Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

In the context of modified gravity, at the linear level, the growth of structure in the Universe will be affected by modifications to the Poisson equation and by the background expansion rate of the Universe. It has been shown that these two effects lead to a degeneracy which must be properly accounted for if one is to place reliable constraints on new forces on large scales or, equivalently, modifications to general relativity. In this paper we show that current constraints are such that assumptions about the background expansion have little impact on constraints on modifications to gravity. We do so by considering the background of a flat, Λ cold dark matter universe, a universe with a more general equation of state for the dark energy, and finally, a general, model-independent, expansion rate. We use Gaussian processes to model modifications to Poisson's equation and, in the case of a general expansion rate, to model the redshift-dependent Hubble rate. We identify a degeneracy between modifications to Poisson's equation and the background matter density, ωM, which can only be broken by assuming a model-dependent expansion rate. We show that, with current data, the constraints on modifications to the Poisson equation via measurements of the growth rate range between 10-20% depending on the strength of our assumptions on the Universe's expansion rate.

Abstract:

We use the SPHINX²⁰ cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR₁₀) or 100 Myr (SFR₁₀₀). Both have f_esc>20 per cent and experienced an extreme burst of star formation, but Bursty Leakers have SFR₁₀ > SFR₁₀₀, while Remnant Leakers have SFR₁₀ < SFR₁₀₀. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H II region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H II region densities. Both types of leaker exhibit [C II]_158μm deficits on the [C II]–SFR₁₀₀ relation, while only Bursty Leakers show deficits when ₁₀ is used. We predict that [C II] luminosity and SFR indicators such as Hα and M_1500Å can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C II] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with f_esc>20 per cent dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with f_esc<5 per cent becomes significant at the tail-end of reionization.

Abstract:

The design and experimental demonstration are reported of an active convective heat switch suitable for use at cryogenic temperatures. The switch is mechanically simple, relatively inexpensive, and requires no moving parts, instead being operated entirely using heaters. The working gas used is <4> He in a closed cycle and, as such, the switch requires no external gas connections. Closed conductances on the order of 50 mW/K have been demonstrated with residual open conductances on the order of 0.4 mW/K. Novel modelling is presented which shows excellent agreement with the experimental data and significant improvement over existing models.

Abstract: