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

For decades, H0 and q0 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 q0 cannot be directly measured using distance indicators with both accuracy (without offset away from its true value) and precision (small error bar). While H0 can be measured with accuracy and precision, to avoid a small bias in its direct measurement (of order 5%) we demonstrate that the pair H0 and ΩM (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 q0 = −0.57 ± −0.04. Finally, we show that it may be possible to directly determine q0 with both accuracy and precision using the time dependence of redshifts (“redshift drift”).