Black holes seem to be detectable only when they are gobbling up surrounding matter. Is it possible that there are a large number of small isolated black holes. If so, could they be detected by transient deflections of light from background stars.

I’m not a scientist, nor do I have high-level knowledge of physics, but I’ve been thinking about something that doesn’t make sense to me.

We’re told that the universe came from “nothing”—no space, no time, no physics. But if that’s true, how did inflation even start?

For anything to happen, there has to be: 1. A place for it to happen (meaning space existed). 2. Some kind of rule or force that allowed it to happen (meaning physics existed).

If there was truly nothing—no time, no laws, no forces—then what caused inflation to begin? What was it expanding into?

This makes me think that something had to exist before the Big Bang. Maybe space was already there. Maybe there was a different kind of physics before our universe’s physics took over?

i mean I may sound crazy but this is what i have been thinking about lately

I’m currently in my undergrad and am looking towards doing my postgraduate in cosmology as I find it fascinating.

I do however, have a question: how alive is mathematical cosmology?

Looking at recent papers it would seem like majority of modern cosmology involves very little “hard core” maths and mainly consists of observational cosmology. I love mathematical physics and applied mathematics and hence want to know whether modern cosmology research will allow for a more theoretical and mathematical approach?

Whenever I look at a black hole, and whenever I think about the state the universe was in before the Big bang, I can't help but see similarities between the two. So I was wondering if they could be related somehow? Like could our universe have been a black hole?

So we all know about the basic physical constants that seem to be finely tuned to make atoms and life, like the cosmological constant and vacuum permittivity and things like that, but one I don't see often mentioned is this Theta Vacuum angle.

https://en.m.wikipedia.org/wiki/Theta_vacuum

Apperently it could take any value between 0 and 1 (or is it 0 and 2*pi?) but it seems to be unbelievably close to 0, which leads to very little CP violation which allows for stable atoms and such.

But the problem is I just cannot understand that wiki page and what the Theta vacuum represents physically. It's something like all the possible vaccum states and how they interact or something like that? Seeing it can also be resolved by changing it to be a dynamic field using axions but not likely since we aren't finding axions?

So looking for help understanding Theta vacuum, what it represents physically, and how it relates to the greater universal structure of spacetime.

I feel there are a lot of similarities between hyperspheres and black holes. And if theoretical white holes are just the inverse of black holes would that not also mean their shape is also inverted mathematically?

edit: or rather, if not, could a black hole be an inverted hypersphere, given an inverted hypersphere would curve inwards, and also have a singularity??

I’m trying to make code to simulate the Boltzmann equation for two species A,B that interaction through A+X->B where X is some other species that has a known distribution. I assume a fermi dirac distribution for both and by computing the collision terms I can find how both species number and energy density changes, and therefore how the temperature and chemical potential change. The code I have looks like it gives reasonable results. The problem is it is absurdly slow. I’ve optimized my computations (all in C) to the point where I am unsure if there’s much else I can do and my hardware is pretty solid (7900x, using all processors to do the numerical integration). I’m using CVODE in the SUNDIALS library which seems to be pretty reputable.

I am wondering if there are techniques for speeding up these computations. I don’t really know the best way to approach this since it seems quite difficult to tell which approximations will preserve the accuracy of the computation. I’d appreciate any advice/articles/texts greatly.

*also for clarity I’m just talking about the first order Boltzmann equation here, not necessarily the perturbations

Wikipedia says the star Groombridge 1830 is just 29 light years away, but is located in the galactic halo. I understood the thickness of the Milky Way's disk where we are to be thousands of light years. Are we really so close to the "upper or lower" edge of the disk, that we can be as few as 29 light years away from a star that is outside the disk?

Ask your cosmology related questions in this thread.

Please read the sidebar and remember to follow reddiquette.

https://www.bbc.com/news/articles/c4geldjjge0o

Thought to share this new development.

Early universe observations produce some huge redshift values. The median redshift for the period of last reionization is (according to the Planck team) about z=7.8. The CMB has a redshift of about 1100. The JWST has observed a galaxy with a redshift of 14.32.

However, if you use a flat lambda-CDM model with omega Mass = 0.352 and an H0 of 71.97, then a different story comes out. The lookback time to redshift isn't perfectly linear, but if you use a lookback time of 15 billion years in this model, you only get a redshift of about 1.83.

Why doesn't the lambda-CDM value come anywhere close to early-universe observations?

I built a MOND model using the SPARC Newtonian data set. So far the results are ok. I can get about a third of the galaxies below a reduced X2 of 1.5. The rest are kind of all over the place. I’ve double checked my data, but I think that my handling of mass/light ratio is the biggest problem. The second issue is breaking up bulge v disc. Any tips for working with this data set?

Hi, I want to compute the 2 point correlation function of the temperature map of the CMB, I know there are libraries like CAMB that do that, but they use the theoretical approach where some power spectrum is passed to a function and the expansion in Legendre polynomials is been made.

The thing is that I want to compute the experimental one by just doing the \rangle T(\hat{n_1})T(\hat{n_2})\langle calculation, but I cant find any code that does that.

I have found the treecor package that is more general but it says it can be used for cmb data, but my kernels dies when processing the correlation function (maybe something is bad with my code and I will ask in the repository), but in the meantime, does anyone know any other alternative to compute that?

Thanks for reading

well this might be a dumb question ( again ), but If Einstein's theory of general relativity is held true then earth orbits the sun cause of the curvature the sun causes right? well that means theres no gravity or gravitational field that these planets and stars have, its just space time bending. okay so what prevents the earth from getting pulled by the sun if the earth doesnt have its own gravitational field to balance out the forces? how does it even follow a stable orbit? and i know how the black hole's space time just becomes a kind of a waterfall because of its incredibly high mass density and that explains why it eats out planets and stuff . And so i believe even the sun might pull earth little by little as the earth shoudnt have anything to prevent it from going in

We do a lot of experiments on Earth and looking abroad at the Universe through our satellites. How can we ensure that our theories of thermal dynamics, electromagnetism, gravity, material science, etc. apply to the rest of the Universe or isn't as static/general as we think they are? We know that in quantum mechanics, Bell proved non-locality behavior. Could this non-locality effect the macro world enough where we see things that violate the laws of physics? And I wonder if our gravity well via being on Earth causes an observation bias as well.

There is also weird assertions that I don't agree with. If Energy couldn't be created nor destroyed, then Energy wouldn't exist at all. If systems tend to move towards a high entropy state overtime, then it asks the question has to how anything was made into a state of low entropy to begin with. These fundamental assumptions we have in physics I think are worth challenging because it doesn't make sense to have rules that would make us non-existent.

Either energy can be created and destroyed, or the universe is infinite in size/time and didn't start with the big-bang.

Hi. I was doing research on the big bang and Ive heard that there's one popular theory that before the big bang happened the universe began as an infinitly hot, dense, and small state called the initial singularity. I also found some facts that that the big bang is what started time and without time there's no past or future and everything would just be frozen in the present (or something like that). Since theres no way for anything to change without time does that mean that the initial singularity "always" existed and always was infinitly hot, small, and dense (at least until the big bang happened)?

so i was reading about how the universe at the beginning had a very low entropy i.e in a much ordered state. And then when the big bang happened , the entropy started increasing and matter and stuff were created.

Which led me to question the second law of thermodynamics in the first place. like why does the entropy of the universe tends to a maximum, why would an ordered state try to be less ordered and vastly spread out. I mean Isnt stability the ultimate goal of a system?

maybe i am missing a fundamental reasoning or this is a dumb question and i should know the answer already being in university but idk i dont think i remember anyone justifying the 2nd law of Thermodynamics. so id love someone to explain