Child Universes in the Laboratory

Although cosmology is usually considered an observational science, where there is little or no space for experimentation, other approaches can (and have been) also considered. In particular, we can change rather drastically the above, more passive, observational perspective and ask the following question: could it be possible, and how, to create a universe in a laboratory? As a matter of fact, this seems to be possible, according to at least two different paradigms; both of them help to evade the consequences of singularity theorems. In this contribution we will review some of these models and we will also discuss possible extensions and generalizations, by paying a critical attention to the still open issues as, for instance, the detectability of child universes and the properties of quantum tunnelling processes.Comment: To appear in the proceedings of the "9th Workshop on what comes beyond the standard model", Bled, Slovenia, September 16-26, 2006. LaTe

Unified Dark Energy-Dark Matter model with Inverse Quintessence

We consider a model where both dark energy and dark matter originate from the coupling of a scalar field with a non-conventional kinetic term to, both, a metric measure and a non-metric measure. An interacting dark energy/dark matter scenario can be obtained by introducing an additional scalar that can produce non constant vacuum energy and associated variations in dark matter. The phenomenology is most interesting when the kinetic term of the additional scalar field is ghost-type, since in this case the dark energy vanishes in the early universe and then grows with time. This constitutes an "inverse quintessence scenario", where the universe starts from a zero vacuum energy density state, instead of approaching it in the future.Comment: 13 pages; minor changes with references adde

Universes out of almost empty space

Baby universes (inflationary or non--inflationary) are regions of spacetime that disconnect from the original ambient spacetime, which we take to be asymptotically flat spacetime. A particular kind of baby universe solution, involving string--like matter, is studied to show that it can be formed by ``investing'' an arbitrarily small amount of energy, i.e. it can appear from an almost flat space at the classical level. Since this possibility has not yet been clearly recognized in the literature, we then discuss in detail its properties, relevance and possible generalizations.Comment: LaTeX (RevTeX), 4 pages, 1 figure. A few typos corrected and some references added; structure streamlined (changes reflected in a slightly modified abstract); small, non-substantial modifications in the figur

Towards the compression of parton densities through machine learning algorithms

One of the most fascinating challenges in the context of parton density function (PDF) is the determination of the best combined PDF uncertainty from individual PDF sets. Since 2014 multiple methodologies have been developed to achieve this goal. In this proceedings we first summarize the strategy adopted by the PDF4LHC15 recommendation and then, we discuss about a new approach to Monte Carlo PDF compression based on clustering through machine learning algorithms.Comment: 4 pages, 4 figures, to appear in the proceedings of 50th Rencontres de Moriond, QCD and High Energy Interactions, La Thuile, Italy, March 201

Helix surfaces in the Berger Sphere

We characterize helix surfaces in the Berger sphere, that is surfaces which form a constant angle with the Hopf vector field. In particular, we show that, locally, a helix surface is determined by a suitable 1-parameter family of isometries of the Berger sphere and by a geodesic of a 2-torus in the 3-dimensional sphere.Comment: The main theorem has been modified and improved. Final version to appear in Israel Journal of Mathematic

A proof of the irreversibility of renormalization group flows in four dimensions

We present a proof of the irreversibility of renormalization group flows, i.e. the c-theorem for unitary, renormalizable theories in four (or generally even) dimensions. Using Ward identities for scale transformations and spectral representation arguments, we show that the c-function based on the trace of the energy-momentum tensor (originally suggested by Cardy) decreases monotonically along renormalization group trajectories. At fixed points this c-function is stationary and coincides with the coefficient of the Euler density in the trace anomaly, while away from fixed points its decrease is due to the decoupling of positive--norm massive modes.Comment: 22 pages, 2 figures, plain tex with harvmac and epsf; several typos corrected; final version, to be published in Nucl. Phys.

Solitons as Key Parts to Produce a Universe in the Laboratory

Cosmology is usually understood as an observational science, where experimentation plays no role. It is interesting, nevertheless, to change this perspective addressing the following question: what should we do to create a universe, in a laboratory? It appears, in fact, that this is, in principle, possible according to at least two different paradigms; both allow to circumvent singularity theorems, i.e. the necessity of singularities in the past of inflating domains which have the required properties to generate a universe similar to ours. The first of them is substantially classical, and is built up considering solitons which collide with surrounding topological defects, generating an inflationary domain of space-time. The second is, instead, partly quantum and considers the possibility of tunnelling of past-non-singular regions of spacetime into an inflating universe, following a well-known instanton proposal. We are, here, going to review some of these models, as well as highlight possible extensions, generalizations and the open issues (as for instance the detectability of child universes and the properties of quantum tunnelling processes) that still affect the description of their dynamics. In doing so we will remark how the works on this subject can represent virtual laboratories to test the role that fundamental principles of physics (particularly, the interplay of quantum and general relativistic realms) played in the formation of our universe.Comment: Based on a talk given at the 2006 Biennial Meeting of the International Association for Relativistic Dynamics (IARD06). 11 pages, LaTe

Geodesics on an invariant surface

We study the geodesics on an invariant surface of a three dimensional Riemannian manifold. The main results are: the characterization of geodesic orbits; a Clairaut's relation and its geometric interpretation in some remarkable three dimensional spaces; the local description of the geodesics; the explicit description of geodesic curves on an invariant surface with constant Gauss curvature.Comment: 14 pages, 1 figur