We construct and analyze the Standard Model of electroweak and strong interactions in multiscale spacetimes with (i) weighted derivatives and (ii) q-derivatives. Both theories can be formulated in two different frames, called fractional and integer picture. By definition, the fractional picture is where physical predictions should be made. (i) In the theory with weighted derivatives, it is shown that gauge invariance and the requirement of having constant masses in all reference frames make the Standard Model in the integer picture indistinguishable from the ordinary one. Experiments involving only weak and strong forces are insensitive to a change of spacetime dimensionality also in the fractional picture, and only the electromagnetic and gravitational sectors can break the degeneracy. For the simplest multiscale measures with only one characteristic time, length and energy scale t à , l à and E à , we compute the Lamb shift in the hydrogen atom and constrain the multiscale correction to the ordinary result, getting the absolute upper bound t à < 10 −23 s. For the natural choice α 0 ¼ 1=2 of the fractional exponent in the measure, this bound is strengthened to t à < 10 −29 s, corresponding to l à < 10 −20 m and E à > 28 TeV. Stronger bounds are obtained from the measurement of the fine-structure constant.(ii) In the theory with q-derivatives, considering the muon decay rate and the Lamb shift in light atoms, we obtain the independent absolute upper bounds t à < 10 −13 s and E à > 35 MeV. For α 0 ¼ 1=2, the Lamb shift alone yields t à < 10

We use a top-down holographic model for strongly interacting quark matter to study the properties of neutron stars. When the corresponding equation of state (EOS) is matched with state-of-the-art results for dense nuclear matter, we consistently observe a first-order phase transition at densities between 2 and 7 times the nuclear saturation density. Solving the Tolman-Oppenheimer-Volkov equations with the resulting hybrid EOSs, we find maximal stellar masses in excess of two solar masses, albeit somewhat smaller than those obtained with simple extrapolations of the nuclear matter EOSs. Our calculation predicts that no quark matter exists inside neutron stars.

We derive the covariant equations of motion for Maxwell field theory and electrodynamics in multiscale spacetimes with weighted Laplacian. An effective spacetime-dependent electric charge of geometric origin naturally emerges from the theory, thus giving rise to a varying fine-structure constant. The theory is compared with other varying-coupling models, such as those with a varying electric charge or varying speed of light. The theory is also confronted with cosmological observations, which can place constraints on the characteristic scales in the multifractional measure. We note that the model considered here is fundamentally different from those previously proposed in the literature, either of the varying-e or varying-c persuasion.

It has been conjectured that the speed of sound in holographic models with UV fixed points has an upper bound set by the value of the quantity in conformal field theory. If true, this would set stringent constraints for the presence of strongly coupled quark matter in the cores of physical neutron stars, as the existence of two-solar-mass stars appears to demand a very stiff Equation of State. In this article, we present a family of counterexamples to the speed of sound conjecture, consisting of strongly coupled theories at finite density.The theories we consider include N = 4 super Yang-Mills at finite R-charge density and non-zero gaugino masses, while the holographic duals are Einstein-Maxwell theories with a minimally coupled scalar in a charged black hole geometry. We show that for a small breaking of conformal invariance, the speed of sound approaches the conformal value from above at large chemical potentials.

We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different compact star solutions: traditional neutron stars, strange quark stars, as well as two non-standard solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the stationary properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo. The assumptions underlying the viability of the different star types, in particular those corresponding to absolutely stable quark matter, are finally discussed at some length.

We study electroweak interactions in the multiscale theory with q-derivatives, a framework where spacetime has the typical features of a multifractal. In the simplest case with only one characteristic time, length, and energy scale t à , l à , and E à , we consider (i) the muon decay rate and (ii) the Lamb shift in the hydrogen atom, and constrain the corrections to the ordinary results. We obtain the independent absolute upper bounds (i) t à < 10 −13 s and (ii) E à > 35 MeV. Under some mild theoretical assumptions, the Lamb shift alone yields the even tighter ranges t à < 10 −27 s, l à < 10 −19 m, and E à > 450 GeV. To date, these are the first robust constraints on the scales at which the multifractal features of the geometry can become important in a physical process. DOI: 10.1103/PhysRevD.93.025005 The variety of theories of quantum gravity proposed in the last 30 years has highlighted two facts. First, that there are many languages and mathematical tools with which one can describe consistent quantum geometries and that, while some of these approaches are mutually exclusive, others can be related in nontrivial ways or even embedded into one another. Second, that despite their differences these approaches (including causal dynamical triangulations in the "de Sitter" phase, asymptotic safety, Hořava-Lifshitz gravity, nonlocal gravity, loop quantum gravity and spinfoams, noncommutative spacetimes, quantum black holes, and more [1]) share some distinct features. One of the most striking phenomena one comes across the landscape of quantum-gravity models is dimensional flow, the change of the dimension of spacetime (whenever a notion of spacetime emerges meaningfully in each approach) with the probed scale [2,3]. In any approach to quantum gravity, the effective dimension flows to four at low energies and large scales, where general relativity is an impeccable description of geometry. At small scales, however, the spectral dimension d S of spacetime can attain a completely different value, usually equal to or smaller than 2. The transition between the two regimes varies depending on the model but it is usually difficult to have it under full analytic control. For this and other reasons, most of the physical consequences of dimensional flow in contexts as diverse as quantum field theory (QFT) and cosmology remain elusive.Nevertheless, it is surprisingly easy to reproduce ad hoc the dimensional flow found in various quantum-gravity theories [4]. This is achieved by placing a field theory L½ϕ μν… on a geometry with action S ¼ R d 4 x v L, where vðxÞ ¼ v à ðtÞv à ðxÞ is a nondynamical profile (unrelated to the volume density ffiffiffiffiffiffi −g p in curved spacetimes) withThe parameter 0 < α < 1 is called fractional exponent (it can be different along different directions, but here this complication is not necessary). This geometry has Hausdorff dimension d H ¼ 4 at large scales Δl ≫ l à and late times Δt ≫ t à , while d H ¼ 4α < 4 at small scales and early times. The spectral dimension d S has a similar behavio...

According to common lore, Equations of State of field theories with gravity duals tend to be soft, with speeds of sound either below or around the conformal value of v s = 1/ √ 3. This has important consequences in particular for the physics of compact stars, where the detection of two solar mass neutron stars has been shown to require very stiff equations of state. In this paper, we show that no speed limit exists for holographic models at finite density, explicitly constructing examples where the speed of sound becomes arbitrarily close to that of light. This opens up the possibility of building hybrid stars that contain quark matter obeying a holographic equation of state in their cores.

A current challenge in condensed matter physics is the realization of strongly correlated, viscous electron fluids. These fluids can be described by holography, that is, by mapping them onto a weakly curved gravitational theory via gauge/gravity duality. The canonical system considered for realizations has been graphene. In this work, we show that Kagome systems with electron fillings adjusted to the Dirac nodes provide a much more compelling platform for realizations of viscous electron fluids, including non-linear effects such as turbulence. In particular, we find that in Scandium Herbertsmithite, the fine-structure constant, which measures the effective Coulomb interaction, is enhanced by a factor of about 3.2 as compared to graphene. We employ holography to estimate the ratio of the shear viscosity over the entropy density in Sc-Herbertsmithite, and find it about three times smaller than in graphene. These findings put the turbulent flow regime described by holography within the reach of experiments.