One of the great privileges of astronomy is that there’s always something new and surprising to discover. It’s a big universe. There’s a lot in it. In appreciation of this simple joy, I thought I’d share a nifty discovery made by the Rubin Observatory*. This is a big ground-based telescope with an absolutely mammoth camera that is conducting a decade-long survey to unprecedented depth. It basically images the sky over and over again every night.

The stability of spiral galaxies was a foundational motivation to invoke dark matter: a thin disk of self-gravitating stars is unstable unless embedded in a dark matter halo. Modified dynamics can also stabilize galactic disks.

If a title is posed as a question, the answer is usually No. There has been a little bit of noise that dark matter might have been detected near the center of the Milky Way. The chatter seems to have died down quickly, for, as usual, this claim is greatly exaggerated. Indeed, the claim isn’t … Continue reading Has dark matter been detected in the Milky Way?

The mass distribution of dark matter halos that we infer from observations tells us where the dark matter needs to be now. This differs form the mass distribution it had to start, as it gets altered by the process of galaxy formation. It is the primordial distribution that dark matter-only simulations predict most robustly.

I have always been interested in the extremes of galaxy properties, especially to low surface brightness (LSB). LSB galaxies are hard to find and observe, so they present an evergreen opportunity for discovery. They also expose theories built to explain bright galaxies to novel tests. Fundamental properties of galaxies include their size and luminosity.

This is a long post. It started focused on ultrafaint dwarfs, but can’t avoid more general issues. In order to diagnose non-equilibrium effects, we have to have some expectation for what equilibrium would be. The Tully-Fisher relation is a useful empirical touchstone for that. How the Tully-Fisher relation comes about is itself theory-dependent.

Previously, we discussed non-equilibrium dynamics in tidal dwarf galaxies. These are the result of interactions between giant galaxies that are manifestly a departure from equilibrium, a circumstance that makes TDGs potentially a decisive test to distinguish between dark matter and MOND, and simultaneously precludes confident application of that test.

There are a number of galaxies that have been reported to lack dark matter. This is weird in a universe made of dark matter. It is also weird in MOND, which (if true) is what causes the inference of dark matter. So how can this happen?

A strange and interesting aspect of MOND is the External Field Effect (EFE). If physics is strictly local, it doesn’t matter what happens outside of an experimental apparatus, only inside it. Examples of gravitational experiments include an Eötvös-style apparatus in a laboratory or a dwarf galaxy in space: in each case, test masses/stars respond to … Continue reading The minimum acceleration in intergalactic space →

My colleague Jim Schombert pointed out a nifty new result published in Nature Astronomy which you probably can’t access so here is a link to what looks to be the preliminary version. The authors use the Deep Synoptic Array (DSA) to discover some new Fast Radio Bursts (FRBs), many of which are apparently in galaxies … Continue reading The baryons are mostly in the intergalactic medium. Mostly.

Currently, English is the lingua franca of science. It wasn’t always that way, and there’s no reason to expect it always will be. A century ago, all the great physicists who wanted to be part of the quantum revolution went to study in Germany.