Some recent papers...

https://arxiv.org/abs/1906.02764

Enhancement of the Axion Decay Constant in Inflation and the Weak Gravity Conjecture

Pran Nath, Maksim Piskunov

Models of axion inflation based on a single cosine potential require the axion decay constant f to be super-Planckian in size. However, f>MPlis disfavored by the Weak Gravity Conjecture (WGC). It is then pertinent to ask if one can construct axion inflation models in conformity with WGC. In this work we assume that WGC holds for the microscopic Lagrangian so that f<MPl. However, inflation is controlled by an effective Lagrangian much below the Planck scale where the inflaton is an effective axionic field associated with an effective decay constant fe which could be very different from f. In this work we propose a Coherent Enhancement Mechanism (CEM) for slow roll inflation controlled by flat potentials which can produce fe≫MPl while f<MPl. In the analysis we consider a landscape of chiral fields charged under a U(1) global shift symmetry and consider breaking of the U(1) symmetry by instanton type symmetry breaking terms. In the broken phase there is one light pseudo-Nambu-Goldstone-Boson (pNGB) which acts as the inflaton. We show that with an appropriate choice of symmetry breaking terms the inflaton potential is a superposition of many cosines and the condition that they produce a flat potential allows one to enhance fe so that fe/MPl≫1. We discuss the utility of this mechanism for a variety of inflaton models originating in supersymmetry and supergravity. The Coherent Enhancement Mechanism allows one to reduce an inflation model with an arbitrary potential to an effective model of natural inflation, i.e. with a single cosine, by expanding the potential near a field point where horizon exit occurs, and matching the expansion coefficients to those of natural inflation.

https://arxiv.org/abs/1906.00986

Direct anthropic bound on the weak scale from supernovae explosions

Guido D'Amico, Alessandro Strumia, Alfredo Urbano, Wei Xue

(Submitted on 3 Jun 2019)

Core-collapse supernovae presumably explode because trapped neutrinos push the material out of the stellar envelope. This process is directly controlled by the weak scale v: we argue that supernova explosions happen only if fundamental constants are tuned within a factor of few as v∼Λ3/4QCDM1/4Pl, such that neutrinos are trapped in supernovae for a time comparable to the gravitational time-scale. We provide analytic arguments and simulations in spherical approximation, that need to be validated by more comprehensive simulations. The above result can be important for fundamental physics, because core-collapse supernova explosions seem anthropically needed, as they spread intermediate-mass nuclei presumably necessary for `life'. We also study stellar burning, finding that it does not provide anthropic boundaries on v.

_________________________________________________________________________________

https://arxiv.org/abs/1905.12421

Hunting for extra dimensions in the shadow of M87*

Sunny Vagnozzi, Luca Visinelli

(Submitted on 29 May 2019)

The Event Horizon Telescope has recently provided the first image of the dark shadow around the supermassive black hole M87*. The observation of a highly circular shadow provides strong limits on deviations of M87*'s quadrupole moment from the Kerr value. We show that the absence of such a deviation can be used to constrain the physics of extra dimensions of spacetime. Focusing on the Randall-Sundrum AdS5brane-world scenario, we show that the observation of M87*'s dark shadow sets the limit ℓ≲170AU, where ℓ is the AdS5 curvature radius. This limit is among the first quantitative constraints on exotic physics obtained from the extraordinary first ever image of the dark shadow of a black hole.

_________________________________________________________________________________

https://arxiv.org/abs/1905.10614

Gravitational waves and extra dimensions: a short review

Hao Yu, Zi-Chao Lin, Yu-Xiao Liu

(Submitted on 25 May 2019)

We give a brief review on the recent development of gravitational waves in extra-dimensional theories of gravity. Studying extra-dimensional theories with gravitational waves provides a new way to constrain extra dimensions. After a flash look at the history of gravitational waves and a brief introduction to several major extra-dimensional theories, we focus on the sources and spectra of gravitational waves in extra-dimensional theories. It is shown that one can impose limits on the size of extra dimensions and the curvature of the universe by researching the propagations of gravitational waves and the corresponding electromagnetic waves. Since gravitational waves can propagate throughout the bulk, how the amplitude of gravitational waves decreases determines the number of extra dimensions for some models. In addition, we also briefly present some other characteristics of gravitational waves in extra-dimensional theories.

_________________________________________________________________________________
https://arxiv.org/abs/1903.11820

Small field models of inflation that predict a tensor-to-scalar ratio r=0.03

Ira Wolfson, Ram Brustein

(Submitted on 28 Mar 2019)

Future observations of the cosmic microwave background (CMB) polarization are expected to set an improved upper bound on the tensor-to-scalar ratio of r≲0.03. Recently, we showed that small field models of inflation can produce a significant primordial gravitational wave signal. We constructed viable small field models that predict a value of r as high as 0.01. Models that predict higher values of r are more tightly constrained and lead to larger field excursions. This leads to an increase in tuning of the potential parameters and requires higher levels of error control in the numerical analysis. Here, we present viable small field models which predict r=0.03. We further find the most likely candidate among these models which fit the most recent Planck data while predicting r=0.03. We thus demonstrate that this class of small field models is an alternative to the class of large field models. The BICEP3 experiment and the Euclid and SPHEREx missions are expected to provide experimental evidence to support or refute our predictions.

_________________________________________________________________________________

https://arxiv.org/abs/1902.05559

Supersymmetric Inflation from the Fifth Dimension

Kaustubh Deshpande, Raman Sundrum

(Submitted on 14 Feb 2019)

We develop a supersymmetric bi-axion model of high-scale inflation coupled to supergravity, in which the axionic structure originates from, and is protected by, gauge symmetry in an extra dimension. While local supersymmetry (SUSY) is necessarily Higgsed at high scales during inflation we show that it can naturally survive down to the ∼ TeV scale in the current era in order to resolve the electroweak hierarchy problem. We show how a suitable inflationary effective potential for the axions can be generated at tree-level by charged fields under the higher-dimensional gauge symmetry. The inflationary trajectory lies along the lightest direction in the bi-axion field space, with periodic effective potential and an effective super-Planckian field range emerging from fundamentally sub-Planckian dynamics. The heavier direction in the field space is shown to also play an important role, as the dominant source of super-Higgsing during inflation. This model presents an interesting interplay of tuning considerations relating the electroweak hierarchy, cosmological constant and inflationary superpotential, where maximal naturalness favors SUSY breaking near the electroweak scale after inflation. The scalar superpartner of the axionic inflaton, the "sinflaton", can naturally have ∼ Hubble mass during inflation and sufficiently strong coupling to the inflaton to mediate primordial non-Gaussianities of observable strength in future 21-cm surveys. Non-minimal charged fields under the higher-dimensional gauge symmetry can contribute to periodic modulations in the CMB, within the sensitivity of ongoing measurements.

_________________________________________________________________________________