We have in MOND a formula that has had repeated predictive successes. Many of these have been true a priori predictions, like the absolute nature of the baryonic Tully-Fisher relation, the large mass discrepancies evinced by low surface brightness galaxies, and the velocity dispersions of many individual dwarf Spheroidal galaxies like Crater 2. I don’t see how these can be an accident.
Recently I have been complaining about the low standards to which science has sunk. It has become normal to be surprised by an observation, express doubt about the data, blame the observers, slowly let it sink in, bicker and argue for a while, construct an unsatisfactory model that sort-of, kind-of explains the surprising data but not really, call it natural, then pretend like that’s what we expected all along.
The arXiv brought an early Xmas gift in the form of a measurement of the velocity dispersion of Crater 2. Crater 2 is an extremely diffuse dwarf satellite of the Milky Way. Upon its discovery, I realized there was an opportunity to predict its velocity dispersion based on the reported photometry. The fact that it is very large (half light radius a bit over 1 kpc) and relatively far from the Milky Way (120 kpc) make it a unique and critical case.
Vincent: Want to talk about MOND? Jules: No man, I don’t consider MOND. Vincent: Are you biased? Jules: Nah, I ain’t biased, I just don’t dig MOND, that’s all. Vincent: Why not? Jules: MOND is an ugly theory. I don’t consider ugly theories. Vincent: MOND makes predictions that come true. Fits galaxy data gooood.
So the always humorous, unabashedly nerdy xkcd recently published this comic: {.alignnone .size-full .wp-image-2940 loading=“lazy” attachment-id=“2940” permalink=“http://tritonstation.com/2016/11/12/xkcdd/astrophysics/” orig-file=“https://tritonstation.files.wordpress.com/2016/11/astrophysics.png” orig-size=“579,314” comments-opened=“1”
Last time, I addressed some of the problems posed by the radial acceleration relation for galaxy formation theory in the LCDM cosmogony. Predictably, some have been quick to assert there is no problem at all. The first such claim is by Keller &
People often ask for a straight up comparison between ΛCDM and MOND. This is rarely possible because the two theories are largely incommensurable. When one is eloquent the other is mute, and vice-versa. It is possible to attempt a comparison about how bad the missing baryon problem is in each.
A long standing problem in cosmology is that we do not have a full accounting of all the baryons that we believe to exist. Big Bang Nucleosynthesis (BBN) teaches us that the mass density in normal matter is Ω b ≈ 5%. One can put a more precise number on it, but that’s close enough for our purposes here. Ordinary matter fails to account for the closure density by over an order of magnitude.
OK, I’m not even going to try to answer that one. But I am going to do some comparison exploration. A complaint often leveled against MOND is that it is not a theory. Or not a complete theory. Or somehow not a proper one. Sometimes people confuse MOND with the empirical observations that display MONDian phenomenology. I would say that MOND is a hypothesis, as is dark matter.
I find that my scientific colleagues have a variety of attitudes about what counts as a theory. Some of the differences amount to different standards. Others are simply misconceptions about specific theories. This comes up a lot in discussions of MOND. Before we go there, we need to establish some essentials. What is empirical? I consider myself to be a very empirically-minded scientist. To me, what is empirical is what the data show. Hmm.
