Day 1416

Wednesday.

1805.08252
Tale of stable interacting dark energy, observational signatures, and the $H_0$ tension
Yang, et al

Investigate the observational consequences of a novel class of stable interacting dark energy (IDE) models, featuring interactions between DM and DE.  In the first part of the work, start by considering two IDE models which are known to present early-time linear perturbation instabilities.  Applying a transformation depending on the DE EoS to the DM-DE coupling, then obtain two novel stable IDE models.  Subsequently, derive robust and accurate constraints on the parameters of these models, assuming a constant EoS $w_x$ for the DE fluid, in light of some of the most recent publicly available cosmological data.  These include CMB temperature and polarization anisotropy measurements from the Planck satellite, a selection of BAO measurements, SNIa luminosity distance measurements from the JLA sample, and measurements of the Hubble parameter up to redshift 2 from cosmic chronometers.  The analysis displays a mild preference for the DE fluid residing in the phantom region (w_x<-1), with significance up to 95% confidence level, while obtaining new upper limits on the coupling parameter between the dark components.  The preference for a phantom DE suggests a coupling function Q<0, thus a scenario where energy flows from the DM to the DE.  Also examine the possibility of addressing the H_0 and sigma_8 tensions, finding that only the former can be partially alleviated.  Finally, perform a Bayesian model comparison analysis to quantify the possible preference for the two IDE models against the standard concordance LCDM model, finding that the latter is always preferred with the strength of the evidence ranging from positive to very strong.


1805.08682
The peculiar shapes of Saturn's small inner moons as evidence of mergers of similar-sized moonlets
Leleu, Jutzi, Rubin

The Cassini spacecraft revealed the spectacular, highly irregular shapes of the small inner moons of Saturn, ranging from the unique "ravioli-like" forms of Pan and Atlas to the highly elongated structure of Prometheus.  Closest to Saturn, these bodies provide important clues regarding the formation process of small moons in close orbits around their host planet, but their range of irregular shapes has not been explained yet.  Here, show that the spectrum of shapes among Saturn's small moons is a natural outcome of merging collisions among similar-sized moonless possessing physical properties and orbits that are consistent with those of the current moons.  A significant fraction of such merging collisions take place either at the first encounter or after 1-2 hit-and-run events, with impact velocities in the range of 1-5 times the mutual escape velocity.  Close to head-on mergers result in flattened objects with large equatorial ridges, as observed on Atlas and Pan.  With slightly more oblique impact angles, collisions lead to elongated, Prometheus-like shapes.  These results suggest that the current forms of the small moons provide direct evidence of the processes at the final stages of their formation, involving pairwise encounters of moonlets of comparable size.  Finally, show that this mechanism may also explain the formation of Iapetus' equatorial ridge, as well as its oblate shape.