No Center to the Expansion in 3D Space
General Relativity describes gravity as a warping or distortion of space and time near a massive
object. In General Relativity, fourdimensional spacetime is curved. You may want to refresh
your memory of these concepts by reading the
Relativity chapter
.
To help you understand what curved spacetime means, let's use the analogy of a twodimensional
world curving into the third dimension. Pretend you are confined to the surface of a balloon and
you only know about ``front'', ``back'', ``left'', and ``right'', but not ``up'' and ``down''. In your 2D
universe you cannot see the third dimension. Your universe appears flat. Yet you know that your
2D universe must be curved because if you walk in a straight line, you eventually arrive back at
where you started! The balloon universe has a finite size but
no edge.
You also know that the
angles of large triangles add up to a number larger than 180°! For example, on the balloon the
lines of longitude running northsouth intercept the equator at a 90° angle and converge at the
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 Fall '10
 EmilyHoward
 Astronomy, Space, General Relativity, Spacetime, large triangles

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