What Does it Mean When We Say the Universe is 'Flat'?

[MikeeMiracle]

TL;DR

Flat Universe Definition

When we say the universe is "flat," do we mean:

1) If you could hypothetically "zoom" out to the edge of the universe it would appear roughly as a flat like a sheet of paper with local fluctuations / bumps to indicate the presence of matter.

2) If we draw 2 parallel lines from 1 location they will never move towards each other or further apart? (Assuming discounting the effect of cosmic inflation and curvatures caused by matter itself?)

I am assuming it is the 2nd option?

Thanks

 

[PeroK]

Summary:: Flat Universe Definition

When we say the universe is "flat," do we mean:

1) If you could hypothetically "zoom" out to the edge of the universe it would appear roughly as a flat like a sheet of paper with local fluctuations / bumps to indicate the presence of matter.

2) If we draw 2 parallel lines from 1 location they will never move towards each other or further apart? (Assuming discounting the effect of cosmic inflation and curvatures caused by matter itself?)

I am assuming it is the 2nd option?

Thanks

Essentially it's number 2). Number 1) is simply absurd. The universe has 3 spatial dimensions.

 

[MikeeMiracle]

You know, in all honesty number 1 sounds absurd to me too but my time on this forum has demonstrated to me that just because initially appears to be absurd and /or completely counter intuative, that does not necesarily mean it is wrong, hence why I felt the need to post asking for a clarification.

 

[PeterDonis]

When we say the universe is "flat,"

We mean it is spatially flat. In other words, spacelike slices of constant time, where "time" here means time in the standard FRW coordinates used in cosmology, are flat Euclidean 3-spaces.