Ptolemaic epicycles were “used to address discrepancies in the observed motions of the planets” in an Earth-centered system.
Alex Filippenko, astrophysicist and professor of astronomy at Berkeley, worries that that might be the case for dark matter and dark energy:
Filippenko describes “wake up in the middle of the night screaming” worries that dark energy and dark matter might be theory-driven devices to accommodate the data. “Band-aids to explain the data,” he wonders, “but they’re just completely wrong.”
Well, recall that we were told recently at Science mag that “really weird” gravity would be needed to explain away dark matter. We asked, what do we do when the situation is untenable but so are the alternatives?
Our physics color commentator Rob Sheldon offers,
“Dark matter” is not nearly as mysterious as all these breathless pressers say. After 30 years of experiments we know it isn’t an exotic particle, which was a promising guess some 40 years ago, the so-called “WIMP miracle”. Going back to non-exotic particles, we need a cold, non-emitting particle larger than a dust-mote but smaller than, say, a skyscraper which possesses negative viscosity. The study of negative viscosity matter is known as “active particles” because they possess motive power. Many such particles are known, for example, comets that jet away from hot stars. The reason this isn’t widely known, is that 1000’s of unemployed particle physicists have had a second career building “Dark Matter” detectors looking for (naturally) exotic particles and dominating the very public search for dark matter.
This article is about fine-tuning gravity as a wiggly, high-order polynomial (instead of 1/r2) to possess all the density properties of dark matter (active particles) filling a galaxy. It’s hard to say which is more doomed–the search for exotic particles or the search for exotic polynomials.
So we asked Rob, why can’t they find dark matter, despite much search? He replied,
The old joke is that a man is looking under a lamppost one night. The policeman asks what he is doing. “Looking for my keys” he replies. “Did you lose them here?” “No, but the light is better over here.”
The reason people have not been finding Dark Matter is because the “WIMP Miracle” convinced everyone that WIMPs were the answer. This was convenient, because quite a number of particle accelerators had been cancelled–Superconducting Supercollider, Tevatron at Fermilab, etc, and unemployed particle physicists began searching for WIMP dark matter. That was 30 years ago. And they still haven’t found it. But the funding is better over here.
Yet astronomers gave them lots of hints. “It’s matter that is dark” they said, “like comets, boulders, and black holes”. The search for black holes, using microlensing, did turn up a few, but not enough. The search for smaller objects using “femtolensing” has not been funded, (yup, another failed proposal of mine), probably because the money was all going toward WIMPs. The theorists haven’t been too helpful either—“Big Bang Nucleosynthesis models all say that it can’t be made of ordinary matter. So it has to be an exotic particle.” My rejoinder is the BBN models fit 5 elements with 4 free parameters and manage to miscalculate the fifth element. Maybe it is the BBN model that needs fixing. But that appears to be a bridge too far.
Rob Sheldon is the author of Genesis: The Long Ascent and The Long Ascent, Volume II.