arXiv subjects

Gia Dvali

Publications and source records attributed to Gia Dvali.

At least 55 records · Page 3Linked to original sources

arXiv2022

Strong-$CP$ with and without gravity

Conventionally, the strong-$CP$ problem is assumed to be a naturalness puzzle, with the axion solution sometimes viewed as an ad hoc fix. Gravity is either ignored or taken as a threat for the global Peccei-Quinn symmetry. We explain that the situation is fundamentally different. In gravity, axion is a matter of consistency imposed by the $S$-matrix: Each gauge sector must include axion with exact relaxation of the corresponding $\barθ$. We show that this favors an alternative and remarkably simple formulation of the axion, fully fixed by the gauge redundancy of QCD, without involvement of a global symmetry. The axion mechanism is a Higgs effect for the QCD $3$-form, ensuring that physics is independent of $\barθ$ to all orders in operator expansion. A near-future experimental detection of the neutron EDM will be an unambiguous signal of $CP$-violating physics beyond the Standard Model. The axion coupling is tied to the scale of gravity.

hep-ph↗

arXiv2022

Black Hole Metamorphosis and Stabilization by Memory Burden

Systems of enhanced memory capacity are subjected to a universal effect of memory burden, which suppresses their decay. In this paper, we study a prototype model to show that memory burden can be overcome by rewriting stored quantum information from one set of degrees of freedom to another one. However, due to a suppressed rate of rewriting, the evolution becomes extremely slow compared to the initial stage. Applied to black holes, this predicts a metamorphosis, including a drastic deviation from Hawking evaporation, at the latest after losing half of the mass. This raises a tantalizing question about the fate of a black hole. As two likely options, it can either become extremely long lived or decay via a new classical instability into gravitational lumps. The first option would open up a new window for small primordial black holes as viable dark matter candidates.

hep-th↗

arXiv2022

De Sitter space as BRST invariant coherent state of gravitons

The $S$-matrix formulation indicates that a consistent embedding of de Sitter state in quantum gravity is possible exclusively as an excited quantum state constructed on top of a valid $S$-matrix vacuum such as Minkowski. In the present paper we offer such a construction of de Sitter in the form of a coherent state of gravitons. Unlike previous realizations of this idea, we focus on BRST invariance as the guiding principle for physicality. In order to establish the universal rules of gauge consistency, we study the BRST-invariant construction of coherent states created by classical and quantum sources in QED and in linearized gravity. Introduction of $N$ copies of scalar matter coupled to gravity allows us to take a special double scaling limit, a so-called species limit, in which our construction of de Sitter becomes exact. In this limit, the irrelevant quantum gravitational effects vanish whereas the collective phenomena, such as Gibbons-Hawking radiation, are calculable.

hep-th↗

arXiv2021

Primordial Black Holes from Confinement

A mechanism for the formation of primordial black holes is proposed. Here, heavy quarks of a confining gauge theory produced by de Sitter fluctuations are pushed apart by inflation and get confined after horizon re-entry. The large amount of energy stored in the colour flux tubes connecting the quark pair leads to black-hole formation. These are much lighter and can be of higher spin than those produced by standard collapse of horizon-size inflationary overdensities. Other difficulties exhibited by such mechanisms are also avoided. Phenomenological features of the new mechanism are discussed as well as accounting for both the entirety of the dark matter and the supermassive black holes in the galactic centres. Under proper conditions, the mechanism can be realised in a generic confinement theory, including ordinary QCD. We discuss a possible string-theoretic realisation via $D$-branes. Interestingly, for conservative values of the string scale, the produced gravity waves are within the range of recent NANOGrav data. Simple generalisations of the mechanism allow for the existence of a significant scalar component of gravity waves with distinct observational signatures.

hep-ph↗

arXiv2021

How Special Are Black Holes? Correspondence with saturons in generic theories

Black holes are considered to be exceptional due to their time evolution and information processing. However, it was proposed recently that these properties are generic for objects, the so-called saturons, that attain the maximal entropy permitted by unitarity. In the present paper, we verify this connection within a renormalizable $SU(N)$ invariant theory. We show that the spectrum of the theory contains a tower of bubbles representing bound states of $SU(N)$ Goldstones. Despite the absence of gravity, a saturated bound state exhibits a striking correspondence with a black hole: Its entropy is given by the Bekenstein-Hawking formula; semi-classically, the bubble evaporates at a thermal rate with a temperature equal to its inverse radius; the information retrieval time is equal to Page's time. The correspondence goes through a trans-theoretic entity of Poincaré Goldstone. The black hole/saturon correspondence has important implications for black hole physics, both fundamental and observational.

hep-th↗

arXiv2021

Black-Hole-Like Saturons in Gross-Neveu

It has been argued recently that objects of maximal microstate entropy permitted by unitarity, the so-called saturons, have properties similar to black holes. We demonstrate the existence of such objects in Gross-Neveu model. From the large-$N$ scaling of $S$-matrix, we deduce the connection between the entropy of the bound-state and the unitarity of scattering. We observe that upon saturation of unitarity, the bound state exhibits a remarkable correspondence with a black hole. The scaling of its entropy is identical to Bekenstein-Hawking entropy. The saturon decays via Hawking's thermal rate of temperature given by the inverse size. The information retrieval time from the Gross-Neveu saturon is isomorphic to Page's time. Our observations indicate that black hole properties are exhibited by saturated states in simple calculable models.

hep-th↗

arXiv2021

Domestic Axion

We attempt to identify a phenomenologically viable solution to the strong $CP$ problem in which the axion is composed entirely out of Standard Model fermion species. The axion consists predominantly of the $η'$ meson with a minuscule admixture of a pseudoscalar bilinear composite of neutrinos, $η_ν$. The Peccei-Quinn symmetry is an axial symmetry that acts on the up quark and the neutrino species and is spontaneously broken by the QCD condensate of quarks as well as the condensate of neutrinos triggered by chiral gravitational anomaly. The up-quark mass is spontaneously generated by the neutrino condensate which plays the role of an additional composite Higgs doublet with the compositeness scale of the order of the neutrino masses. Such a scenario is highly economical: it solves the strong $CP$ problem, generates the up-quark and neutrino masses from fermion condensates and simultaneously protects the axion shift symmetry against gravitational anomaly. The phenomenology is different from the standard hidden axion case. One of the experimental signatures is the existence of a gravity-competing isotope-dependent attractive force among nucleons at (sub)micron distances.

hep-ph↗

arXiv2021

Bounds on Quantum Information Storage and Retrieval

We present certain universal bounds on the capacity of quantum information storage and on the time scale of its retrieval for a generic quantum field theoretic system. The capacity, quantified by the microstate entropy, is bounded from above by the surface area of the object measured in units of a Goldstone decay constant. The Goldstone bosons are universally present due to the spontaneous breaking of Poincare and internal symmetries by the information-storing object. Applied to a black hole, the bound reproduces the Bekenstein-Hawking entropy. However, the relation goes beyond gravity. The minimal time-scale required for retrieving the quantum information from a system is equal to its volume measured in units of the same Goldstone scale. For a black hole this reproduces the Page time as well as the quantum break-time. The same expression for the information retrieval time is shared by non-gravitational saturated states in gauge theories, including QCD. The saturated objects exhibit some universal signatures such as the emission of ultra-soft radiation. Similar bounds apply to non-relativistic many-body systems.

hep-th↗

arXiv2021

Classicalization and unitarization of wee partons in QCD and Gravity: The CGC-Black Hole correspondence

We discuss a remarkable correspondence between the description of Black Holes as highly occupied condensates of $N$ weakly interacting gravitons and that of Color Glass Condensates (CGCs) as highly occupied gluon states. In both cases, the dynamics of "wee partons" in Regge asymptotics is controlled by emergent semi-hard scales that lead to perturbative unitarization and classicalization of $2\rightarrow N$ particle amplitudes at weak coupling. In particular, they attain a maximal entropy permitted by unitarity, bounded by the inverse coupling $α$ of the respective constituents. Strikingly, this entropy is equal to the area measured in units of the Goldstone constant corresponding to the spontaneous breaking of Poincar{é} symmetry by the corresponding graviton or gluon condensate. In gravity, the Goldstone constant is the Planck scale, and gives rise to the Bekenstein-Hawking entropy. Likewise, in the CGC, the corresponding Goldstone scale is determined by the onset of gluon screening. We point to further similarities in Black Hole formation, thermalization and decay, to that of the Glasma matter formed from colliding CGCs in ultrarelativistic nuclear collisions, which decays into a Quark-Gluon Plasma.

hep-th↗

arXiv2021

On $S$-Matrix Exclusion of de Sitter and Naturalness

The cosmological constant puzzle, traditionally viewed as a naturalness problem, is evidently nullified by the $S$-matrix formulation of quantum gravity/string theory. We point out an implication of this fact for another naturalness puzzle, the Hierarchy Problem between the weak and Planck scales. By eliminating the landscape of de Sitter vacua and eternal inflation, the $S$-matrix formulation exhibits an obvious tension with the explanations based on anthropic selection or cosmological relaxation of the Higgs mass. This sharpens the Hierarchy Problem in a profound way. On one hand, it strengthens the case for explanations based on new physics not far from the weak scale. At the same time, it opens up a question, whether instead the hierarchy is imposed by the $S$-matrix consistency between the Standard Model and gravity.

hep-th↗

arXiv2021

Quantum Gravity in Species Regime

A large number of particle species allows to formulate quantum gravity in a special double-scaling limit, the species limit. In this regime, quantum gravitational amplitudes simplify substantially. An infinite set of perturbative corrections, that usually blur the picture, vanishes, whereas the collective and non-perturbative effects can be cleanly extracted. Such are the effects that control physics of black holes and of de Sitter and their entanglement curves. In string theory example, we show that the entropy of open strings matches the Gibbons-Hawking entropy of a would-be de Sitter state at the point of saturation of the species bound. This shows, from yet another angle, why quantum gravity/string theory cannot tolerate a de Sitter vacuum. Finally, we discuss various observational implications.

hep-th↗

arXiv2020

$S$-Matrix and Anomaly of de Sitter

$S$-matrix formulation of gravity excludes de Sitter vacua. In particular, this is organic to string theory. The $S$-matrix constraint is enforced by an anomalous quantum break-time proportional to the inverse values of gravitational and/or string couplings. Due to this, de Sitter can satisfy the conditions for a valid vacuum only at the expense of trivializing the graviton and closed-string $S$-matrixes. At non-zero gravitational and string couplings, de Sitter is deformed by corpuscular $1/N$ effects, similarly to Witten-Veneziano mechanism in QCD with $N$ colors. In this picture, an $S$-matrix formulation of Einstein gravity, such as string theory, nullifies an outstanding cosmological puzzle. We discuss possible observational signatures which are especially interesting in theories with large number of particle species. Species can enhance the primordial quantum imprints to potentially observable level even if the standard inflaton fluctuations are negligible.

hep-th↗

arXiv2020

Absence of $μ$-Problem in Grand Unification

Using properties of Goldstino, we show that in generic grand unified theories with gravity-mediated supersymmetry breaking the $μ$-problem is non-existent. What happens is that supersymmetry breaking universally induces the shifts of the heavy fields that generate $μ$ and $B_μ$ terms. In the leading order, these are given by the mass of gravitino and are insensitive to the scale of grand unification. The mechanism works regardless whether doublet-triplet splitting is achieved via fine-tuning or not. Moreover, we illustrate this general phenomenon on explicit examples of theories that achieve doublet-triplet splitting dynamically. These include the theories with Higgs doublet as a pseudo-Goldstone boson, as well as, the approach based on spontaneous decoupling of the light color-triplet from quarks and leptons.

hep-ph↗

arXiv2020

Inflation and Decoupling

Decoupling of heavy modes in effective low energy theory is one of the most fundamental concepts in physics. It tells us that modes must have a negligible effect on the physics of gravitational backgrounds with curvature radius larger than their wavelengths. Despite this, there exist claims that trans-Planckian modes put severe bound on the duration of inflation even when the Hubble parameter is negligible as compared to the Planck mass. If true, this would mean that inflation violates the principle of decoupling or at least requires its reformulation. We clarify the fundamental misconception on which these bounds are based and respectively refute them. Our conclusion is that inflation fully falls within the validity of a reliable effective field theory treatment and does not suffer from any spurious trans-Planckian problem.

hep-th↗

arXiv2020

Compact Dark Matter Objects via $N$ Dark Sectors

We propose a novel class of compact dark matter objects in theories where the dark matter consists of multiple sectors. We call these objects $N$-MACHOs. In such theories neither the existence of dark matter species nor their extremely weak coupling to the observable sector represent additional hypotheses but instead are imposed by the solution to the Hierarchy Problem and unitarity. The crucial point is that particles from the same sector have non-trivial interactions but interact only gravitationally otherwise. As a consequence, the pressure that counteracts the gravitational collapse is reduced while the gravitational force remains the same. This results in collapsed structures much lighter and smaller as compared to the ordinary single-sector case. We apply this phenomenon to a dark matter theory that consists of $N$ dilute copies of the Standard Model. The solutions do not rely on an exotic stabilization mechanism, but rather use the same well-understood properties as known stellar structures. This framework also gives rise to new microscopic superheavy structures, for example with mass $10^8\,$g and size $10^{-13}\,$cm. By confronting the resulting objects with observational constraints, we find that, due to a huge suppression factor entering the mass spectrum, these objects evade the strongest constrained region of the parameter space. Finally, we discuss possible formation scenarios of $N$-MACHOs. We argue that, due to the efficient dissipation of energy on small scales, high-density regions such as ultra-compact mini-halos could serve as formation sites of $N$-MACHOs.

astro-ph.CO↗

arXiv2020

Entropy Bound and Unitarity of Scattering Amplitudes

We establish that unitarity of scattering amplitudes imposes universal entropy bounds. The maximal entropy of a self-sustained quantum field object of radius R is equal to its surface area and at the same time to the inverse running coupling evaluated at the scale R. The saturation of these entropy bounds is in one-to-one correspondence with the non-perturbative saturation of unitarity by 2-to-N particle scattering amplitudes at the point of optimal truncation. These bounds are more stringent than Bekenstein's bound and in a consistent theory all three get saturated simultaneously. This is true for all known entropy-saturating objects such as solitons, instantons, baryons, oscillons, black holes or simply lumps of classical fields. We refer to these collectively as "saturons" and show that in renormalizable theories they behave in all other respects like black holes. Finally, it is argued that the confinement in SU(N) gauge theory can be understood as a direct consequence of the entropy bounds and unitarity.

hep-th↗

arXiv2019

Cosmological Relaxation of Higgs Mass Before and After LHC and Naturalness

In post LHC era the old idea of cosmological vacuum relaxation of the Higgs mass that does not require any new physics in the vicinity of LHC energies acquires a new meaning. I discuss how this concept of naturanless differs from the standard one by 't Hooft. Here the observed value of the Higgs mass corresponds to a vacuum of infinite degeneracy and infinite entropy. Therefore, it represents and attractor point of cosmic inflationary evolution. This information is unavailable for a low energy observer living in one of such vacua. By not seeing any stabilizing physics at LHC such an observer is puzzled and creates an artificial problem of naturalness which in reality does not exist. We explain why this solution is fully compatible with the concept of Wilsonian decoupling.

hep-ph↗

arXiv2019

Finding Critical States of Enhanced Memory Capacity in Attractive Cold Bosons

We discuss a class of quantum theories which exhibit a sharply increased memory storage capacity due to emergent gapless degrees of freedom. Their realization, both theoretical and experimental, is of great interest. On the one hand, such systems are motivated from a quantum information point of view. On the other hand, they can provide a framework for simulating systems with enhanced capacity of pattern storage, such as black holes and neural networks. In this paper, we develop an analytic method that enables us to find critical states with increased storage capabilities in a generic system of cold bosons with weak attractive interactions. The enhancement of memory capacity arises when the occupation number $N$ of certain modes reaches a critical level. Such modes, via negative energy couplings, assist others in becoming effectively gapless. This leads to degenerate microstates labeled by the occupation numbers of the nearly-gapless modes. In the limit of large $N$, they become exactly gapless and their decoherence time diverges. In this way, a system becomes an ideal storer of quantum information. We demonstrate our method on a prototype model of $N$ attractive cold bosons contained in a one-dimensional box with Dirichlet boundary conditions. Although we limit ourselves to a truncated system, we observe a rich structure of quantum phases with a critical point of enhanced memory capacity.

quant-ph↗