In our cosmic neighborhood, inflation ended 13.7 billion years ago, but it still continues in remote parts of the universe, and other “normal” regions like ours are constantly being formed. The end of inflation is triggered by quantum, probabilistic processes and does not occur everywhere at once. At the end of inflation, the energy that drove the expansion ignites a hot fireball of particles and radiation. It is so fast that in a fraction of a second a tiny subatomic speck of space is blown to dimensions much greater than the entire currently observable region. Inflation is a period of super-fast, accelerated expansion in early cosmic history.

It is the subject of the theory of inflation, which was developed in the early 1980s by Alan Guth, Andrei Linde and others, and has led to a radically new global view of the universe. The nature of the bang itself has come into focus only relatively recently. For nearly a century cosmologists have been studying the aftermath of this explosion: how the universe expanded and cooled down, and how galaxies were gradually pulled together by gravity. The universe as we know it originated in a great explosion that we call the big bang.

Here, multiverse proponents Alexander Vilenkin and Max Tegmark offer counterpoints, explaining why the multiverse would account for so many features of our universe-and how it might be tested. Editor's note: In the August issue of Scientific American, cosmologist George Ellis describes why he's skeptical about the concept of parallel universes.

Max tegmark mathematical universe