Day 1339

Friday.

1711.05761
Interstellar communication. III. Optimal frequency to maximize data rate
Hippke, Forgan

The optimal frequency for interstellar communication, using "Earth 2017" technology, was derived in papers I and II of this series.  The framework included models for the loss of photons from diffraction (free space), interstellar extinction, and atmospheric transmission.  A major limit of current technology is the focusing of wavelengths lambda < 300 nm (UV).  When this technological constraint is dropped, a physical bound is found at lambda ~ 1 nm (E~keV) for distances out to kpc.  While shorter wavelengths may produce tighter beams and thus higher data rates, the physical limit comes from surface roughness of focusing devices at the atomic level.  This limit can be surpassed by beam-forming with EM fields, e.g. using a free electron laser, but such methods are not energetically competitive.  Current lasers are not yet cost efficient at nm wavelengths, with a gap of two orders of magnitude, but future technological progress may converge on the physical optimum.  Recommend expanding SETI efforts towards targeted (at us) monochromatic (or narrow band) X-ray emission at 0.5-2 keV energies.


1711.06029
Measuring the hydrostatic mass bias in galaxy clusters by combining Sunyaev-Zel'dovich and CMB lensing data
Hurier, Angulo

The cosmological parameters preferred by the CMB primary anisotropies predict many more galaxy clusters than those that have been detected via the tSZ effect.  This tension has attracted considerable attention since it could be evidence of physics beyond the simplest LCDM model.  However, an accurate and robust calibration of the mass-observable relation for clusters is necessary for the comparison, which has been proven difficult to obtain so far.  Here, present new constraints on the mass-pressure relation by combining tSZ and CMB lensing measurements about optically-selected clusters.  Consequently, the galaxy cluster sample is independent from the data employed to derive cosmological constraints.  Estimate an average hydrostatic mass bias of b=0.26±0.07, with no significant mass nor z evolution.  This value greatly reduces the tension between the predictions of LCDM and the observed abundance of tSZ clusters while being in agreement with recent estimations from tSZ clustering.  On the other hand, the value for b is higher than the predictions from hydro-dynamical simulations.  This suggests the existence of mechanisms driving large departures from hydrostatic equilibrium and that are not included in state-of-the-art simulations, and/or unaccounted systematic errors such as biases in the cluster catalogue due to the optical selection.