The next phase

When I put on water for tea, something peculiar happens.

Of course, it happens for everyone, but a lot of people probably don't think about it.  For a while, the water quietly heats.  It undergoes convection -- the water in contact with the element at the bottom of the pot heats up, and since warmer water is less dense, it rises and displaces the cooler layers above.  So there's a bit of turbulence, but that's it.

Then, suddenly, a bit of the water at the bottom hits 100 C and vaporizes, forming bubbles.  Those bubbles rapidly rise, dispersing heat throughout the pot.  Very quickly afterward, the entire pot of water is at what cooks call "a rolling boil."

This quick shift from liquid to gas is called a phase transition.  The most interesting thing about phase transitions is that when they occur, what had been a smooth and gradual change in physical properties (like the density of the water in the teapot) undergoes an enormous, abrupt shift -- consider the difference in density between liquid water and water vapor.

The reason this comes up is that some physicists in Denmark and Sweden have proposed a phase transition mechanism to account for the evolution of the (very) early universe -- and that proposal may solve one of the most vexing questions in astrophysics today.

A little background.

As no doubt all of you know, the universe is expanding.  This fact, discovered through the work of astronomer Edwin Hubble and others, was based upon the observation that light from distant galaxies was significantly red-shifted, indicating that they were moving away from us.  More to the point, the farther away the galaxies were, the faster they are moving.  This suggested that some very long time in the past, all the matter and energy in the universe was compressed into a very small space.

Figuring out how long ago that was -- i.e., the age of the universe -- depends on knowing how fast that expansion is taking place.  This number is called the Hubble constant.

[Image licensed under the Creative Commons Munacas, Big-bang-universo-8--644x362, CC BY-SA 4.0]

This brings up an issue with any kind of scientific measurement, and that's the difference between precision and accuracy.  While we use those words pretty much interchangeably in common speech, to a scientist they aren't the same thing at all.  Precision in an instrument means that every time you use it to measure something, it gives you the same answer.  Accuracy, on the other hand, means that the value you get from one instrument agrees with the value you get from using some other method for measuring the same thing.  So if my car's odometer tells me, every time I drive to my nearby village for groceries, that the store is exactly eight hundred kilometers from my house, the odometer is highly precise -- but extremely inaccurate.

The problem with the Hubble constant is that there are two ways of measuring it.  One is using the aforementioned red shift; the other is using the cosmic microwave background radiation.  Those two methods, each taken independently, are extremely precise; they always give you the same answer.

But... the two answers don't agree.  (If you want a more detailed explanation of the problem, I wrote a piece on the disagreement over the value of the Hubble constant a couple of years ago.)

Hundreds of measurements and re-analyses have failed to reconcile the two, and the best minds of theoretical physics have been unable to figure out why. 

Perhaps... until now.

Martin Sloth and Florian Niedermann, of the University of Southern Denmark and the Nordic Institute for Theoretical Physics, respectively, just published a paper in Physics Letters B that proposes a new model for the early universe which makes the two different measurements agree perfectly -- a rate of 72 kilometers per second per megaparsec.  Their proposal, called New Dark Energy, suggests that very quickly after the Big Bang, the energy of the universe underwent an abrupt phase transition, a bit like the water in my teapot suddenly boiling.  At this point, these "bubbles" of rapidly dissipating energy drove apart the embryonic universe.

"If we trust the observations and calculations, we must accept that our current model of the universe cannot explain the data, and then we must improve the model," Sloth said.  "Not by discarding it and its success so far, but by elaborating on it and making it more detailed so that it can explain the new and better data.  It appears that a phase transition in the dark energy is the missing element in the current Standard Model to explain the differing measurements of the universe's expansion rate.  It could have lasted anything from an insanely short time -- perhaps just the time it takes two particles to collide -- to 300,000 years.  We don't know, but that is something we are working to find out...  If we assume that these methods are reliable -- and we think they are -- then maybe the methods are not the problem.  Maybe we need to look at the starting point, the basis, that we apply the methods to.  Maybe this basis is wrong."

It's this kind of paradigm shift in understanding -- itself a sort of phase transition -- that triggers great leaps forward in science.  To be fair, some of them fizzle.  Most of them, honestly.  But sometimes, there are visionary scientists who take previously unexplained knowledge and turn our view of the universe on its head, and those are the ones who revolutionize science.  Think of how Galileo and Copernicus (heliocentrism), Kepler (planetary motion), Darwin (biological evolution), Mendel (genetics), Einstein (relativity), de Broglie and Schrödinger (quantum physics), Watson, Crick, and Franklin (DNA), and Matthews and Vine (plate tectonics) changed our world.

Will Sloth and Niedermann join that list?  Way too early to know.  But just the fact that one shift in the fundamental assumptions about the early universe reconciled measurements that heretofore had stumped the best theoretical physicists is a hopeful sign.

Time will tell if this turns out to be the next phase in cosmology.

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Behind the mirror

I know I've snarked before about the how unbearably goofy the old 1960s television show Lost in Space was, but I have to admit that every once in a (long) while, they nailed it.  And one of the best examples is the first-season episode "The Magic Mirror."

Well, mostly nailed it.  The subplot about how real girls care about makeup and hair and being pretty is more than a little cringe-inducing.  But the overarching story -- about mirrors being portals to a parallel world, and a boy who is trapped behind them because he has no reflection -- is brilliant.  And the other-side-of-the-mirror world he lives in is hauntingly surreal.

I was thinking about this episode because of a paper that appeared in Physical Review Letters last week entitled, "Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant," by Francis-Yan Cyr-Ravine, Fei Ge, and Lloyd Knox, of the University of New Mexico.  What this paper does is offer a possible solution to the Hubble constant problem -- that the rate of expansion of the universe as predicted by current mathematical models is significantly smaller than the actual measured expansion rate.

What Cyr-Racine, Ge, and Knox propose is that there is an unseen "mirror world" of particles that coexists alongside our own, interacting only through gravity but otherwise invisible to detection.  At first, I thought they might be talking about something like dark matter -- a form of matter that only (very) weakly interacts with ordinary matter -- but it turns out that what they're saying is even weirder.

"This discrepancy is one that many cosmologists have been trying to solve by changing our current cosmological model," Cyr-Racine told Science Daily.  "The challenge is to do so without ruining the agreement between standard model predictions and many other cosmological phenomena, such as the cosmic microwave background...  Basically, we point out that a lot of the observations we do in cosmology have an inherent symmetry under rescaling the universe as a whole.  This might provide a way to understand why there appears to be a discrepancy between different measurements of the universe's expansion rate.  In practice, this scaling symmetry could only be realized by including a mirror world in the model -- a parallel universe with new particles that are all copies of known particles.  The mirror world idea first arose in the 1990s but has not previously been recognized as a potential solution to the Hubble constant problem.  This might seem crazy at face value, but such mirror worlds have a large physics literature in a completely different context since they can help solve important problem in particle physics.  Our work allows us to link, for the first time, this large literature to an important problem in cosmology."

The word "important" is a bit of an understatement.  The Hubble constant problem is one of the biggest puzzles in physics; why theory and observation are so different on this one critical point, and how to fix the theory without blowing to smithereens everything that the theory does predict correctly.  "It went from two and a half Sigma, to three, and three and a half to four Sigma. By now, we are pretty much at the five-Sigma level," said Cyr-Racine.  "That's the key number which makes this a real problem because you have two measurements of the same thing, which if you have a consistent picture of the universe should just be completely consistent with each other, but they differ by a very statistically significant amount.  That's the premise here, and we've been thinking about what could be causing that and why are these measurements discrepant?  So that's a big problem for cosmology.  We just don't seem to understand what the universe is doing today."

I know that despite my background in science, I can have a pretty wild imagination.  It's an occupational hazard of being a speculative fiction writer.  I hear some new scientific finding, and immediately start putting some weird spin on it that, though it might be interesting, is completely unwarranted by the actual research.  But look at Cyr-Racine's own words: a parallel universe with new particles that are all copies of known particles.  I think I'm to be excused for thinking of "The Magic Mirror" and other science fiction stories about ghostly worlds coexisting, unseen, with our own.

I'm not going to pretend to understand the math behind the Cyr-Racine et al. paper; despite having a B.S. in physics, academic papers in the discipline usually lose me in the first paragraph (if not the abstract).  But it's a fascinating and spooky idea.  I doubt if what's going on has anything to do with surreal worlds behind mirrors and boys who are trapped because they have no reflection, but the reality -- if it bears up under analysis -- isn't a whole lot less weird.

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