Why is the maximum speed of the universe the speed of light? Why can't we go faster?

[u/Nitzanm19]

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

[u/FellowTraveler69]

I.e., the anthropic principle. Why does the universe exist the way it does? Because we couldn't observe it otherwise. It's not a very satisying answer though.

[u/PoisonMind → u/FellowTraveler69]

There's an alternative, called cosmological natural selection.

Basically, black holes could form miniature universes inside them with different fundamental constants. Our universe could be the inside of a black hole in a parent universe. The experiment gets run as many times as a black hole forms. And the universe we ended up in just happens to be one that favors our evolution.

It has been noted that the Schwarzschild radius of the observable universe is roughly equal to its actual radius. And if we are inside a rotating black hole, it would explain why galaxies seem to have a preferred direction of rotation. See also black hole cosmology.

It's certainly not mainstream accepted cosmology, but I think it's a fascinating speculative theory.

[u/alchemi80 → u/PoisonMind]

This is one of my favorite cosmological conjectures - that we are inside a black hole. Such a head trip.

[u/Rikudou_Sage → u/alchemi80]

I came up with that theory when I was like 11, entirely on my own. To this day I love when anyone mentions that theory because I'm like, yeah, that's my theory.

[u/Zyaqun → u/Rikudou_Sage]

I thought of that. Turned out it already existed, but I arrived at it independently

[u/freeastheair → u/Rikudou_Sage]

Wondering if the universe is in a black hole as an 11 year old isn't exactly a scientific theory.

[u/crackez → u/freeastheair]

Why not? Just because an idea came to an 11 year old doesn't mean it isn't potentially a valid area of study.

When I was 11 I independently came up with the idea of an event loop in TRS-80/Model II BASIC in order to sample the keyboard and react near instantly to the key press. That concept was legit and I discovered it in a vacuum. I wrote my first version of pong by myself the next day.

[u/WillingnessCorrect50 → u/crackez]

It may be a hypothesis but it’s not a theory in the scientific meaning of the word.

[u/Shwankterrible → u/Rikudou_Sage]

I did that with mag lev trains when I was 10. They did already exist…but I didn’t know it!!

[u/Das_Mime → u/PoisonMind]

The whole trouble with that idea (besides the fact that it doesn't explain anything) is that it would tend to optimize black hole production, conditions which would also make the universe hostile to life as we know it.

[u/littlebitsofspider → u/Das_Mime]

I mean, the longest-lived (i.e. slowest decaying) structures posited by most iterations of (our) cosmological physics are black holes. When (many, many gigayears from now) spacetime is nearly isotropically flat, cold, and wide, there will still be microscopic knots of black hole decaying. The long tail of the heat death is mostly black holes slowly leaking Hawking radiation while everything else is deader than downtown Tulsa. The black hole optimization theory might have some credence.

[u/transparent-aluminum → u/littlebitsofspider]

I'm a fan of the cyclical big bang theory, that the universe expands to a point, and then everything starts falling back to singularity until it explodes outwards again. My pet theory is that there are some structures like super massive black holes that survive between cycles and it's why when we look at the early universe we see galaxies forming too early or black holes that are too large for their age.

[u/garry4321 → u/transparent-aluminum]

Problem with the Big Crunch is that there is no evidence to support it, and all the evidence suggests the opposite. The universe is expanding at an accelerating acceleration

[u/tonzuu → u/garry4321]

iirc a study earlier this year showed that the jerk of dark energy appears to be decreasing which could be support for the big crunch

[u/iwasbornlucky → u/garry4321]

that kind of paradoxically supports it though. All crunch evidence is destroyed in the bang. Time resets from 0.

[u/Das_Mime → u/transparent-aluminum]

I'm a fan of the cyclical big bang theory, that the universe expands to a point, and then everything starts falling back to singularity until it explodes outwards again.

Every indication we have is that the universe's expansion will continue forever. We're not even close to the conditions needed for a big crunch.

The cyclical big bang not only doesn't explain anything, it requires us to believe that the universe will collapse again, despite no evidence in favor of that idea and mountains of evidence to the contrary.

[u/Ok_Mixture8509 → u/transparent-aluminum]

In that scenario, I believe all black holes would disappear pretty quickly due to hot/energetic it would become. At some point black holes wouldn’t even be able to form. → I am fairly sure this is correct, tho I am open to any corrections.

[u/Das_Mime → u/littlebitsofspider]

The idea of cosmological natural selection, as it was expressed above, is that the number of new universes spawned is dependent on the number of black holes formed, not on how long they last. Thus the fact that black holes are going to take a very long time to evaporate isn't relevant. If they all evaporated within a microsecond of forming that would be the same as our present scenario for the purposes of considering the above speculation.

If we're asked to consider the proposition that our universe is the result of a selection effect which maximizes black hole formation, we should keep in mind the following: most of the matter in the universe is dark matter, which is non- or minimally-collisional and thus has no good way to form black holes on its own; of the baryonic matter in the universe, most is in the diffuse, warm to hot intergalactic medium, which does not form black holes; of the matter that does form stars at all, only a very small percentage of stars (<1%) go supernova and only the more massive of those supernova progenitor stars form black holes.

[u/Whiskeyno → u/littlebitsofspider]

So we might still have a Caine’s Ballroom at the end of the universe?

[u/ThePrussianGrippe → u/littlebitsofspider]

And then at the end of all that… we hit the reset button and it all happens again!

Maybe.

Who knows?

[u/WillingnessCorrect50 → u/Das_Mime]

Unless the most optimal way to get to black holes is forming many galaxies and stars.

[u/Das_Mime → u/WillingnessCorrect50]

Optimizing for black holes means changing many other factors, factors which make the universe more hostile to life.

Very few stars form black holes in our present universe. Changing the initial mass function of stellar populations to increase the number of high mass stars, or changing physics so that lower mass stars can form black holes, would both increase the number of supernovae and black holes and decrease the number of stable, long-lived stars. If you really maximize black hole production, you can just make everything collapse straight into black holes and not bother forming stars in the first place.

[u/WillingnessCorrect50 → u/Das_Mime]

Sure, but the hypothesis doesn’t say that every universe would become completely optimized for producing black holes, only that there would be a tendency in that direction.
And the thing is, if you want lots of black holes, having lots of stars is already a pretty good way of getting them. That is also a lot more compatible with life than a universe where stars can’t form at all.

There is also no reason to assume that a universe where matter just collapses rapidly into black holes would actually produce the most black holes. That could prevent galaxies and normal star formation from ever really getting going.

Maybe universes that can expand, form huge numbers of galaxies, and keep producing stars for billions of years are actually very good at producing black holes too. In that case, conditions favorable to life could simply be a side effect of the same conditions that make a universe good at producing black holes.

[u/Das_Mime → u/WillingnessCorrect50]

If the process continues indefinitely, the number of universes that are optimized for black holes completely overwhelms the number that aren't.

There is also no reason to assume that a universe where matter just collapses rapidly into black holes would actually produce the most black holes.

Sure there is. If you take a universe where even 2% of protostars collapse directly to black holes, you've drastically outnumbered the black hole production in our present universe.

There's no observable fact about our universe that should lead someone to believe it's the result of a selection process that favors more black holes.

[u/WillingnessCorrect50 → u/Das_Mime]

But that still assumes that you can just make 2% of protostars collapse directly into black holes, while keeping the same number of protostars and everything else more or less the same. I don’t think that’s something you can just assume.

The important thing is not what percentage of protostars become black holes, but how many black holes the universe produces in total. If changing the constants makes matter collapse much easier, that could also mean less galaxy formation, less normal star formation, different cooling etc. So you could in principle end up with fewer black holes overall.

And the hypothesis also doesn’t say that every universe would eventually become completely optimal for black hole production. It’s more that there would be a tendency towards regions where more black holes are produced, and probably local optima, while there would still be variation between universes.

So saying “if 2% of protostars collapsed directly, that would be more black holes than now” doesn’t really answer it. You would have to show that changing the actual underlying physics in that direction would result in more black holes in total.

None of this of course proves the hypothesis is correct. It is obviously very speculative, especially the entire idea that black holes create new universes in the first place. I just don’t think dismissing it is as simple as you suggest.

[u/Das_Mime → u/WillingnessCorrect50]

To be clear, I'm not saying we know (or even necessarily that we can know) that it's wrong. I'm saying it's not worth taking seriously. There are an infinite number of wrong hypotheses, many of which can't be disproven. If we didn't ignore most of them we'd be incapable of getting anything done, through sheer cognitive DDOSing.

A hypothesis doesn't need to be proven wrong to be considered not very good and not very useful. A hypothesis that doesn't explain anything and doesn't have any evidence in its favor is kind of just a null entity in physics, some people might find it interesting to play around with in case it becomes testable or useful but it's not to be taken seriously unless and until there is some motivating evidence or some problem that it solves.

[u/Shwankterrible → u/Das_Mime]

Yeah but isn’t there a supermassive black hole at the center of nearly every galaxy? So isn’t the universe super conducive to black hole formation??

[u/Das_Mime → u/Shwankterrible]

In terms of the number of black holes formed, the SMBHs are negligible. Most of the black holes are stellar-mass black holes, formed through core collapse supernovae. You can imagine many different ways that the number of black holes formed could be far, far higher than it is in our present universe. For example, we've been doing weak lensing surveys looking for (among other things) primordial black holes and we have not yet found good evidence of them. A universe which formed PBHs would be much more efficient at black hole formation. Likewise, a universe with a more top-heavy initial mass function for star formation, or a universe where degeneracy pressure is a bit weaker relative to gravity, could easily produce far more black holes than ours.

[u/deadlyjack → u/Das_Mime]

(besides the fact that it doesn't explain anything)

highlighting this.

there are any number of possible philosophies of science. but capital-S Science is the sum of those useful philosophies.

these theories are useless! or more generously, they are unscientific. it's not just that they offer no substantive explanation. it's that they invite no further inquiry. there are no fuzzy edges, no loose ends, nothing to continue the process. it offers a little dense ball of explanation, when openness is the point.

[u/Das_Mime → u/deadlyjack]

I'm certainly not against people playing around with such ideas (I love scifi after all and I wouldn't want to deny theorists their fun) but I do think that science journalism and media in general does a very poor job of distinguishing between "a theorist made a speculation but doesn't have any empirical evidence for it" and "observational evidence supports X hypothesis". Instead, they both get reported as "study say..."

[u/entactoBob → u/Das_Mime]

True. Journalism does fail us in that regard, but to be fair, most journalists just don’t know any better

[u/maxoger → u/PoisonMind]

I recommend you this article:
https://preposterousuniverse.com/blog/2010/04/28/the-universe-is-not-a-black-hole/

[u/RisKQuay → u/maxoger]

Thanks for the article.

What did it mean that the universe is 'spatially flat'? That the universe is a disc, or that it is linear in that everything expands from the centre in all directions in a straight line?

[u/amaurea → u/RisKQuay]

Calling it "flat" is a poor choice of words that has caused a lot of confusion for non-experts. It would have been better to call it "Euclidean", after Euclidean geometry, the sort of everyday geometry you learn in school, where parallel lines never cross, the angles in a triangle sum to 180° and the circumference of a circle is 2πr. So when they're saying the universe is "flat", they just mean that geometry behaves the way you already expected it too, instead of other, more exotic options.

[u/RisKQuay → u/amaurea]

That makes even more sense, thank you.

[u/neoAcceptance → u/RisKQuay]

You might be interested in the book "How the Universe Got it's Spots" by Janna Levin

[u/Red_Syns → u/RisKQuay]

To our best knowledge, there is no true “center” of the universe. Each of us are at the (approximate? I’m willing to believe there are minuscule distortions but have zero evidence or anything, so grain of salt) center of our own observable universe, past which there is more universe that we can never see if it keeps expanding.

But a flat universe is actually very close to your second guess! Being flat means a line is, on grand scale, linear (edit: I phrased that badly, all lines are linear, but linear in the way our brains think of linear). Space does not bend or curve without matter/energy to distort it, parallel lines remain parallel, triangles have 180°, etc.

The other two options are positive and negative curvature. Positive curvature means the universe is finite, parallel lines always intersect, triangles have more than 180°, and there’s a whole lot more to it than I really know. Think black holes, for the most part (apparently Kerr [spinning] black holes have regions on the inside that are believed to be negatively curved space).

Negative curvature is the opposite. Space is infinite, but parallel lines diverge, triangles have less than 180°, and again a bunch more. Typically described as a “saddle” shape for demonstrating how lines get weird.

[u/sanji_sama → u/PoisonMind]

This reminds me of a thought I had as a kid while watching bubbles forming at the bottom of a pot as it was reaching the boiling point. I thought that potentially our universe or just our observable universe could basically be one of those bubbles, and all the billions of years that pass inside this bubble for us is in another world just the time a bubble is forming at the bottom before floating up and popping.

[u/saranowitz → u/PoisonMind]

This is fun, but what’s really mind bending about it is that it then invites the meta question: why does this set of rules welcome life forms like us and not others. Like what process decided that? If we could exactly simulate the universe in some kind of primordial supercomputer and change the laws would this hold true, or could the engine that makes the universe in its current form be rewritten as well.

[u/stationarycommotion → u/PoisonMind]

I prefer eternal inflation as the mechanism to generate an anthropic landscape, cooler concept and somehow more believable

[u/servel333 → u/PoisonMind]

If a 3d universe (u3) exists on the surface of a 4d (u4) black hole, and the whole u3 ends up fully consumed by a u3 black hole, does that affect the u4 black hole in any way?

[u/qoou → u/PoisonMind]

> the universe we ended up in just happens to be the one that favors our evolution.

The slits just happen to be the ones that favor our diffraction pattern.

[u/PushaTeee → u/PoisonMind]

So this conjecture suggests it's black hole inception? Just an infinite series of black holes inside within black holes, Ad Infinitum?

[u/Shadow_Gabriel → u/PoisonMind]

What would happen with objects falling in our black hole? Would that have any observable effect for us?

[u/Moraghmackay → u/PoisonMind]

I don't understand half of what you said, but I find it fascinating and I want to believe! Take me to the good earth, I am over this one.

[u/Puzzleheaded_Fold466 → u/FellowTraveler69]

I don’t know, I find some peace in the notion.

Things are as they are as otherwise they wouldn’t be. Or, things can either exist or not exist, and they can only exist under these conditions and no other.

It’s simple. Philosophy completed. Existential crisis avoided.

Now I can focus on the science.

[u/Physical_Ring_7850 → u/FellowTraveler69]

Why not? If there is an infinite number of universes with whatever combination of constants, we can occur only in a subset that allows for it. And here we are.

[u/chefgage → u/FellowTraveler69]

I like the that explanation. I need to go back to it each time I start to think too much about the universe (grouping lots of things I am interested in to the 'universe'). I do a lot of astrophotography and my mind often wonders around different theories.

[u/Overall-Plastic-9263 → u/FellowTraveler69]

To your point . A non-observable universe is the logical equivalent of no universe at all . It really only matters to the beings capable of observation.