Questions tagged [cosmology]

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

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48
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2answers
2k views

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 ...
31
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5answers
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. ...
30
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1answer
6k 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 ...
26
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7answers
5k views

The laws of physics through out the universe

How do we know 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 equations ...
25
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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 ...
23
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3answers
1k 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 ...
23
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1answer
641 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 ...
22
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1answer
406 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 ...
21
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1answer
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 ...
20
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3answers
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 ...
16
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1answer
292 views

Which came first: black holes or galaxies?

In other terms, did galaxies grow around black holes at their center?
16
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1answer
412 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 ...
15
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3answers
1k 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 ...
15
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1answer
328 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 ...
15
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1answer
215 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 ...
14
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1answer
746 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 ...
14
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3answers
724 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"...
14
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2answers
667 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 ...
13
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1answer
557 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 ...
13
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0answers
520 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 ...
12
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2answers
496 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 ...
12
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2answers
1k 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 ...
12
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1answer
301 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 ...
12
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1answer
217 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 ...
11
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2answers
2k 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}{...
11
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2answers
539 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 ...
11
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3answers
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 ...
11
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1answer
877 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 ...
10
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1answer
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?
10
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1answer
5k 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"...
10
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1answer
312 views

If we “turned universe upside-down” and changed all matter to anti-matter instantly

...what would be the immediate consequences? Or putting it better, if (from big-bang or whathever), the universe were always made of what we call Anti-matter what would it be? The scientists of this ...
10
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0answers
128 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 ...
9
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6answers
935 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 ...
9
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2answers
502 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 ...
9
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3answers
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Why do we continue to find a discrepancy in the Hubble Constant?

After taking a look at some recent papers (examples below) that attempt to derive the value of the Hubble Constant, they all conclude with dramatically different values (speaking in context). Why is ...
9
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4answers
621 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. ...
9
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2answers
4k 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 ...
9
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1answer
331 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?
9
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2answers
181 views

How Good Are the Upper Limits on Heavy Elements?

There are between 90 and 254 stable nuclei all the way up to element number 82. In discussions and graphs about big bang nucleosynthesis nothing above lithium is even mentioned. It's a pretty safe bet ...
9
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1answer
182 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, ...
9
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1answer
161 views

How to find the distance between two quasars

I want to calculate the distance between two quasars of which I know the angular position and the red shift. Let $Q_1=(\alpha_1,\delta_1, z_1)$ and $Q_2=(\alpha 2,\delta 2, z_2)$ and suppose $z_2 > ...
9
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1answer
1k 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 ...
8
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3answers
537 views

Is there a natural process by which hydrogen is generated from heavier elements in the cosmos?

we know that stars fuse hydrogen into helium starting at 3 MK; 13 MK in the Sun's core; carbon fusion starts at above 500 million K, and silicon fusion starts at over 2700 million K for comparison; ...
8
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2answers
7k views

Is everything quantum entangled?

So, I am learning about quantum entanglement in my quantum course and had a thought. If the Big Bang is the creation of everything - time included, and this came from a single "source". Would it not ...
8
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1answer
167 views

How do we know so many details about Big Bang, but we do not know if Universe is finite/infinite?

I understand that this question may be too vague, but I am confused about the following: On one hand we know a lot of "details" about Big Bang, like really precise details, evolution of universe, age ...
8
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3answers
816 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 ...
8
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1answer
106 views

Neutrino producton rate over time

Has the rate of neutrino production in the universe varied over time? Are there more or less being created now against the early days?
8
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1answer
649 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, ...
8
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2answers
379 views

Is the diameter of the observable universe a relative quantity?

The diameter of my observable universe is 90 billion ly measured in proper distance. But isn't lenght a relative quantity in the theory of Relativity? Could an observer moving at a different velocity ...
7
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2answers
498 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 ...

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