Beyond H_0 and q_0: Cosmology is no longer just two numbers

摘要

For decades, H_0 and q_0 were the quest of cosmology, as they promised tocharacterize our "world model" without reference to a specific cosmologicalframework. Using Monte Carlo simulations, we show that q_0 cannot be directlymeasured using distance indicators with both accuracy (without offset away fromits true value) and precision (small error bar). While H_0 can be measured withaccuracy and precision, to avoid a small bias in its direct measurement (oforder 5%) we demonstrate that the pair H_0 and Omega_M (assuming flatness and w= -1) is a better choice of two parameters, even if our world model is notprecisely LambdaCDM. We illustrate with analysis of the Constitution set ofsupernovae and indirectly infer q_0 = -0.57+/-0.04. Finally, we show that itmay be possible to directly determine q_0 with both accuracy and precisionusing the time dependence of redshifts ("redshift drift").