Cosmology

Abstract:

The pristine underdense patches of the Universe, cosmic voids, are powerful cosmological laboratories, uniquely sensitive to dark energy, modified gravity, and neutrino masses, yet their baryonic content remains uncharacterized. We present the first constraint on baryon underdensity in voids, exploiting the dispersion measures (DMs) of fast radio bursts (FRBs) as tracers of the electron column. By stacking 3455 sight lines from CHIME/FRB with ∼15′ localizations on 1228 Sloan Digital Sky Survey (SDSS) BOSS voids over redshifts 0.2 < z < 0.7, we measure a DM deficit toward void centers at 3.2σ significance, indicating that diffuse baryons inhabit the emptiest corners of the cosmic web at a suppressed level. The measured signal amplitude is consistent with an effective Universe model built directly from the observed galaxy underdensity in these voids, and a baryonic model calibrated to the FRB DM–redshift relation (αv = 1.80 ± 0.87). A uniform-density void model yields an electron density contrast of δe,v = −0.58 ± 0.30, implying a tentative ∼60% ± 30% underdensity of baryons in void interiors relative to the cosmic mean. Jointly interpreting our FRB measurement with existing stacks of the thermal Sunyaev–Zel’dovich effect on voids further constrains the mean gas temperature to Te ≲ (1.1 ± 0.7) × 10⁶ K, pointing to a warm-hot diffuse phase, consistent with hydrodynamical simulations. With forthcoming FRB and galaxy surveys, this approach opens a new window onto baryon mapping, with direct implications for feedback models governing gas expulsion into low-density environments, and for the use of cosmic voids to extract cosmological constraints.

Abstract:

Abstract We study the influence of fuzzy-dark-matter cores on the orbits of stars at the Galactic centre. This dark matter candidate condenses into dense, solitonic cores, and, if a super-massive black hole is present at the centre of such a core, its central part forms a ‘gravitational atom’. Here, we calculate the atom’s contribution to the gravitational potential felt by a Galactic-centre star, for a general state of the atom. We study the angular-momentum dynamics this potential induces, and show that it is similar to vector resonant relaxation. Its influence is found to be potentially sufficiently strong that such a dynamical component should be accounted for in Galactic-centre modelling. For the Milky Way, the atom is expected to have some spherical asymmetry, and we use this to derive a stability condition for the disc of young, massive stars at the Galactic centre—if the atom’s mass is too large, then the disc would be destroyed. Thus, the existence of this disc constrains the mass of the particles comprising the solitonic core. We study an example model of the core, where all of the rotation of the core’s inner region is assumed to come from an l = 1 state, and its amplitude is determined by the halo’s spin parameter; such a core is found to be in tension with the stability of the clockwise stellar disc for 4.2 × 10−20 eV ≤ ma ≤ 5.4 × 10−20 eV at 2σ. Other core models could vary the constrained values of ma. These constraints will tighten significantly with future, improved data.

Abstract:

There is tentative evidence for a mismatch between the rest frames of matter and the cosmic microwave background, the “quasar dipole anomaly.” We consider such a dipole in tilted anisotropic models, for a range of scenarios and sources: spatial curvature, cosmic heat flux, large scale electromagnetic fields, and a Khronon field. Crucially, we determine the ancillary effects on other cosmological observables in each of these models, and we show that, apart from the case of the Khronon field, it is unlikely that one can obtain a dipole with the amplitude that is being observed unless one considers additional exotica.

Abstract:

Abstract The galaxy luminosity and stellar mass function (LF, SMF), and halo mass function (HMF), are fundamental quantities in astrophysics and crucial inputs to a range of astrophysical and cosmological analyses. They are typically parametrised by fitting functions that have been chosen ‘by eye’ to match observed or simulated data. We apply symbolic regression—specifically the Exhaustive Symbolic Regression (ESR) algorithm—to automate the search for optimal LF, SMF and HMF functional forms. ESR scores all functions up to a maximum complexity composed of a user-defined basis set of operators using the description length, an approximation to the Bayesian evidence that balances accuracy with complexity. We find many functions that outperform the Schechter and double Schechter functions for the LF and SMF, and that outperform all investigated literature functions (that outperform the Press–Schechter, Warren, Tinker, Sheth–Tormen and Jenkins) for the HMF. By additionally imposing ‘physicality checks’ on functions’ extrapolation and integration properties, we identify the optimal, low-complexity functional forms in terms of accuracy, simplicity and behaviour beyond the data range. As well as providing drop-in replacements for literature LF, SMF and HMF fitting functions, and identifying robust behaviour across well-fitting functions, we present a framework with which symbolic regression may be used to automate the discovery of optimal functions for any astrophysical dataset.

Abstract:

Abstract Remnant radio galaxies, whose lobes are no longer replenished by jets from the active galactic nucleus (AGN), offer key constraints on AGN duty cycles and the timescales of radio jets. We present a spectral-ageing study of 14 candidate remnant radio galaxies in the XMM−LSS field, combining new broad-band data from the MeerKAT MIGHTEE (L-band) and uGMRT superMIGHTEE (band-3 and band-4) surveys with complementary observations from LOFAR, GMRT, and JVLA, covering 144 MHz−1.5 GHz. Spectral modeling confirms 12 sources as genuine remnants, while two are reclassified as active, emphasising the importance of sensitive, multi-frequency coverage for robust remnant identification. Pixel-based spectral age maps yield results (~3-43 Myr) broadly consistent with integrated estimates, revealing relatively short spectral ages (~8-42 Myr). These ages likely reflect enhanced inverse-Compton losses at higher redshifts (0.35 ≤ z ≤ 2.85; median z = 1.25) and possible rapid lobe expansion in low-density environments. The ratios of remnant to total source ages (tOFF/ts) span 0.04-0.83, indicating that the sample traces a broad range of evolutionary stages. Our findings reveal a previously underrepresented population of faint, rapidly fading remnants, suggesting that the remnant phase may be shorter and more dynamic than previously thought. This study highlights the crucial role of MIGHTEE and superMIGHTEE surveys in reliably classifying genuine remnants and provides a framework for constraining AGN life cycles in preparation for forthcoming SKA surveys.