ABSTRACT
The presence of an irrelevant field in a quantum field theory is usually not good news, as far as understanding the high-energy physics of the model is concerned. In two space-time dimensions, the T-Tbar deformation is solvable. We can describe physical observables of interest, such as the S-matrix and the finite-volume spectrum, in terms of the corresponding undeformed quantities. For this irrelevant perturbation, we can reverse the renormalization group trajectory and gain exact information about ultraviolet physics. The outcome is stunning: low-energy physics resembles that of a conventional local quantum field theory while at high-energy the density of states on a cylinder shows Hagedorn growth similar to that of a string theory.

BIO
Marc Kamionkowski is a theoretical physicist. His research is in cosmology, astrophysics, and elementary-particle theory. His main focus has been on particle dark matter, inflation and the cosmic microwave background, and cosmic acceleration. He also worked on neutrino and nuclear physics and astrophysics, large-scale-structure and galaxy formation, intrinsic galaxy alignments and gravitational lensing, gravitational waves, phase transitions in the early Universe, alternative-gravity theories, the first stars and the epoch of reionization, and a bit in stellar and high-energy astrophysics.

ABSTRACT
We’ve known since the late 1920s that the Universe is expanding. However, the expansion rate currently inferred from measurements of the cosmic microwave background now disagrees with that obtained from supernova measurements. Over the past few years, theorists have been exploring the possibility that this Hubble tension is explained by some new “early dark energy”: a new component of matter that may have been dynamically important several hundred thousand years after the Big Bang.