Plenty Dark Out There

And It’s Way Darker Than Expected

A pair of articles on Dark Matter has demonstrated to me that I’m terribly, terribly behind the curve in astronomical knowledge. Gee – I used to know it all! That was the good ol’ days, when Saturn had only 9 moons, Jupiter and Uranus didn’t have rings, Mars had canals and Pluto was a planet.

Things change – sigh.

Dark matter may not exist, but it seems to be about 26% of everything that’s out there (including the totally not-understood “Dark Energy” which you should not confuse with the merely confusing Dark Matter). At first, Dark Matter was a speculative artifice, put there by astronomers interested in the problem of balancing the gravitational books. It was known quite some time ago that there wasn’t enough matter (not in the stars and interstellar gas we see, anyway) to hold galaxies like our own Milky Way together for very long. By the ’60s, it became clear that something unseen was also holding clusters of galaxies together too. And lots of it.

Astronomers didn’t know what it was, but they were pretty confident that it was in an invisible halo surrounding a galaxy. That pretty much accounted for stars not flying away from the galaxy like bees on a mission. Now two U. of Arizona astronomers have determined that the stuff is right here in our solar system too.

Dark matter isn’t just far off in the Milky Way or somewhere on the other side of the Universe, though: it’s right here at home in our Solar System. In a recent paper submitted to Physical Review D, Ethan Siegel and Xiaoying Xu of the University of Arizona analysed the distribution of dark matter in our Solar System, and found that the mass of dark matter is 300 times more than that of the galactic halo average, and the density is 16,000 times higher than that of the background dark matter.

So can we identify it? Maybe, but maybe not. They theorize that there’s only enough inside the solar system that, if you could scoop it up in one place, you’d have only enough to equal the mass of the largest asteroid.

Don’t get me wrong – that’s a lot of mass. But it’s nothing close to the mass of a planet (and it’s spread out more or less uniformly over the vast area of space between the sun and the orbit of Neptune). Does this solve the so-called Pioneer Anomaly?

And, alas, the mystery of the Pioneer anomaly is not going to be solved by this revelation, as the mass of the captured dark matter is not enough to explain the odd motions of that spacecraft.

The second article, (see also this article by Ker Than), posits that the primordial stars born into the universe, the so-called population III stars, which were very different from stars today – and which should no longer exist – may indeed be around for us to see. The reason? Massive clouds of – you guessed it – dark matter that inhibited the normal course of their evolution and death.

When I saw that, my initial reaction was – and I think I can quote myself exactly – “huh???”  They explained:

This amazing theory comes from research carried out by Gianfranco Bertone and his team at the Paris Institute of Astrophysics in France. The thought that the first stars, born over 14 billion years ago, could possibly inhabit the Universe today is a very impressive idea. These primordial stars are thought to have been seeded inside dense clouds of dark matter, where gravity caused dark matter compression. As the matter became concentrated, non-baryonic particles may have begun annihilating, stopping natural hydrogen fusion (the mechanism commonly associated with star creation). “Normal” stellar evolution was therefore paused and the “dark star” phase began as dark matter annihilation heated the stellar cores.

Non-baryonic matter, huh. Interesting. What they’re saying here, is that this mysterious matter is made from a kind of normal, but rare (in nature) type of matter. The stuff we’re used to seeing – made of protons, neutrons (and electrons, which are different) – is made from quarks, held together in groups of 3, in an elegant formation described by quantum dynamics. Non-baryonic matter is everything else, including neutrinos, free electrons, and the “super-symmetric” particles.

Oh boy! These theoretical entities (also known as sparticles), whose existence come from attempts to combine the fundamental forces of nature (electro-magnetism, the strong and weak nuclear forces and gravity) into one, go a long way towards explaining the universe that we observe. BUT NOBODY’S SEEN THEM. And there may be scads of this stuff around us, even in our own solar system.

Consider me boggled.

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