Observations from the WMAP satellite show with a 0.5% margin of error that the universe has a flat geometry, which suggests that its size is unbounded.[i]
Director of the Institute for Cosmology at Tufts University, Alexander Vilenkin says, “The theory of inflation is by far the best explanation we have for the big bang. If we accept this theory, and refuse to mutilate it by adding any ad hoc, unnecessary features, then we have no choice but to accept eternal inflation.”[ii] The theory of inflation solves the horizon problem, the flatness problem and the magnetic monopole problem of the traditional Big Bang Theory.[iii]
According to eternal inflation, the multiverse will grow forever, spawning bubble universes like holes in a gigantic block of Swiss cheese. But if our bubble universe is infinitely big, one might wonder, how can it be contained within the grand block? A professor of physics at Columbia University, Brian Green explains: “Much as Hamlet famously declares, ‘I could be bounded in a nutshell, and count myself a king of infinite space,’ each of the bubble universes appears to have finite spatial extent when examined from the outside, but infinite spatial extent when examined from the inside.” This is far from obvious, but it is due to the observers’ vastly different frameworks of time. Greene says, “what appears as endless time to an outsider appears as endless space, at each moment of time, to an insider.”[iv]
What we call “the observable universe” is simply one light cone. But within each bubble universe, there are infinitely many light cones, also called Hubble volumes. “A generic prediction of cosmological inflation is an infinite ‘ergodic’ space, which contains Hubble volumes realizing all initial conditions—including an identical copy of you about 1010^29m away,”[v] says Max Tegmark, a professor of math/physics at MIT. Due to quantum fluctuations during the Big Bang, all of the Hubble volumes became causally disconnected from each other.
For a summary of the successes, mechanisms and implications of eternal inflation, you can see this paper[vi] by MIT physics professor Alan Guth, who helped develop it.
[i]Wollack, Edward. “Will the Universe Expand Forever?” NASA. 2011. http://map.gsfc.nasa.gov/universe/uni_shape.html [ii] Vilenkin, Alexander. Many Worlds in One. New York: Hill and Wang. 2006. (p. 117) [iii] Wollack, Edward. “What is the Inflation Theory?” NASA. 2010. http://map.gsfc.nasa.gov/universe/bb_cosmo_infl.html [iv] Greene, Brian. The Hidden Reality. New York: Random House. 2011. (p. 68-70) [v] Tegmark, Max. “The Multiverse Hierarchy”, 2007, Cambridge University Press. http://arxiv.org/abs/0905.1283 (p. 1) [vi] Guth, Alan. “Eternal inflation and its implications”. Journal of Physics, 40:6811-6826. (2007) http://arxiv.org/abs/hep-th/0702178