The mystifying unknown of dark matter, used to explain the movement of faraway galaxies, has begun to annoy some physicists. NewScientist (22 August 2015, paywall) publishes an article by Sabine Hossenfelder and Naomi Lubick on the matter:
But quarks are quirky. They never float around freely thanks to a peculiar property of the strong nuclear force that binds them. When the distance between quarks is small, the force is weak. But as that distance increases, it gets stronger and the quarks are pulled back together. Another strange quality of the strong force is that it is weaker at high energies, such as those produced in collisions at the LHC. Physicists can calculate how quarks interact at these energies but not at lower ones, where the force keeping quarks together strengthens. As a result, physicists still struggle to explain how quarks form mesons and baryons, a process which occurs at lower energies.
This uncertainty has led to proposals that other forms of matter might exist. As early as the 1980s, Edward Witten, a mathematical physicist at Princeton University, suggested that light quarks could combine with their heavier cousins, such as strange quarks, in unusual ways.
Unlike in ordinary matter, these combinations of quarks would not form atomic nuclei. Instead they would develop into large amorphous blobs, gathering ever more particles in a small space. Witten called them “quark nuggets”. Bryan Lynn, a theoretical physicist at University College London, and others later expanded this to more examples such as “strange baryon matter” and “chiral liquid drops”.
Such exotic clumps of familiar elementary particles would not contain the enormous spaces between atomic nuclei that we see in normal matter. This would make them as dense as neutron stars, a teaspoon of which weighs as much as a mountain. So even though they might be extremely heavy, they could also be tiny. Some researchers have dubbed them “macros” – a reference to the need to measure their masses in kilograms and tonnes rather than the vanishingly small units usually employed for particles.
And because macros are entirely made up of nuclear matter, without any circulating electrons or empty spaces, they would not be capable of sustaining fusion and therefore could not shine. The high density of the clumps would also make them less likely to interact with incoming light. In short, macros would be diminutive, massive and extremely hard to spot, if not entirely invisible.
While it’s not unheard of to theorize about some particle and have it discovered years later, such as the neutrino, dark matter has been somewhat unsatisfying without any signal beyond gravity to show for it, so this alternative has a certain attraction to it – just quarks in some new configurations. No sign these actually exist, yet, but new tests are being devised and they have not yet been ruled out.
