Day 1474

Wednesday.

1810.00269
The unphysical character of dark energy fluids
Duarte, et al

In the context of general relativity, an unknown kind of matter must violate the strong energy condition is required to explain the current accelerated phase of expansion of the Universe.  This unknown component is called dark energy and is characterized by an equation of state parameter w=p/rho<-1/3.  Thermodynamic stability requires that 3w-d ln(|w|)/d ln(a) >= 0 and positiveness of entropy that w>=-1.  In this paper, prove that one cannot obtain a differentiable function w(a) to represent the dark energy that satisfies these conditions through the entire history of the Universe.


1810.00885
Broadband intensity tomography: spectral tagging the cosmic UV background
Chiang, Ménard, Schiminovich

Cosmic photons can be efficiently collected by broadband intensity mapping but information on their emission redshift and frequency is largely lost.  Introduce a technique to statistically recover these otherwise collapsed dimensions by exploiting information in spatial fluctuations and apply it to the GALEX All Sky and Medium Imaging Surveys.  By spatially cross-correlating photons in the GALEX far-UV (1500 A) and near-UV (2300 A) bands with a million spectroscopic objects in SDSS as a function of redshift, robustly detect the z-dependent intensity of the UV background (UVB) modulated by its clustering bias up to z~2.  These measurements clearly reveal the imprints of UVB spectral features redshirting through the filters.  Using a simple parameterization, simultaneously fit a UVB emissivity and clustering bias factor to these observations and constrain the main spectral features of the UV background spectrum: (i) the Lyman break, (ii) the non-ionizing UV continuum, which agrees with the Haardt & Madau model but does not rely on any assumption regarding the nature of the sources, and (iii) the Ly-alpha emission, whose luminosity density is consistent with estimates of the combined galaxy and AGN contributions at z~1.  Since the technique probes the total background including low surface brightness emission, place constraints on the amount of UV light originating from the diffuse intergalactic medium (IGM).  Finally, the clustering bias of UV photons is found to be chromatic and evolving.  The frequency- and z-dependent UVB measurement delivers a summary statistic of the universe's net radiation output from stars, black holes and the IGM combined.


1810.01287
Testing the speed of light over cosmological distances: the combination of strongly lensed and unlensed supernova Ia
Cao et al

Probing the speed of light is an important test of GR but the measurements of c using objects in the distant universe have been almost completely unexplored.  In this letter, propose an idea to use the multiple measurements of galactic-scale strong GL systems with type Ia SN acting as background sources to estimate the speed of light.  This provides an original method to measure the speed of light using objects located at different redshifts which emitted their light in a distant past.  Moreover, predict that strongly lensed SNe Ia observed by the LSST would produce robust constraints on Delta c/c at the level of 1e-3.  Also discuss whether the future surveys such as LSST may succeed in detecting any hypothetical variation of c predicted by theories in which fundamental constants have dynamical nature.