/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

1

As far as I understand, the classical big bang model is not used much by physicists today. The hot big bang and the inflation that preceded it are more accepted. Also, it is not known how long inflation lasted. So where do you put the gut epoch and the planck epoch in the modern model, before or after the inflation? Or do these epochs not exist in the inflation model?

6 Comments

2024/08/28

07:34 UTC

07:34 UTC

0

We seem to assume that the universe emanates from a point and expands in a globe - that is what any models that I have look like ? Why is this assumption cast in stone ?

There are two other shapes that the universe could be. 1 bilobed like many of the planetary nebula. 2 Torus shape.

The torus shape is the more interesting because it allows for more complex spatial interactions. As the torus expands stars appear to drift apart just like the sphere model. The torus model also admits spiraling strands within the torus, so that stars may be moving away from the origin or towards it at different times.

How would a torus form ? Some axis in the multiverse coul be spinning relative to other dimensions. An energy burst at some point along the axis would generate a torus shape rather than a sphere. Indeed the chance of the Big Bang originating from a single point is unlikely compared to a point on an axis. The sphere requires the congruence of three dimensions, but the torus only requires the congruency of two dimensions with a theoretically infinite axis.

Since the torus appears to be a viable model why are we not disproving its existence before we assume the universe is a sphere ?

How would we do this. We can detect movement within the universe on a large scale by taking repeated snapshots of gravitational lens effects. If thew universe is a shpere the sequence of snapshots should stay roughly the same. But if the universe is a torus we should see changes over time.

5 Comments

2024/08/27

23:18 UTC

23:18 UTC

10

I have basically zero knowledge of cosmology, but I find the general ideas really interesting. If these are stupid questions, sorry in advance. I tried to do some internet digging but I didn't really find answers, or they were contradictory.

I know that we know dark matter exists because of gravitational effects, but how do we know that most matter is dark matter? And can we find patterns where dark matter exists, versus where it doesn't (i.e., can we "map" dark matter)? Also, from what I've read, it's basically undetectable, so how are scientists working on studying it? Or is technology not yet advanced enough?

Also, what exactly are "gravitational effects"?

Thanks! 😊

26 Comments

2024/08/27

19:41 UTC

19:41 UTC

0

I want to use the data in this graph but can’t find a set of numerical data anywhere—I’m wondering if anyone knows a source I could get this from?

2 Comments

2024/08/27

04:15 UTC

04:15 UTC

21

Many phycisists become upset at the idea of an infinite universe, deriding the idea as unscientific hogwash. So why is it so prevelent? Is it just meta-physics that sells pop-science books? Or does it deserve serious discussion? Is it suggested by the data? Or just philosophical speculation?

100 Comments

2024/08/26

23:50 UTC

23:50 UTC

7

Suggest me some good textbooks or video lectures for cosmological perturbation theory, which make the subject for approachable for an undergraduate with an introductory background in general relativity.

3 Comments

2024/08/26

09:07 UTC

09:07 UTC

0

I want to know what you think is outside of the universe

28 Comments

2024/08/26

04:04 UTC

04:04 UTC

83

We have seen an uptick in non-scientific content, AI generated content, and people arguing. Repeated engagements with these people only makes the situation worse. There are lots of people asking genuine questions, but it's easy to sucked into an argument with someone saying nonsensical things. Don't.

0 Comments

2024/08/25

21:12 UTC

21:12 UTC

11

22 Comments

2024/08/25

19:18 UTC

19:18 UTC

1

I understand that most cosmologists view Conformal Cyclic Cosmology as just an unproven conjecture, but still a consistent one if granting some premises. My question isn't about CCC specifically, but rather about how the conformal rescaling used in CCC is considered consistent.

An example I've seen used multiple times to explain the rescaling in CCC is that a universe that is mere centimeters across can be conformally rescaled to a universe that is many lightyears across and vice versa, if both universes consist of only massless particles at similar angles tracing a similar pattern.

But if dark energy exists in those universes, a sufficiently large universe would have photons that would never reach the other side. Wouldn't rescaling also cause otherwise causally disconnected particles to interact (if the photon energies are sufficient for photon-photon interactions in vacuum)?

How can such a universe be conformal with one that is centimeters across and doesn't have that happen?

4 Comments

2024/08/25

18:00 UTC

18:00 UTC

19

According to this article, the recent discoveries show that dark energy is evolving (and is currently weakening). If I understand that correctly, that means that the speed of acceleration of the expansion of the universe is slowing down and will at some point become negative, leading to Big Crunch. Am I correct?

10 Comments

2024/08/25

05:39 UTC

05:39 UTC

0

Imagine discovering that our current theories about the universe’s origin and structure are incorrect. How would this shift impact our knowledge of existence, our place in the cosmos, and our future exploration of space?

19 Comments

2024/08/24

23:58 UTC

23:58 UTC

22

Imagine we had magical physics powers or magical physics device, doesn’t particularly matter, just something to bend/break the laws of physics at will. Now let’s take a star the size of the sun and magically cause it to undergo gravitational collapse at a constant, relatively slow rate, maybe over the course of a day, until it forms a black hole. What would that look like?

I know that neutron stars are so dense that they can “bend” light such that you can see parts of the neutron star you would be unable to see otherwise, and of course black holes warp spacetime so much that you would have to travel faster than light to escape the event horizon. Would we see this theoretical collapsing mass get darker and darker and space becoming more and more warped around it until the event horizon “fully forms”? Or would the event horizon just spontaneously appear once the theoretical object becomes dense enough and exceeds the Schwarzschild radius (assuming it’s not charged and non-spinning)?

This is just a fun question for me, and I have a very rudimentary understanding of General Relativity and of physics in general, so I apologize in advance if the question isn’t quite clear in what I’m asking. Thank you!

7 Comments

2024/08/24

05:27 UTC

05:27 UTC

0

10 Comments

2024/08/23

08:33 UTC

08:33 UTC

8

Given a sufficiently small black hole in terms of mass with enough positive electric charge, are there any solutions in which there are orbits around the black hole where an electron could orbit due to the gravitational attraction and electrostatic repulsion being the same?

I guess it should be possible (after all, black holes do get accretion disks of gas matter and gravity is much weaker than electromagnetism), but how would that work? Electric interaction would need the exchange of virtual photons between the electron and the black hole, but how could virtual photons “escape” the black hole? Is it because they are “virtual” and thus only a mathematical tool?

9 Comments

2024/08/23

08:16 UTC

08:16 UTC

0

I had an epiphany today regarding the ‘Big Bang’. When the question of what happened before it comes up I usually go with the we can’t test this so it doesn’t matter explanation (and but it wasn’t your god!).

To comprehend this event it helps to run the Universe backwards. Everything gets closer together as time winds back 13.7 B yrs. As it gets closer and more compressed it creates a larger and larger gravity well in space time. That volume of space is getting smaller and the stuff in it more and more compressed. Eventually we reach the singularity - a point containing everything that is so small that there is no actual space left.

The epiphany was: Einstein tells us that the closer you get to a large gravitational source the slower time passes. therefore as the tiny universe’s gravity well grows stronger and space becomes smaller, time slows down until eventually it must stop.

With no space and no time, the question of what came before is made redundant.

Does this make sense? Am I correct, or do I have more to learn?

30 Comments

2024/08/22

13:38 UTC

13:38 UTC

3

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

1 Comment

2024/08/22

09:00 UTC

09:00 UTC

71

It bothers me to no end that some questions in physics and cosmology seem to only have the answer "because that's just how it works"

The speed of light, generally considered to be around 299,792,458 m/s, is the most well known example. The speed at which particles with no mass travel. Ask something like "why do massless particles travel at that speed?" and the answer is just "because they do" or "we don't know"

Other constants, like the Gravitational constant, Sommerfeld constant, Planck constant all do the same thing! Why is the gravitational constant 6.673X10^(-11)? Just because that's what it is.

Is there a way we could actually explain, or give a reason besides "because that's how the universe works" for why these values are what they are? If so, how? and if not, does this annoy anybody else?

I cant find any other things asking people if it annoys them and I just want to know if I'm weird for it annoying me, but I like to know why things do what they do, and the fact that the only answer we will likely ever have for questions like "why is the speed of light 299,792,458 m/s" is "because that's how the universe works" is endlessly frustrating to me.

129 Comments

2024/08/22

00:19 UTC

00:19 UTC

0

11 Comments

2024/08/21

13:45 UTC

13:45 UTC

15

Does spacetime have materialistic properties that we can relate to similar to "rubbery" (oppposed to it bring rigid, like a hard plastic or wood)? In a situation where there is a large mass with spacetime curvature, if the mass disappeared, and the curvature flattened out, would there be a wobble / over compensation (like a pendulum)? And in that part, then spacetime is momentarily curved in the opposite direction? So normal gravity well has curvature that slows down time, but when it wobbles, its like a momentary negative gravity well where time goes faster?

Similar for gravity waves after massive mergers, theres the "compression" part of the wave, but also a decompressed which gives an idea that spacetime is elastic/ rubbery and not rigid. Is this "texture" of spacetime expressed in any models / theories?

(In this short clip, wten the sun disappears, could spacetime carry momentum and bounce upwards in this perspective before teturning to flatness? understanding that this is just a visual representation. https://youtube.com/shorts/VMI3T-VQcO0?si=JaaARL0oYqP6uLpF)

20 Comments

2024/08/21

04:23 UTC

04:23 UTC

0

What is the learn roadmap to learn astrophysics, black holes, quantum physics, etc.

Also reference books please

6 Comments

2024/08/20

22:57 UTC

22:57 UTC

2

Pretty simple question: after decoupling we can compute neutrino temperature as 1.95k/a. Is there a simple analytical way to compute neutrino temperature before this?

9 Comments

2024/08/20

22:46 UTC

22:46 UTC

3

Do all the theories or ideas that make use of tired light also reject the fact the universe is expanding?

Asking cause whenever I see someone bring up light loosing energy and being redshifted by expansion, others bring up tired light.. but I was always under the impression that shift proposed in tired light also comes with a denial of expansion.

12 Comments

2024/08/18

16:07 UTC

16:07 UTC

59

86 Comments

2024/08/18

01:31 UTC

01:31 UTC

0

This is probably an unpopular and reductive hill to die on but I'm tired of hearing about the "tension" and the "crisis" in cosmology. It's melodramatic and inaccurate. Following some rough modeling and, apparently, Occam's razor, it follows that the 2 different measurements of expansion, CMB and type 1a "standard candle", are both equally valid and accurate.

Moreover, they HAVE to be. Call me a simpleton if you will but the true "crisis" would result from finding an agreeing measurement for the 2 methods. Without even applying degrees of freedom, the difference in the 2, which accounts for the acceleration of expansion, produces a universe of size and scale like the one we live in.

It's really that easy. Natural evolution for which we already have accurate measurements and data. Of course the 2 values are different because if they weren't, there would be no acceleration which would ACTUALLY damage LambdaCDM. I have no love of inflationary cosmology, I seek to upend it one day, but in this case, if it ain't broke, don't fix it.

8 Comments

2024/08/17

14:03 UTC

14:03 UTC