3.2 The Evidence of Inflation Review

summary

Back to course

The visible universe is extraordinarily uniform and homogeneous.

The cosmic background radiation is the strongest evidence of this uniformity.
By examining local temperature fluctuations, we can see that the cosmic microwave background is uniform to 1 part in 100,000.

For the first ~380,000 years of the universe, photons hardly traveled anywhere.

Photons were constantly scattered to and fro by the plasma of electrically charged particles, bound in a small region.
Eventually, the universe cooled down sufficiently, allowing electrons to be captured by protons and form electrically neutral atoms.
The photons no longer scattered off these neutral particles, and instead were allowed to pass right by them.
This essentially “released” the photons, which then escaped isotropically outward, forming the cosmic microwave background radiation.

The observed uniformity of the cosmic background radiation must mean that when the photons were released, the environment was already extremely homogeneous.

Consider some mechanism that could have established this uniformity.
The distance between uniform photons on opposite ends of the universe was so large that in order for them to establish this uniformity, information would have had to travel at $100c$.
This impossibility is what is known as the Horizon problem.

Inflation proposes a solution to the Horizon problem.

The inflationary solution theorizes that the universe began incredibly small—so small that uniformity could be easily established.
Then inflation stretches the region to be large enough to include the visible universe, all while maintaining uniformity.
Consider drawing a grid of dots on the surface of a deflated balloon, and then inflating it.
The dots always remain uniformly spaced, but quickly spread apart, just as the matter in the universe would have done.