OK, this is for all you physics geeks. You already know that there have been debates, theories, and conjectures over dark matter for decades. In February of this year an informational release of a coming paper was issued. It said in part:
The predictions of a theory proposed as an alternative to dark matter have been verified in a new class of objects, according to a study currently in press. The results seem unlikely to convince astrophysicists to abandon dark matter as one of the cornerstones of the standard model of cosmology; but suggest a direction in which new work is urgently needed.
The alternative theory is Modified Newtonian Dynamics, or MOND, proposed in 1983 by astrophysicist Moti Milgrom of the Weizmann Institute of Science in Israel. Like dark matter, MOND was hypothesized to explain why galaxies remain in one piece when observations of the rotation speeds and estimates of the mass of their luminous contents suggest they should fly apart. Rather than assuming the universe contains 5/6 invisible dark matter, Milgrom proposed a modification to Newton’s laws that kicked in at the low acceleration scales of stars in galaxies, and that strengthened the attractive force of gravity, having the effect of dark matter without imagining a mysterious extra substance. MOND successfully predicted the observed rotation speeds of galaxies and gained supporters but its adherents dropped off dramatically after observations of the cosmic microwave background (CMB) – the so-called “echo of the Big Bang” — first by the balloon experiment BOOMERanG and then by NASA’s Wilkinson Anisotropy Probe (WMAP), provided stunning confirmation of predictions from the dark matter theory.
One astronomer who still hasn’t given up on MOND is Stacy McGaugh of the University of Maryland in College Park. In his latest paper, in press at Physical Review Letters, McGaugh compares the prediction of the theory to the rotation speeds of spiral galaxies that are gas-rich. While estimates of the mass of most galaxies are uncertain; because it’s not known exactly how to convert measurements of the light from the stars into the mass of the galaxy, the mass of gas-rich galaxies can be measured precisely from the intensity of a radio emission from the hydrogen atoms that make up the gas. That enabled McGaugh to test MOND more rigorously than has ever been done before. He found that, in detail, the galaxies obey the Tully-Fischer relation, the observation that more massive galaxies rotate proportionally faster than less massive ones. “MOND predicted this naturally ahead of time,” says McGaugh. In contrast a dark matter model that assumed the proportion of dark matter in the galaxies matched the cosmic value of 5/6 gave a prediction that was some way off the observations.
On the other hand, the MOND model does not work as well with galaxy clusters, and with the universe as a whole. On top of that, MOND appears to have some complicated relationships with the general theory of relativity. This is troublesome, because science tends to follow the Occam’s Razor principle that the simplest explanation is the best. The simpler an explanation is and the more encompassing it is, the more “elegant” it is considered to be. So, you physics geeks, is it Newtonian gravity plus dark matter or MOND without dark matter? Here is a hint by way of a quote from another blog:
Neil Degrass Tyson in an interview that everyone needs to see call Cosmic Quandaries, addresses this issue by saying that “If you are a hammer, all of your problems look like nails.” He goes on to say that if you are gravitationalist you might decide that the solution to the “dark matter” problem is that there is some error with the current calculations of gravitational force, or that if you are a particle physicist, you might think that there is a particle out in space that we just aren’t able to currently detect.
I am being nice and posting the interview below. Please note that you have to skip the first 15 minutes of the video to get to the talk. The dark matter discussion begins about the 31 minute mark. And, by the way, there is a lot more discussed on this video than just dark matter. So, sit back and enjoy!
Very interesting Father!