/r/cosmology

**/r/cosmology** - a community for questions, discussions, and articles about cosmology.

Welcome to r/cosmology, the subreddit for questions, discussions, and articles about cosmology. Cosmology is the study of the early universe and the universe on the largest scales. Things galactic size and smaller generally belong elsewhere.

Reddiquette is enforced. Disrespectful and irrelevant comments will be removed.

If you claim an alternative model of the universe and ignore known data or have no equations or calculations, then your post will be removed. It isn't the responsibility of experts to review your "what if the universe was...?" idea. Homework questions will be removed. Repeated infractions will result in a ban.

Rude behavior is not tolerated.

If you do not see your submission in the new queue, please message the moderators as it may have been sent to the spam filter.

Find today's cosmology preprints on arXiv:astro-ph.CO.

Check out some other great subreddits:

Common questions:

The universe is not expanding faster than light. Expansion has units of inverse time.

The evidence for dark matter is overwhelming and reaches far beyond rotation curves.

**Anything not related to cosmology will be removed.**

/r/cosmology

4

One of the big open questions in cosmology is:

"What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe?"

My question is:

How is it possible for the entire comoving space to be described as a manifold in cosmological models, considering that not all regions within it are causally connected? Essentially, why do cosmologists use a manifold to describe the entire universe, including regions that have never interacted or influenced each other?

1 Comment

2023/11/18

20:00 UTC

20:00 UTC

0

I have read that dark energy can avoid a true virial equilibrium to ever be established. How can it do so? What does it exactly mean?

1 Comment

2023/11/18

13:54 UTC

13:54 UTC

6

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

2 Comments

2023/11/16

10:00 UTC

10:00 UTC

3

In November of 2022 the General Conference on Weights and Measures (CGPM), the inter-governmental body responsible for measurement, designated new prefixes to the International System of measurement (SI) for the first time since the prefixes zetta (1021) and yotta (1024) were added in 1991. The new prefixes are "ronna," 1027 and "quetta," 1030. https://www.bipm.org/documents/20126/64811223/Resolutions-2022.pdf/281f3160-fc56-3e63-dbf7-77b76500990f

These will be used to describe extremely large quantities such as electronic data. For example, it has been estimated that sometime in the next decade the world will produce a yottabyte of data every year. That is a million trillion megabytes. A thousand times that will be in the ronnabyte realm, and a million times that will be a quettabyte of data. We are a long way from that, but that’s what apparently what they said about yottabytes 30 years ago.

Likewise, for very small numbers there is now the "ronto," 10-27 (a decimal point followed by 26 zeros and then a 1), and "quecto," 10-30(with 29 zeros before the 1). These are used to describe unimaginably small quantities. For example, a proton has a rest mass of approximately 1.67x 10-24 grams, which I guess means 1.67 yoctograms (yocto- being the reciprocal of yotta-), which are a million times heavier than quectograms. (An electron weighs 1/1836 of a proton so yes, there are uses for the new prefixes.)

This all got me wondering while I am home sick with a cold. Several years ago I heard of a unit of measurement someone had come up with known at the “attoparsec.” It is supposedly only used humorously, but it is found in several places on the internet, including Wikipedia. The prefix atto- denotes 10-18, or in other words, one quintillionth of something, or a millionth of a billionth. A parsec is a unit of measurement used in astronomy to measure the distance to nearby stars using stellar parallax. It is equal to about 3.26 light years, with a light year being the distance light travels in one year at 299,792,458 kilometers per second, or roughly 6 trillion miles. (For example, according to Wikipedia, the nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun, and most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand.)

So an attoparsec is a unit of measure equal to 10-18 x 3.26 light years, which comes out to about 3.085 centimeters or 1.215 inches. I checked to see if anyone has come up with a similar unit of measurement using parsecs and the new prefixes and I am unable to find anything on Wikipedia or Google. I assume it is because since the new prefixes were announced, nobody has had too much free time on their hands, (let alone any practical reason), to even think about this.

Nevertheless, I am going to do stake my claim as the person who first coined the following terms:

**Yoctomegaparsec** – A megaparsec is a term used by astronomers which describes a distance equal to one million parsecs, or 3.26 million light years. A yoctomegaparsec would 1/10-24 of a megaparsec. Since a megaparsec is a million times larger than the parsec, and “Yocto-” is a million times smaller than “Atto-“, a yoctomegaparsec should equal the same as an attoparsec – 1.215 inches.

**Rontogigaparsec** – The largest unit of distance I have been able to find that is actually in use is the gigaparsec. It is also used by astronomers. It is a billion parsecs, or 3.26 billion light years, which is about one-fourteenth the distance to the horizon of the observable universe, or about 1/28 of the estimated present diameter of the observable universe of 93 billion light years. So since a Ronto- is one thousand times smaller than a Yocto- , and a gigaparsec is a thousand times larger than a megaparsec, then a rontogigaparsec should be the same as a yoctomegaparsec or an attoparsec—1.215 inches.

You heard it here first.

3 Comments

2023/11/14

01:27 UTC

01:27 UTC

14

thank you

5 Comments

2023/11/13

11:24 UTC

11:24 UTC

6

In a nutshell, you run the experiment a number of times and average the results. If the average is below a critical value then hidden variables are true, where as if it is above the critical value then hidden variables are false.

My question is, what exactly determined that the lower value was for hidden variables to be true? What defined the difference between the upper and lower limit?

9 Comments

2023/11/12

20:17 UTC

20:17 UTC

11

Or it's a totally unknown territory for science at current stage of it's development?

45 Comments

2023/11/12

18:50 UTC

18:50 UTC

6

As far as I understand it, dark energy can affect bound systems at cosmological scales (https://physics.stackexchange.com/questions/781404/how-does-dark-energy-affect-the-dynamics-of-galaxy-clusters) effectively modifying their orbits.

This phenomenon and this thesis dissertation (https://louis.uah.edu/uah-dissertations/31/) made me wonder...

Could dark energy make a bound system (like a large galaxy or a satellite galaxy orbiting a bigger one) be "less bounded" so that orbits towards the central point of mass are larger?

And if dark energy would help to make orbits be further away from the central point of mass, does it mean that dark energy could add orbital energy to bodies orbiting the center of the galaxy at the outskirts of it?

7 Comments

2023/11/11

11:53 UTC

11:53 UTC

11

32 Comments

2023/11/09

17:18 UTC

17:18 UTC

8

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

4 Comments

2023/11/09

10:00 UTC

10:00 UTC

4

I was reading this interesting article about possible effects of dark energy in the formation of large-scale structures which should have an impact on the Sunyaev-Zeldovich effect (*"Dark energy imprints on the kinematic Sunyaev-Zel'dovich signal"* (https://arxiv.org/abs/1309.1163))

There, the authors indicate that the signal from the kinematic Sunyaev-Zeldovich effect is increased if the equation of state (ω) is more negative than -1 (which is the usual value standarised in ΛCDM), which would mean as far as I understand it that if Dark Energy was stronger or had a bigger density (making ω more negative than -1) then the kSZ effect would be enhanced. However, if ω was less negative than -1 (between 0 and -1) then that would be in a universe with a smalled dark energy density and you say that in such scenario the kSZ effect would be supressed. I have a few question about this:

When they say that the kSZ signal is enhanced or supressed what do they exactly mean? That the photons would be blueshifted (in the case where the kSZ in enchanced) or redshifted (in the case where it is surpressed)?

If I understood it correctly, if ω was 0, there would be no accelerated expansion and therefore no dark energy of any form. Then in that case, the kSZ effect should be very surpressed, correct?

Finally, and just to confirm, does their work show that in a universe with a negative ω (and therefore an accelerated expansion caused by e.g. dark energy) the kSZ effect is enchanced (compared to a universe with ω=0, or no accelerated expansion) and therefore the photons are scattered, blueshifting them in the process?

5 Comments

2023/11/07

13:21 UTC

13:21 UTC

7

Hey there, I've got a bit of a curious question.

I know that there isn't a specific 'center' of the universe like where the Big Bang happened, and it's not really a practical concept in cosmology.

But I was wondering, in a theoretical sense, if we were to calculate it, where would the 'center of mass of the universe' be?

Just out of pure curiosity, no practical implications here.

Any insights or thoughts would be appreciated!

40 Comments

2023/11/06

18:37 UTC

18:37 UTC

5

I am in no way smart enough to talk about cosmology and the talk about our universe and what beyond. But I’ve been on a non stop quest for knowledge about what beyond us and why were here. And I need to know if any books or articles that can at least give the right questions to ask and maybe some answers, I don’t even know if this would be the right subreddit.

11 Comments

2023/11/06

14:12 UTC

14:12 UTC

2

Could the Universe have ended somehow but we'd be oblivious to it until some effect of its end reached us at the speed of light?

9 Comments

2023/11/05

22:30 UTC

22:30 UTC

0

Hi, I have got a question, I have just done wave-particle duality in my Physics class (alevel). We were told that all particle could be thought of as waves. So, I wondered whether this meant we could assign 'wave properties' to all particles (i.e a wavelength, and frequency). If so, does that mean that galaxies and even the universe could be assigned these properties? If not, why? I have a very basic grasp of physics so please explain as simply as possible.

4 Comments

2023/11/05

12:43 UTC

12:43 UTC

6

So I get that the problem has to do with black holes and timespace, but that's about it. I find cosmology super interesting, but my grasp of it is admittedly not great.

Can someone explain in simple terms what this problem is, and how/why we would solve it?

Bonus points to someone who can expand(heh) on the idea of infinitely expandable space. Does that mean that our entire universe could just be like a spec in a much larger universe? Like....is our universe just expanding to fill the space of some expanding Planck in another universe infinitely bigger than ours?

3 Comments

2023/11/04

17:40 UTC

17:40 UTC

0

6 Comments

2023/11/04

00:22 UTC

00:22 UTC

8

Sorry for stupid question

63 Comments

2023/11/03

06:11 UTC

06:11 UTC

2

2 Comments

2023/11/03

00:31 UTC

00:31 UTC

5

If matter and energy cannot be destroyed but the universe is constantly expanding, then what is that new distance between celestial bodies filled by? I hope this is the right subreddit, I didn’t know wether to go for astronomy or cosmology

16 Comments

2023/11/02

13:44 UTC

13:44 UTC

4

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

6 Comments

2023/11/02

09:00 UTC

09:00 UTC

4

I was watching a documentary and Kip Thorne said that during the formation of a black hole, the matter is crushed to nothing. So if there is no matter, how can a black hole have mass?

Edit : the documentary is a spark documentary on YouTube. Called unravelling the mysteries of black hole. Go to 8.55 min in and that's the section with kip throne I was referring to.

32 Comments

2023/10/31

23:44 UTC

23:44 UTC

11

In fact, is there a list of elementary particles that never decay?

28 Comments

2023/10/31

16:33 UTC

16:33 UTC

7

I'm currently doing my Masters degree in Cosmology in the UK and was tensed about my future. My thesis supervisor told me that there aren't many FUNDED pHDs available. I was originally looking for a career in academia but now I'm not so sure. I'd say I'm pretty average when it comes to exams and assessments , but research is one of my strong suites. I might take a little time to come up with solutions to certain problems that might need looking up previous work or references but I'm pretty sure I can do it. So should I work in any industry for a while and then do a phD? If so , what would anyone recommend me to do?

6 Comments

2023/10/31

15:56 UTC

15:56 UTC

9

Like ever, with infinite time?

4 Comments

2023/10/31

02:46 UTC

02:46 UTC