BEYOND H[subscript 0] AND q[subscript 0]: COSMOLOGY IS NO LONGER JUST TWO NUMBERS

Abstract

For decades, H [subscript 0] and q [subscript 0] were the quest of cosmology, as they promised to characterize our "world model" without reference to a specific cosmological framework. Using Monte Carlo simulations, we show that q [subscript 0] cannot be directly measured using distance indicators with both accuracy (without offset away from its true value) and precision (small error bar). While H [subscript 0] can be measured with accuracy and precision, to avoid a small bias in its direct measurement (of order 5%) we demonstrate that the pair H [subscript 0] and Ω [subscript M] (assuming flatness and w = –1) is a better choice of two parameters, even if our world model is not precisely ΛCDM. We illustrate this with analysis of the Constitution set of supernovae and indirectly infer q [subscript 0] = –0.57 ± –0.04. Finally, we show that it may be possible to directly determine q [subscript 0] with both accuracy and precision using the time dependence of redshifts ("redshift drift").