Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

Cosmological data has opened up new vistas on fundamental physics yet it is limited in its scope. While it has given us tantalizing hints at how the Universe might be expanding, it is unclear whether it can ever be used to find the microphysical structure of whatever is driving this expansion. I will discuss the evidence for one proposal, dynamical dark energy driven by a scalar field, pointing out what we can and cannot say about its fundamental nature. I will argue that it is unlikely to be thawing quintessence, that there is strong evidence that there is some type of non-minimal coupling but which brings with it a host of undesirable consequences. I will try to look at the various loopholes and present what I think is the current status of scalar fields as a source of dark energy.