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Comment by maaaaattttt | Hacker News Reader

This is a question I've been asking myself (probably due to a lack of understanding and knowledge) and this theory seems to hint at something similar: why do we assume spacetime to be homogenous in its nature? Here they seem to say that on some quantum level spacetime could vary drastically. Couldn't then dark matter be explained by some variations on a large scale? Like giant "wrinkles" in spacetime? They would then behave as pseudo black holes, or more like "black trenches", that lead to the structures we see today in the universe. The structures seem to have more mass than what we can actually see, but are in fact just placed in locations that behave differently gravity wise?

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"Here they seem to say that on some quantum level spacetime could vary drastically."

This is not a result of this theory. Our best current theories say this too: https://en.wikipedia.org/wiki/Quantum_foam Spacetime getting distinctly different at very small scales is a vital part of all modern theories and I doubt it's going away. Even if spacetime is shown to be continuous itself, which I tend to personally doubt [1], getting close to the scales where the uncertainty principle becomes a major influence rather than an almost academic curiosity will inevitably have major impacts on the behavior of things.

"Like giant "wrinkles" in spacetime?"

Giant wrinkles in space time don't act like dark matter. They either smooth themselves out basically instantly (on cosmological scales) or form something other than a giant wrinkle, such as a cosmic string: https://en.wikipedia.org/wiki/Cosmic_string

In fact one of the low-key mysteries extant in the universe today is that our best theories say cosmic strings really ought to exist, but we don't see them. Though I think the confidence in our theories being pushed to that extent is low enough that this is not generally considered the biggest problem, and very unlikely to be the thing that cracks the mystery, so it isn't something that gets talked about a lot. So you could say that we actually don't "expect" space time to be homogeneous and one of the mysteries is why it observationally is!

[1]: https://news.ycombinator.com/item?id=38433917

Thank you for the link to cosmic strings! This is actually exactly what I had in mind when, as you quoted, I was mentioning wrinkles and “black trenches”.

So, you don’t think structures like galaxy superclusters are a second hand visualization of the existence of cosmic strings? I know these formations could be explained by the effect of gravity of just visible matter. But maybe it’s both. Like a valley could exist because of a river and a river could exist because of a valley.

Since this concept has been around for so long I have some reading to do to understand why cosmic strings are not a possible explanation for dark matter. To me high density and invisible fit the bill for dark matter quite well.

Edit: never mind, continued reading the article and saw this

> in the past it was thought that their gravity could have been responsible for the original clumping of matter into galactic superclusters. It is now calculated that their contribution to the structure formation in the universe is less than 10%.

If you look at the various ways people have tried to interpret thermodynamics and the observation that our local universe started in a state of low entropy (not exactly spacetime, I know, but it's related to the problem), then you'll see that a variation of your question has been very thoroughly considered already.

Namely, the possibility of the low-entropy start of our observable universe being merely a local statistical fluke on a cosmic scale across infinite time and space.

The problem is that this often leads to weird paradoxes, like the Boltzmann brain universe[0], and from what I understand it's still not entirely clear if that's just a sign we're doing statistics wrong or if there's something missing in the theory of cosmology.

Personally, I'm wondering if the issue isn't that these theories are assuming randomness, but in reality the next "state" of the universe depends on the previous state, so it's actually pseudo-random. But that sounds like too obvious an explanation so that has probably been ruled out somehow.

Anyway, just to be explicit about it again: what I'm talking about relates to thermodynamics and entropy. You're question is about spacetime and the laws of physics, which is something else.

[0] https://en.wikipedia.org/wiki/Boltzmann_brain

I've had the same question and I don't have the answer, but what you're asking about might be referred to as the cosmological principle

https://en.wikipedia.org/wiki/Cosmological_principle

Seems like so far our observations are that the universe is pretty homogeneous. Cosmological principle being the default position, my best understanding is that it's just a lack of evidence otherwise:

> The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized in accordance with the Cosmological Principle. At this scale, no pseudo-random fractalness is apparent.

https://en.wikipedia.org/wiki/Observable_universe#End_of_Gre...

Small nitpick on choice of words here - homogenous probably isn't the right word to use, instead the correct term is probably invariant.

The universe appears homogenous at various scales, meaning it's self-same within some amount of tolerance across itself. The distribution is homogenous. That's different from invariance, the assumption that a variable (in this case an intrinsic property such as mass) remains unchanged over time for a given subject of study, such as a particle.

Even if a given property can vary from subject to subject (even wildly), the sum of all subjects could still appear homogeneous in their distribution.

>why do we assume spacetime to be homogenous in its nature?

Because it's distribution appears to be consistent, at least as far as we can measure. That's not the same thing as objects at the quantum scale being invariant (or variable) in some way that we currently think is the opposite.

>Couldn't then dark matter be explained by some variations on a large scale? Like giant "wrinkles" in spacetime?

Considering the scales at which these fluctuations would have to occur for us to have not already measured them I'm doubtful they could build up to anything like that, even in aggregate and random patterns.

>They would then behave as pseudo black holes, or more like "black trenches", that lead to the structures we see today in the universe. The structures seem to have more mass than what we can actually see, but are in fact just placed in locations that behave differently gravity wise?

I don't think I'm really qualified to answer on how possible this interpretation is, but with the confirmation of gravitational waves a few years back we confirmed another piece of Einstein's theories and seem to have a good grasp on the observational effects of gravity at least. There was a recent announcement about mapping of the Gravitational Wave Background [0] using pulsars spread across the galaxy that was super cool too you might be interested in that I would say is related to this question.

The real answer is that theories like this require quite a bit of work to manipulate existing and complex math, and that you'd need someone who can translate an idea like yours into a mathematical model that fits with our observational data. That's an exceedingly difficult thing to do, as evidenced by the last 70ish years of physics.

[0]: https://www.space.com/gravitational-wave-background-universe...

Science had fared quite well so far with the assumption that the laws of physics are largely the same no matter where and when we are. There would have to be very strong evidence to throw that out. And the immediate next problem is that it actually wouldn't explain much. The immediate next question would be "why is spacetime behaving this way".

Ultimately it's just a way to say, "well, there is simply more gravity there". I was also thinking of the "why" and I'd say because when the universe expends, or initially expended, it didn't happen homogeneously. In my opinion it's more realistic that spacetime isn't homogeneous than that it is.

If there is some reason over here isn’t the same as over there then there will be a mechanism as to why that is the case and you will want an explanation.

You want your physics to apply everywhere and that’s why you start with the assumption that the same laws, constants, etc apply everywhere.

There’s no “ope stuff is just different over there” in physics, you look for those inconsistencies and then you try to explain them.

I have a similar question:

What happens to quantum tangles[0] that get inflated? — do they melt or is it possible for them to absorb energy in a hard to interact with state?

Eg, is the reason dark matter looks like a braid that it is? — and inflated foam turned into a foamy-web?

[0] - https://en.wikipedia.org/wiki/Anyon

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