Andromeda galaxy (credit: NASA)
“We therefore conclude that the universe is not a fluctuation, and that the order is a memory of conditions when things started. This is not to say that we understand the logic of it. For some reason, the universe at one time had a very low entropy for its energy content, and since then the entropy has increased. So that is the way toward the future. That is the origin of all irreversibility, that is what makes the processes of growth and decay, that makes us remember the past and not the future, remember the things which are closer to that moment in the history of the universe when the order was higher than now, and why we are not able to remember things where the disorder is higher than now, which we call the future.” – Richard Feynman (https://www.feynmanlectures.caltech.edu/I_46.html)
Physicists have no clue why the entropy of the universe was very low at the beginning of the Big Bang.
Sean Carroll’s FAQ to his book “From Eternity to Here” (a short read) provides a very good introduction to this problem.
Sean Carroll’s article at the Discover magazine is also very readable. Regarding the question “Could the universe just be a statistical fluctuation?” he says:
“No. This was a suggestion of Bolzmann’s and Schuetz’s, but it doesn’t work in the real world. The idea is that, since the tendency of entropy to increase is statistical rather than absolute, starting from a state of maximal entropy we would (given world enough and time) witness downward fluctuations into lower-entropy states. That’s true, but large fluctuations are much less frequent than small fluctuations, and our universe would have to be an enormously large fluctuation. There is no reason, anthropic or otherwise, for the entropy to be as low as it is; we should be much closer to thermal equilibrium if this model were correct. The reductio ad absurdum of this argument leads us to Boltzmann Brains — random brain-sized fluctuations that stick around just long enough to perceive their own existence before dissolving back into the chaos.” – Sean Carroll
In the same article he answers the question “What is the entropy of the universe?” as follows:
“We’re not precisely sure. We do not understand quantum gravity well enough to write down a general formula for the entropy of a self-gravitating state. On the other hand, we can do well enough. In the early universe, when it was just a homogenous plasma, the entropy was essentially the number of particles — within our current cosmological horizon, that’s about 10^88. Once black holes form, they tend to dominate; a single supermassive black hole, such as the one at the center of our galaxy, has an entropy of order 10^90, according to Hawking’s famous formula. If you took all of the matter in our observable universe and made one big black hole, the entropy would be about 10^120. The entropy of the universe might seem big, but it’s nowhere near as big as it could be.” – Sean Carroll
Sabine Hossenfelder wrote an interesting article titled “How did the universe begin?” which is an excellent summary of cosmological models. She touches on the other idea (other than “Boltzmann Brains”) that talks about universe being a fluctuation, namely the idea of eternal cosmic inflation (quantum fluctuations of the inflaton field). On this she comments:
“How scientific is this idea? Well, we have zero evidence that the forces were ever unified and have equally good evidence, namely none, that the inflaton field exists. The idea that the early universe underwent a phase of rapid expansion fits to some data, but the evidence is not overwhelming, and in any case, what the cause of this rapid expansion would have been – an inflaton field or something else – the data don’t tell us. ” – Sabine Hossenfelder
Regarding the cosmic inflation theory Sean Carroll answers the question “Does inflation explain the low entropy of the early universe?” as follows:
“Not by itself, no. To get inflation to start requires even lower-entropy initial conditions than those implied by the conventional Big Bang model. Inflation just makes the problem harder.”
He then answers the follow-up question “Does that mean that inflation is wrong?”
“Not necessarily. Inflation is an attractive mechanism for generating primordial cosmological perturbations, and provides a way to dynamically create a huge number of particles from a small region of space. The question is simply, why did inflation ever start? Rather than removing the need for a sensible theory of initial conditions, inflation makes the need even more urgent.”