Beyond H_0 and q_0: Cosmology is no longer just two numbers
For decades, H_0 and q_0 were the quest of cosmology, as they promised to characterize our "world model" in a model-independent way. Using simulated data, we show that q_0 cannot be both accurately and precisely determined using distance indicators. While H_0 can be both accurately and precisely determined, to avoid a small bias in its direct measurements (of order -5 %) we demonstrate that H_0/Omega_M (assuming flatness and w=-1) is a better choice of two parameters, even if our world model is not precisely Lambda CDM. We illustrate with the analysis of the Constitution set of supernovae and indirectly infer q_0 = -0.57 +/- 0.04. Finally, we show that it may be possible to directly determine q_0 using the time dependence of redshifts, a method far less susceptible to the biases that plague measurements using distance indicators.