Okay, the universe is flat.
From astrophysicist Paul Sutter at Space.com:
The universe has all sorts of deformations in space-time where it varies from the perfectly flat. Any place where there’s mass or energy, there’s a corresponding bending of space-time — that’s General Relativity 101. So a couple light beams would naturally collide inside a wandering black hole, or bend along weird angles after encountering a galaxy or two.
But average all those small-scale effects out and look at the big picture. When we examine very old light — say, the cosmic microwave background — that has been traveling the universe for more than 13.8 billion years, we get a true sense of the universe’s shape. And the answer, as far as we can tell, to within an incredibly small margin of uncertainty, is that the universe is flat.
But, he warns, there is a lot of subtlety packed into that flatness: “You heard it here first: Cylinders are flat.”
Also:
And here’s a bonus fact: not only can we not determine the topology of the universe from observations, but there are also no laws of physics that predict or restrict the topology. More.
Surely something does, however, restrict the topology if the universe appears to be the same in every direction.
See also: Once more: What if dark matter doesn’t exist?
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