Higher-spin

There are important observational indications that our universe underwent a period of accelerated expansion – the primordial inflationary era – at an early stage. During this period, the universe is described by an approximately de Sitter spacetime, with the Hubble scale H that could be as high as 10^{14} GeV. From the high energy point of view, inflation is therefore an appealing playground as it may happen at energies much larger than any energy we can expect to achieve with future particle accelerators, and thus it provides the most powerful collider to test high energy physics. It is therefore interesting to understand how to extract information about the laws of physics and particle contents at those scales. One of the most appealing possibilities is the presence of spinning particles.

Higher-spin theories are field theories containing an infinite tower of interacting gauge fields with increasing spin. In Minkowski space, one can devise no-go theorems which render the S-matrix of a theory with higher-spin conserved charges essentially trivial. These no-go theorems can be avoided if the spacetime is no longer asymptotically flat, like a de Sitter spacetime. Indeed, non-linear classical equations of motion for interacting massless higher-spin fields in a curved background were obtained by Vasiliev, whose theory admits asymptotically de Sitter configurations, that may be of cosmological interest. Consequently, we are prompted to ask whether the inflationary era could be of the higher-spin type, and if so, what would be the observational imprints for such a scenario.

Some other possibilities are provided by massive higher-spin particles (with masses comparable to the Hubble scale) coupled to the inflaton or partially massless fields. In fact, one line of research consists in studying the distinctive signature of these particles on the primordial cosmological correlators of the comoving curvature perturbation, that may be characterised by the presence of non-gaussianity originated from the exchange of these higher-spin fields.