Quantum Gravity Scale Research Articles

Point-like sources and the scale of quantum gravity

We review the General Relativistic model of a (quasi) point-like particle represented by a massive shell of neutral matter which has vanishing total energy in the small-volume limit. We then show that, by assuming a Generalised Uncertainty Principle, which implies the existence of a minimum length of the order of the Planck scale, the total energy instead remains finite and equal to the shell's proper mass both for very heavy and very light particles. This suggests that the quantum structure of space–time might be related to the classical Equivalence Principle and possible implications for the late stage of evaporating black holes are briefly mentioned.

High-energy γ-ray astronomy and string theory

There have been observations, first from the MAGIC Telescope (July 2005) and quite recently (September 2008) from the FERMI Satellite Telescope, on non-simultaneous arrival of high-energy photons from distant celestial sources. In each case, the highest energy photons were delayed, as compared to their lower-energy counterparts, by clearly observable time intervals. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrival might be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently some scenarios in quantum gravity, predicting Lorentz violating space-time "foam" backgrounds with a non-trivial subluminal vacuum refractive index suppressed linearly by a quantum gravity scale of order of the reduced Planck mass (∼ 1018 GeV). In this talk, I discuss the MAGIC and FERMI findings in this context. First, I review the high-energy astrophysics models for cosmic acceleration at celestial sources, stressing that currently there is no consensus as regards the observed delays. Then I derive estimates/bounds on the quantum gravity scale that reproduces the observed time delays on the assumption of a vacuum refractive index for photons with linear suppression, and argue on the consistency of such bounds with measurements from other Gamma Ray Telescopes, such as H.E.S.S. I then explain under which circumstances the MAGIC and FERMI findings could be accommodated in such models in agreement with all the other, currently available, astrophysics constraints of Lorentz Violation. The key features are: (i) transparency of the foam to electrons, (ii) absence of birefringence effects and (iii) a breakdown of the local effective lagrangian formalism. In contrast to other Quantum Gravity (field-theoretic) models with non-trivial optical properties available to date, a string model based on brane-worlds with the bulk space being punctured by space-time point-like D0-brane defects, that provide the seeds for Lorentz violating foamy structures, seems to respect all three requirements. The model provides an explanation for the observed photon time delays in a natural range of the string coupling and mass scale.

Influence of generalized and extended uncertainty principle on thermodynamics of FRW universe

The influence of the generalized uncertainty principle (GUP) and extended uncertainty principle (EUP) on the thermodynamics of the Friedmann–Robertson–Walker (FRW) universe has been investigated. It is shown that the entropy of the apparent horizon of the FRW universe gets a correction if one considers the effect of the GUP or EUP. Moreover, starting with the modified entropy and applying the first law of thermodynamics, dE=TdS, to apparent horizon of the FRW universe, we obtain the modified Friedmann equations. The influence of the GUP or EUP on the thermodynamics of the FRW universe provides a deep insight into the understanding the quantum gravity or large length scale corrections to the dynamics of the FRW universe.

Gravitational contributions to the running Yang-Mills coupling in large extra-dimensional brane worlds

We study the question of a modification of the running gauge coupling of Yang-Mills theories due to quantum gravitational effects in a compact large extra dimensional brane world scenario with a low energy quantum gravity scale. The ADD scenario is applied for a D=d+\delta dimensional space-time in which gravitons freely propagate, whereas the non-abelian gauge fields are confined to a d-dimensional brane. The extra dimensions are taken to be toroidal and the transverse fluctuation modes (branons) of the brane are taken into account. On this basis we have calculated the one-loop corrections due to virtual Kaluza-Klein graviton and branon modes for the gluon two- and three-point functions in an effective field theory treatment. Applying momentum cut-off regularization we find that for a d=4 brane the leading gravitational divergencies cancel irrespective of the number of extra dimensions \delta, generalizing previous results in the absence of extra-dimensions. Hence, again the Yang-Mills \beta-function receives no gravitational corrections at one-loop. This is no longer true in a `universal' extra dimensional scenario with a d>4 dimensional brane. Moreover, the subleading power-law gravitational divergencies induce higher-dimensional counterterms, which we establish in our scheme. Interestingly, for d=4 these gravitationally induced counterterms are of the form recently considered in non-abelian Lee-Wick extensions of the standard model -- now with a possible mass scale in the TeV range due to the presence of large extra dimensions.

Constraining theories of low-scale quantum gravity by nonobservation of the bulk vector boson signal from the Sun

In this experiment we aim to detect Kaluza-Klein (KK) excitations of the bulk gauge field, emitted in a bremsstrahlung process on solar plasma constituents, by looking at a process analogous to the photoelectric effect inside the HPGe detector. Using a generic feature of the underlying effective theory that the unknown four-dimensional gauge coupling is independent of the number of extra large dimensions $\ensuremath{\delta}$, we show that the expected number of events in the detector is insensitive to the true scale of quantum gravity for $\ensuremath{\delta}=2$. With the entire data collection time of 202 days in the energy interval 1.7--3.8 keV, the number of events detected was as low as $1.1\ifmmode\times\else\texttimes\fi{}{10}^{6}$, compared to $2.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ from the expected high multiplicity of the solar KK excitations for $\ensuremath{\delta}=2$. Hence, our bound from the presumed existence of new forces associated with additional gauge bosons actually conforms with very stringent bounds set from various astrophysical considerations. Baring any modifications of the infrared part of the KK spectrum, this bound would therefore rule out the possibility of observing any signal at the LHC for $\ensuremath{\delta}=2$. Although a dependence on the fundamental scale referring to $(4+\ensuremath{\delta})$-dimensional gravity turns on again for $\ensuremath{\delta}=3$, the experimental sensitivity of the present setup proves insufficient to draw any constraint for $\ensuremath{\delta}g2$.

Operational definition of (brane-induced) space–time and constraints on the fundamental parameters

First we contemplate the operational definition of space–time in four dimensions in light of basic principles of quantum mechanics and general relativity and consider some of its phenomenological consequences. The quantum gravitational fluctuations of the background metric that comes through the operational definition of space–time are controlled by the Planck scale and are therefore strongly suppressed. Then we extend our analysis to the braneworld setup with low fundamental scale of gravity. It is observed that in this case the quantum gravitational fluctuations on the brane may become unacceptably large. The magnification of fluctuations is not linked directly to the low quantum gravity scale but rather to the higher-dimensional modification of Newton's inverse square law at relatively large distances. For models with compact extra dimensions the shape modulus of extra space can be used as a most natural and safe stabilization mechanism against these fluctuations.

Top production from black holes at the LHC

In theories with large extra dimension and with low quantum gravity scale near a TeV, it is expected that TeV-scale black holes to be produced in proton-proton collisions at the LHC with the center of mass energy of 14 TeV. Since the black holes temperature can be around 1 TeV, top quark production is expected from them via Hawking radiation. Within the Standard Model of particle physics top quarks are produced via strong interaction in $t\bar{t}$ pairs or via electroweak interaction singly. Therefore, black holes can be the new source of top quark production. In this article we present the total cross sections and transverse momentum distributions of top quark production from black holes at the LHC. We find that the top quarks from black holes tend to reside at very high transverse momentum region so it can be a very useful signature for the black holes at the LHC.

Quantum gravity at a TeV and the renormalization of Newton’s constant

We examine whether renormalization effects can cause Newton's constant to change dramatically with energy, perhaps even reducing the scale of quantum gravity to the TeV region without the introduction of extra dimensions. We examine a model that realizes this possibility and describe experimental signatures from the production of small black holes.

TIME MACHINE AT THE LHC

Recently, black hole and brane production at CERN's Large Hadron Collider (LHC) has been widely discussed. We suggest that there is a possibility to test causality at the LHC. We argue that if the scale of quantum gravity is of the order of few TeVs, proton-proton collisions at the LHC could lead to the formation of time machines (spacetime regions with closed timelike curves) which violate causality. One model for the time machine is a traversable wormhole. We argue that the traversable wormhole production cross section at the LHC is of the same order as the cross section for the black hole production. Traversable wormholes assume violation of the null energy condition (NEC) and an exotic matter similar to the dark energy is required. Decay of the wormholes/time machines and signatures of time machine events at the LHC are discussed.

Black holes and quantum gravity at the LHC

We argue that the highly studied black hole signatures based on thermal multiparticle final states are very unlikely and only occur in a very limited parameter regime if at all. However, we show that if the higher-dimensional quantum gravity scale is low, it should be possible to study quantum gravity in the context of higher dimensions through detailed compositeness-type searches.

Study of Time Lags inHETE‐2Gamma‐Ray Bursts with Redshift: Search for Astrophysical Effects and a Quantum Gravity Signature

The study of time lags between spikes in gamma-ray burst light curves in different energy bands as a function of redshift may lead to the detection of effects due to quantum gravity. We present an analysis of 15 gamma-ray bursts with measured redshift, detected by the HETE-2 mission, in order to measure time lags related to astrophysical effects and search for a quantum gravity signature in the framework of an extradimensional string model. The wavelet transform method is used both for denoising the light curves and for detecting sharp transitions. The use of photon-tagged data allows us to consider various energy ranges and to evaluate systematic effects due to selections and cuts. The analysis of maxima and minima of the light curves leads to no significant quantum gravity effect. A lower limit at the 95% confidence level on the quantum gravity scale parameter of 2 × 1015 GeV is set.

Black hole bound on the number of species and quantum gravity at CERN 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/root N. This bound suggests a novel solution to the hierarchy problem in which there are N approximate to 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 Lambda(G)approximate to M-Planck/root N and a new description is needed around this scale. In particular, black holes smaller than Lambda(-1)(G) are already no longer semiclassical. The nature of the completion is model dependent. One natural possibility is that Lambda(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.

Cargèse Lectures on Brane Induced Gravity

A brief introduction is given to the subject of brane induced gravity. The 5D example is discussed in detail. The 4D laws of gravity are obtained on a brane embedded in an infinite volume extra space, where the problem of stabilization of the volume modulus is absent. The theory has two classically disjoint branches of solutions -- the conventional and self-accelerated one. The conventional branch gives a perturbatively stable model of a metastable graviton, with potentially testable predictions within the Solar system. The self-accelerated branch, on the other hand, provides an existence proof for an idea that the accelerated expansion of the Universe could be due to modified gravity. The issue of perturbative stability of the self-accelerated branch is obscured by a breakdown of the conventional perturbative expansion. However, a certain exact non-perturbative solution found in hep-th/0612016 exhibits a net negative gravitational mass, while this mass is positive on the conventional branch. This suggest that the self-accelerated solution must be non-perturbatively unstable. A proposal to overcome this problem in an extension of the original model, that also allows for the quantum gravity scale to be unrestricted, is briefly discussed.

Phenomenological description of quantum gravity inspired modified classical electrodynamics

We discuss a large class of phenomenological models incorporating quantum gravity motivated corrections to electrodynamics. The framework is that of electrodynamics in a birefringent and dispersive medium with non-local constitutive relations, which are considered up to second order in the inverse of the energy characterizing the quantum gravity scale. The energy-momentum tensor, Green functions and frequency dependent refraction indices are obtained, leading to departures from standard physics. The effective character of the theory is also emphasized by introducing a frequency cutoff Ω. The analysis of its effects upon the standard notion of causality is performed, showing that in the radiation regime (Ω R ≫ 1) the expected corrections of the order (ω/Ω) n get further suppressed by highly oscillating terms proportional to $$\sin(\Omega R), \cos(\Omega R)$$ , thus forbiding causality violations to show up in the corresponding observational effects.

Decoherence in quantum gravity: issues and critiques

An increasing number of papers have appeared in recent years on decoherence in quantum gravity at the Planck energy. We discuss the meaning of decoherence in quantum gravity starting from the common notion that quantum gravity is a theory for the microscopic structures of spacetime, and invoking some generic features of quantum decoherence from the open systems viewpoint. We dwell on a range of issues bearing on this process including the relation between statistical and quantum, noise from effective field theory, the meaning of stochasticity, the origin of non-unitarity and the nature of nonlocality in this and related contexts. To expound these issues we critique on two representative theories: One claims that decoherence in quantum gravity scale leads to the violation of CPT symmetry at sub-Planckian energy which is used to explain today's particle phenomenology. The other uses this process in place with the Brownian motion model to prove that spacetime foam behaves like a thermal bath. A companion paper will deal with intrinsic and fundamental decoherence which also bear on issues in classical and quantum gravity.

Baryogenesis from gravitational decay of TeV particles in theories with low scalegravity

In models with the fundamental gravity scale in the TeV range, early cosmology is quitedifferent from the standard picture, because the universe must have arisen at a muchlower temperature and the electroweak symmetry was probably never restored. Inthis context, baryogenesis appears to be problematic: if the physics involved isessentially that of the standard model, ‘conventional’ non-conserving baryon numberprocesses are completely negligible at such low temperatures. In this paper weshow that the observed matter–antimatter asymmetry of the universe may begenerated by gravitational decay of TeV mass particles: such objects can be out ofequilibrium after inflation and, if their mass is of the same order of magnitude asthe true quantum gravity scale, they can quickly decay through a black holeintermediate state, violating global symmetries, in particular, baryon number.In this context, we take advantage of the fact that the ‘Sakharov conditions’for baryogenesis can be more easily satisfied with a low fundamental scale ofgravity.

Spacetime foam at a TeV

Motivated by recent interest in TeV-scale gravity and especially by the possibility of fast baryon decay mediated by virtual black holes, we study another dangerous aspect of spacetime foam interactions: lepton flavor violation. We correlate existing limits on gravity- induced decoherence in the neutrino sector with a lower bound on the scale of quantum gravity, and find that if spacetime foam interactions do not allow an S-matrix description the UV cutoff is well beyond the electroweak scale. This suggests that string theory provides the appropriate framework for description of quantum gravity at the TeV-scale.

Black holes at the IceCube neutrino telescope

If the fundamental Planck scale is about a TeV and the cosmic neutrino flux is at the Waxman-Bahcall level, quantum black holes are created daily in the Antarctic ice cap. We reexamine the prospects for observing such black holes with the IceCube neutrino-detection experiment. To this end, we first revise the black hole production rate by incorporating the effects of inelasticty, i.e., the energy radiated in gravitational waves by the multipole moments of the incoming shock waves. After that we study in detail the process of Hawking evaporation accounting for the black hole's large momentum in the lab system. We derive the energy spectrum of the Planckian cloud which is swept forward with a large, O(10{sup 6}), Lorentz factor. (It is noteworthy that the boosted thermal spectrum is also relevant for the study of near-extremal supersymmetric black holes, which could be copiously produced at the Large Hadron Collider.) In the semiclassical regime, we estimate the average energy of the boosted particles to be less than 20% the energy of the {nu} progenitor. Armed with such a constraint, we determine the discovery reach of IceCube by tagging on soft (relative to what one would expect from charged current standard model processes)more » muons escaping the electromagnetic shower bubble produced by the black hole's light descendants. The statistically significant 5{sigma} excess extends up to a quantum gravity scale {approx}1.3 TeV.« less

Can Pulsars be used to Probe Quantum Gravity?

Several radio pulsars have now been shown to emit giant pulses that align in phase with high energy emission rather than with their integrated pulse components. We investigate whether timing of giant and γ-ray pulses can be used to improve limits on the energy scale of quantum gravity by placing bounds on the energy dependence of the speed of light. We find good sources for this experiment are putative Crab-like pulsars in M31, which should in principle be detectable by the current generation of radio telescopes. However, the detection of pulsed emission from their γ-ray counterparts seems unlikely in the near future.

Why Black Hole Production in Scattering of Cosmic Ray Neutrinos Is Generically Suppressed

It has been argued that neutrinos originating from ultrahigh energy cosmic rays can produce black holes deep in the atmosphere in models with TeV-scale quantum gravity. Such black-hole events could be observed at the Auger Observatory. However, any phenomenologically viable model with a low scale of quantum gravity must explain how to preserve protons from rapid decay. We argue that the suppression of proton decay will also suppress lepton-nucleon scattering and hence black-hole production by scattering of ultrahigh energy cosmic ray neutrinos in the atmosphere. We discuss explicitly the split fermion solution to the problem of fast proton decay.