Federico Piazza

Rapidly-varying speed of sound, scale invariance and non-Gaussian signatures

We show that curvature perturbations acquire a scale invariant spectrum for any constant equation of state, provided the fluid has a suitably time-dependent sound speed. In order for modes to exit the physical horizon, and in order to solve the usual problems of standard big bang cosmology, we argue that the only allowed possibilities are inflationary (albeit not necessarily slow-roll) expansion or ekpyrotic contraction. Non-Gaussianities offer many distinguish features. As usual with a small sound speed, non-Gaussianity can be relatively large, around current sensitivity levels. For DBI-like lagrangians, the amplitude is negative in the inflationary branch, and can be either negative or positive in the ekpyrotic branch. Unlike the power spectrum, the three-point amplitude displays a large tilt that, in the expanding case, peaks on smallest scales. While the shape is predominantly of the equilateral type in the inflationary branch, as in DBI inflation, it is of the local form in the ekpyrotic branch. The tensor spectrum is also generically far from scale invariant. In the contracting case, for instance, tensors are strongly blue tilted, resulting in an unmeasurably small gravity wave amplitude on cosmic microwave background scales.

Sub-eV scalar dark matter through the super-renormalizable Higgs portal

The Higgs portal of the standard model provides the opportunity for coupling to a very light scalar field

Glimmers of a Pre-geometric Perspective

\ensuremath{\phi}

Supernova Legacy Survey data are consistent with acceleration at z ≈ 3

via the super-renormalizable operator

Renormalized thermal entropy in field theory

\ensuremath{\phi}({H}^{\ifmmode\dagger\else\textdagger\fi{}}H)

MODIFYING GRAVITY IN THE INFRARED BY IMPOSING AN "ULTRASTRONG" EQUIVALENCE PRINCIPLE

. This allows for the existence of a very light scalar dark matter that has coherent interaction with the standard model particles and yet has its mass protected against radiative corrections. We analyze ensuing constraints from the fifth force measurements, along with the cosmological requirements. We find that the detectable level of the fifth force can be achieved in models with low inflationary scales, and by a certain amount of fine-tuning in the initial deviation of

Constraints on modified gravity from Planck 2015: when the health of your theory makes the difference

\ensuremath{\phi}

The universe is accelerating. Do we need a new mass scale

from its minimum.

Limits on the Anomalous Speed of Gravitational Waves From Binary Pulsars

Spacetime measurements and gravitational experiments are made by using objects, matter fields or particles and their mutual relationships. As a consequence, any operationally meaningful assertion about spacetime is in fact an assertion about the degrees of freedom of the matter (i.e. non gravitational) fields; those, say for definiteness, of the Standard Model of particle physics. As for any quantum theory, the dynamics of the matter fields can be described in terms of a unitary evolution of a state vector in a Hilbert space. By writing the Hilbert space as a generic tensor product of “subsystems” we analyse the evolution of a state vector on an information theoretical basis and attempt to recover the usual spacetime relations from the information exchanges between these subsystems. We consider generic interacting second quantized models with a finite number of fermionic degrees of freedom and characterize on physical grounds the tensor product structure associated with the class of “localized systems” and therefore with “position”. We find that in the case of free theories no spacetime relation is operationally definable. On the contrary, by applying the same procedure to the simple interacting model of a one-dimensional Heisenberg spin chain we recover the tensor product structure usually associated with “position”. Finally, we discuss the possible role of gravity in this framework.

Bimetric structure formation: Non-Gaussian predictions

We point out that the Type Ia supernovae in the SNLS dataset are consistent with an early beginning of the cosmic acceleration if dark energy interacts strongly with dark matter. We find that the acceleration could have started at redshift z =3 and higher.