# The Inflationary Universe and Fine Tuning

In the "Origin of the Universe" thread, Angelo made some claims about cosmology and fine tuning. I want to respond to those comments here, because it is a good example of how further understanding sometimes removes the need for "fine tuning."

Back in the 1970s, when I was a mere undergrad, the Big Bang Model (BB) of the early universe had scored numerous successes. However, there were still some puzzles, including:

The flatness problem: A space can be positively curved, negatively curved, or flat (zero curvature). This is easiest to picture with two-dimensional space: the surface of a sphere is positively curved, a saddle is negatively curved, and a sheet of paper is flat. But the same idea can be applied to three-dimensional space. The BB allows for any of the three types of curvature for our universe, yet astronomical measurements seemed to show that the universe was spatially flat. Why should the universe be so close to this special value (0)? Worse, when you extrapolate back to the very early universe, you find that the curvature had to be really really close to 0, or else the universe would have either expanded too fast for galaxies to form, or re-collapsed almost immediately. So, the parameters of the BB had to be extremely fine tuned to get a universe in which complex structures like galaxies, stars, planets, and, of course, us, could form.

There were other puzzles: the uniformity of the cosmic microwave background and the absence of monopoles among them.

Then, in 1980, Alan Guth came up with the idea of the inflationary universe model (IM). He noticed that a particle like the Higgs particle could drive the early universe's expansion into a supercharged phase, during which a very small region would expand to astronomical size in a tiny fraction of a second. This would solve the flatness problem: if you take even a very bumpy surface, focus down on a tiny portion of it, and then blow that portion up to the size of the universe, the bumpiness will get smoothed out. In addition, the IM solved the uniformity problem and the monopole problem.

Today we have a lot more evidence for the IM. In particular, the details of the cosmic microwave background conform very closely to the predictions of the IM. So the IM is now considered to be pretty firmly established.

This does not, of course, prove that other apparent fine tunings can be resolved in a similar manner. However, it does show that something that at one time is an apparent fine tuning of the physical parameters of the universe can, on further investigation, turn out to be a result of a very generic, and un-tuned, physical process.

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