This recent press release on some of the latest galaxy formation simulations has reignited an old debate about dark matter versus modified theories of gravity. Sadly, many commentators seem to be missing the point. (A thread, 1/N.)
quantamagazine.org/coder-physicis…
quantamagazine.org/coder-physicis…
In mod. grav. theories, all the force comes from the visible stars & gas. Given good observations, we can accurately predict the gravitational force field without knowing the first thing about how galaxies form. Such theories are easy to test in disc galaxies & work well. (2/N.)
By contrast, dark matter theories exist in the context of a cosmological model that explains the formation and evolution of all structure in the Universe. For these theories to predict the force field in a given galaxy, we are required to understand how galaxies form. (3/N.)
Despite tremendous progress, we still do not understand how galaxies form and evolve. Whatever someone may tell you, we cannot make "ab-initio" calculations at the scale of galaxies, of the sort we can for the standard model of particle physics. (4/N.)
Indeed, the latest simulations have a wide dispersion of results at the scale of dwarf galaxies. Some form too few stars, some too many. One recent one from the FIRE team has too few massive isolated dwarfs (the opposite of the missing sat. problem). We have much to learn! (5/N.)
Sadly, these puzzles are often over sold in our community as "crises", "calamities", "catastrophies". In reality, the simulations rely on phenomenological "sub-grid" models and often lack the resolution to model all of the important physics. (6/N.)
Thus, if the simulations do not match the data, it could mean that something is wrong with the whole cosmological model. Or it could just mean that something is wrong with the simulation. Indeed, when the simulations fail, this is how we learn. It's a good thing. (7/N.)
So, should we just throw our hands in the air and give up? No! If we want to test dark matter models, we should find corners of the Universe where the uncertain physics of galaxy formation are less important. (8/N.)
This is exactly what we've been trying to do. The Draco dwarf, for example, stopped forming stars over 10Gyrs ago; it should have a near-pristine dark matter (DM) halo. We have recently shown that its DM distribution agrees beautifully with predictions from CDM models. (9/N.)
The bottom line is that some theories can be well-tested in disc galaxies, like modified gravity models. Other theories like LCDM are much more challenging to test in discs since we must first understand galaxy formation and evolution. LCDM is better tested elsewhere. (End.)
