Eli5 If the universe expanded from a single point why do scientists say its flat and not spherical?

Question

Why would it only expand in one plane not every direction like you’d expect?

Also how is a flat universe even possible? Surely since we live in 3 dimensions the universe needs to be a 3 dimensional shape.

Im probably misunderstanding what physicists are trying to say but that’s why im here.

In: 229

Answer 1

The thing is that the “spherical” you are thinking is a volume, a flat volume. Let me give you an analogy in 2d: imagine expanding from a point in the plane (like in a table). The point would expand as a circle, a flat circle. Geometry inside the circle is perceived as how would be the fastest way of getting from one point to another point in that circle. Since that circle is on the table, the fastest way is a straight line, and we call that a planar (or euclidean) geometry.

Now, imagine that instead of expanding the point on a plane, it expands on the surface of a sphere (but still imagine we still live on the surface). The closest path between 2 points on that surface is not a straight line, but a “geodesic” (that is the term that mathematician give to “straight lines” on non-planar geometries). This geodesic is actually illustrated in how planes fly long distances (because they want to find the shortest path between two points) on the surface of the earth. The geometry on the surface of the earth is in fact a spherical geometry.

Now, for the fabric of space-time itself (i’m not a physician, so i hope i dont get most things wrong here), you are talking about a 4 dimensional “surface” that could “live” in a higher dimensional “table” (if planar geometry satisffies) or “sphere” (if spherical geometry satisffies). There’s also a third option which is hyperbolical geometry but i dont want to get into many details.

This is hard to visualize even for physicists (i cannot visualize myself) but most scientists use analogies. There are however methods to measure “straight lines” in order to verify the geometry of the universe, and it seems that these measures have shown until today that the universe geometry is flat.

I hope the explanation is not so obscure and makes things clear.