Tomi Sebastian Koivisto

The parallel theory of relativity predicts conserved energy-momentum currents for an arbitrary metric, without invoking Killing symmetries. By treating the reference frame as an independent variational field and requiring it to carry no energy, the theory naturally unifies Einstein's two formulations of gravity and yields uniquely defined covariant charges. In isotropic and homogeneous cosmology, the canonical time direction selected by the reference frame coincides with the Kodama vector, and the associated Noether energy reproduces the Misner-Sharp mass.

A bstract: Theories formulated in the arena of teleparallel geometries are generically plagued by ghost-like instabilities or other pathologies that are ultimately caused by the breaking of some symmetries. In this work, we construct a class of ghost-free theories based on a symmetry under Transverse Diffeomorphisms that is naturally realised in symmetric teleparallelism. We explicitly show their equivalence to a family of theories with an extra scalar field plus a global degree of freedom and how Horndeski theories and healthy couplings to matter fields can be readily accommodated.

Motivated by cosmological applications for interacting matters, an extension of the action functional for relativistic fluids is proposed to incorporate the physics of non-adiabatic processes and chemical reactions. The former are characterised by entropy growth, while the latter violate particle number conservation. The relevance of these physics is demonstrated in the contexts of self-interacting fluids, fluids interacting with scalar fields, and hyperhydrodynamical interactions with geometry. The possible cosmological applications range from early-universe phase transitions to astrophysical phenomena, and from matter creation inflationary alternatives to interacting dark sector alternatives to the ΛCDM model that aim to address its tensions. As an example of the latter, a single fluid model of a unified dark sector is presented. The simple action of the model features one field and one parameter, yet it can both reproduce the ΛCDM cosmology and predict new phenomenology.

We explore a new action formulation of hyperfluids, fluids with intrinsic hypermomentum. Brown's Lagrangian for a relativistic perfect fluid is generalised by incorporating the degrees of freedom encoded in the hypermomentum tensor, namely by including connection-matter couplings. Quite interestingly, generic hyperfluids are imperfect, since hypermomentum induces such effects as bulk and shear viscosities as well as heat fluxes. The various coefficients that appear in the first order expansion of hydrodynamics can now be deduced from a Lagrangian formulation, given a geometrical interpretation and a suggested microscopic description in terms of hypermomentum. This connection between hypermomentum and dissipative fluids could shed new light on the physics of relativistic hydrodynamics. The applicability of the new formalism is demonstrated by exact cosmological solutions.

We explore a new action formulation of hyperfluids, fluids with intrinsic hypermomentum. Brown's Lagrangian for a relativistic perfect fluid is generalised by incorporating the degrees of freedom encoded in the hypermomentum tensor, namely by including connection-matter couplings. Quite interestingly, generic hyperfluids are imperfect, since hypermomentum induces such effects as bulk and shear viscosities as well as heat fluxes. The various coefficients that appear in the first order expansion of hydrodynamics can now be deduced from a Lagrangian formulation, given a geometrical interpretation and a suggested microscopic description in terms of hypermomentum. This connection between hypermomentum and dissipative fluids could shed new light on the physics of relativistic hydrodynamics. The applicability of the new formalism is demonstrated by exact cosmological solutions.