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petarv
2008-Oct-27, 08:51 PM
Simplistically, I have difficulty accepting that the Universe can be flat. Any Big Bang should eject debris in all directions, should it not? And how would a 'flat axis' be preferentially determined and matter attracted to any particular plane?

Jeff Root
2008-Oct-27, 09:23 PM
Hello, petarv!

The idea that the Universe is "flat" sounds preposterous because it doesn't
mean what you think it means.

The Universe is pretty much the same in all directions, as you expect.

Gravity can be described as "warping" or "curving" spacetime. Spacetime
is just the combination of the three dimensions of space with the dimension
of time. Mass or energy warps or curves spacetime. The more densely the
mass or energy is concentrated, the stronger the curvature. A small amout
of curvature causes effects like your body being attracted toward the Earth.
A larger amount of curvature holds Mercury in orbit around the Sun. A very
large amount of curvature causes a black hole at the center of our galaxy.

The masses that cause this curvature of spacetime are distributed in lumps:
Earth, the Sun, the stars that became the Milky Way's supermassive black
hole. The curvature is great close to the mass and becomes less and less
the farther you are from the mass. But there is an overall effect -- an
overall curvature.

How much overall curvature? Measurements of the cosmic microwave
background radiation and counts of galaxies of different sizes suggest that
the curvature is very small -- less than a couple of percent. It is close to
zero curvature. Zero curvature means that overall, ignoring the local bumps
or pits that are things like planets and stars and galaxies, spacetime can be
described as "Euclidean". That is, it follows the rules of Euclidean geometry.
Euclidean geometry describes space which is not curved. In other words,
"flat" space. The geometry which describes curved space is "Reimann"
geometry.

The fact that the geometry of the Universe is so close to "flat" overall means
either that it is much, much larger than we can see, or it is infinite in extent,
or there is something interesting going on that makes it "flatter" than we
would otherwise expect.

-- Jeff, in Minneapolis

spratleyj
2008-Oct-27, 09:24 PM
"Flat" doesn't mean perfectly flat just relatively like 2-3% give or take positive or negative curvature.

speedfreek
2008-Oct-27, 09:49 PM
This website - The Shape of Space (http://www.etsu.edu/physics/etsuobs/starprty/120598bg/startit.htm) is a good primer for this subject. The site is a little old, so the given age for the universe is a little high, but the principles are all sound.