arXiv subjects

Gia Dvali

Publications and source records attributed to Gia Dvali.

At least 145 records · Page 8Linked to original sources

arXiv2007

Strong CP Problem with 10^{32} Standard Model Copies

We show that a recently proposed solution to the Hierarchy Problem simultaneously solves the Strong CP Problem, without requiring an axion or any further new physics. Consistency of black hole physics implies a non-trivial relation between the number of particle species and particle masses, so that with ~10^{32} copies of the standard model, the TeV scale is naturally explained. At the same time, as shown here, this setup predicts a typical expected value of the strong-CP parameter in QCD of theta ~ 10^{-9}. This strongly motivates a more sensitive measurement of the neutron electric dipole moment.

hep-th↗

arXiv2007

Cascading DGP

We present a higher codimension generalization of the DGP scenario which, unlike previous attempts, is free of ghost instabilities. The 4D propagator is made regular by embedding our visible 3-brane within a 4-brane, each with their own induced gravity terms, in a flat 6D bulk. The model is ghost-free if the tension on the 3-brane is larger than a certain critical value, while the induced metric remains flat. The gravitational force law `cascades' from a 6D behavior at the largest distances followed by a 5D and finally a 4D regime at the shortest scales.

hep-th↗

arXiv2007

Black Hole Bound on the Number of Species and Quantum Gravity at LHC

In theories with a large number N of particle species, black hole physics imposes an upper bound on the mass of the species equal to M_{Planck}/\sqrt{N}. This bound suggests a novel solution to the hierarchy problem in which there are N \approx 10^{32} gravitationally coupled species, for example 10^{32} copies of the Standard Model. The black hole bound forces them to be at the weak scale, hence providing a stable hierarchy. We present various arguments, that in such theories the effective gravitational cutoff is reduced to Λ_G \approx M_{Planck}/\sqrt{N} and a new description is needed around this scale. In particular black-holes smaller than Λ_G^{-1} are already no longer semi-classical. The nature of the completion is model dependent. One natural possibility is that Λ_G is the quantum gravity scale. We provide evidence that within this type of scenarios, contrary to the standard intuition, micro black holes have a (slowly-fading) memory of the species of origin. Consequently the black holes produced at LHC, will predominantly decay into the Standard Model particles, and negligibly into the other species.

hep-th↗

arXiv2007

Black Holes and Large N Species Solution to the Hierarchy Problem

We provide the perturbative and non-perturbative arguments showing that theories with large number of species of the quantum fields, imply an inevitable hierarchy between the masses of the species and the Planck scale, shedding a different light on the hierarchy problem. In particular, using the black hole physics, we prove that any consistent theory that includes N number of the Z_2-conserved species of the quantum fields of mass Λ, puts a lower bound on the Planck mass, which in large N limit is given by NΛ^2. An useful byproduct of this proof is that any exactly conserved quantum charge, not associated with a long-range classical field, must be defined maximum modulo N, bounded by the the ratio of the Planck to the unit charge masses squared. For example, a continuous global U(1) `baryon number' symmetry, must be explicitly broken by gravity, at least down to a Z_N subgroup, with N bounded by the ratio of the Planck to baryon masses squared. The same constraint applies to any discrete gauge symmetry, as well as to other quantum-mechanically detectable black hole charges that are associated with the massive quantum hair of the black hole. We show that the gravitationally-coupled N-species sector that solves the gauge hirearchy problem, should be probed by LHC.

hep-th↗

arXiv2007

Consistent Lorentz Violation in Flat and Curved Space

Motivated by the severity of the bounds on Lorentz violation in the presence of ordinary gravity, we study frameworks in which Lorentz violation does not affect the spacetime geometry. We show that there are at least two inequivalent classes of spontaneous Lorentz breaking that even in the presence of gravity result in Minkowski space. The first one generically corresponds to the condensation of tensor fields with tachyonic mass which in turn is related to ghost-condensation. In the second class, realized by the DGP model or theories of massive gravitons, spontaneous Lorentz breaking is induced by the expectation value of sources. The generalization to de-Sitter space is also discussed.

hep-th↗

arXiv2007

Degravitation of the Cosmological Constant and Graviton Width

We study the possibility of decoupling gravity from the vacuum energy. This is effectively equivalent to promoting Newton's constant to a high-pass filter that degravitates sources of characteristic wavelength larger than a certain macroscopic (super) horizon scale L. We study the underlying physics and the consistency of this phenomenon. In particular, the absence of ghosts, already at the linear level, implies that in any such theory the graviton should either have a mass 1/L, or be a resonance of similar width. This has profound physical implications for the degravitation idea.

hep-th↗

arXiv2006

Domain Walls As Probes Of Gravity

We show that domain walls are probes that enable one to distinguish large-distance modified gravity from general relativity (GR) at short distances. For example, low-tension domain walls are stealth in modified gravity, while they do produce global gravitational effects in GR. We demonstrate this by finding exact solutions for various domain walls in the DGP model. A wall with tension lower than the fundamental Planck scale does not inflate and has no gravitational effects on a 4D observer, since its 4D tension is completely screened by gravity itself. We argue that this feature remains valid in a generic class of models of infrared modified gravity. As a byproduct, we obtain exact solutions for super-massive codimension-2 branes.

hep-th↗

arXiv2006

Predictive Power of Strong Coupling in Theories with Large Distance Modified Gravity

We consider theories that modify gravity at cosmological distances, and show that any such theory must exhibit a strong coupling phenomenon, or else it is either inconsistent or is already ruled out by the solar system observations. We show that all the ghost-free theories that modify dynamics of spin-2 graviton on asymptotically flat backgrounds, automatically have this property. Due to the strong coupling effect, modification of the gravitational force is source-dependent, and for lighter sources sets in at shorter distances. This universal feature makes modified gravity theories predictive and potentially testable not only by cosmological observations, but also by precision gravitational measurements at scales much shorter than the current cosmological horizon. We give a simple parametrization of consistent large distance modified gravity theories and their predicted deviations from the Einsteinian metric near the gravitating sources.

hep-th↗

arXiv2006

Black Holes with Flavors of Quantum Hair?

We show that black holes can posses a long-range quantum hair of super-massive tensor fields, which can be detected by Aharonov-Bohm tabletop interference experiments, in which a quantum-hairy black hole, or a remnant particle, passes through the loop of a magnetic solenoid. The long distance effect does not decouple for an arbitrarily high mass of the hair-providing field. Because Kaluza-Klein and String theories contain infinite number of massive tensor fields, we study black holes with quantum Kaluza-Klein hair. We show that in five dimensions such a black hole can be interpreted as a string of `combed' generalized magnetic monopoles, with their fluxes confined along it. For the compactification on a translation-invariant circle, this substructure uncovers hidden flux conservation and quantization of the monopole charges, which constrain the quantum hair of the resulting four-dimensional black hole. For the spin-2 quantum hair this result is somewhat unexpected, since the constituent `magnetic' charges have no `electric' counterparts. Nevertheless, the information about their quantization is encoded in singularity.

hep-th↗

arXiv2006

Black Holes with Quantum Massive Spin-2 Hair

We show that black holes can posses a long range quantum mechanical hair associated with a massive spin-2 field, which can be detected by a stringy generalization of the Aharovon-Bohm effect, in which a string loop lassoes the black hole. The long distance effect persist for arbitrarily high mass of the spin-2 field. An analogous effect is exhibited by a massive antisymmetric two-form field. We make a close parallel between the two and the ordinary Aharonov-Bohm phenomenon, and also show that in the latter case the effect can be experienced even by the electrically-neutral particles, provided some boundary terms are added to the action.

hep-th↗

arXiv2005

Topological Mass Generation in Four Dimensions

Schwinger's mechanism for mass generation relies on topological structures of a 2-dimensional gauge theory. In the same manner, corresponding 4-dimensional topological entities give rise to topological mass generation in four dimensions.

hep-th↗

arXiv2005

Cosmic D-strings as Axionic D-term Strings

In this work we derive non-singular BPS string solutions from an action that captures the essential features of a D-brane-anti-D-brane system compactified to four dimensions. The model we consider is a supersymmetric abelian Higgs model with a D-term potential coupled to an axion-dilaton multiplet. The strings in question are axionic D-term strings which we identify with the D-strings of type II string theory. In this picture the Higgs field represents the open string tachyon of the D-Dbar pair and the axion is dual to a Ramond Ramond form. The crucial term allowing the existence of non-singular BPS strings is the Fayet-Iliopoulos term, which is related to the tensions of the D-string and of the parent branes. Despite the presence of the axion, the strings are BPS and carry finite energy, due to the fact that the space gets very slowly decompactified away from the core, screening the long range axion field (or equivalently the theory approaches an infinitely weak 4D coupling). Within our 4D effective action we also identify another class of BPS string solutions (s-strings) which have no ten dimensional analog, and can only exist after compactification.

hep-th↗

arXiv2005

A Vacuum Accumulation Solution to the Strong CP Problem

We suggest a solution to the strong CP problem in which there are no axions involved. The superselection rule of the θ-vacua is dynamically lifted in such a way that an infinite number of vacua are accumulated within the phenomenologically acceptable range of θ< 10^{-9}, whereas only a measure-zero set of vacua remains outside of this interval. The general prediction is the existence of membranes to which the standard model gauge fields are coupled. These branes may be light enough for being produced at the particle accelerators in form of the resonances with a characteristic membrane spectrum.

hep-th↗

arXiv2005

Three-Form Gauging of axion Symmetries and Gravity

Nonlinearly realized Abelian global symmetries can be reformulated as local shift symmetries gauged by three-form gauge fields. The anomalous symmetries of the Standard Model (such as Peccei-Quinn or $B+L$) can be dualized to local symmetries gauged by the Chern-Simons three-forms of the Standard Model gauge group. In this description the strong CP problem can be reformulated as the problem of a massless three-form field in QCD, which creates an arbitrary CP-violating constant four-form electric field in the vacuum. Both the axion as well as the massless quark solutions amount to simply Higgsing the three-form gauge field, hence screening the electric field in the vacuum. This language gives an alternative way for visualizing the physics of the axion solution as well as the degree of its vulnerability due to gravitational corrections. Any physics that can jeopardize the axion solution must take the QCD three-form out of the Higgs phase. This can only happen if the physics in question provides an additional massless three-form. The axion then Higgses one combination of the three-forms and the QCD electric field gets partially unscreened, reintroducing the strong CP problem. Gravity provides such a candidate in form of the Chern-Simons spin connection three-form, which could un-Higgs the QCD three-form in the absence of additional chiral symmetries. We also discuss analogous effects for the baryon number symmetry.

hep-th↗

arXiv2005

Infrared Lorentz Violation and Slowly Instantaneous Electricity

We study a modification of electromagnetism which violates Lorentz invariance at large distances. In this theory, electromagnetic waves are massive, but the static force between charged particles is Coulomb not Yukawa. At very short distances the theory looks just like QED. But for distances larger than 1/m the massive dispersion relation of the waves can be appreciated, and the Coulomb force can be used to communicate faster than the speed of light. In fact, electrical signals are transmitted instantly, but take a time ~ 1/m to build up to full strength. After that, undamped oscillations of the electric field are set in and continue until they are dispersed by the arrival of the Lorentz-obeying part of the transmission. We study experimental constraints on such a theory and find that the Compton wavelength of the photon may be as small as 6000 km. This bound is weaker than for a Lorentz-invariant mass, essentially because in our case the Coulomb constraint is removed.

hep-th↗

arXiv2004

Large Hierarchies from Attractor Vacua

We discuss a mechanism through which the multi-vacua theories, such as String Theory, could solve the Hierarchy Problem, without any UV-regulating physics at low energies. Because of symmetry the number density of vacua with a certain hierarchically-small Higgs mass diverges, and is an attractor on the vacuum landscape.The hierarchy problem is solved in two steps. It is first promoted into a problem of the super-selection rule among the infinite number of vacua (analogous to theta-vacua in QCD), that are finely scanned by the Higgs mass. This rule is lifted by heavy branes, which effectively convert the Higgs mass into a dynamical variable. The key point is that a discrete "brane-charge-conjugation" symmetry guarantees that the fineness of the vacuum-scanning is set by the Higgs mass itself. On a resulting landscape in all, but a measure-zero set of vacua the Higgs mass has a common hierarchically-small value. In minimal models this value is controlled by the QCD scale and is of the right magnitude. Although in each particular vacuum there is no visible UV-regulating low energy physics, the realistic models are predictive. For example, we show that in the minimal case the "charge conjugation" symmetry is automatically a family symmetry, and imposes severe restrictions on quark Yukawa matrices.

hep-th↗

arXiv2004

Formation and evolution of cosmic D-strings

We study the formation of D and F-cosmic strings in D-brane annihilation after brane inflation. We show that D-string formation by quantum de Sitter fluctuations is severely suppressed, due to suppression of RR field fluctuations in compact dimensions. We discuss the resonant mechanism of production of D and F-strings, which are formed as magnetic and electric flux tubes of the two orthogonal gauge fields living on the world-volume of the unstable brane. We outline the subsequent cosmological evolution of the D-F string network. We also compare the nature of these strings with the ordinary cosmic strings and point out some differences and similarities.

hep-th↗

arXiv2004

Infrared Modification of Gravity

In this lecture I address the issue of possible large distance modification of gravity and its observational consequences. Although, for the illustrative purposes we focus on a particular simple generally-covariant example, our conclusions are rather general and apply to large class of theories in which, already at the Newtonian level, gravity changes the regime at a certain very large crossover distance $r_c$. In such theories the cosmological evolution gets dramatically modified at the crossover scale, usually exhibiting a "self-accelerated" expansion, which can be differentiated from more conventional "dark energy" scenarios by precision cosmology. However, unlike the latter scenarios, theories of modified-gravity are extremely constrained (and potentially testable) by the precision gravitational measurements at much shorter scales. Despite the presence of extra polarizations of graviton, the theory is compatible with observations, since the naive perturbative expansion in Newton's constant breaks down at a certain intermediate scale. This happens because the extra polarizations have couplings singular in $1/r_c$. However, the correctly resummed non-linear solutions are regular and exhibit continuous Einsteinian limit. Contrary to the naive expectation, explicit examples indicate that the resummed solutions remain valid after the ultraviolet completion of the theory, with the loop corrections taken into account.

hep-th↗