Twinkle, twinkle, little antistar

It's a big mystery why anything exists.

I'm not just being philosophical, here.  According to the current most widely-accepted cosmological model, when the Big Bang occurred, matter and antimatter would have formed in equal quantities.  As anyone who has watched Star Trek knows, when matter and antimatter come into contact, they mutually annihilate and all of the mass therein is converted to a huge amount energy in the form of gamma rays, the exact quantity of which is determined by Einstein's law of E = mc^2.

So if we started out with equal amounts of matter and antimatter, why didn't it all eventually go kablooie, leaving a universe filled with nothing but gamma rays?  Why was there any matter left over?

The answer is: we don't know.  Some cosmologists and astrophysicists think that there may have been a slight asymmetry in favor of matter, driven by random quantum fluctuations early on, so while most of the matter and antimatter were destroyed by collisions, there was a little bit of matter left, and that's what's around today.  (And "a little bit" is honestly not an exaggeration; the vast majority of the universe is completely empty.  An average cubic meter of space is very unlikely to have much more than an atom or two in it.)

One question this sometimes brings up is whether the stars and galaxies we see in the night sky are matter; if, perhaps, some entire galaxies are made of antimatter, and there really are equal amounts of the two.  After all, antimatter is predicted to act exactly like matter except that its fundamental particles have the opposite charges -- its protons are negative, its electrons positive, and so forth.  So a planet entirely formed of antimatter would look (from a safe distance) exactly like an ordinary planet.

And just to throw this out there, an antiplanet wouldn't have copies of all of us except for having the opposite personalities, for example some people who are good guys being evil and/or having beards, as outlined in the highly scientific Lost in Space episode "The Antimatter Man:"

Nor would there be a creepy bridge between the two universes, covered with fog and backed by eerie music:

Which is a shame, because I always kinda liked that episode.

Considerations of evil Major Don West with a beard notwithstanding, here are two arguments why most physicists believe that the stars we see, even the most distant, are made of ordinary matter.  The first is that there is no known process that would have sorted out the matter from the antimatter early in the universe's life, leaving isolated clumps of each to form their respective stars and galaxies.  Secondly, if there were antistars and antigalaxies, then there'd be an interface between them and the nearest clump of ordinary stars and galaxies, and at that interface matter and antimatter would be constantly meeting and mutually annihilating.  This would produce a hell of a gamma ray source -- and we haven't seen anything out there that looks like a matter/antimatter interface (although I will return to this topic in a moment with an interesting caveat).

A paper last year found that the key to understanding why matter prevailed might lie in the mysterious "ghost particles" called neutrinos.  There are three kinds of neutrinos -- electron neutrinos, muon neutrinos and tau neutrinos -- and one curious property they have is that they oscillate, meaning they can convert from one type to another.  The rate at which they do this is predicted from current theories, and it's thought that antineutrinos do exactly the same thing at exactly the same rate.

The experiment described in the paper took place in Japan, and found that there is an unexpected asymmetry between neutrinos and antineutrinos.  Beams of muon neutrinos and muon antineutrinos were sent on a six-hundred-kilometer journey across Japan, and upon arriving at a detector, were analyzed to see how many had converted to one of the other two "flavors."  The surprising result was that the neutrinos had oscillated a lot more than predicted, and the antineutrinos a lot less -- something called a "CP (charge-parity) violation" that shows antimatter doesn't, in fact, behave exactly like matter.  This asymmetry could lie at the heart of why the balance tipped in favor of matter.

But now a new analysis of data from the Fermi Gamma-ray Space Telescope has thrown another monkey wrench into the works.  The study was undertaken because of a recent puzzling detection by an instrument on the International Space Station of nuclei of antihelium, which (if current models are correct) should be so rare in the vicinity of ordinary matter that they'd be entirely undetectable.  But what if the arguments against antistars and antigalaxies I described earlier aren't true, and there are such odd things out there?  Antistars would be undergoing fusion just like the Sun does, and producing antihelium (and other heavier antielements), which then would be shed from the surface just like our Sun sheds helium.  And some of it might arrive here, only to fall into one of our detectors.

But what about the whole gamma-rays-at-the-interface thing?  Turns out, the study in question, the subject of a paper last week in the journal Physical Review D, found that there are some suspicious gamma-ray sources out there.

Fourteen of them, in fact.

These gamma-ray sources are producing photons with an energy that's hard to explain from known sources of gamma rays -- pulsars and black holes, for example.  In fact, the energy of these gamma rays is perfectly consistent with the source being ordinary matter coming into contact with an antistar.

Curiouser and curiouser.

It doesn't eliminate the problem of why the universe is biased toward matter; even if these are antistars, their frequency in the universe suggests that only one in every 400,000 stars is an antistar.  So we still have the imbalance to explain.

But it's a strange and fascinating finding.  Astrophysicists are currently re-analyzing the data from every angle they can think of to try and account for the odd gamma-ray sources in any way other than it being evidence of antistars, so it may be that the whole thing will fizzle.  But for now, it's a tantalizing discovery.  It brings to mind the famous quote from J. B. S. Haldane -- "The universe is not only queerer than we imagine, it's queerer than we can imagine."

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When people think of mass extinctions, the one that usually comes to mind first is the Cretaceous-Tertiary Extinction of 66 million years ago, the one that wiped out all the non-avian dinosaurs and a good many species of other types.  It certainly was massive -- current estimates are that it killed between fifty and sixty percent of the species alive at the time -- but it was far from the biggest.

The largest mass extinction ever took place 251 million years ago, and it destroyed over ninety percent of life on Earth, taking out whole taxa and changing the direction of evolution permanently.  But what could cause a disaster on this scale?

In When Life Nearly Died: The Greatest Mass Extinction of All Time, University of Bristol paleontologist Michael Benton describes an event so catastrophic that it beggars the imagination.  Following researchers to outcrops of rock from the time of the extinction, he looks at what was lost -- trilobites, horn corals, sea scorpions, and blastoids (a starfish relative) vanished completely, but no group was without losses.  Even terrestrial vertebrates, who made it through the bottleneck and proceeded to kind of take over, had losses on the order of seventy percent.

He goes through the possible causes for the extinction, along with the evidence for each, along the way painting a terrifying picture of a world that very nearly became uninhabited.  It's a grim but fascinating story, and Benton's expertise and clarity of writing makes it a brilliant read.

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

The symmetrical universe

I try to avoid writing about topics I don't fully understand, because that's just too great an opportunity for my sticking my foot in my mouth (and having to write a retraction afterwards).  Because of this reluctance, and because I'm pretty up-front about it when I don't know something, I don't get caught out very often, and I'd like to keep it that way.

So I'm gonna put a disclaimer right here at the beginning of this post: today's topic is one I have only a shallow understanding of.  If you ask me for more information, I'm likely to give you a puzzled head tilt, the same look my dog gives me when I ask him questions he doesn't have a good answer to, like why he chewed up my magazine before I had a chance to read it.  And if you are an expert in this field, and I get some of the facts wrong, let me know so I can fix 'em.

Okay, that being said: have you heard of CPT symmetry?

The initials stand for "charge," "parity," and "time," and the idea goes something like this: if you take any physical process, and reverse the charges (replace particles with their antiparticles), reverse the parity (reverse everything left-to-right), and run time backwards, the two would be indistinguishable.  Such a mirror universe would proceed according to exactly the same physical laws as ours does.

(As far as I know, it would not generate the scientific result elucidated in the Lost in Space episode "The Antimatter Man," wherein the mirror universe had an evil Don West with a beard.)

Initially, physicists thought that there was also CP symmetry -- that processes needed only charge and parity reversal to maintain symmetry, but that was found to be false when CP violations were found, most notably the decay of the particle called a neutral kaon.  The fact that symmetry is not preserved with reversal of charge and parity is thought to be the key to why there were unequal amounts of matter and antimatter produced in the Big Bang.  Fortunately for us.  If the matter/antimatter ratio had been exactly 1:1, ultimately it would all have mutually annihilated, and the universe would now be devoid of matter -- just space filled with photons zinging merrily about.

So CPT symmetry and CP violations are apparently fundamental to the nature of matter.  Which is why physicists have been pushing on the CPT symmetry idea, trying to find out if it holds -- or if there are circumstances, as there were with CP symmetry, where CPT symmetry is not preserved.

The latest test, described in a paper this week in Nature Physics, finds that even one of the oddest particles ever created in a laboratory preserves CPT symmetry.  In "Measurement of the Mass Difference and the Binding Energy of the Hypertriton and Antihypertriton," written by a team of particle researchers at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, we read about bizarre particles that instead of the "up" and "down" quarks (and antiquarks) found in ordinary matter (and antimatter, if there's such a thing as "ordinary antimatter"), additionally have "strange" quarks (and antiquarks), which have higher mass and only form under extremely high energy conditions.  These particles -- the hypertritons and antihypertritons in the title -- have never had their masses calculated accurately before, and the theory is that if the masses are different, it would break CPT symmetry and require a huge rethinking of how matter works on the smallest scales.

The result?  Hypertritons and antihypertritons have exactly the same mass.  CPT symmetry -- the fact that a charge reversed, mirror-image, time-running-backwards universe would look exactly the same as ours -- is preserved.  "It is conceivable that a violation of this symmetry would have been hiding in this little corner of the universe and it would never have been discovered up to now," said study co-author Declan Keane of Kent State University.  "But CPT symmetry was upheld even in these high-energy conditions."

This discovery gives physicists a clue about what might be happening in some of the most extreme and hostile spots in the universe -- the interiors of neutron stars.  The heat and crushing pressure in the core of a neutron star is thought to have enough energy to produce strange quarks and antiquarks, and therefore if those quarks (and the particles made from them) broke CPT symmetry, it would be a lens into a place where the known laws of physics do not hold.

But the symmetrical models won out.  Also, the measured energy of the hypertriton and antihypertriton were higher than expected, which squares with known neutron star masses.  "The presence of hyperons would soften the matter inside neutron stars," said Morgane Fortin, of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences in Warsaw.  "Softer neutron stars would more easily collapse into black holes, so neutron stars couldn’t become as massive.  That feature makes hyperons’ potential presence difficult to reconcile with the largest neutron stars seen in the cosmos — which range up to about two solar masses.  But the newly measured, larger binding energy of the hyperon helps keep alive the idea of a hyperon-filled center to neutron stars.  The result suggests that hyperons’ interactions with neutrons and protons are stronger than previously thought. That enhanced interaction means neutron stars with hyperons are stiffer and could reach higher masses.  So neutron stars may still have strange hearts."

Strange indeed.  Mirror universes, neutron stars, and symmetry preserved to the smallest scales and highest energies.  Amazingly cool stuff, even if (1) I don't understand it all that well, and (2) it doesn't involve evil Don West with a beard.

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This week's Skeptophilia book-of-the-week is brand new: Brian Greene's wonderful Until the End of Time.

Greene is that wonderful combination, a brilliant scientist and a lucid, gifted writer for the scientifically-inclined layperson.  He'd already knocked my socks off with his awesome The Elegant Universe and The Fabric of the Cosmos (the latter was made into an equally good four-part miniseries).

Greene doesn't shy away from difficult topics, tackling such subjects as relativity, quantum mechanics, and the nature of time.  Here, Greene takes on the biggest questions of all -- where the universe came from, how it has evolved and is evolving, and how it's going to end.

He begins with an observation that as a species, we're obsessed with the ideas of mortality and eternity, and -- likely unique amongst known animals -- spend a good part of our mental energy outside of "the now," pondering the arrow of time and what its implications are.  Greene takes a lens to this obsession from the standpoint of physics, looking at what we know and what we've inferred about the universe from its beginnings in the Big Bang to its ultimate silent demise in the "Heat Death" some billions or trillions of years in the future.

It's definitely a book that takes a wide focus, very likely the widest focus an author could take.  And in Greene's deft hands, it's a voyage through time you don't want to miss.

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