Diego Chialva

Warped Reheating in Multi-Throat Brane Inflation

We investigate in some quantitative details the viability of reheating in multi-throat brane inflationary scenarios by estimating and comparing the time scales for the various processes involved. We also calculate within perturbative string theory the decay rate of excited closed strings into KK modes and compare with that of their decay into gravitons; we find that in the inflationary throat the former is preferred. We also find that over a small but reasonable range of parameters of the background geometry, these KK modes will preferably tunnel to another throat (possibly containing the Standard Model) instead of decaying to gravitons due largely to their suppressed coupling to the bulk gravitons. Once tunneled, the same suppressed coupling to the gravitons again allows them to reheat the Standard Model efficiently. We also consider the effects of adding more throats to the system and find that for extra throats with small warping, reheating still seems viable.

Signatures of very high energy physics in the squeezed limit of the bispectrum

We investigate the general signatures of modifications of the standard theory at high energy in the squeezing limit of the primordial scalar bispectrum. Our results are obtained through the analysis in field theory and not using the (approximated) templates for the bispectrum as done in previous works. In this way we can obtain the precise dependence on the probed scale and quantify the presence of enhancements of the non-Gaussianities, which are relevant, for example, for measurements of the halo bias. It is shown that the signatures are distinctive and the results are different from those obtained using the templates proposed in the literature, where existing, in particular because of the interplay between the scale of the observations and the scale of the high-energy physics when taking the large-scale limit. Several pieces of information regarding high energy physics could be obtained in case of detection of these signals, especially bounds on the scales of new physics.

Multiple dark matter scenarios from ubiquitous stringy throats

We discuss the possibility of having multiple Kaluza-Klein (KK) dark matter candidates which arise naturally in generic Type-IIB string theory compactification scenarios. These dark matter candidates reside in various throats of the Calabi-Yau manifold. In principle, they can come with varied range of masses in four-dimensions depending upon the hierarchical warping of the throats. We show that consistency with cosmological bounds and four-dimensional effective theory description imposes strong constraints on the parameter space and the geometry of the throats. With a rather model-independent approach, we find that the mass scales allowed for the KK dark matter particles in various throats can vary between 0.1 eV and 10 TeV, depending upon the throat geometry. Thus, there could be simultaneously more than one kind of cold (and possibly warm and hot) dark matter components residing in the Universe. This multiple dark matter scenario could weaken the bound on a conventional supersymmetric dark matter candidate and could also account for extra relativistic degrees of freedom in our Universe.

Cross sections for production of closed superstrings at high energy colliders in brane world models

In brane world string models with large extra dimensions, there are processes where fermion and antifermion (or two gluons) can annihilate producing a light particle (e.g. gluon) carrying transverse momentum and a Kaluza-Klein graviton or an excited closed string that propagates in the extra dimensions. In high energy colliders, this process gives a missing-momentum signature. We compute the total cross section for this process within the context of type II superstring theory in the presence of a D-brane. This includes all missing-energy sources for this string-theory model up to s=8M{sub s}{sup 2}, and it can be used to put new limits on the string scale M{sub s}.

Cosmological implications of quantum corrections and higher-derivative extension

We discuss the challenges for the early universe cosmology from quantum corrections, and in particular higher-derivative terms, in the gravitational and inflaton sectors of the models. The work is divided in two parts. In the first one we review the already well-known issues due to quantum corrections to the inflaton potential, in particular focusing on chaotic/slow-roll single-field models. We will point out some issues concerning the proposed mechanisms to cope with the corrections, and also argue how the presence of higher-derivative corrections could be problematic for those mechanisms. In the second part we will more directly focus on higher-derivative corrections. We will show how, in order to discuss a number of high-energy phenomena relevant to inflation (such as its actual onset) one has to deal with energy scales where the derivative expansion breaks down, presenting problems such as quantum vacuum instability and ghosts. To discuss such phenomena in the convenient framework of the effective theory, one must then abandon the derivative expansion and resort to the full nonlocal formulation of the theory, which is in fact equivalent to re-integrating back the relevant physics, but with the benefit of using a more compact single-field formalism. Finally, we will briefly discuss possible advantages offered by the presence of higher derivatives and a nonlocal theory to build better controlled UV models of inflation. Read More: http://www.worldscientific.com/doi/abs/10.1142/S0217732315400088Models of inflation with super-Planckian excursion seem well in agreement with the recent observations of Bmode polarization in the cosmic microwave background (CMB) radiation by the BICEP2 data. In this note, we highlight the challenges faced by such models from ultraviolet (UV) completion. In particular, we will discus radiative corrections to the inflaton Lagrangian and to the gravitational sector. We will emphasize why we would require an UV complete theory of gravity to tackle some of the issues for the super-Planckian excursion. In particular, we will highlight how higher derivative terms in the inflaton and gravity sectors cause problems from non-locality and ghosts, if considered order by order, and thus prompt us to take into account infinite series of such terms. We will also stress how the presence of a scale of new physics below the Planck scale would make some of the UV related problems more compelling and invalidate some of the remedies that have been proposed in the literature. Finally, we will briefly speculate on possible ways of curing some of the challenges.

Deforming, revolving and resolving?new paths in the string theory landscape

In this paper we investigate the properties of series of vacua in the string theory landscape. In particular, we study minima to the flux potential in type IIB compactifications on the mirror quint ...

Long lived large type-II strings: decay within compactification

Motivated also by recent revival of interest about metastable string states (as cosmic strings or in accelerator physics), we study the decay, in presence of dimensional compactification, of a particular superstring state, which was proven to be remarkably long-lived in the flat uncompactified scenario. We compute the decay rate by an exact numerical evaluation of the imaginary part of the one-loop propagator. For large radii of compactification, the result tends to the fully uncompactified one (lifetime ≡const. gs−2M5), as expected, the string mainly decaying by massless radiation. For small radii, the features of the decay (emitted states, initial mass dependence, ...) change, depending on how the string wraps on the compact dimensions.

Quantum effects in gravitational wave signals from cuspy superstrings

We study the gravitational emission, in Superstring Theory, from fundamental strings exhibiting cusps. The classical computation of the gravitational radiation signal from cuspy strings features strong bursts in the special null directions associated to the cusps. We perform a quantum computation of the gravitational radiation signal from a cuspy string, as measured in a gravitational wave detector using matched filtering and located in the special null direction associated to the cusp. We study the quantum statistics (expectation value and variance) of the measured filtered signal and find that it is very sharply peaked around the classical prediction. Ultimately, this result follows from the fact that the detector is a low-pass filter which is blind to the violent high-frequency quantum fluctuations of both the string worldsheet, and the incoming gravitational field.

High-energy physics and cosmological perturbations: observing new physics at large scales.

Correlators of primordial perturbations could provide us with the signatures of physics at earlier times/higher momentum scales than inflation. The key-mechanisms are the interference and cumulation in time related to the interplay of negative- and positive-frequency components of fields and energy density generated by the high-momentum scale physics. Here, we discuss which signatures are universal for such scenarios, and which ones instead would distinguish the specific cases (for example modified initial states for inflationary perturbations or modified dispersion relations). We also discuss the scale dependence of the correlators in presence of these signatures, especially for some scenarios, and how this could be interesting for observations.

NON-BPS BLACK HOLES AND SELF-INTERACTING FUNDAMENTAL STRINGS

The string-black hole correspondence principle can be investigated in the non-BPS scenario by studying the string configuration and entropy when the string coupling is slowly increased. Through a rigorous analysis, it is shown how an ensemble of string states at fixed mass and Neveu-Schwarz charges gets dominated in any dimension by compact states for which the one-loop corrections are important (possibly signaling the transition to a black hole regime/description) and with a size (spread) within the horizon radius of the expected correspondent black holes.