Is the Universe "Flat"?

[Heyo]

Cosmologists tell us that the universe is most probably "flat", i.e. there is no universal curvature.

I have problems to wrap my brain around that.
1. If space was curled up into a singularity at the Big Bang, when did it expand to infinity? Is there even a mathematical possibility to go from curled up to infinite in finite time? Can the universe be infinite without space having always existed?
2. Every point of mass makes a "dent" into the fabric of spacetime. I.e. even if space is infinite, shouldn't at least the local universe in a (curved) gravity well? The only way to get a local (and universal) flat space, I can think of, is the existence of local regions of "anti-gravity", i.e. regions of space with a negative curvature that balance out the regions of positive curvature. Would those regions "repel" mass and stretch spacetime? (I.e. is that the Dark Energy?)
3. Would that be, at least theoretically, evidence for the existence of negative mass?

Where do I go wrong?

 

[ChristineM]

It's not known whether space has always existed or our universe created its own space as it expands.

Given the time is finite then so must be the universe (about 93 billion lighy years across and expanding). So I've always considered the infinate bit as potentially. I.e. it will keep expanding for ever,

I can't really answer the second part but i don't think there is enough mass to really effect it

See also https://www.forbes.com/sites/startswithabang/2021/03/05/ask-ethan-why-is-the-universe-flat/amp/

 

[The Spirit of Truth]

All that exists in the Nothing is energy. Nothing is exactly that, nothing, so it is endless. The super-computer equivalent Creator merely created matter from energy, which cannot be created nor destroyed, whereas matter can. All but our spirit is held in existence by God/Creator so can cease to exist or be returned at His will. God bless. The Branch growing out of the Rod i.e. the Comforter.

 

[Brickjectivity]

At minute 10:20 this presenter (who is talking about the expansion rate of the universe) says that some of the variant measurements suggests that 'Dark energy' may vary with time. Would this affect the curvature of the universe? It seems like it might, because it would affect the mass of the universe. Its really just shooting in the dark though, because her comment is based upon a slight difference from the expected expansion rate.

Spoiler: Universe Expansion Rate Measurement

Oh...another thing is I have no idea what we are talking about. That might matter, too.

 

[Heyo]

At minute 10:20 this presenter

Dr. Becky is one of my favourite science communicators. I rarely miss any of her videos.

(who is talking about the expansion rate of the universe) says that some of the variant measurements suggests that 'Dark energy' may vary with time. Would this affect the curvature of the universe?

Yes, it would. The cosmological constant (which probably isn't constant) gives us an estimate of the size of the universe at any given time.

The yellow line is for a closed universe, positive curvature, gravitation overcomes expansion, and we end in a "Big Crunch".
The red line is for an open universe, negative curvature, expansion was always stronger than gravity, and we end in a "Big Rip".

But that is only if expansion rate and gravity are constant. If, as it seems the case, the universe can gain expansion energy from the vacuum, we could have had a closed universe in the past but switch to an open universe.
And I think that answers my first question. There is a mathematical possibility to go from curved to flat, but it doesn't have to be universal. When the transition is only local, there is no reason that it can't happen. Thanks.

It seems like it might, because it would affect the mass of the universe. Its really just shooting in the dark though, because her comment is based upon a slight difference from the expected expansion rate.

Spoiler: Universe Expansion Rate Measurement

Oh...another thing is I have no idea what we are talking about. That might matter, too.