Dark Matters

On a regular basis, I read a variety of scientific papers in myriad disciplines.  That’s how I learn about new ideas like using enzymes to edit our memories, and lacing cement with a specific kind of charcoal to make it work like a battery, along with keeping up with research pertinent to my “real” job in astronautical engineering.  Most of the papers I read, unless I’m doing in-depth research on something specific, I read as one-offs, without doing a lot of contextual research or diving into the topic, much like reading the news, but once every few weeks or months I come upon a paper that proves the gaping maw of a rabbit hole down which I dive.  Most recently, that was one about dark matter.

Well, I say it was about dark matter, because that’s where it eventually led, but the original paper was addressing a very specific problem arising from some of the new, early universe observations coming from the JWST.  Apparently, that data is revealing far more, and far larger, stars than the current models of the universe’s development and of star formation can support, and this paper suggests that the solution is something called a dark star.

This led me to an earlier paper from the same authors in which they first proposed the possibility of “dark stars,” which are not really stars at all, at least not in the conventional sense.  Conventional stars are powered by fusion reactions of baryonic matter, particularly hydrogen nuclei, which generates an outward pressure that balances the inward force of gravity.  Dark stars would not experience nuclear fusion, or would experience it only minimally; instead, they would maintain their stability (that outward force), and emit observable energy, from the self-annihilation of dark matter.

As you might expect, this gave me pause.  Dark matter physics has come a long way from the WIMPs and super-WIMPs I remember reading about in seventh grade, and I admit that I haven’t kept up with all of the permutations, but I thought I understood that the whole idea of dark matter is that it is weakly interacting.  How could something that doesn’t really interact much, on a physical level, be self-annihilating?  From dark stars, therefore, I jumped to reading about “self-heating dark matter via semi-annihilation.”

Annihilation is the process by which a particle and an antiparticle interact, and both are converted to energy.  At a very basic level, anyway.  Some particles in the standard model are their own antiparticles, which leads to interesting physics, but most particles have their opposite number: for the electron there is the positron, for the quark there’s the antiquark, and so forth.  The universe is dominated (or so we think) by ordinary matter, with very minimal antimatter, probably because of a slight imbalance between the production of matter and antimatter in reactions during the formation of the universe.  If dark matter particles, whatever they are, are their own antiparticle, they can engage in mutually annihilating reactions that would release energy and could power these “dark stars,” which are somewhere between a normal star and a nebula – big clouds of dust and gas loosely bound together and heated by annihilating dark matter.

I could have stopped there, but the paper on self-heating dark matter mentioned two additional concepts which defied my then-understanding and challenged me to learn more: dark photons and dark radiation.  Like a traveler without a map just going where the locals tell him might be interesting, I set off to learn about these dark concepts.

Dark photons in particular struck me as counterintuitive.  Not because of the whole light thing – I know enough about dark matter physics to know that the “dark” doesn’t mean that it’s literally dark the way a room is dark when the lights are off, but rather that it’s matter that doesn’t really interact with conventional matter, that is difficult to detect save by its silhouette (as it were), and that we don’t fully understand and cannot yet fully describe or define.  But if dark matter is supposed to not interact with ordinary matter, how could there be a dark equivalent of the energy carrying particle for the electromagnetic force, precisely the force with which we cannot directly detect what we think is dark matter?

After reading a 70-page piece devoted to the dark photon…I still don’t entirely understand what it is, much less am I able to describe what it is to you in a coherent way, but I will do my best.  Physicists have invented an entire “dark sector” of physics, involving particles that cannot interact or interact only minimally with ordinary matter described in the Standard Model.  The dark photon is a gauge boson proposed under this new physics which would interact with dark matter much the same way the photon interacts with ordinary matter, but would not interact with ordinary matter.

What about dark radiation?  With a name like that, it certainly sounds frightening, although I suspect if I’d tried to invoke “dark radiation” in my The Abyss Stares Back (previously titled Voyage into the Dark), it would have come across as lazy writing.  It shouldn’t be frightening, though, because it would obey the same conceit as the dark photon, and refers simply to other kinds of force fields which would interact only in the “dark realm” and not effect the physics of the standard model and our everyday reality.

At times, it is difficult to take some of these proposals seriously.  When I first read about dark matter in the context of WIMPs and super-WIMPs, the mainstream understanding was that the universe was 4% conventional matter, 23% dark matter that was a cold, collisionless relic of the early universe with minimal interactivity with ordinary matter, and 73% mysterious and undefined dark energy.  As interesting as I found these proposals for 96% of the universe being something that we’ve never directly detected and have no substantial, empirical evidence to support, I sometimes wondered if we were falling into a Ptolemaic trap.  When Copernicus proposed his heliocentric model, it was in large part because the Ptolemaic model introduced so many complications and contrivances to make a geocentric model work that it seemed a simpler solution must exist.  Perhaps what we perceive as dark matter and dark energy is similar, a sign, not of undetectable elements, but of inadequacy or incompleteness in our current physics.

Reading about these ideas for “dark realm” physics, I find them fascinating, imaginative, and wonderful.  I love considering the possibilities of an entire parallel reality of sorts consisting of the dark realm, with its own physics running alongside conventional physics but quite undetectable.  If some of the more elaborate proposals are true, it would mean there really could be dark matter lifeforms, like the ones I invoke in The Abyss Stares Back.  A sober perspective, though, requires that I tame my imagination.  A rationalist approach would suggest that either there is something missing or wrong about our current physics that results in the inference of dark matter/dark energy, like the arbitrary cosmological constant that was introduced at one point in relativity to provide for a static universe instead of an expanding one, or that dark matter, if it does exist, is relatively cold, static, and non-interacting.

I am not physicist enough to come up with whatever the next theory is that would solve these problems and not require us to invoke mysterious “dark” physics to make the math line up with observations.  And reality does not always obey Occam’s Razor – sometimes, a monkey really did break out of the local zoo.  Maybe there really is a dark realm out there, with its own physics, its own radiation and matter and interactions.  I don’t know if I’ll live to read anything approaching a conclusion to this story, but for now, I know it makes for excellent science fiction.