Galaxies contain fewer baryons than the cosmic average — the ratio of baryonic to dark matter in galactic halos is systematically lower than the universal ratio. The standard explanation is feedback: supernovae and active galactic nuclei inject energy that expels gas from the halo. But feedback operates locally — it pushes gas out of the galaxy. Where that gas ends up depends on whether the universe is pulling it back or pushing it further away.
Veenema, Sorini, and Bose (arXiv:2603.13095) build an analytical model that balances gravitational collapse, hydrostatic pressure, and cosmic expansion to predict the “closure radius” — the distance at which the baryon fraction equals the cosmic average. Beyond this radius, baryons have been expelled; within it, they're retained or accreting.
The result: dark energy significantly influences baryon expulsion. Closure radii are approximately 30% larger in the standard ΛCDM cosmology compared to an Einstein-de Sitter universe (same matter content, no dark energy) at redshifts below 2. Dark energy doesn't just accelerate the universal expansion — it makes it harder for expelled baryons to fall back. Gas pushed out by a supernova in a universe with dark energy travels further and stays away longer than the same gas in a universe without it.
In simulations with dark energy amplified to 10 or 100 times the observed value, dark energy becomes the dominant mechanism driving baryon removal from massive halos — more important than feedback itself. The cosmological constant isn't just a background parameter; it's an active participant in galaxy formation, setting the gravitational potential well that determines whether feedback-expelled gas returns or escapes permanently.