Signatures of Galactic expansion in Gaia DR3: implications for the JWST early-Galaxy observations

ABSTRACT Recent James Web Space Telescope observations reveal that massive galaxies were already in place within the first ${\sim} 1$ Gyr of cosmic history, challenging models in which galaxies assemble across many billions of years. One possible explanation is that active galactic nuclei drive large-scale outflows that accelerate early galaxy growth. This context motivates a search for signatures of dynamical non-equilibrium and possible expansion in the Milky Way. We investigate Galactocentric radial stellar motions within the range $1 \le r_{\rm GC} \le 16~\mathrm{kpc}$ from the Galactic centre. Gaia Data Release 3 stars with high-quality astrometric and spectroscopic measurements (signal-to-noise ratios $\gt\,5$) were selected. The Galaxy was divided into 27 spatial sectors, together with global samples covering all Galactic quadrants and vertical subsystems. Galactocentric radial velocities were derived using the Astropy package. Mean motions were computed in radial bins of width 0.25 kpc, with uncertainties estimated from standard errors and statistical significance assessed using t-tests. We find that in 21 of 27 Galactic sectors, as well as in the global samples, the mean Galactocentric radial velocities are statistically significantly positive. The global Galaxy, disc, and halo samples exhibit coherent outwards motions, with a mean radial velocity of ${\sim} 3.95~\mathrm{km\, s^{-1}}$ ($p \lt\, 0.000\,01$). These results indicate that the Milky Way may not be in complete dynamical equilibrium on kiloparsec scales and may exhibit a weak global expansion trend which, if applicable to high-redshift galaxies, could provide a natural explanation for their accelerated formation on time-scales of a few hundred million years.