Graviton time delay and a speed limit for small black holes in Einstein-Gauss-Bonnet theory

Camanho, Edelstein, Maldacena and Zhiboedov have shown that gravitons can experience a negative Shapiro time delay, i.e. a time advance, in Einstein-Gauss-Bonnet theory. They studied gravitons propagating in singular "shock-wave" geometries. We study this effect for gravitons propagating in smooth black hole spacetimes. For a small enough black hole, we find that gravitons of appropriate polarisation, and small impact parameter, can experience time advance. Such gravitons can also exhibit a deflection angle less than $\pi$, characteristic of a repulsive short-distance gravitational interaction. We discuss problems with the suggestion that the time advance can be used to build a "time machine". In particular, we argue that a small black hole cannot be boosted to a speed arbitrarily close to the speed of light, as would be required in such a construction.This work was supported by ERC grant No. ERC-2011-StG 279363-HiDGR and by an STFC studentshipThis is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/JHEP11(2015)10

On the local well-posedness of Lovelock and Horndeski theories

We investigate local well-posedness of the initial value problem for Lovelock and Horndeski theories of gravity. A necessary condition for local well-posedness is strong hyperbolicity of the equations of motion. Even weak hyperbolicity can fail for strong fields so we restrict to weak fields. The Einstein equation is known to be strongly hyperbolic in harmonic gauge so we study Lovelock theories in harmonic gauge. We show that the equation of motion is always weakly hyperbolic for weak fields but, in a generic weak-field background, it is not strongly hyperbolic. For Horndeski theories, we prove that, for weak fields, the equation of motion is always weakly hyperbolic in any generalized harmonic gauge. For some Horndeski theories there exists a generalized harmonic gauge for which the equation of motion is strongly hyperbolic in a weak-field background. This includes “k-essence” like theories. However, for more general Horndeski theories, there is no generalized harmonic gauge for which the equation of motion is strongly hyperbolic in a generic weak-field background. Our results show that the standard method used to establish local well-posedness of the Einstein equation does not extend to Lovelock or general Horndeski theories. This raises the possibility that these theories may not admit a well-posed initial value problem even for weak fields

Causality violation, gravitational shockwaves and UV completion

The effective actions describing the low-energy dynamics of QFTs involving gravity generically exhibit causality violations. These may take the form of superluminal propagation or Shapiro time advances and allow the construction of "time machines", i.e. spacetimes admitting closed non-spacelike curves. Here, we discuss critically whether such causality violations may be used as a criterion to identify unphysical effective actions or whether, and how, causality problems may be resolved by embedding the action in a fundamental, UV complete QFT. We study in detail the case of photon scattering in an Aichelburg-Sexl gravitational shockwave background and calculate the phase shifts in QED for all energies, demonstrating their smooth interpolation from the causality-violating effective action values at low-energy to their manifestly causal high-energy limits. At low energies, these phase shifts may be interpreted as backwards-in-time coordinate jumps as the photon encounters the shock wavefront, and we illustrate how the resulting causality problems emerge and are resolved in a two-shockwave time machine scenario. The implications of our results for ultra-high (Planck) energy scattering, in which graviton exchange is modelled by the shockwave background, are highlighted.Comment: 42 pages, 15 figures, updated reference

Reverse Engineering and Additive Manufacturing towards the design of 3D advanced scaffolds for hard tissue regeneration

3D Printing and Additive Manufacturing technologies represent powerful tools for the direct fabrication of lightweight structures with improved and tunable properties. In current research, Fused Deposition Modeling (FDM)/3D fiber deposition technique was considered to design 3D multifunctional scaffolds with complex morphology, tailored biological, mechanical and mass transport properties. Polymeric and nanocomposite materials were used for scaffold design and optimization, with a particular focus on bone tissue engineering. As an example, poly(epsilon-caprolactone) (PCL), and PCL-based nanocomposite scaffolds were fabricated and analyzed. The effects of structural and morphological features (i.e., sequence of stacking, fiber spacing, pore size and geometry) as well as of nanoparticle inclusion on the mechanical performances were reported. Furthermore, the possibility to design 3D customized scaffolds for mandibular defect regeneration (i.e., symphysis and ramus) was also considered

Analyzing the Role of Magnetic Features in Additive Manufactured Scaffolds for Enhanced Bone Tissue Regeneration

The concept of magnetic guidance has opened a wide range of perspectives in the field of tissue regeneration. Accordingly, the aim of the current research is to design magnetic responsive scaffolds for enhanced bone tissue regeneration. Specifically, magnetic nanocomposite scaffolds are additively manufactured using 3D fibre deposition technique. The mechanical and magnetic properties of the fabricated scaffolds are first assessed. The role of magnetic features on the biological performances is properly analyzed