I promised more results from SPARC. Here is one. The dynamical mass surface density of a disk galaxy scales with its central surface brightness. This may sound trivial: surface density correlates with surface brightness. The denser the stars, the denser the mass. Makes sense, yes? Turns out, this situation is neither simple nor obvious when dark matter is involved.
We have a new paper that introduces SPARC: Spitzer Photometry & Accurate Rotation Curves. SPARC is a database of 175 galaxies with measured HI rotation curves and homogeneous near-infrared [3.6 micron] surface photometry obtained with the Spitzer Space Telescope. It provides the largest cohesive dataset currently available of disk galaxy mass models. SPARC represents all known types of rotating galaxies.
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.
Do not be too proud of this technological terror you’ve constructed. The ability to simulate the formation of large scale structure is insignificant next to the power of the Force. – Darth Vader, Lord of the Sith The now standard cosmology, ΛCDM, has a well developed cosmogony that provides a satisfactory explanation of the formation of large scale structure in the universe.
I should perhaps explain a little about the title of the last post. It is perfectly obvious to me. But probably not to anyone else. Our brains work in subtly different ways. One thing that mine does, whether I like it or not, is memorize lines and make obscure links between them.
I’m just back from the World Science Festival where I participated in the discussion shaking up the dark universe. It was an interesting experience, and mostly positive despite some offstage diva behavior. One thing it drove home to me was how hard it is to communicate across a gulf of different training and experience. Not just from scientists to the public, but between scientists.
I said I would occasionally talk about scientific papers. Today’s post is about the new paper Testing Feedback-Modified Dark Matter Haloes with Galaxy Rotation Curves: Estimation of Halo Parameters and Consistency with ΛCDM by Harley Katz et al. I’ve spent a fair portion of my career fitting dark matter halos to rotation curves, and trying to make sense of the results.
With four parameters I can fit an elephant, and with five I can make him wiggle his trunk. – John von Neumann The simple and elegant cosmology encapsulated by the search for two numbers has been replaced by ΛCDM. This is neither simple nor elegant. In addition to the Hubble constant and density parameter, we now also require distinct density parameters for baryonic mass, non-baryonic cold dark matter, and dark energy.
Cosmology used to be called the hunt for two numbers. It was simple and elegant. Nowadays we need at least six. It is neither simple nor elegant. So how did we get here? The two Big Numbers are, or at least up till the early-90s were, the Hubble constant H 0 and the density parameter Ω. These told us Everything. Or so we thought. The Hubble constant is the expansion rate of the universe.
There has been some debate of late over the role of falsifiability in science. Falsifiability is the philosophical notion advocated by Popper as an acid test to distinguish between ideas that are scientific and those that are not. In short, for a theory to be scientific, it has to be subject to falsification.