How can the universe be flat

Can a flat universe collapse?
A spatially closed universe can expand forever if the vacuum energy density is not zero.
Yes, a universe without dark energy will expand decelerated and collapse into a big crunch.
This is still true if small amounts of vacuum energy, respectively ΩΛ is added..
How can the universe be flat if we live in 3 dimensions?
The fact that our Universe exists with the properties we observe tells us that, very early on, the Universe had to be at least very close to flat.
A Universe with too much matter-and-energy for its expansion rate will have positive curvature, while one with too little will have negative curvature..
How could the universe be flat?
Most cosmological evidence points to the universe's density as being just right — the equivalent of around six protons per 1.3 cubic yards — and that it expands in every direction without curving positively or negatively.
In other words, the universe is flat._{Feb 23, 2021}.
How is the universe flat if we are 3d?
the term “Flat” doesn't mean it's .
1. D flat surface like the table top, but it means that it's an euclidean space that every two parallel line will remain parallel if they extended to infinity, also any triangle stretched across the universe its angle always add up to 180 degrees
An angular size smaller than 1 degree would indicate a negatively curved Universe, while an angular size larger than 1 degree would indicate a positively curved Universe.
Observations of CMB show that the spots are actually around 1 degree in diameter, further supporting the fact that the Universe is flat.
The best evidence for a flat universe comes from measuring the size of irregularities in the cosmic background radiation.
The small size of these irregularities is predicted by a flat, inflationary universe theory, not by open (negative curvature) or closed (positive curvature) universe theories.

If all the matter and energy in the universe, including dark matter and dark energy, adds up to exactly the concentration at which the energy of the outward expansion balances the energy of the inward gravitational pull, space will extend flatly in all directions.

Does a flat universe have to be infinite?

The surface of the torus is also spatially flat, but it is finite.
So you have two possibilities for a flat Universe:

one infinite

like a plane

and one finite

like a torus

which is also flat.
ESA:‘Flat' seems to have a different meaning to non-scientists.
By 'flat' we understand to be like a table, which has width.

How flat is our universe really?

Yes, it’s entirely possible that the Universe is only almost flat on large scales, as is acknowledged by the community.
There is a cosmological parameter, Ω k, that relates to the amount of large-scale curvature, and observations can constrain it to be within a small range including:

zero

but can never show it to be exactly zero.

What does the term "flat universe" mean?

In cosmology, "flat universe" means that space is flat, not spacetime.
Space is the three-dimensional hypersurface that results when cosmic time is held fixed.
The spacetime metric induces a spatial metric on this three-dimensional hypersurface.

How can the universe be flat

Can a flat universe collapse?