Patches of ultra dense dark energy, arrow of time

This is in reference to an article in this month’s Scientific American by Sean M. Carroll.

There are two main things that confuse me about the article.

Firstly, Mr. Carroll states that when the universe cools and reaches maximum entropy, in empty space, in the presence of dark energy, particles can fluctuate into and out of existence. Also (I didn’t know this), substantial collections of particles can fluctuate into and out of existence. According to Mr. Carroll, “Among the things that can fluctuate into existence are small patches of ultra dense dark energy…” which can undergo inflation – that is start each start its own big bang and baby universe.

That is counterintuitive to me. A “small patch of ultra dense patch of dark energy” seems like a complex thing to spring out of the vacuum and into existence. Why not a Volkswagen, or (better yet) a beautiful woman? Is a small patch of ultra dense dark energy with the potential to start a new universe a simple thing that could just fluctuate into existence?

The second is the idea that when the (or a) universe contracts, time runs backwards. If that were true, it would collapse into itself, using the same time that was expended in its expansion. How would that be different than if it just expanded and nothing else happened? I hope my question makes sense. At any point during the expansion, or contraction (which happens during the same time) would anything be different than if the contraction never occurred? What if we assume that in “backwards time” we are contracting right now? Who would know?

Wow, I’m glad that I canceled my SciAm subscription a few years ago… Their standards have really gone downhill.

First, there’s a lot we don’t know about dark energy. But so far as anyone knows, it can’t vary in space at all. And while particles can, in layman’s terms, fluctuate in and out of existence, there ain’t no such thing as a free lunch. When particles fluctuate into existence, the Universe goes into debt, and that debt is paid off quickly. You can’t get an increase in the density just from such fluctuations.

Second, there are three known arrows of time in physics: The only one that’s ever important for people is the thermodynamic arrow, which defines “future” as the time when entropy is higher. Any distinction between “past” and “future” you care to think of (for instance, you can remember the past but not the future) ultimately falls back on that one. There’s also an arrow of time in fundamental particle physics, which (presumably; it’s never been measured directly) leads some reactions to proceed slightly differently in reverse than forwards. But that’s an effect that only shows up in a very small number of interactions, and only to a very small degree in even those. Finally, there’s what’s called the cosmological arrow of time, which defines “future” as the time when the Universe is bigger. There is no known reason to believe in any connection between these three arrows of time, and in fact it would be absurd to believe in a connection between the cosmological arrow of time and the other two. We’ve never seen the Universe as a whole contracting, but how are small-scale phenomena supposed to know about what the Universe as a whole is doing? Small portions of the Universe, however, can and have collapsed, and physics doesn’t seem to have behaved at all differently for those portions.

I definitely wouldn’t go so far as Chronos in saying that Sci. Am.'s standards have gone downhill. Sean Carroll’s a smart guy (although the fact that I’m on a first-name basis with him may mean that I’m biased), and I don’t doubt that he’s given these ideas a fair amount of thought.

The name for this theory is “chaotic” or “eternal” inflation; it was invented by Linde, shortly after the idea of plain-vanilla inflation came about, and now seems to be well-accepted as a possibility (if not a certainty) by the prominent proponents of inflationary cosmology. The basic idea is basically as you’ve described it. As far as your intuition about “complex things”, it’s a good idea to keep in mind; one of the tenets of thermodynamics is that systems can, in principle, fluctuate into highly unlikely states; it’s just that the more ordered & finely tuned a state is, the less likely it is to occur. However, this is quantified in physics via the entropy of an state, and it so happens that “a small patch of ultradense cosmological constant” is a state of higher entropy than, say, Scarlett Johansson. So a fluctuation that causes Scarlett to appear in my living room is much, much less likely to occur than something that gives rise to a baby Universe.

Of course, if you carry this line of thinking too far, you’re led to the idea of the Boltzmann brain, which makes a lot of physicists (myself included) view the whole exercise as suspect.

Quintessence is hardly a new idea — the byline on the article I just linked is seven years old. Unproven, to be sure, but to the best of my knowledge it hasn’t been disproven either (feel free to smack me down here if this is not the case.) Still, to say that there are no models in which “dark energy” varies spatially & temporally is false.

Energy is not conserved in an expanding cosmos; you can’t define the energy of a system in general relativity without having an “asymptotically flat” region, meaning (roughly speaking) that you have to be able to get far enough away from the system such that the influence of gravity dies off. No such region exists when you’re talking about the entire Universe, and inflationary cosmology can therefore fudge things a lot more than you might think from simple energy conservation models. Or am I missing something here, too?

Quintessence (if that’s what the dark energy is) varies in time, but it still doesn’t vary in space. And I’m aware that conservation of energy is a local property, not global, but the OP refered to small pockets of concentrated dark energy, and energy should still be conserved in those regions (more precisely, any change of the energy in that region should correspond to a flow of energy through the boundaries of the region).

The words “beautiful woman” in my post spawned a Google Ad for “Beautiful Russian girls.” I clicked it, and none appeared, lending credence to your statement that the more ordered and finely tuned a state is the less likely it is to occur. (Fortunately, either did a new universe.)

Thank you for your reply. And thank you, Chronos.