Beecroft Institute for Particle Astrophysics and 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 In the context of maximum-likelihood parametric component separation for next-generation full-sky cosmic microwave background (CMB) polarization experiments, we study the impact of fitting different spectral parameters of Galactic foregrounds in distinct subsets of pixels on the sky, with the goal of optimizing the search for primordial B modes. Using both simulations and analytical arguments, we highlight how the post-component separation uncertainty and systematic foreground residuals in the cleaned CMB power spectrum depend on spatial variations in the spectral parameters. We show that allowing spectral parameters to vary across subsets of the sky pixels is essential to achieve competitive signal-to-noise ratio on the reconstructed CMB after component separation while keeping residual foreground bias under control. Although several strategies exist to define pixel subsets for the spectral parameters, each with its advantages and limitations, we show using current foreground simulations in the context of next-generation space-borne missions that there are satisfactory configurations in which both statistical and systematic residuals become negligible. The exact magnitude of these residuals, however, depends on the mission’s specific characteristics, especially its frequency coverage and sensitivity. We also show that the post-component separation statistical uncertainty is only weakly dependent on the properties of the foregrounds and propose a semi-analytical framework to estimate it. On the contrary, the systematic foreground residuals highly depend on both the properties of the foregrounds and the chosen spatial resolution of the spectral parameters.

Abstract:

We present constraints on the amplitude of matter fluctuations from the clustering of galaxies and their cross-correlation with the gravitational lensing convergence of the cosmic microwave background (CMB), focusing on low redshifts (z ≲ 0.3), where potential deviations from a perfect cosmological constant dominating the growth of structure could be more prominent. Specifically, we make use of data from the 2MASS photometric survey (2MPZ) and the WISE×SuperCOSMOS galaxy survey, in combination with CMB lensing data from Planck. Using a hybrid effective field theory (HEFT) approach to model galaxy bias we obtain constraints on the combination S 8 = σ 8 √(Ω m /0.3), where σ 8 is the amplitude of matter fluctuations, and Ω m is the non-relativistic matter fraction. Using a prior on Ω m based on the baryon acoustic oscillation measurements of DESI, we find S 8 = 0.79 ± 0.06, in reasonable agreement with CMB constraints. We also find that, in the absence of this prior, the data favours a value of Ω m = 0.245 ± 0.024, that is 2.8σ lower than Planck. This result is driven by the broadband shape of the galaxy auto-correlation, and may be affected by theoretical uncertainties in the HEFT power spectrum templates. We further reconstruct the low-redshift growth history, finding it to be compatible with the Planck predictions, as well as existing constraints from lensing tomography. Finally, we study our constraints on the HEFT bias parameters of the galaxy samples studied, finding them to be in reasonable agreement with coevolution predictions.

Abstract:

A primary target of the Euclid space mission is to constrain early-universe physics by searching for deviations from a primordial Gaussian random field. A significant detection of primordial non-Gaussianity would rule out the simplest models of cosmic inflation and transform our understanding of the origin of the Universe. This paper forecasts how well field-level inference of galaxy redshift surveys can constrain the amplitude of local primordial non-Gaussianity, f NL^local, within a Bayesian hierarchical framework, in the upcoming Euclid data. We designed and simulated mock datasets and performed Markov chain Monte Carlo analyses using a full-field forward modelling approach. By including the formation history of the cosmic matter field in the analysis, the method takes into account all available probes of primordial non-Gaussianity, and goes beyond statistical summary estimators of f NL^local. Probes include, for example, two-point and higher-order statistics, peculiar velocity fields, and scale-dependent galaxy biases. Furthermore, the method simultaneously handles systematic survey effects, such as selection effects, survey geometries, and galaxy biases. The forecast shows that, using simulated Euclid data, the method can achieve a precision of σ(f NL^local) = 2.6 (68.3% confidence level), assuming a grid resolution of ΔL = 31.25 h ⁻¹ Mpc and a cut-off scale of k NF = 0.1 h Mpc⁻¹. We also provide data products, including realistic simulations with non-zero values of f NL^local and maps of adiabatic curvature fluctuations. The results underscore the feasibility and advantages of field-level inference to constrain f NL^local in galaxy redshift surveys. Our approach consistently captures all the information available in the large-scale structure to constrain f NL^local, and resolves the degeneracy between early-universe physics and late-time gravitational effects, while mitigating the impact of systematic and observational effects.

Abstract:

Massive early-type galaxies (ETGs; M > 10¹¹ M⊙) are believed to form primarily through mergers of less massive progenitors, which leave behind numerous traces of violent formation histories, such as stellar streams and shells. A particularly striking signature of these mergers is the formation of supermassive black hole (SMBH) binaries, which can create depleted stellar cores through interactions with stars on radial orbits – a process known as core scouring. The secondary SMBH in such systems may still carry a dense stellar envelope and thereby remain observable for some time as a secondary nucleus while it sinks towards the shared gravitational potential of the merged galaxy. Direct observations of secondary nuclei on sub-kiloparsec scales remain rare, with only a few notable cases, such as NGC 5419. Investigating such features and building up statistics requires both high spatial resolution and wide-field coverage, a capability uniquely provided by Euclid. In this study, we leverage Euclid’s Q1 Early Release data to systematically search for secondary nuclei in ETGs. We present a preliminary sample of 666 candidate systems distributed over 504 hosts (some of which contain multiple secondary nuclei). The vast majority of these fall at separations of 3 kpc to 15 kpc, indicative of normal mergers. However, 44 fall at projected separations of less than 2 kpc. We argue that this most interesting subset of secondary nucleus candidates – those at very close angular separations – are unlikely to be a consequence of chance alignments. We show that their stellar masses are mostly too large for them to be globular clusters and that a significant subset are unresolved even at Euclid’s spatial resolution, rendering them too small to be dwarf galaxies. These objects may represent the highest-density nuclei of a previously merged galaxy currently sinking into the centre of the new common gravitational potential, and thus they likely host a secondary SMBH. We also demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the 30 times larger sky coverage of the future Euclid DR1. Finally, we argue that our method can detect the remnants of a recoil event from two merged SMBHs, as two of our secondary nuclei candidates are unresolved at the Euclid spatial resolution, appear at projected physical separations of less than 2 kpc, and appear in hosts of M > 10¹¹ M⊙, which makes them viable candidates.