So the universe won't eventually cool down and shrink? Why is this theory outdated...

So the universe won't eventually cool down and shrink? Why is this theory outdated? I learned this is school only about 5 years ago

>bbc holes succ all matter
>bbc becomes 2big4u
>succs everything including other bbc holes and the gravity succs in the space fabric
>collapses
>sends out all mass again
>mass creates stars n shit
>star dies and becomes a smol bbc
>repeat

>So the universe won't eventually cool down and shrink? Why is this theory outdated?
Because the universe has existed for an infinite amount of time, and so we can deduce that if this theory was correct then the universe should have already reached its heat death, which it clearly has not

>what is entropy

Because observations have concluded that the rate of expansion is increasing from forces unknown to us in detail and generalized as "dark matter" and "dark energy" at this point in time.

Unless other forces completely unknown to us are also at work, the universe will expand infinitely and eventually end up as a collection of black holes or as a collection of nothing but photons depending on whether protons decay.

Your school is behind the times.
Before Dark Energy, all the matter in the universe had less than the critical density which would cause a re-collapse. It would expand forever, just more and more slowly.
Now the expansion seems to be accelerating.

Whether the accelerating force will continue to grow, will remain steady, or even reverse -- is unknown.

We know that the universe is flat to within some specification. If the deviation is on one side, then collapse is a sure thing. If the universe deviates from flatness globally on the other side of flatness then it will not collapse.

I'm completely lost on visualizing the geometry of the universe. If it's flat, why do there seem to be three dimensions going infinitely into the distance? If it's not flat, wouldn't it mean that we're either on the inside or the outside of a sphere and could circle it by setting off in a random direction?

"Flat" is math speak. You can tell you're living on the surface of a globe by painting larger and larger circles around yourself. On a plane, the perimeter of each circle is 2 pi times its radius. On a globe, the circles stop growing after awhile. Then they shrink down to a point on the opposite side of the globe from where you started.

In 3 dimensions, you draw larger and larger spheres around yourself and see it their surface area keeps increasing. We're not in a position to actually paint spheres, but we can do the equivalent by noting if the number of galaxies X billion lightyears away varies as the square of X.

I'm aware of basic geometry. The spherical shape of the Earth was determined using trigonometry in ancient Greece. It doesn't answer any of my questions though. Is our "flat" universe infinite in all directions? Would a "curved" universe eventually meet up with itself? What happens to the third dimension in a "curved" universe, would it theoretically pass through the middle of it starting from any given point?