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Questions tagged [cosmology]

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

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52 votes
2 answers
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Two species of dark matter?

At this point in time, evidence for the existence of dark matter has accumulated in many ways: it affects galactic rotation curves plays a major role in cosmology, and the evolution of structure in ...
Alexey Bobrick's user avatar
45 votes
1 answer
50k views

Shouldn't very very distant objects appear magnified?

My understanding of the expansion of space itself is very shaky, but if space itself is expanding, then shouldn't very very distant objects appear magnified? For example these James Webb Space ...
orion elenzil's user avatar
31 votes
1 answer
7k views

Why didn't the Big Bang produce heavier elements?

Shortly after the Big Bang, temperatures cooled from the Planck temperature. Once temperatures lowered to 116 gigakelvins, nucleosynthesis took place and helium, lithium and trace amounts of other ...
Sir Cumference's user avatar
30 votes
6 answers
6k views

Is the universe considered to be flat?

I've read various articles and books (like this one) stating that we are not certain about the geometry of the universe, but there were experiments on-going or planned that would help us find out. ...
ivanmp's user avatar
  • 403
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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26 votes
7 answers
6k views

How do we know the laws of physics are the same throughout the universe?

How do we know the laws of physics are the same throughout the universe? Intuitively I would say they would vary in two natural ways: the constants in the equations may vary or the math in the ...
qwerty10's user avatar
  • 395
25 votes
3 answers
2k views

How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
user2346333's user avatar
25 votes
2 answers
903 views

Future of CMB observations: How will our knowledge of the early universe change?

The Planck satellite has been presented and awaited for a long time as the ultimate experiments for measuring temperature fluctuations in the cosmic microwave background (CMB) over the full sky. One ...
Francesco Montesano's user avatar
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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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
3k views

Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?

Dark matter is described as being spread not only throughout a galaxy, but also around it in a halo of some sort that extends far beyond the visible parts of the galaxy... In fact, dark matter haloes ...
Kurt Hikes's user avatar
  • 5,177
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
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
17 votes
11 answers
4k views

Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

When we look in all directions, we see distant objects red-shifted, with the size of the red-shift correlated with the distance from us. As I understand it, the consensus among cosmologists is that ...
Brionius's user avatar
  • 331
17 votes
1 answer
3k views

Why do cosmologists assume that inflation began shortly after the Big Bang, rather than at the exact start?

Somehow, I have never come across an explanation of why cosmologists claim that the alleged inflation of the very early universe occurred not at the Big Bang, but very shortly afterwards (~10^-36 to ...
Kurt Hikes's user avatar
  • 5,177
16 votes
9 answers
11k views

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

It strikes me as contradictory that the scientific community will say that we don't know what dark matter is, but be happy to state things like "dark matter makes up about 85% of the cosmos" ...
Zinn's user avatar
  • 405
16 votes
5 answers
7k views

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
  • 493
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
16 votes
1 answer
632 views

A Sky Map of Gravity - what would it look like?

PREAMBLE Sky Maps exist which show the distribution and intensity of electromagnetic radiation received on Earth at various wavelengths. An obvious form is a map of the stars (Star Chart). Maps ...
steveOw's user avatar
  • 1,444
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
  • 946
16 votes
1 answer
533 views

How exactly does inflation convert random gravity fluctuations into coherent gravitational waves?

In the course of this very enjoyable press announcement, it is mentioned that inflation can create gravity waves by amplifying gravity fluctuations. I do not properly understand this statement. I ...
Dilaton's user avatar
  • 809
15 votes
6 answers
4k views

What's the issue with Olbers' paradox?

I'm not quite grasping the reasoning behind Olbers' Paradox, or why an eternal, non-expanding and spatially infinite universe would be incompatible with a dark sky. For simplicity, let's suppose that ...
Alex Popescu's user avatar
15 votes
3 answers
982 views

What is the ultimate fate of a cluster of galaxies?

We're fairly aware clusters of galaxies drift apart due to space expansion, which will drive them out of each other's cosmic event horizon eventually, leaving them separate, alone, each a single "body"...
SF.'s user avatar
  • 6,269
15 votes
1 answer
269 views

How Does Cosmology Constrain the Number of Neutrino Species?

I've asked this question already on the Physics SE, but I felt it would be worth posting here as well. I know that based upon theories of structure formation cosmologists can constrain the sum of the ...
astromax's user avatar
  • 5,953
15 votes
1 answer
723 views

How to determine scalar-to-tensor ratio r from CMB polarization spectrum?

CMB polarization spectrum can tell us about the primordial scalar and tensor perturbation. By analyze B and E mode angular spectrum power spectrum and temperature power spectrum we can determine the ...
Ziang Yan's user avatar
  • 151
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
14 votes
1 answer
914 views

What happened to the reemitted photons during recombination?

To my understanding, the Cosmic Microwave Background Radiation (CMBR) is light released during the recombination epoch where the formation of neutral hydrogen atoms allowed for a sudden drop in the ...
YiFan's user avatar
  • 558
13 votes
2 answers
4k views

Why do gravitational waves travel "only" at the speed of light but the gravitational scalar potential is instantaneous?

To my old Space SE question Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch? @DavidHammen's excellent answer replies that one should not add a ...
uhoh's user avatar
  • 30.6k
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
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
  • 5,177
13 votes
2 answers
9k views

Why is there a supermassive black hole at the center of every galaxy?

According to https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole) the gravitational sphere of influence of a supermassive black hole is really limited compared to the size of its hosting ...
betolink's user avatar
  • 143
13 votes
1 answer
698 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
  • 6,290
13 votes
1 answer
424 views

The recent results on Hubble constant measurements

In recent news there is this announcement In the introduction they say: Distance measurement discrepancy: a $4.4σ$ tension on the value of $H_0$ and I understand the discrepancy is with the value ...
anna v's user avatar
  • 233
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
  • 5,177
12 votes
2 answers
3k views

Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. And FOF depends on only one parameter, linking length, $l_\mathrm{link}=b \left(\frac{V_u}{...
Wang Yun's user avatar
  • 431
12 votes
0 answers
217 views

Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection

I would like to know how I can estimate the tangential and cross component of the galaxy's shear using Gnomonic projection of the right ascension and declination the galaxies relate to the center of ...
Dalek's user avatar
  • 261
11 votes
1 answer
1k 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?
Haris Ansari's user avatar
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
11 votes
3 answers
1k views

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
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
1 answer
1k views

Inflation cosmology: slow-roll inflation versus tunneling between two vacua

In slow-roll inflation models, the early inflation of the universe is driven by the flat non-zero part of the inflaton potential, and it ends as the ball rolls down the cliff and the potential energy ...
Dilaton's user avatar
  • 809
11 votes
1 answer
319 views

Dark Flow: statistical limits on existence

Dark Flow has been thrown around as a term a lot lately, especially with the latest Planck results. New Scientist reported last year that it was now statistically improbable, but the paper by Atrio ...
christopherlovell's user avatar
10 votes
1 answer
2k views

Is the amount of dark matter the same as you look back through time (further away from earth)?

In the hope that it may provide information on the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large ...
Zinn's user avatar
  • 405
10 votes
1 answer
353 views

Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?

I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored ...
Swike's user avatar
  • 3,926
10 votes
2 answers
2k 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!...
uhoh's user avatar
  • 30.6k
10 votes
1 answer
658 views

Are black holes considered part of baryonic content of universe?

Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?
Leos Ondra's user avatar
  • 1,074
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

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