4.2 A Radical Alternative
Do we need a complicated explanation for our 'simple' universe?
- We live in an extraordinarily simple universe with very special properties.
- On a macrophysical scale, our universe is incredibly uniform. It has this beautiful, scale-invariant spectrum of hot spots and cold spots.
- On the microphysical scale, we’ve learned that the physics that underlies our universe can be reduced to incredibly simple forms.
- Do we need the complex explanation that we are one universe within an infinite multiverse, or should we look for a better idea that avoids this kind of breakdown?
It's time to rethink the assumptions that got us to our present situation.
- One assumption you might change is the notion that coming out of the big bang came something random and unpredictable. Maybe the big bang is some process that produces the very simply universe we observe. Maybe when we understand quantum gravity properly, we’ll discover that what you end up with is a flat, smooth universe with tiny ripples of hot spots and cold spots.
- How about the idea that the big bang is the beginning of time and space? Even the fathers of big bang theory—people like Friedmann and Lemaître—didn’t believe that the big bang was the beginning of space and time. When Friedmann wrote down his solutions to Einstein’s equations, he actually had the idea that what we call the big bang today was actually a “big bounce,” a transition from a state of contraction to a state of expansion; one that can happen more than once.
- In a cyclic model of the universe, these bounces would occur at regular intervals, say once every trillion years or so. In each bounce, new matter and radiation would be created—the seeds for the next round of galaxies and stars and planets. Then the universe would enter a phase in which dark energy takes over the universe, causing the expansion of the universe to accelerate, which is what we are observing.
- But this dark energy would not be stable. It would eventually become a negative form of energy which would cause the universe to contract—the “big crunch.” And yet, that doesn’t have to be the end of the story. It could be a big crunch into a big bang, and then the process would begin again.
What are the benefits of considering a bounce?
- The moment that you consider this bounce, you discover that there’s a simple solution to the flatness and horizon problems that don’t require inflation at all. The period of contraction takes care of both of these issues.
- In the original big bang picture, the horizon problem depended on the big bang defining the beginning of time—there wasn’t enough time for information (temperature) to be exchanged from one end of the universe to the other. If we now say the big bang is a bounce, we now have all the time before the bounce for those regions to have interacted.
- The period of contraction also flattens the curvature of the universe, so by the time you reach the bounce, the universe is flat.
- We also want to make sure the universe is isotropic and that you have fluctuations that form the hot spots and cold spots in the CMB, as well as the seeds of galaxies, stars, and planets. For that, we end up introducing elements that look a little like inflation: scalar fields with different kinds of energy curves. When you introduce those, you end up with a universe that has dark energy domination, with that dark energy eventually decaying into a form of energy that causes a slow contraction, in such a way to keep the universe smooth and isotropic.
- While the universe is contracting, not every part of it can stay in perfect sync due to quantum fluctuations—some regions will contract faster than others. Some will bounce and reheat faster than others, and that will lead to temperature variations across the sky, producing hot spots and cold spots in the CMB, in a pattern with the same statistical properties as from inflation, but with a different mechanism.
The bounce also negates certain problems inherent with inflation.
- Another nice property: inflation is very unlikely to start from arbitrary initial conditions, but a slowly contracting universe begins at very low energies, where it’s much more likely and much easier to start.
- More importantly, you don’t end up with eternal inflation or the multiverse.
- Recall our earlier problem: parts of the universe kept inflating while others had finished rolling down the hill. In the contracting universe, there will also be rare procrastinating regions, left behind while other regions have already finished their contraction, bounced, and expanded.
- But those rare regions are contracting regions, not expanding regions. Contracting regions don’t compete for volume—they’ll just shrink away while most of the universe continues on its way, so you don’t end up with rare procrastinating regions taking over the volume of the universe and producing a multiverse.