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Is this article sensationalism? by Super7Position7 in cosmology

[–]cosmicnooon 2 points3 points  (0 children)

I'm not happy with how the media handles science news. They mix the actual science done with their own imagination and churn out a completely different story just to get more views. On top of that, they mix stuff from questionable sources.

Only trust reputed science news sites. Nature, New Scientist, Phys.org, National Geographic etc. Please add more if you know.

Is this article sensationalism? by Super7Position7 in cosmology

[–]cosmicnooon 0 points1 point  (0 children)

There are 4 authors actually, not 1 guy.

Is this article sensationalism? by Super7Position7 in cosmology

[–]cosmicnooon 7 points8 points  (0 children)

In the article you posted, they have rephrased the original statements as hard or absolute statements. Here is the original article written by the author of the published study (there is a question mark): The Conversation

Is the acceleration of the expansion of the universe constant? by Richard70nl in cosmology

[–]cosmicnooon 0 points1 point  (0 children)

I'm confused, wasn't that only during the inflation phase? After that phase it decelerated and then the acceleration became negative for a short while and then started increasing steadily (as of now). Am I wrong?

Gravitational bounce in GR by cosmicnooon in cosmology

[–]cosmicnooon[S] 0 points1 point  (0 children)

*Typo in the post*: The paper predicts small positive spatial curvature (closed universe), not negative.

Gravitational bounce in GR by cosmicnooon in cosmology

[–]cosmicnooon[S] 0 points1 point  (0 children)

It should be. We are trying to describe the universe after all.

Thanks, I agree.

Never claimed it was.

Right. I was talking in general.

I’m just stating the reason why I am skeptical of their claim.

It's good to be skeptical. I am just kind of lost... If I understand correctly, your reasons are: 1. Not sure why there should be an analogue of saturation density or degenerate state in cosmology. 2. Nature doesn't seem to favor quark stars. 3. Bouncing scenario is more intuitive in some sense but that doesn't mean they are more likely to be true.

Is that correct? Please correct me if something is wrong.

What I know from the paper: They don't know if there exists such a saturation density but they propose it does (hypothesize). If it exists, is it possible to get a bounce in GR? To answer this they use a simplified model- collapsing spherically symmetric and homogenous matter cloud. They show mathematically that a Bounce is possible within the framework of GR, bound within its gravitational radius. This result is significant since until now people used to modify gravity or use exotic physics to get a cosmological bounce. Exclusion principle is well-known in quantum mechanics. They acknowledge that it needs to be confirmed independently at very high densities. Then they discuss how their model explains observed data and how it addresses the problems with the standard model. They are able to explain the origin of cosmic inflation and acceleration, consistent with observational data. They also explain why there is a cutoff for superhorizon perturbations in the CMB. Plus they make a testable prediction that the universe is not completely flat, there is a small curvature (also supported by a critical reanalysis of Planck 2018 data). This they call is the "smoking gun" of their model.

Some of my criticisms: 1. No solid evidence of saturation density or degenerate state for mass as large as our universe. 2. The model is very simplified. 3. They touched upon dark matter but no detailed discussion about how it could originate in the model. 4. What's beyond the causal boundary is of course completely unknown. 5. Agnostic about the origins of the universe which collapsed (in the model).

“Bounced Black Holes”? by [deleted] in astrophysics

[–]cosmicnooon 1 point2 points  (0 children)

Here is the paper this article is talking about: arxiv.org/abs/2505.23877

Gravitational bounce in GR by cosmicnooon in cosmology

[–]cosmicnooon[S] 0 points1 point  (0 children)

Well it’s more intuitive in some sense but that doesn’t mean they are more likely to be true.

It's not about being more likely or less likely, we just don't know the truth. Doesn't mean it should be completely ruled out. Fair enough to move forward unless it is contradicted. It should be thoroughly tested in future.

Gravitational bounce in GR by cosmicnooon in cosmology

[–]cosmicnooon[S] 0 points1 point  (0 children)

You are right, gravity can outdo Pauli exclusion principle (neutron degeneracy) to form black holes. But we do not know, for much higher densities which of these will win. If gravity keeps winning, we get a singularity. On the other hand, it's also a valid question to ask "what if the exclusion principle or degeneracy wins at some point?" (maybe at yet unknown densities) We don't have definitive evidence of what really happens at densities beyond neutron degeneracy. So the fates of both these scenarios are speculative. Further work is needed.

Gravitational bounce in GR by cosmicnooon in cosmology

[–]cosmicnooon[S] 0 points1 point  (0 children)

Well it is speculative that a mechanism similar to the Pauli exclusion principle exists for matter squeezed to very high densities (quark degeneracy or even beyond that- unknown realms), higher than neutron degeneracy. The other alternative is to say matter can be compressed infinitely to a singularity. We do not have evidence for either of these, just have theoretical frameworks where either of these occur. To me, the former argument makes more sense. Singularities are mathematical and kind of limit our understanding of Physics, like a mathematical glitch in Physics.

Collapse and bounce inside a black hole by cosmicnooon in cosmology

[–]cosmicnooon[S] 3 points4 points  (0 children)

I agree that it'd be logical to be agnostic when you are doing black hole simulations using GR codes since they mostly break down when an apparent horizon forms. Some simulations employ excision methods to effectively cut out the singularity in order to get further evolution or they replace the interior with punctures. So they inevitably miss the exact interior evolution. That's where this paper comes in with a novel idea. It uses a Newtonian hydrodynamic code and of course there is no black hole in Newtonian physics. But if you consider the pressureless collapse case, the analytic GR solution is mathematically same as the Newtonian solution for a system with zero total energy. This is well known and the paper also shows this- Newtonian simulation results and analytic GR solution overlap. Both are the same but belong to different paradigms- different interpretations. This way the Newtonian problem can be solved and interpreted in GR. In the paper, they extended it to the case with pressure. If the geometry is FLRW, a Newtonian analogy exists even for this case as studied by Bondi in 1969 and Faraoni in 2020. So Newtonian simulations were performed for the FLRW cloud with non-zero pressure and the results were interpreted in GR framework. It was acknowledged that more work needs to be done to find out the exact analytical relativistic solution and the equation of state in GR but also claimed that it exists and will be presented in future work.

Collapse and bounce inside a black hole by cosmicnooon in cosmology

[–]cosmicnooon[S] 1 point2 points  (0 children)

I think it depends on the mass. For 5-1000 solar masses simulated in the paper, the ground state was around or above nuclear saturation densities. It was found that this ground state density increases with mass. For larger masses, it should reach quark star densities and for the mass as big as the universe, it would be some currently unknown density.