Two new diagnostics of dark energy

TL;DR

This paper introduces two new diagnostics, Om and q-probe, to test dark energy models and determine the onset of cosmic acceleration using observational data.

Contribution

The paper presents novel diagnostics for dark energy that do not require precise matter density and can distinguish between different dark energy models.

Findings

Om(z) remains constant for a cosmological constant

Slope of Om(z) indicates Phantom or Quintessence

q-probe identifies the redshift when acceleration began

Abstract

We introduce two new diagnostics of dark energy (DE). The first, Om, is a combination of the Hubble parameter and the cosmological redshift and provides a "null test" of dark energy being a cosmological constant. Namely, if the value of Om(z) is the same at different redshifts, then DE is exactly cosmological constant. The slope of Om(z) can differentiate between different models of dark energy even if the value of the matter density is not accurately known. For DE with an unevolving equation of state, a positive slope of Om(z) is suggestive of Phantom (w < -1) while a negative slope indicates Quintessence (w > -1). The second diagnostic, "acceleration probe"(q-probe), is the mean value of the deceleration parameter over a small redshift range. It can be used to determine the cosmological redshift at which the universe began to accelerate, again without reference to the current value of the matter density. We apply the "Om" and "q-probe" diagnostics to the Union data set of type Ia supernovae combined with recent data from the cosmic microwave background (WMAP5) and baryon acoustic oscillations.