arXiv subjects

Fabio Briscese

Publications and source records attributed to Fabio Briscese.

28 records · Page 2Linked to original sources

arXiv2015

Origin of nonlinear contribution to the shift of the critical temperature in atomic Bose-Einstein condensates

We discuss a possible origin of the experimentally observed nonlinear contribution to the shift $ΔT_{c}=T_c-T_{c}^{0}$ of the critical temperature $T_{c}$ in an atomic Bose-Einstein condensate (BEC) with respect to the critical temperature $T_{c}^{0}$ of an ideal gas. We found that accounting for a nonlinear (quadratic) Zeeman effect (with applied magnetic field closely matching a Feshbach resonance field $B_0$) in the mean-field approximation results in a rather significant renormalization of the field-free nonlinear contribution $b_{2}$, namely $ΔT_{c}/T_{c}^{0}\simeq b_{2}^{\ast }(a/λ_{T})^{2}$ (where $a$ is the s-wave scattering length, $λ_{T}$ is the thermal wavelength at $T_{c}^{0}$) with $b_{2}^{\ast }=γ^{2}b_{2}$ and $γ=γ(B_0)$. In particular, we predict $b_{2}^{\ast }\simeq 42.3$ for the $B_{0}\simeq 403G$ resonance observed in the $\ ^{39}K$ BEC.

cond-mat.quant-gas↗

arXiv2014

On the role of hyperfine-interactions-mediated Zeeman effect in the condensation temperature shift of trapped atomic Bose-Einstein condensates

We discuss the effect of interatomic interactions on the condensation temperature $T_c$ of a laboratory atomic Bose-Einstein condensate under the influence of an external trapping magnetic field. We predict that accounting for hyperfine interactions mediated Zeeman term in the mean-field approximation produces, in the case of the $403 \, G$ Feshbach resonance in the $|F,m_F> = |1,1>$ hyperfine state of a $^{39}K$ condensate, with $F$ the total spin of the atom, an experimentally observed (and not yet explained) shift in the condensation temperature $ΔT_{c}/T_{c}^{0}=b^{*}_0+b^{*}_1 (a/λ_{T}) + b^{*}_2 (a/λ_{T})^2$ with $b^{*}_0 \simeq 0.0002$, $b^{*}_1 \simeq -3.4$ and $b^{*}_2 \simeq 47$, where $a$ is the s-wave scattering length, and $λ_T$ is the thermal wavelength at $T_{c}^{0}$. Generic expressions for the coefficients $b^*_0$, $b^*_1$ and $b^*_2$ are also obtained, which can be used to predict the temperature shift for other Feshbach resonances of $^{39}K$ or other atomic condensates.

cond-mat.quant-gas↗

arXiv2014

Isochronous Spacetimes

gr-qc↗

arXiv2014

Isochronous Cosmologies

The possibility has been recently demonstrated to manufacture (nonrelativistic, Hamiltonian) many-body problems which feature an isochronous time evolution with an arbitrarily assigned period $T$ yet mimic with good approximation, or even exactly, any given many-body problem (within a large, physically relevant, class) over times $\tilde{T}$ which may also be arbitrarily large (but of course such that $\tilde{T}<T$). Purpose and scope of this paper is to explore the possibility to extend this finding to a general relativity context. For simplicity we restrict our consideration to the case of homogeneous and isotropic metrics and show that, via an approach analogous to that used for the nonrelativistic many-body problem, a class of homogeneous and isotropic cyclic solutions of Einstein's equations may be obtained. For these solutions the duration of the cycles does not depend on the initial conditions, so we call these models isochronous cosmologies. We give a physical interpretation of such metrics and in particular we show that they may behave arbitrarily closely, or even identically, to the Friedman-Robertson-Walker solutions of Einstein's equations for an arbitrarily long time (of course shorter than their period, which can also be assigned arbitrarily), so that they may reproduce all the satisfactory phenomenological features of the standard cosmological $Λ$-CDM model in a portion of their cycle; while these isochronous cosmologies may be geodesically complete and therefore singularity-free.

gr-qc↗

arXiv2014

Super-renormalizable or finite completion of the Starobinsky theory

The recent Planck data of Cosmic Microwave Background (CMB) temperature anisotropies support the Starobinsky theory in which the quadratic Ricci scalar drives cosmic inflation. We build up a multi-dimensional quantum consisted ultraviolet completion of the model in a phenomenological "bottom-up approach". We present the maximal class of theories compatible with unitarity and (super-)renormalizability or finiteness which reduces to the Starobinsky theory in the low-energy limit. The outcome is a maximal extension of the Krasnikov-Tomboulis-Modesto theory including an extra scalar degree of freedom besides the graviton field. The original theory was afterwards independently discovered by Biswas-Gerwick-Koivisto-Mazumdar starting from first principles. We explicitly show power counting super-renormalizability or finiteness (in odd dimensions) and unitarity (no ghosts) of the theory. Any further extension of the theory is non-unitary confirming the existence of at most one single extra degree of freedom, the scalaron. A mechanism to achieve the Starobinsky theory in string (field) theory is also investigated at the end of the paper.

hep-th↗

arXiv2013

Interaction effects on atomic laboratory trapped Bose-Einstein condensates

We discuss the effect of inter-atoms interactions on the condensation temperature $T_c$ of an atomic laboratory trapped Bose-Einstein condensate. We show that, in the mean-field Hartree-Fock and semiclassical approximations, interactions produce a shift $ΔT_{c}/T_{c}^{0} \approx b_1 (a/λ_{T_c}) + b_2 (a/λ_{T_c})^2 + ψ[a/λ_{T_c}]$ with $a$ the s-wave scattering length, $λ_T$ the thermal wavelength and $ψ[a/λ_{T_c}]$ a non-analytic function such that $ψ[0] = ψ'[0] = ψ"[0] = 0$ and $|ψ"'[0]| = \infty$. Therefore, with no more assumptions than Hartree-Fock and semiclassical approximations, interaction effecs are perturbative to second order in $a/λ_{T_c}$ and the expected non-perturbativity of physical quantities at critical temperature appears only to third order. We compare this finding with different results by other authors, which are based on more than the Hartree-Fock and semiclassical approximations. Moreover, we obtain an analytical estimation for $b_2 \simeq 18.8$ which improves a previous numerical result. We also discuss how the discrepancy between $b_2$ and the empirical value of $b_2 = 46 \pm 5$ may be explained with no need to resort to beyond-mean field effects.

cond-mat.quant-gas↗

arXiv2013

Inflation in (Super-)renormalizable Gravity

We investigate a (super-)renormalizable and ghost-free theory of gravity, showing that under a natural (exponential) ansatz of the form factor and a suitable truncation it can give rise to the Starobinsky inflationary theory in cosmological frameworks, and thus offering a theoretical justification of its origin. We study the corresponding inflationary evolution and we examine the generation of curvature perturbations, adapting the $f(R)$-like equations in a symmetry-reduced FLRW metric. Furthermore, we analyze how the ultraviolet regime of a simply renormalizable and unitary theory of gravity is also compatible with the Starobinsky action, and hence we show that such a theory could account for an inflationary phase of the Universe in the ultraviolet regime.

hep-th↗

arXiv2012

On the true nature of renormalizability in Horava-Lifshitz gravity

We argue that the true nature of the renormalizability of Horava-Lifshitz gravity lies in the presence of higher order spatial derivatives and not in the anisotropic Lifshitz scaling of space and time. We discuss the possibility of constructing a higher order spatial derivatives model that has the same renormalization properties of Horava-Lifshitz gravity but that does not make use of the Lifshitz scaling. In addition, the state-of-the-art of the Lorentz symmetry restoration in Horava-Lifshitz-type theories of gravitation is reviewed.

hep-th↗

arXiv2011

Viability of complex self-interacting scalar field as dark matter

We study the viability of a complex scalar field $χ$ with self-interacting potential $ V = m^χ_0/2 \, |χ|^2 + h \, |χ|^4$ as dark matter. The scalar field is produced at reheating through the decay of the inflaton field and then, due to the self-interaction, a Bose-Einstein condensate of $χ$ particles forms. The condensate represents dark matter in that model. We analyze the cosmological evolution of the model, stressing how, due to the presence of the self-interaction, the model naturally admits dark matter domination at late times, thus avoiding any fine tuning on the energy density of the scalar field at early times. Finally we give a lower bound for the size of dark matter halos at present time and we show that our model is compatible with dark matter halos greater than $0.1 \, Kpc$ and with BBN and CMB bounds on the effective number of extra neutrinos $Δ_ν^{eff}$. Therefore, the model is viable and for $h\simeq 10^{-4}-10^{-12}$ one obtains a mass $m^χ\simeq m^χ_0 \simeq 1-10^{-2} \, eV$ for dark matter particles from radiation-matter equality epoch to present time, but at temperatures $T_γ\gg 10 \, eV$, where $T_γ$ is the photons temperature, thermal corrections to $m^χ_0$ due to the self-coupling $h$ are dominant.

astro-ph.CO↗

arXiv2007

Twisted Hopf symmetries of canonical noncommutative spacetimes and the no-pure-boost principle

We study the twisted-Hopf-algebra symmetries of observer-independent canonical spacetime noncommutativity, for which the commutators of the spacetime coordinates take the form [x^{mu},x^{nu}]=i theta^{mu nu} with observer-independent (and coordinate-independent) theta^{mu nu}. We find that it is necessary to introduce nontrivial commutators between transformation parameters and spacetime coordinates, and that the form of these commutators implies that all symmetry transformations must include a translation component. We show that with our noncommutative transformation parameters the Noether analysis of the symmetries is straightforward, and we compare our canonical-noncommutativity results with the structure of the conserved charges and the "no-pure-boost" requirement derived in a previous study of kappa-Minkowski noncommutativity. We also verify that, while at intermediate stages of the analysis we do find terms that depend on the ordering convention adopted in setting up the Weyl map, the final result for the conserved charges is reassuringly independent of the choice of Weyl map and (the corresponding choice of) star product.

hep-th↗