When you look at the horizon from the beach it appears flat , could Einsteins cross be a similar phenomenon?

Question

Actually i want to know why they think the universe is flat?

Answer 1

OK, so there are three questions here.

The horizon appears to be flat because we're so close to it that it's hard to see the curve.  From just 10 miles up, the curve is evident, and from 100 miles up, it's painfully obvious.

Einstein's cross has to do with the gravity of a supercluster of galaxies bending the light of a background object - typically a quasar - which is the jet of a black hole pointed right at us.

The idea that the Universe is "flat" means something totally different.  This is astronomers using the terminology of topology in multiple dimensions to get across a very simple idea.  The expansion of the Universe seems to be very nearly exactly the speed that causes the matter in it to go out forever, but slow to a speed approaching zero as time goes to eternity.  Flatness is normally a two dimensional concept, but space-time is four dimensional at least.