I spent my morning nerd-out working on Peebles's and my synthesis of galaxy evolution observations. I was reminded of my philosophical position on all this, with which I have no-doubt bored my legions of blog fans before:
The primary goal of observational astrophysics—as distinct from, say, pure astronomy—ought to be to rule out physical models. Note that all important physical experiments have the property that they ruled out—or could have ruled out—one of the fundamental, dominant theories of their day. In physical cosmology, we have a fundamental, dominant theory for the growth of structure (CDM). Our role as observers is not to bolster this model, or find ad-hoc parameters we can add to the model that make it consistent with the data. Our role is to perform experiments that have the power, even in the face of uncertainties (about how galaxies form, for example), to rule out or substantially modify the fundamental assumptions of this theory. If an experiment does not have the power to rule out the theory, then it can hardly be said to provide substantial support when the results end up in agreement! Thus my primary experimental design criterion is that my observations be capable of falsifying CDM, even after marginalization over unknowns.