9,220 research outputs found
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
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
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
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
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