Abstract
For as long as astrophysicists have considered the large-scale structure of the cosmos, discoveries on the subject have been taken to provide critical empirical insights relevant to theorizing in fundamental physics. This close connection between the two subjects is most familiar in the context of the relativistic hot 'Big Bang' model of the expanding universe that first rose to prominence in the 1930s, and which later developed into the standard Lambda-CDM model familiar today. In that hot 'Big Bang' model, the developmental origins of large-scale cosmic structure present today in our observable universe are understood in terms of high-energy fundamental physical processes far beyond our reach. And in Lambda-CDM, this role for fundamental physics beyond our reach is supplemented by additional roles, via theorizing about a dark sector to be incorporated into future physics. But through history, competitor accounts of the large-scale structure of the cosmos have likewise included empirical upshots that were to be taken as clues about new high-energy fundamental physics: witness the particle basis for the 'C-field' in Hoyle’s approach to Steady State cosmology in the 1950s, and even the earlier call in the 1930s to revise our fundamental understanding of classical gravitational dynamics within the context of Milne’s 'kinematic' model of the receding nebulae. In light of this consistent close connection between the empirical claims of large-scale cosmology and ongoing theorizing in fundamental physics, it is unsurprising that researchers interested in quantum gravity have often tried to mine the Lambda-CDM model for empirical clues that pertain specifically to their own domain of theorizing: looking for explicit cosmic imprints of quantum gravity. In this talk, after arguing in brief for the historical point above, I will then carefully pull apart (and critically assess) what I take to be two distinct recent programs of empirical QG research that fit the historical pattern. The first program treats the dynamics of Lambda-CDM as an autonomous structural theory of our present-day observable universe at the largest accessible scales (in terms of a nearby portion of the large-scale cosmos), to explore how detailed processes in quantum gravity might lead to 'trans-Planckian' physics in that same target system, which almost mimics familiar expectations about the large-scale cosmos derived from known fundamental physics. The second program treats Lambda-CDM, the model, as a low-energy effective description of a future model of quantum cosmology, so as to consider whether a sustained commitment to the descriptive accuracy of the former may ultimately be constraining on the topic of how to quantize gravity (i.e. in order to eventually construct the latter). I will conclude by noting a curious feature of the new 'trans-Planckian censorship' conjecture that has been advertised as a general principle governing possible low-energy physical descriptions in a universe where gravity is quantized: that the two empirical programs just pulled apart would seem, perhaps, to collapse into one --- albeit contingent on very particular speculations about what may come of our future understanding of the quantum nature of gravity within our present-day observable universe.
