Photograph via snooOG

/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:

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

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

Anything not related to cosmology will be removed.


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Question about Dark Energy

At a relatively early age I came to understand that the space itself between distant objects in the universe was expanding. I don’t really have a more nuanced understanding of this than what I’ve said above.

My question is this. Is Dark Energy just a thermodynamic perspective on the aforementioned phenomena? Like the objects in the universe are attracted by gravity, so the energy to overcome that force has to come from somewhere, cause otherwise they would come together, or at least slow down, looking at it from a thermodynamics perspective. Is that what Dark Energy is?

12:09 UTC


Rate of black hole evaporation and speed of in-falling object

To a distant observer, object falling toward black hole appears to slow down. Asymptotically toward stand still as it approach the event horizon. The distant observer would also see the black hole evaporate at a very slow speed. Taking perhaps orders of magnitude longer than the expected life time of the universe. But evaporate it must. However, if a black hole is evaporating at any non-zero rate from the view point of the distant observer, then relative to the object at its threshold it must have been evaporating at a fantastic speed. For the observer would see the object approaching stand still while the black hole evaporates. So how fast is the black hole evaporating relative to the object? Could it be that the object can never fall in because the black hole is evaporating away at a faster speed than the object can fall? And therefore both the object and the distant observer would see the same narrative?

06:34 UTC


I want to be an astrophysicist.

I want to be an astrophysicist, but I don't have any idea what that path looks like. I'm currently a junior in high school and I've been researching what colleges are good and what is good to major in. I was thinking computer science and physics because I want a backup just in case the astrophysics path doesn't go as planned. Does computer science go with astrophysics? What is it even like to be an astrophysicist?

02:16 UTC


Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

09:00 UTC


Do we know if dark matter and dark energy actually exist?

Neither dark matter nor dark energy have been directly detected. It is possible that one or both of them don't actually exist? Could they be today's luminous aether?

19:24 UTC


Everett's many worlds probability

What is the response by Everett, or anyone else arguing for many worlds, to the question about the inherent probabilistic nature of the wave function? If everything will be realized, then where does the probabilty factor in?

19:22 UTC


Could the universe actually be expanding at different speeds in different areas? Could this explain the Hubble tension?

Could the universe be expanding at different speeds in different areas?

02:38 UTC


Can an infinite universe contract?

And if so, would it keep contracting forever?

21:34 UTC


If Dark Matter is matter, can it be projected ?

Does cosmological physics allow the possibility of a naturally occurring ray, or a projected stream or vent, of dark matter, emanating from a source of dark matter, on any scale.

20:38 UTC


Some questions on galactic haloes...?

#1 Can there be a galactic halo made from baryonic matter (not dark matter)?

#2 And if it can, do galactic haloes (both from baryonic and dark matter) emmit gravitational waves as they rotate around their galaxies?

#3 Can they cause tidal forces in the galaxy?

#4 Can these haloes have high amounts of angular momentum?

20:24 UTC


Why black holes increase their volume?

Sorry if it is a stupid question.

If all the matter is condensed into the center (point) of the black hole, shouldn't the volume remain the same?

17:56 UTC


Why does the power spectrum change at matter-radiation domination equivalence and why is there a difference between Hot, warm, and cold dark matter?

I’m referring to the graph of the following form.

I’m confused on why the power spectrum changes at matter-radiation equality. I think it has something to do with a difference in large scale density fluctuations (where P(k) ~ k^n ) and small scale density fluctuations (where P(k) ~ k^-3 ) and modes entering the Hubble horizon?

I’m also confused on why we get different power spectra for hot, cold, and warm dark matter?

17:25 UTC


How does inflation predict an initial power spectrum to be P(k) ~ k^n?

  1. I’m having some difficulty interpreting what is meant by the “power spectrum”. Is it just defined as: P(k) = <|δ_k|^2 > where δ is just the density fluctuations and < > is some average?

  2. Why does inflation predict then that P(k) ~ k^n ? I think this is called the Harrison-Zeldovich power spectrum and it’s supposed to be scale invariant, right? Apparently n is also denoted n_s for scalar, why is that?

  3. I was told we can measure n_s using the CMB and have found n_s ~ 1. How can we measure that?

17:11 UTC


What happens near a charged black hole?

In the 1970’s, before the significance of Hawking radiation reached the general public, the science fiction writer Larry Niven wrote the novelette “The Borderland of Sol.”  One of the plot points is a micro black hole which had been given a net electric charge so it could be maneuvered by electric fields. In reading this today I had a thought question.

Assume a black hole of large enough mass to be stable and a MASSIVE linear accelerator shooting electrons at it from beyond the photon sphere. (Note: The manufacture of this device will be left to the interested student.)

As the black hole’s net charge increases, at some point the repulsive Coulomb force on the incoming electrons will exceed the gravitational attraction. Assuming enough initial energy, classically the electrons could run out of kinetic energy past the event horizon. This would prevent them from collapsing into the singularity but leave them inside the event horizon with a net outward force. This looks like a paradox as it implies they would then be pushed back out of the black hole.

As the above is a naïve extrapolation of classical physics, my guess is that general relativity or quantum effects will prevent the paradox. Frame dragging comes to mind, as does the effect of the net charge on Hawking radiation; are there others?

It also occurred to me that given a sufficient charge on the black hole, the Coulomb force could be enough to ionize atoms of neutral matter outside the event horizon, repelling the electrons and attracting the naked nuclei in a sort of classical analogy to Hawking radiation as this would decrease the net charge. Could this be the answer?

Thanks for any comments.

16:58 UTC


Thought experiment with Hubble horizon

It is common sense that whatever is beyond the Hubble horizon will never interact with us. But we need to be careful with words. Here is a simple thought experiment that seems to make a dent in this common sense.

1- Take 2 particules A and B, entangle them and separate them in space: A remains with you, and B is at some distance from you.

2- Wait. As the universe expands, A remains with you but B is taken away from you. If you wait long enough, as B’s velocity relative to you increases there will be a finite point in time when B reaches c and goes beyond the Hubble horizon relative to you.

3- Look at what you (or your descendants) have now: a particle in your lab that is entangled with another one that is beyond the Hubble horizon. Now if B interacts with whatever is beyond the Hubble horizon and its wave function collapses you would immediately know. You’ve made a probe that gives you information about what’s behind the horizon.


1- Of course, the information you got (that there was something beyond the horizon that made B’s wave function collapse) is extremely simple and one shot. But still, it’s a matter of principle. It’s like one bit of information. It’s the huge difference between knowing nothing and knowing something. And after all, in this experiment you could very well replace A and B by two sets of entangled particles. You would then get more complex info.

2- there is a practical difficulty in making such a probe: it takes time, a lot of time, much more than your own lifetime. But consider this: first, this time is finite and certainly you could pass the experiment to your descendants and second, once the probe is setup it provides instantaneous real time info.

Questions: 1- is there anything wrong in this reasoning? 2- what are the theoretical limits as to what kind of info can be gotten using this probe, if any 3- could a similar experiment be used to probe what’s behind a black hole’s event horizon?

14:04 UTC


Question(s) about depictions of the observable universe


Just curious... since we are seeing light with wide variations in distance/age then the apparent position of a galaxy or groups of distant (ie billions of years old) galaxies then are we computing their observed trajectories to guess at what the universe is shaped like now? I know that is a tall order but if we are just looking to make "maps of the universe" then from our perspective without knowing directions things are moving how can we know how the universe appears at this time?

01:16 UTC


YT Playlist covering all the basics of Big Bang Cosmology

00:32 UTC


Can "voids" just be "lightless light"

[english isnt my first language]
So me and my boyfriend were just chatting about about space and voids and how it all could be, we eventually came across the though of emptyness/voids just being "lightless light" like a infinite patch of just empty lightless light. We then googled a bit about it and didnt find anything, but we are no professionals and its simly just a interesting theroy that came to our minds. If anyone could explain a bit of why it would/wouldnt be possible we'd appreciate it!

18:49 UTC


When was Virgo Supercluster formed? When was Laniakea formed?

19:37 UTC


I still struggle with time after the Big Bang.

Posted this as a reply in another post but decided to post anew here:

Since time is relative and all those primordial particles, radiation, energy up to hydrogen atoms are buzzing around near the speed of light, how the heck do we clock that?

Doesn’t this skew our timekeeping of the early universe?

16:04 UTC


Big band at the edge of our universe


I’m an amateur layperson cosmologist and I have a burning question for the experts

So as we look deeper and deeper out to the observation Universe we , of course, are looking further and further back in time also . So the further distance we observe the closer we get to the moment of the big band. I know of course that there was no light until about 300,000 years after and so we could never see that instant.

My question, therefore, is: is the Big Bang effectively smeared out along the entire edge of our Universe?

I hope I’m making sense. Many thanks in advance

14:57 UTC


Book recommendations for a philosophy of physics student

I'm studying philosophy of physics where I have a section on cosmology. I need to have a working understanding of the field before starting my philosophy material.

These are the topics I have to read:

  • Copernican model of universe
  • Newtonian model
  • Big bang
  • Expansion of the universe
  • Einsteinian Universe
  • Oscillating Universe
  • Steady State Universe
  • Multiverse
  • Theories of end of universe

I would really appreciate if you could help me out by suggesting some books that are a level above books like Astrophysics for People in Hurry, and delve into some maths behind the theory.

13:19 UTC


What happens to your body after you die in a black hole?

Does it just stay there?

04:45 UTC


I need help with people who don’t study astronomy

So I just asked r/astronomy and someone redirected me here, but my friend and my brother are trying to argue that space is NOT infinite and that we don’t truly know that the universe is expanding. Can you guys explain and provide evidence to support my claims that space is infinite and the universe is expanding

21:10 UTC

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