Stefano Foffa

Cosmological dynamics and dark energy from nonlocal infrared modifications of gravity

We study the cosmological dynamics of a recently proposed infrared modification of the Einstein equations, based on the introduction of a nonlocal term constructed with m2gμν□-1R, where m is a mass parameter. The theory generates automatically a dynamical dark energy component, that can reproduce the observed value of the dark energy density without introducing a cosmological constant. Fixing m so to reproduce the observed value ΩDE≃0.68, and writing wDE(a) = w0+(1-a)wa, the model provides a neat prediction for the equation of state parameters of dark energy, w0≃-1.042 and wa≃-0.020, and more generally provides a pure prediction for wDE as a function of redshift. We show that, because of some freedom in the definition of □-1, one can extend the construction so to define a more general family of nonlocal models. However, in a first approximation this turns out to be equivalent to adding an explicit cosmological constant term on top of the dynamical dark energy component. This leads to an extended model with two parameters, ΩΛ and m. Even in this extension the EOS parameter w0 is always on the phantom side, in the range -1.33 ≲w0≤-1, and there is a prediction for the relation between w0 and wa.

Apparent ghosts and spurious degrees of freedom in non-local theories

Recent work has shown that non-local modifications of the Einstein equations can have interesting cosmological consequences and can provide a dynamical origin for dark energy, consistent with existing data. At first sight these theories are plagued by ghosts. We show that these apparent ghost-like instabilities do not describe actual propagating degrees of freedom, and there is no issue of ghost-induced quantum vacuum decay.

Dynamics of the gravitational two-body problem at fourth post-Newtonian order and at quadratic order in the Newton constant

We derive the conservative part of the Lagrangian and the energy of a gravitationally bound two-body system at fourth post-Newtonian order, up to terms quadratic in the Newton constant. We also show that such terms are compatible with Lorentz invariance and we write an ansatz for the center-of-mass position. The remaining terms carrying higher powers of the Newton constant are currently under investigation.

Loop corrections and graceful exit in string cosmology

Abstract We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a Z N orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e., the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we center a D-brane dominated phase.

CFT, holography, and causal entropy bound

The causal entropy bound (CEB) is confronted with recent explicit entropy calculations in weakly and strongly coupled conformal field theories (CFTs) in arbitrary dimension

The shortest scale of quantum field theory

D

Non-local gravity and comparison with observational datasets. II. Updated results and Bayesian model comparison with ΛCDM

. For CFTs with a large number of fields,

Nonlocal gravity with a Weyl-square term

N

Conformal symmetry and nonlinear extensions of nonlocal gravity

, the CEB is found to be valid for temperatures not exceeding a value of order

Sensitivity of a small matter-wave interferometer to gravitational waves

M_P/N^{{1over D-2}}