Knocking on Heaven's Door - Lisa Randall [168]
We really don’t know what exists beyond the horizon—the boundary of the observable universe. The limits to our observations allow for the possibility of new and exotic phenomena beyond. Different structures, different dimensions, and even different laws of physics can in principle apply so long as they don’t contradict anything that has been observed. That doesn’t mean every possibility is realized in nature, as my astrophysics colleague Max Tegmark sometimes asserts. However, it does mean there are many possibilities for what can be out there.
We don’t yet know if other dimensions or other universes exist. Really, we can’t even say with certainty whether the universe as a whole is finite or infinite, though most of us think it’s likely to be the latter. No measurement shows any sign of its ending, but measurements only reach so far. In principle, the universe could end, or even have the shape of a ball or balloon. But no theoretical or experimental clue leads us in that direction at present.
Most physicists prefer not to think too much about the regime beyond the visible universe, since we are unlikely to ever know what is there. However, any theory of gravity or quantum gravity gives us the mathematical tools to contemplate the geometry of what might exist. Based on theoretical methods and ideas about extra dimensions of space, physicists sometimes consider exotic other universes, which are not in contact with us over the lifetime of our universe or are only in contact via gravity. As discussed in Chapter 18, string theorists and others contemplate the existence of a multiverse that contains many disconnected independent universes that are consistent with string theory’s equations, sometimes combining these ideas with the anthropic principle that exploits the possible riches of universes that might exist. Some even try to find observable signatures of such multiverses for the future. As we saw in Chapter 17, in one distinct scenario, a two-brane “multiverse” might even help us understand questions in particle physics and in that case have testable consequences. But most additional universes, though conceivable and maybe even likely, will probably remain beyond the realm of experimental testability for the foreseeable future. They will then remain theoretical abstract possibilities.
THE BIG BANG: FROM SMALL TO LARGE THROUGH TIME
Now that we’ve ventured out to the largest sizes we can observe or discuss in the context of the observable universe, and reached the outer limits as to what we can see (and contemplate with our imagination), let’s explore how the universe we do live in and observe evolved over time to create the enormous structures we see today. The Big Bang theory tells us how the universe grew during its 13.75-billion-year life span from its small initial size to the current extent, 100 billion light-years across. Fred Hoyle facetiously (and skeptically) named the theory after the initial explosion when a hot dense fireball began to expand into the massive extent of stars and structures we now observe: growing, diluting matter, and cooling as it evolved.
However, the one thing we certainly don’t know is what banged in the beginning and how it appened—or even the precise size it had been when it did. Despite our understanding of the universe’s late evolution, its beginnings remain shrouded in mystery. Nonetheless, although the Big Bang theory does not tell us anything about the universe’s initial moment, it is a very successful theory that tells us much about its subsequent history. Current observations combined with