in documentaries that at the center of each galaxy is a super massive black hole which holds the galaxy together. Since black holes have such a strong pull, are we slowly being pulled in ...
Rich's user avatar
  • 609
11 votes
2 answers
671 views

How long would it take for a rogue planet to evaporate in the late stages of the Universe?

I've read once a popular science book, in which the author calculated how long would it take for the most massive black holes to evaporate because of Hawking radiation. He claimed that after that time ...
user avatar
9 votes
4 answers
1k views

Is the Universe really expanding at an increasing rate?

Here's what I just read from Wikipedia's page on the Hubble Space Telescope: While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. ...
DrZ214's user avatar
  • 1,970
8 votes
1 answer
1k views

What percentage of the hydrogen today has never been in a star

It stands to reason that some of the hydrogen and helium that formed directly as a product of the big bang might never have fallen into a star to re-ejected when that star explodes. My question is, ...
AgilePro's user avatar
  • 183
7 votes
1 answer
849 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 ...
Kurt Hikes's user avatar
  • 5,177
7 votes
2 answers
764 views

How did UV from the earliest stars 'alter the state of the 21 cm line' such that it shows up in CMB today?

In this question I discuss the recent (open access) paper in Nature An absorption profile centred at 78 megahertz in the sky-averaged spectrum at length. The abstract begins: After stars formed in ...
uhoh's user avatar
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