Cal
Tech physicist Sean Carroll just released a scientific paper entitled
In What Sense Is the Early Universe Fine-Tuned? Since Carroll
has taken on a part-time role as a worldview warrior, trying to throw
cold water on anyone suspecting that the universe is not the result
of pure blind chance, I suspect that some with similar views must
have started to read the paper hopefully, thinking to themselves: now
finally here is someone who will debunk all that talk about the early
universe being extremely fine-tuned.
Fine-tuning
basically means “fitness for an end,” and the term is used by
both scientists and philosophers nowadays in a religiously neutral
sense. Using the term fine-tuning does not semantically imply belief
in a fine-tuner (although if one is discussing a sufficiently
improbable case of fine-tuning that might, arguably, bring to mind the notion that
such a fine-tuner once existed). When people talk about fine-tuning
of the early universe, they mean the idea that the early universe had
some arrangement of matter and energy (or evolutionary trajectory)
that was highly improbable, something that was conducive to the
eventual appearance of creatures such as us. Discussions of the
fine-tuning of the early universe are discussions of the way the
universe was about 13 billion years ago, around the time of the Big Bang, when the universe was very, very hot and dense.
Cosmic fine-tuning is also discussed in a different context, the
observation that the universe's fundamental constants and laws are
improbably suitable for the existence of intelligent life. That's a
fascinating topic discussed here, but in this post I will discuss
only the first type of fine-tuning.
Rather
than thinking of fine-tuning in the early universe in terms of a
horizon problem or a flatness problem, Carroll suggests that it
should be considered in terms of smoothness and trajectories. We know
that the early universe was incredibly smooth. About 380,000 years
after the Big Bang, the universe was uniform to 1 part in 100,000.
We know that from the cosmic background radiation, which has no lumps
greater than 1 part in 100,000. You may get the wrong idea by
looking at one of those maps of the cosmic background radiation that
show different colors. Those maps are amplifying differences of only
1 part in 100,000. A map of the cosmic background radiation that does
not use such an amplification would consist of a single color (see
the visual below).
But
such almost perfect smoothness, Carroll points out, would not occur
in more than the tiniest fraction of the trajectories that the
universe might have had after an event such as the Big Bang. How
small is that fraction? On page 21 of the paper Carroll estimates
that “the total fraction of the trajectories that are smooth at
early times” is very roughly 1 in 10 to the 66 millionth power. That's a
fraction equal to 1 in x, where x is 10 followed by 66 million
zeroes.
How
low is this probability of 1 in 10 to the 66 millionth power? It's a
probability much less than the probability of you filling up a huge
dump truck with many thousands of dice, having the truck dump the
dice gradually along a road, and then finding that each and every one
of the dice landed on the road coming up showing the number 6 (with
no 1's, 2's, 3's, 4's, or 5's showing anywhere on the road). It's
also a probability much less than the probability of you guessing the
birth date (month, day, and year) of every person you ever met, and you
guessing the correct birth date each and every time throughout your
life.
Does
the theory of cosmic inflation eliminate this fine-tuning? Carroll
says:
Carroll
refers to the famous cosmologist Sir Roger Penrose, who made an
argument (based on entropy) that the early universe required
fine-tuning even more improbable than the microscopic probability
listed by Carroll.
Carroll
also concludes (page 21):
I
can give a very crude analogy of how mind-boggling this is, one involving a trajectory and smoothness.
Imagine you are walking along carrying a white poster board. You walk by a
big muddy hole in the street. A big truck speeds by, driving over
the muddy hole. Splat! A big blob of mud hits your poster board.
Later you analyze the mud splat, and find that the distribution of
mud is uniform to one part in 100,000. It's as if someone very
carefully sprayed on the mud with a spray can, but even more orderly.
How the hell could that have happened?
The
fact that this result (1 chance in 10 to the 66 millionth power) has
been advanced by Sean Carroll may make it all the more compelling, as
Sean Carroll (a champion of naturalism) is the type of thinker one
might expect to be hostile towards any conclusion of cosmic
fine-tuning. Professor Carroll's naturalism forbids him from philosophically
connecting the dots in regard to this and other cases of cosmic
fine-tuning, but it is not obvious that others should conform to such a prohibition.
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