21cm Forest with the SKA

An alternative to both the tomography technique and the power spectrum approach is to search for the 21cm forest, that is the 21cm absorption features against high-z radio loud sources caused by the intervening cold neutral intergalactic medium (IGM) and collapsed structures. Although the existence of high-z radio loud sources has not been confirmed yet, SKA-low would be the instrument of choice to find such sources as they are expected to have spectra steeper than their lower-z counterparts. Since the strongest absorption features arise from small scale structures (few tens of physical kpc, or even lower), the 21cm forest can probe the HI density power spectrum on small scales not amenable to measurements by any other means. Also, it can be a unique probe of the heating process and the thermal history of the early universe, as the signal is strongly dependent on the IGM temperature. Here we show what SKA1-low could do in terms of detecting the 21cm forest in the redshift range z = 7.5-15.Comment: Accepted for publication in the SKA Science Book 'Advancing Astrophysics with the Square Kilometre Array', to appear in 2015; 10 pages, 5 figures; the manuscript is based on Ciardi et al., 2013, MNRAS, 428, 175

Foregrounds for 21cm Observations of Neutral Gas at High Redshift

We investigate a number of potential foregrounds for an ambitious goal of future radio telescopes such as the Square Kilometer Array (SKA) and Low Frequency Array (LOFAR): spatial tomography of neutral gas at high redshift in 21cm emission. While the expected temperature fluctuations due to unresolved radio point sources is highly uncertain, we point out that free-free emission from the ionizing halos that reionized the universe should define a minimal bound. This emission is likely to swamp the expected brightness temperature fluctuations, making proposed detections of the angular patchwork of 21cm emission across the sky unlikely to be viable. An alternative approach is to discern the topology of reionization from spectral features due to 21cm emission along a pencil-beam slice. This requires tight control of the frequency-dependence of the beam in order to prevent foreground sources from contributing excessive variance. We also investigate potential contamination by galactic and extragalactic radio recombination lines (RRLs). These are unlikely to be show-stoppers, although little is known about the distribution of RRLs away from the Galactic plane. The mini-halo emission signal is always less than that of the IGM, making mini-halos unlikely to be detectable. If they are seen, it will be only in the very earliest stages of structure formation at high redshift, when the spin temperature of the IGM has not yet decoupled from the CMB.Comment: submitted to MNRA

Bounds on extra dimensions from micro black holes in the context of the metastable Higgs vacuum

We estimate the rate at which collisions between ultrahigh-energy cosmic rays can form small black holes in models with extra dimensions. If recent conjectures about false vacuum decay catalyzed by black hole evaporation apply, the lack of vacuum decay events in our past light cone may place new bounds on the black hole formation rate and thus on the fundamental scale of gravity in these models. For theories with fundamental scale E∗ above the Higgs instability scale of the Standard Model, we find a lower bound on E∗ that is within about an order of magnitude of the energy where the cosmic-ray spectrum begins to show suppression from the Greisen-Zatsepin-Kuzmin effect. Otherwise, the abundant formation of semiclassical black holes with short lifetimes would likely initiate vacuum decay. Assuming a Higgs instability scale at the low end of the range compatible with experimental data, the excluded range is approximately 1017  eV≲E∗≤1018.8  eV for theories with n=1 extra dimension, narrowing to 1017  eV≲E∗≤1018.1  eV for n=6. These bounds rule out regions of parameter space that are inaccessible to collider experiments, small-scale gravity tests, or estimates of Kaluza-Klein processes in neutron stars and supernovae