Positivity bounds are one of the most reliable tools which allow us to identify effective field theories (EFTs) which cannot be embedded into UV complete theories respecting unitarity, causality, Lorentz invariance, and locality. In the first half of this talk, we will consider non-gravitational theories and show that the locality assumption is unnecessary to derive some of the positivity bounds obtained in the literature. This result enlarges the applicability of the positivity bounds. In the second half of this talk, we will explain that the presence of gravity could invalidate the usual positivity argument. We then clarify how to overcome the obstructions to get useful bounds on EFTs.

Non-singular universe, which avoids the appearance of the singularity of the spacetime at the very early time, has been investigated for many years. Most of the analyses have been done under the assumption that spacetime is homogeneous isotropic for simplicity. However, the existence of anisotropies can easily spoil the achievement of the non-singular universe. We propose a new framework motivated by limiting curvature hypothesis, and, by introducing a mechanism to limit anisotropies in this framework, we obtain a non-singular anisotropic universe. It is remarkable that our framework gives a unified picture of mimetic gravity and cuscuton gravity in the context of limiting curvature theories. References: Theories with limited extrinsic curvature and a nonsingular anisotropic universe, Yuki Sakakihara, Daisuke Yoshida, Kazufumi Takahashi, Jerome Quintin, PRD 102 (2020) 084004 arXiv:2005.10844[gr-qc]

I consider higher derivative, UV modifications to GR. In particular, I will focus on a specific kind of string theory-inspired higher derivative gravity where one includes derivatives to all orders in the action. First, I will discuss how such a non-local theory of gravity admits stable, non-singular bouncing solutions in the absence of matter. Moreover, around this bouncing background, there exists only one propagating (and ghost-free) scalar mode, and no vector or tensor modes. Next, I will discuss the general analysis of scalar-vector-tensor perturbations in non-local gravity - in particular, I will show how non-local gravity is ghost-free around (A)dS and certain non-maximally symmetric backgrounds, and how certain (A)dS backgrounds have special physical spectra in that the propagating degrees of freedom are different from usual expectations. References: Perturbations in higher derivative gravity beyond maximally symmetric spacetimes (https://arxiv.org/abs/1905.03227) Stable, non-singular bouncing universe with only a scalar mode (https://arxiv.org/abs/2005.01762)

Attempts to solve naturalness by having the weak scale as the only breaking of classical scale invariance in 4-dimensional Quantum Field Theories satisfy Total Asymptotic Freedom (TAF): the theory holds up to infinite energy, where all coupling constants flow to zero and is devoid of any Landau poles. Specifically we will discuss a fundamental field theory of the QCD axion in the totally asymptotically free (TAF) scenario, and the dynamics of the Peccei-Quinn (PQ) phase transition there-in. The PQ phase transition can be of strongly first order and produce stochastic gravitational waves (GW) background within the reach of GW detectors, with predictions in a frequency peak in the range 100-1000 Hz with an amplitude that is already within the sensitivity of LIGO \& advanced LIGO. In the second part of the talk, we focus on Primordial GW: particularly we discuss the PGW spectrum in non-standard cosmology and in modified gravity theories, in early Universe cosmology, specifically in scalar-tensor and extra dimensional gravity scenarios, investigating the detection prospects in current and future GW observatories which can be potentially observed by laser interferometers operating in the high-frequency range and at low frequencies with pulsar timing arrays respectively. We will see that data from the planned network of several GW detectors operating across various frequency ranges will be able to distinguish between various modified gravity and non-standard cosmological history scenarios.

In this seminar I will introduce Non-Local Quantum Gravity and I will review its most important features. Then, I will present some recent results related to the ultraviolet asymptotic freedom of the model, as the undetectability of causal violations and the solution of the trans-Planckian problem. Finally, I will discuss future perspectives, e. g. , cosmological applications, and new ideas in this research field.

In last years, the quest for axion dark matter has attracted a large amount of attention from the physics community leading to the development of a diverse search program. Mostly of these searches are based on the axion-photon coupling. Regrettable, such phenomena have not been observed until today encouraging the community to look for novel set up to detect axions, including astrophysical scenarios. In this talk, I will cover most of the astrophysical properties of axion Bose Einstein condensates (or axion clumps) and explain how their presence today in the Milky Way Halo could lead to a possible axion detection. For moderately large axion-photon couplings, axion clumps with masses larger than a critical mass may undergo parametric resonance into photons. To have a clump today in the Milky Way halo above that critical mass is required to have mergers. By performing a full 3-dimensional simulation of pairs of axion clumps, we determine the conditions under which these mergers can take place. Interesting enough, mergers are expected to be kinematically allowed in the galaxy today for high values of the axion decay constant, regime which is strongly suggested by string theory and unification ideas. Axion clump mergers may lead to a non-negligible flux of energy on Earth so that detecting the axion in this novel scenario is an interesting possibility which deserves further studies. Reference: Main: Merger of Dark Matter Axion Clumps and Resonant Photon Emission (https://arxiv.org/abs/2005.02405) Complementary: Analysis of Dark Matter Axion Clumps with Spherical Symmetry (https://arxiv.org/abs/1710.04729) Dark Matter Axion Clump Resonance of Photons(https://arxiv.org/abs/1805.00430)

Gravity can be regarded as a consequence of local Lorentz (LL) symmetry, which is essential in defining a spinor field in curved spacetime. The gravitational action may admit a zero-field limit of the metric and vierbein at a certain ultraviolet cutoff scale such that the action becomes a linear realization of the LL symmetry. Consequently, only three types of term are allowed in the four-dimensional gravitational action at the cutoff scale: a cosmological constant, a linear term of the LL field strength, and spinor kinetic terms, whose coefficients are in general arbitrary functions of LL and diffeomorphism invariants. In particular, all the kinetic terms are prohibited except for spinor fields, and hence the other fields are auxiliary. Their kinetic terms, including those of the LL gauge field and the vierbein, are induced by spinor loops simultaneously with the LL gauge field mass. The LL symmetry is necessarily broken spontaneously and hence is nothing but a hidden local symmetry, from which gravity is emergent. Reference: Dynamically emergent gravity from hidden local Lorentz symmetry (https://arxiv.org/abs/2003.07126)

The so-called Lovelock theorem implies that the Einstein-Hilbert action is unique in four dimensions D=4. Recently, it was claimed that nontrivial dynamics from the Gauss-Bonnet term, which is not dynamical in D=4, can be achieved through a dimensional regularization process if we start from D(>4) dimensions and then take the D -> 4 limit. Taking this nontrivial contribution into account, they found a new theory of D->4 Einstein-Gauss-Bonnet gravity and bypassed the Lovelock theorem. Soon after that, some doubts were raised about the idea. In this talk, in the first part we clarify the source of doubts. In the second part, we provide consistent formulation of D->4 Einstein-Gauss-Bonnet gravity based on the minimally modified gravity scenario. In the third part, we discuss implications of the setup for the gravitational waves.

According to the asymptotic-safety conjecture, the gravitational renormalization group flow features an ultraviolet-attractive fixed point that makes the theory renormalizable and ultraviolet complete. The existence of this fixed point entails an antiscreening behavior of the gravitational interaction at short distances. After a brief introduction to asymptotically safe gravity (ASG), in this talk I will review some of the most important phenomenological consequences of ASG, focusing on the implications of gravitational antiscreening in cosmology. Reference: From renormalization group flows to cosmology (https://arxiv.org/abs/2003.13656 , Front.in Phys. 8 (2020) 188)

In this seminar I will present recent predictions in inflationary cosmology obtained from quantum gravity with purely virtual quanta. In particular, amplitudes and spectral indices derived to the next-to-leading order in the expansion around de Sitter background. The quantum field theory from which these predictions are derived is both renormalizable and unitary. The key ingredient to achieve these properties is the purely virtual quantum, also called fakeon, a degree of freedom that can mediate interactions but cannot appear as external state. The consistency of the approach in curved spacetime puts a lower bound on the mass of the fakeon with respect to the mass of the inflaton. After presenting these results and the features of the theory relevant for their derivation, I will discuss in detail how to implement a fakeon in quantum field theory and show that this operation preserves renormalizability and unitarity at the same time. I will conclude with some phenomenological aspects of the theory from the high-energy physics point of view.

The inflationary origin of primordial black holes (PBHs) relies on a large enhancement of the power spectrum Δζ of the curvature fluctuation ζ at wavelengths much shorter than those of the CMB anisotropies. This is typically achieved in models where ζ evolves without interacting significantly with additional (isocurvature) scalar degrees of freedom. However, quantum gravity inspired models are characterized by moduli spaces with highly curved geometries and a large number of scalar fields that could vigorously interact with ζ (as in the cosmological collider). Here we show that isocurvature fluctuations can mix with ζ inducing large enhancements of its amplitude. This occurs whenever the inflationary trajectory experiences rapid turns in the field space of the model leading to amplifications that are exponentially sensitive to the total angle swept by the turn, which induce characteristic observable signatures on Δζ . We derive accurate analytical predictions and show that the large enhancements required for PBHs demand non-canonical kinetic terms in the action of the multi-field system.

Emergence of R^2 inflation which is the best fit framework for CMB observations till date comes from the attempts to attack the problem of quantization of gravity which in turn have resulted in the trace anomaly discovery. Further developments in trace anomaly and different frameworks aiming to construct quantum gravity indicate an inevitability of non-locality in fundamental physics at small time and length scales. A natural question would be to employ the R^2 inflation as a probe for signatures of non-locality in the early Universe physics. In this talk, I will discuss recent advances of embedding R^2 inflation in a string theory inspired non-local gravity modification and provide very promising theoretical predictions connecting the non-local physics in the early Universe and the forthcoming CMB observations. This talk is based on https://arxiv.org/abs/1711.08864 and https://arxiv.org/abs/2003.00629.

We study cosmological and astrophysical applications of the recently proposed generalized hybrid metric-Palatini gravity theory, which combines features of both the metric and the Palatini approaches to the variational method in $f\left(R\right)$ gravity. This theory arises as a natural generalization of the hybrid metric-Palatini gravity which has been proven to be the first theory to unify the cosmic acceleration with the solar system constraints, without resource to the chameleon mechanism. In the cosmological point of view, we show using reconstruction methods that the usual power-law and exponential scale factor behaviors in FLRW universes exist for various different distributions of matter, along with solutions for collapsing universes. Using the dynamical system approach, we also show that no global attractors can exist in the cosmological phase space and that stable universes can either be described by scale factors that diverge in finite time or asymptotically approach constant values. In the area of astrophysics, we show that using the junction conditions of the theory it is possible to obtain solutions for compact objects supported by thin-shells, such as self-gravitating shells with and without perfect fluids on their exteriors, and also traversable wormhole solutions which satisfy the null energy condition for the whole spacetime, thus not needing the support of exotic matter. Furthermore, we show that there exist specific forms of the action for which the massive scalar degree of freedom of the theory is stable in the scope of rotating black-holes described by the Kerr metric.

Ultralight bosonic dark matters are expected to solve the core-cusp problem in dark matter halos. In this talk, we study the ultralight vector dark matter with a mass around 10?23 eV. The vector field oscillating coherently on galactic scales induces oscillations of the spacetime metric with a frequency around nHz, which is detectable by pulsar timing arrays. We find that the pulsar timing signal due to the vector dark matter has nontrivial angular dependence unlike the scalar dark matter and the maximal amplitude is three times larger than that of the scalar dark matter.

N=2 supergravity in four dimensions naturally appears from higher-dimensional supergravity and string compactifications. For phenomenological applications, we need to consider its breaking mechanism since in N=2 supergravity, there is no chiral-structure which is necessary to describe real world. As regards for the breaking of extended supergravity, there are several breaking patterns to be considered in contrast to N = 1 case. In this talk, I will try to clarify the relations between the breaking patterns and input parameters of the theory, in a systematic way based on the embedding tensor formalism.

Photon surface is a geometrical object which generalizes the photon sphere of Schwarzschild spacetime. Due to its geometrical features, there are interesting phenomena concerning the photon surface. In this talk, after reviewing a photon surface, we see the relation between photon surfaces and wormholes. A thin-shell wormhole spacetime is constructed by truncating two spacetimes at some boundaries and gluing them together along the boundaries. The glued boundary is called a shell or a throat. Such a wormhole spacetime is said to satisfy Einstein equation if Israel's junction conditions are satisfied with the surface stress energy tensor of the shell. If the shell is of pure-tension, the corresponding boundaries of the original spacetimes must be photon surfaces. Applying the uniqueness theorem of photon spheres established by Cederbaum to the wormhole, we prove the uniqueness theorem of pure-tension wormholes.

We construct, for the first time, the time-domain gravitational wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy emitted as a function of width, radius and string tension Gμ. We find that the mass normalized gravitational wave energy displays a strong dependence on the inverse of the string tension E_GW / M_0 ∝ 1/Gμ, with E_GW / M_0 ∼ O(1)% at the percent level, for the regime where Gμ 1e-3 . Conversely, we show that the efficiency is only weakly dependent on the initial string width and initial string radii. Using these results, we argue that gravitational wave production is dominated by kinematical instead of geometrical considerations.

In this talk we give a short review of the generalized uncertainty principle (GUP), then we focus on the bounds recently found for the deformation parameter of GUP, in particular bounds of gravitational origin. Possible violations of the Equivalence Principle are highlighted. Then, we propose a technique to compute the deformation parameter by using the leading quantum corrections to the Newtonian potential. This specific numerical value is discussed and compared with the previously obtained bounds on the deformation parameter.