Investigating Dark Energy Equation of State With High Redshift Hubble Diagram

Abstract

Several independent cosmological data, collected within the last 20 years, revealed the accelerated expansion rate of the Universe, usually assumed to be driven by the so called dark energy, which, according to recent estimates, provides now about 70% of the total amount of matter-energy in the Universe. The nature of dark energy is yet unknown. Several models of dark energy have been proposed: a non-zero cosmological constant, a potential energy of some self interacting scalar field, effects related to the non-homogeneous distribution of matter, or effects due to alternative theories of gravity. Recently, it turned out that the standard flat CDM is disfavored (at 4 σ) when confronted with a high redshift Hubble diagram, consisting of supernovae of type Ia (SNIa), quasars (QSOs), and gamma ray-bursts (GRBs) (Lusso and Risaliti, 2016; Lusso et al., 2019; Risaliti and Lusso, 2019). Here we use the same data to investigate if this tension is confirmed, using a different approach: actually in Lusso and Risaliti (2016), Lusso et al. (2019), and Risaliti and Lusso (2019), the deviation between the best fit model and the CDM model was noticed by comparing cosmological parameters derived from cosmographic expansions of their theoretical predictions and observed high redshift Hubble diagram. In this paper we use a substantially different approach, based on a specific parametrization of the redshift dependent equation of state (EOS) of dark energy component w(z). Our statistical analysis is aimed to estimate the parameters characterizing the dark energy EOS: our results indicate (at > 3σ level) an evolving dark energy EOS, while the cosmological constant has a constant EOS, w = −1. This result not only confirms the tension previously detected but shows that it is not an artifact of cosmographic expansions.

A roadmap to gamma-ray bursts: new developments and applications to cosmology

“…• F X and F 0 are the observed X-ray fluxes respective to t X and τ , whereas the corresponding rest-frame 0.3-10 keV luminosities L X and L 0 are computes as follows 13 • V , the variability of the GRB light curve. It is computed by taking the difference between the observed light curve and its smoothed version, squaring this difference, summing these squared differences over time intervals, and appropriately normalizing the resulting sum.…”

“…Although essential to better understand 12 The time lag is historically computed in these energy bands which are the BATSE energy channels 3 and 1, respectively. 13 As a convention, the X-ray luminosities are computed in a rest-frame energy band with similar extrema with respect to the observed one; with this prescription their expression are simple, as portrayed in Eq. (11).…”

“…Hence, it is hard to relate luminosity to GRB distances as their microphysics is not well understood. Although the above caveats plague the overall GRB scenario, both short and long GRBs have relativistic outflows and share analogous properties 3 and many attempts have been spent to standardize GRBs for both clarifying their nature, internal structure and origin together with employing these objects for cosmological purposes [12,13].…”

Thermodynamic parametrization of dark energy

“…Thus, characterising the Universe evolution without postulating a cosmological model a priori [8,9] might shed light on the dark sector properties whose dynamics could be addressed also by improving the geometry (see e.g. [10][11][12][13]). In addition, requiring thermodynamics to hold throughout the Universe evolution is dutiful for any cosmological model that aims at extending the ΛCDM scenario [14].…”

A Roadmap to Gamma-Ray Bursts: New Developments and Applications to Cosmology

“…Hence, it is hard to relate luminosity to GRB distances as their microphysics is not well understood. Although the above caveats plague the overall GRB scenario, both short and long GRBs have relativistic outflows and share analogous properties 3 and many attempts have been spent to standardize GRBs for both clarifying their nature, internal structure, and origin together with employing these objects for cosmological purposes [12,13].…”