When quantum theory is applied to space at the smallest possible scale, space itself becomes unstable. Rather than remaining perfectly smooth and continuous, space and time destabilize, churning and frothing into a foam of space-time bubbles. Little bubbles of space and time can form spontaneously. If space and time are quantized, they can fluctuate, Virtual space-times can be created just as virtual particles can be.
In quantum physics, if something is not forbidden, it necessarily happens with some non-zero probability, so if it’s possible for these bubbles to form, they certainly will. Just as particles and antiparticles that snap in and out of nothingness, bubbles of space-time can do the same. But how can an infinitesimal space-time bubble convert into a massive universe that hosts 100 billion galaxies?
Cosmic inflation can make such bubble survive. Inflation theory proposes that in the immediate aftermath of the Big Bang, the universe expanded much faster than it did later. A fraction of a second after the Big Bang, the quantum-sized bubble of space expanded extremly fast. In an incredibly brief moment, it went from being smaller than the nucleus of an atom to the size of a grain of sand. When the expansion finally slowed, the force field that had powered it was transformed into the matter and energy that fill the universe today.
That explains why the cosmic microwave background, the faint remnant of radiation left over from the Big Bang, is almost perfectly uniform across the sky. If the universe had not expanded so rapidly, we would expect the radiation to be patchier than it is.
Inflation also gave cosmologists the measuring tool they needed to determine the underlying geometry of the universe. It turns out this is also crucial for understanding how the cosmos came from nothing.
Einstein’s theory of general relativity tells us that the space-time we live in could take three different forms. It could be as flat as a table top. It could curve back on itself like the surface of a sphere, in which case if you travel far enough in the same direction you would end up back where you started. Alternatively, space-time could curve outward like a saddle.
On the largest observable scale our universe is flat. That is crucial, because only a flat universe is likely to have come from nothing.
Everything that exists, from stars and galaxies to the light we see them by, must have sprung from somewhere. We already know that particles spring into existence at the quantum level, so we might expect the universe to contain a few odds and ends. But it takes a huge amount of energy to make all those stars and planets.
Where did the universe get all this energy? Bizarrely, it may not have had to get any. That’s because every object in the universe creates gravity, pulling other objects toward it. This balances the energy needed to create the matter in the first place.
Physicists have calculated that in a flat universe the energy of matter is exactly balanced by the energy of the gravity the mass creates. But this is only true in a flat universe. If the universe had been curved, the two sums would not cancel out.
Quantum mechanics tells us that “nothing” is inherently unstable, so the initial leap from nothing to something may have been inevitable. Then the resulting tiny bubble of space-time could have burgeoned into a massive, busy universe, thanks to inflation. The laws of physics as we understand them make it eminently plausible that our universe arose from nothing – no space, no time, no particles, nothing that we now know of.
If one space-time bubble popped into existence and inflated to form our universe, what kept other bubbles from doing the same? When a new universe stops inflating, it is still surrounded by space that is continuing to inflate. That inflating space can spawn more universes, with yet more inflating space around them. So once inflation starts it should make an endless cascade of universes – an eternal inflation.
Those universes might be profoundly different to ours. The universe next door might have five dimensions of space rather than the three – length, breadth and height – that ours does. Gravity might be ten times stronger or a thousand times weaker, or not exist at all. Matter might be built out of utterly different particles.