Mirror image

One of the hallmarks of science is its falsifiability.  Models should generate predictions that are testable, allowing you to see if they conform to what we observe and measure of the real universe.  It's why science works as well as it does; ultimately, nature has the last word.

The problem is that there are certain realms of science that don't lend themselves all that well to experiment.  Paleontology, for example -- we're dependent on the fossils that happen to have survived and that we happen to find, and the genetic evidence from the descendants of those long-gone species, to piece together what the ancient world was like.  It's a little difficult to run an experiment on a triceratops.

An even more difficult one is cosmology -- the study of the origins and evolution of the universe as a whole.  After all, we only have the one universe to study, and are limited to the bits of it we can observe from here.  Not only that, but the farther out in space we look, the less clear it becomes,  By the time light gets here from a source ten billion light years away, it's attenuated by the inverse-square law and dramatically red-shifted by all the expanding space it traveled through to get here, which is why it takes the light-collecting capacity of the world's most powerful telescopes even to see it.

None of this is meant as a criticism of cosmology, nor of cosmologists.  But the fact remains that they're trying to piece together the whole universe from a data set that makes what the paleontologists have look like an embarrassment of riches.

The result is that we're left with some massive mysteries, one of the most vexing of which is dark energy.  This is a placeholder name for the root cause of the runaway expansion of the universe, which (according to current models) accounts for 68% of the mass/energy content of the universe.  (Baryonic, or ordinary, matter is a mere 5%.)  And presently, we have no idea what it is.  Attempts either to detect dark energy directly, or to create a model that will account for observations without invoking its existence have, by and large, been unsuccessful. 

But that hasn't stopped the theorists from trying.  And the latest attempt to solve the puzzle is a curious one; that dark energy isn't necessary if you assume our universe has a partner universe that is a reflection of our own.  In that universe, three properties would all be mirror images of the corresponding properties in ours; positive and negative charges would flip, spatial "handedness" (what physicists call parity) would be reversed, and time would run backwards.

Couldn't help but think of this, of course.

The idea is intriguing.  Naman Kumar, who authored the paper on the model, is enthusiastic about its potential for explaining the expansion of the universe.  "The results indicate that accelerated expansion is natural for a universe created in pairs," Kumar writes.  "Moreover, studying causal horizons can deepen our understanding of the universe.  The beauty of this idea lies in its simplicity and naturalness, setting it apart from existing explanations."

Which may well be true.  The difficulty, however, is that the partner universe isn't reachable (or even directly detectable) from our own, Lost in Space notwithstanding.  It makes me wonder how this will ever be more than just an interesting possibility -- an idea that, in Wolfgang Pauli's often-quoted words, "isn't even wrong" because there's no way to test whether it accounts for the data any better than the other, less "natural" models do.

In any case, that's the latest from the cosmologists.  Mirror-image universes created in pairs may obviate the need for dark energy.  We'll see what smarter people than myself have to say about whether the claim holds water; or, maybe, just wait for Evil Major West With A Beard to show up and settle the matter once and for all.

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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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Mirror, mirror

The idea of there being a mirror universe has been a staple of science fiction for as long as I can remember.  Many people think immediately of evil Spock with a beard, but my first contact with the idea was when I was seven and first saw the episode of Lost in Space called "The Anti-Matter Man."  The two universes were connected by a creepy, surreal, fog-covered bridge:

The characters from our universe had duplicates in the other -- evil, of course, with an evil, amoral John Robinson, a trash-talking Robot, and a version of Major Don West who looked like he was an escapee from an institute for the criminally insane.

Oh, and the evil Major West had a beard.  Of course.

It was definitely one of their more atmospheric and effective episodes.  Not that there was that much competition in that regard, considering that the previous episode, "Castles in Space," featured a cringe-worthy pseudo-Mexican bounty hunter (complete with a sombrero) who is searching for a runaway princess Dr. Smith had set free from a block of ice by covering it with an electric blanket, and said bounty hunter (did I mention he had silver skin?) gets the Robot drunk by pouring tequila onto his circuit boards.

For the record, I didn't make any of that up.

This whole thing comes up because there are some physicists who are trying to demonstrate the existence of a mirror universe -- an unseen parallel reality coexisting with our own -- and that this might explain the mysterious "dark matter" that seems to permeate all of space, but which no one's been able to detect other than by its gravitational signature.

A team led by physicist Leah Broussard has designed an experiment at Oak Ridge National Laboratories to see if she can find evidence of a mirror reality, looking for neutrons that have been transformed into particles with the opposite parity.  In a paper in arXiv, the authors write:

The theory of mirror matter predicts a hidden sector made up of a copy of the Standard Model particles and interactions but with opposite parity.  If mirror matter interacts with ordinary matter, there could be experimentally accessible implications in the form of neutral particle oscillations.  Direct searches for neutron oscillations into mirror neutrons in a controlled magnetic field have previously been performed using ultracold neutrons in storage/disappearance measurements, with some inconclusive results consistent with characteristic oscillation time of τ∼10~s.  Here we describe a proposed disappearance and regeneration experiment in which the neutron oscillates to and from a mirror neutron state.

What Broussard is doing is to fire a beam of electrons past a powerful magnet and into a wall.  If her idea is correct, the collision will cause some of the electrons to transform into their mirror versions, and appear on the other side of the wall.

She got her idea from a peculiarity that has been observed in neutron decay -- that in a beam such as the one Broussard is using, neutrons have an average life that is ten seconds longer than if they're simply at rest in a laboratory.  There's no Standard-Model explanation of why this should be; all neutrons should behave in exactly the same way.  But Broussard says that this discrepancy is exactly what you'd expect if some of the neutrons in the beam are being transformed into their mirror versions -- and then become invisible to ordinary detectors.

Even Broussard admits that the idea is kind of far-fetched, but given that every other effort to elucidate the nature of dark matter -- or even establish its existence beyond its gravitational effects -- has met with zero success, it's worth looking at.

So it'll be interesting to see what turns up.  I can't help but hope that she gets positive results, because it's about time we finally get something concrete about dark matter.  Besides, the idea of there being a mirror universe is pretty cool.  Even if that means we might have to watch out for evil Major West with a beard.

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This week's Skeptophilia book recommendation is about a subject near and dear to me: sleep.

I say this not only because I like to sleep, but for two other reasons; being a chronic insomniac, I usually don't get enough sleep, and being an aficionado of neuroscience, I've always been fascinated by the role of sleep and dreaming in mental health.  And for the most up-to-date analysis of what we know about this ubiquitous activity -- found in just about every animal studied -- go no further than Matthew Walker's brilliant book Why We Sleep: Unlocking the Power of Sleep and Dreams.

Walker, who is a professor of neuroscience at the University of California - Berkeley, tells us about what we've found out, and what we still have to learn, about the sleep cycle, and (more alarmingly) the toll that sleep deprivation is taking on our culture.  It's an eye-opening read (pun intended) -- and should be required reading for anyone interested in the intricacies of our brain and behavior.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]