Some Astrophysics Quick Hits

Many good theories start from the foundation of patterns in the empirical data.

The standard theory of galaxy formation predicts that all galaxies should contain dark matter, yet a handful of recently discovered galaxies appear to lack it, challenging our understanding of galaxy formation. We investigate whether such dark-matter deficient objects can be identified from their baryonic properties alone, analogously to the radial-acceleration relation, which tightly links baryon and dark matter distributions in spiral galaxies. 

Using a sample of ultra-diffuse and dwarf spheroidal galaxies -- systems whose baryonic properties resemble those of the confirmed dark-matter-deficient galaxies -- we systematically search for a formula to predict baryonic fractions from stellar mass, effective radius, distance to the host, and the host's baryonic mass. We find that baryonic fraction correlates most strongly with the gravitational acceleration expected from baryons alone, a(bar), or equivalently, with mean surface brightness, following an approximately a(bar)^−1 dependence. This scaling resembles the radial-acceleration relation but differs in functional form and applies to a different galaxy population. 

Strikingly, the dark-matter-deficient galaxies occupy the extreme end of the correlation. This suggests that they result from standard formation processes operating at unusual intensities rather than from exotic mechanisms. Importantly, the correlation predicts that all ultra-diffuse galaxies brighter than approximately 25 mag arcsec^−2 in the g-band should have very low dark matter content, offering a straightforward observational criterion for identifying these rare objects.

Michal Bílek, "A correlation predicting galaxies without dark matter" arXiv:2605.11070 (May 11, 2026).

Footnote: "cosmic noon" (i.e. the middle of the age of the universe measured in years) is sometimes operationally defined as "0.5 < z < 3".

Deur's approach to gravity provides a mechanism that explains the seeming observational preference for planar structures over spherical ones at galactic and larger scales.

An update of the evidence that radio galaxies and clusters of galaxies are more common than average near the plane of the de Vaucouleurs Local Supercluster shows that in the distance range 100 to 200Mpc objects whose positions are correlated with the plane of the Local Supercluster include galaxies that are exceptionally luminous at two microns, radio galaxies, and clusters of galaxies. There can be little doubt about this property of cosmic structure. I also argue for detection of this correlation for the galaxies at 400Mpc distance that are exceptionally luminous at two microns. 

It will be interesting to learn whether these results are expected in the standard cosmology.

P. J. E. Peebles, "The Extended Plane of the Local Supercluster" arXiv:2605.11184 (May 11, 2026).

We study how constraints on the abundance of ultralight axions (ULAs) from cosmic microwave background (CMB) data depend on their nonlinear modelling. We focus on the axion mass range 10^−25 ≤ m/eV ≤ 10^−23, where the axion Jeans scale falls in the quasi-linear regime probed by CMB lensing, making constraints highly sensitive to the choice of nonlinear prescription. 

We show that the inferred constraints depend significantly on the choice of nonlinear model, which must therefore be treated carefully. Performing Markov Chain Monte Carlo (MCMC) analyses with Planck 2018, ACT DR6 and DESI DR2 BAO data, we find naive nonlinear modelling of non-cold matter can produce an artificial preference for a subdominant ULA dark matter component with mass m ≈ 10^−24 eV. This arises from a lensing-like enhancement of the CMB power spectrum.

Lauren Gaughan, Anne M. Green, Adam Moss, "Ultra-light axion constraints from Planck and ACT: the role of nonlinear modelling" arXiv:2605.12054 (May 12, 2026).

Noting this paper for future reference and maybe a post of its own.

We present results from the Big Mysteries Survey, a large-scale survey conducted through the American Physical Society's Physics Magazine on foundational and controversial topics in contemporary physics. The survey provides a snapshot of physicists' views on issues in cosmology, black-hole physics, quantum mechanics, quantum gravity, and anthropic coincidences. A central finding is that several positions often described publicly as field-wide ``consensus'' views are, in practice, supported by much narrower majorities or by pluralities rather than majorities.

Niayesh Afshordi, Phil Halper, Matteo Rini, Michael Schirber, "Big Mysteries Survey: Physicists' Views on Cosmology, Black Holes, Quantum Mechanics, and Quantum Gravity" arXiv:2605.11058 (May 11, 2026).