We make a case for setting initial conditions for inflation at the Planck epoch in the kinetically dominated regime. For inflationary potentials with a plateau or a hill, i.e., potentials that are ...bounded from above within a certain region of interest, we cannot claim complete ignorance of the energy distribution between kinetic and potential energy, and equipartition of energy at the Planck epoch becomes questionable. We analyze different classes of potentials in phase space and quantify the fraction of the Planck surface that is kinetically dominated. Considering bounded potentials with very small amplitudes as favored by current data and restricting ourselves to the domains of phase space that are of interest to cosmic inflation, we find that initial conditions of the inflaton field should be set in the kinetically dominated regime regardless of any choice of prior.
We present a novel approach for setting initial conditions on the mode functions of the Mukhanov-Sasaki equation. These conditions are motivated by minimization of the renormalized stress-energy ...tensor and are valid for setting a vacuum state even in a context where the spacetime is changing rapidly. Moreover, these alternative conditions are potentially observationally distinguishable. We apply this to the kinetically dominated universe and compare with the more traditional approach.
We propose a one-parameter extension to Λ CDM, expected to strongly affect cosmological tensions. An effective dark radiation component in the early Universe redshifts away as hot dark matter, then ...quintessence, tracking the dominant equation-of-state parameter and leaving a falsifiable torsion field in the current epoch. This picture results from a new Poincaré gauge theory (PGT), one of the most promising among the latest batch of 58 PGTs found to be both power-counting renormalizable and free from ghosts and tachyons. We systematically categorize the cosmologies of 33 of these PGTs, as special cases of the most general parity-preserving, Ostrogradsky-stable PGT with a purely Yang-Mills action. The theory we consider contains two propagating massless gravitons, which may be JP = 2+ (long-range gravitation and gravitational waves). A conspiracy among the coupling constants eliminates the spatial curvature k ∈ {±1, 0} from the field equations. We show that this " k-screening" is not restricted to conformal gravity theories. The flat Friedmann equations are then emergent, with potentially tension-resolving freedom at the early scale-invariant epoch that reliably gives way to an attractorlike state of modern Λ CDM evolution. We compare with related theories and promising special cases, such as k -screened theories with negative-definite effective k , and more traditional theories with effective Λ and a JP = 0− massive graviton (dark matter candidate). As a bonus, we analyze similarly constrained actions in the new extended Weyl gauge theory (eWGT). We show that in cosmology, PGT and eWGT span exactly the same classical phenomenology up to a linear map between their coupling constants, hinting at a deeper relationship between the two.
The ten-parameter, quadratic Poincaré gauge theory of gravity is a plausible alternative to general relativity. We show that the rich background cosmology of the gauge theory is described by a ...noncanonical biscalar-tensor theory in the Jordan frame: the metrical analogue. This provides a unified framework for future investigation by the broader community. For many parameter choices, the noncanonical term reduces to a Cuscuton field of the form √|Xϕϕ|. The Einstein–Cartan–Kibble–Sciama theory maps to a pure quadratic cuscuton, whereas the teleparallel theory maps to the Einstein–Hilbert Lagrangian. We apply the metrical analogue to novel unitary and power-counting-renormalizable cases of Poincaré gauge theory. These theories support the concordance Λ CDM background cosmology up to an optional, effective dark radiation component, we explain this behavior in terms of a stalled cuscuton. We also obtain two dark energy solutions from one of these cases: accelerated expansion from a negative bare cosmological constant whose magnitude is screened, and emergent dark energy to replace vanishing bare cosmological constant in Λ CDM.
It was recently found that, when linearized in the absence of matter, 58 cases of the general gravitational theory with quadratic curvature and torsion are (i) free from ghosts and tachyons and (ii) ...power-counting renormalizable. We inspect the nonlinear Hamiltonian structure of the eight cases whose primary constraints do not depend on the curvature tensor. We confirm the particle spectra and unitarity of all these theories in the linear regime. We uncover qualitative dynamical changes in the nonlinear regimes of all eight cases, suggesting at least a broken gauge symmetry, and possibly the activation of negative kinetic energy spin-parity sectors and acausal behavior. Two of the cases propagate a pair of massless modes at the linear level, and were interesting as candidate theories of gravity. However, we identify these modes with vector excitations, rather than the tensor polarizations of the graviton. Moreover, we show that these theories do not support a viable cosmological background.
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