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Comment by danielbigham | Hacker News Reader

I wonder if spacetime, in addition to being curvable by mass, is also inherently curved at the scale of the universe.

My personal (lay) theory is that the global curvature of the universe is flat, but that there are local distortions due to the energy of the big bang/rip.

Two (probably completely inaccurate) thought experiments:

Imagine a sheet of aluminum foil. Crumple it up really tightly into a ball then stretch it back out again. The sheet won't be perfectly flat anymore: it will have ridges and valleys introduced when compressing the sheet. Place objects with mass onto the sheet and those will cause further distortions, but there will be areas without mass that are also distorted.

Imagine a sheet of fabric. Place an object with mass on the sheet, then pull on the sheet (like trying to remove a tablecloth without displacing the tableware). Depending on the velocity of the pull, the mass of the object, and a bunch of other factors, you might introduce ripples into the sheet around the object. Again, distortions in areas without mass. (From my very lay understanding, this would be related to, but independent from, gravitational waves, which are caused by motion of mass in space-time rather than the expansion of space-time itself. But then again, motion is all dependent on the frame of reference, so what do I know?)

It's an open question.

https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvatur...

could there be an adjacent sphere of expanding matter? a "parallel universe" or universes, not in the sci-fi alternate dimension sense, but in the "physically adjacent big bangs in space" sense where the light has simply never reached us yet, but could one day

Maybe it's not inherently curved. Maybe it's just curved by a mass that is further from us than tens of billions of light-years.

We can't know if whole universe looks as smooth as observable universe.

I sometimes ask a similar question about matter/antimatter symmetry, if the “tiny asymmetry” that gets used as an explanation for why any baryonic matter exist at all, could be explained by the antiparticles we should’ve annihilated with being on the other side of our cosmic event horizon.

Seems like a not bad idea. Especially if the antiparticles accelerated away before they could interact. But the two types of particles should be mutually attracted. They’d also have to appear in an organized way to avoid collisions with other particles.

Maybe there wasn’t a Big Bang, but instead a “large shearing”. Ie two “sheets” (like higher dimensional objects) started in contact with one another and moved apart/were pulled apart from one another. Before the shearing, lots of energy existed with little or no opportunity for massive objects to form. During the shearing the particles adhered to one sheet (for whatever reason) and the antiparticles the other.

(End of my out there positing)

A question I'm too amateur to resolve on my own is what if the Big Bang simply created two universes? One made primarily of "matter" and the other made of "anti-matter?"

What if the big bang was simply a Casimir event? The particles should have collided and annihilated, but didn't? The result being two new universes pulled into existence by the sudden inversion of entropy?

I've often wondered the same thing. What if the entire universe is finite, but something like 1^(enormously large number) times larger than the observable universe? We could be this tiny speck of a backwater that's completely unrepresentative of the overall universe. The Big Bang itself could have been a minuscule local disturbance that took place in some small part of the overall universe which is trillions of times older.

Surely you mean 10^(enormously large number)

A mass that happens to be around us in all directions?

Is it possible that we are observing effects of mass that is outside the observable universe?

I think that depends on whether big bang was global event that created matter in the entire universe (then no, because it's too far) or local event that affected observable universe (then we could see the spacetime curvature caused by outside objects and even gravitational waves that are older than the age of observable universe).

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