The more I examine the universe and the details of its architecture, the more evidence I find that the universe in some sense must have known we were coming.
When we look out at our Universe today, we see all sorts of beautiful things throughout space, from galaxies and clusters, distributed roughly evenly throughout space:
to the cosmic microwave background (also known as the last scattering surface), radiating at the same temperature in all directions.
We also see, when we try to measure the shape of the Universe, that there are three major possibilities: it could be flat like a plane, curved positively like a sphere, or curved negatively like a saddle.
We learn two important things from making our measurements:
1.) The Universe has, on average, the exact same properties in all directions in space. Same average density, same average temperature, same average galaxy count, etc., despite the fact that these regions are separated by billions upon billions of light years.
2.) The Universe, on the largest scales, appears to be completely spatially flat, without a hint of overall curvature in either the positive or negative directions.
We are curious creatures, and are bound to ask the question of why? Why is the Universe the same in all directions, when there’s no reason for it to have not started with different properties in different places? Why is the Universe spatially flat, when even a slight change in the overall matter density or expansion rate would have given us a curved Universe? Physicists call these problems the horizon problem and the flatness problem, and they are two major observations that the Big Bang cannot explain.
In 1979, a young theoretical physicist named Alan Guth (center, above) came up with a theory that could explain both of these problems, by stating what happened before the big bang! His big idea was the theory of inflation, which says the following. Imagine a Universe with different conditions everywhere. Some regions might be expanding, some might be contracting, and some might be stationary. Some might have positive curvature, some might have negative curvature, and some might be flat. Some might have a lot of matter, some might have little to none. Some could be very, very hot, and some could be practically at absolute zero.
In other words, it doesn’t matter what your initial conditions are to the Universe. What matters is that, at one point in space, in one of these regions, the right conditions to have inflation exist. Inflation takes this one region of space and expands it exponentially.
In just a tiny fraction of a second, inflation can take a region the size of a proton and stretch it to be billions of times the size of the visible Universe. This has three very important effects.
First off, the horizon problem is solved. If you take a very small region with the same temperature, density, and expansion rate within that region and expand it to be as big as the Universe, then all of a sudden everywhere within your Universe should have the same temperature, density, and expansion rate. Problem #1 is solved.
Second, it doesn’t matter what the shape of your Universe was before inflation. Inflation comes along and makes it so big that — with our instruments only able to measure what’s within our visible Universe — our Universe appears to be flat.
This is in the same sense that, if all you could see of Earth was what was within 100 feet of you, you couldn’t tell that it was curved like a sphere; it would look flat to you. The Universe looks flat to us in exactly the same sense.
But the last (important) thing that inflation predicts was so profound, because it wasn’t yet observed when it was predicted. Quantum mechanics tells us that empty space isn’t completely empty. It’s full of particles (matter) and anti-particles (antimatter) that continuously get created, live for a brief while, meet back up together, and annihilate one another again. This happens over very short timescales, and gives rise to the Casimir effect.
When inflation occurs, these particle/antiparticle pairs are suddenly ripped apart across the Universe by the exponential expansion. Instead of space being truly uniform everywhere, there are slight fluctuations — overdensities in some regions and underdensities in others — that arise from these quantum effects in the inflating Universe. Inflation predicted what these fluctuations would look like in the 1980s, and these fluctuations were finally observed to the desired precision in 2003, with WMAP’s measurement of the fluctuations in the cosmic microwave background.
And all of this happened before the big bang ever took place. Was it for the first 10-35 seconds of the Universe? Possibly, but it’s also possible that the Universe existed for billions of years before inflation, or not at all. It’s also conceivable that time didn’t have the same meaning that it does now back then. Regardless of what happened, inflation prevents us from knowing right now; it wiped out any information about the Universe that existed before inflation!
So that’s our starting point in the greatest story ever told. Inflation is the first thing we can really knowledgeably speak about, and it sets up all the conditions we need — including a flat, uniform Universe with the right fluctuations — to start our Universe off with a bang!