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.

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.

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 – John von Neumann The simple and elegant cosmology encapsulated by the search for two numbers has been replaced by ΛCDM.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.

I travel to Cambridge, MA tomorrow to participate in the workshop Rethinking the Dark Matter Paradigm (I had nothing to do with the choice of title). I went to college at MIT in the ’80s, so is a bit back to the future for me in space as well as time. There is a lot to rethink, or nothing at all, depending on who you ask.