A robust upper limit on N_eff from BBN, circa 2011

We derive here a robust bound on the effective number of neutrinos from constraints on primordial nucleosynthesis yields of deuterium and helium. In particular, our results are based on very weak assumptions on the astrophysical determination of the helium abundance, namely that the minimum effect of stellar processing is to keep constant (rather than increase, as expected) the helium content of a low-metallicity gas. Using the results of a recent analysis of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95 % C.L., quite independently of measurements on the baryon density from cosmic microwave background anisotropy data and of the neutron lifetime input. In our approach, we also find that primordial nucleosynthesis alone has no significant preference for an effective number of neutrinos larger than the standard value. The ~2 sigma hint sometimes reported in the literature is thus driven by CMB data alone and/or is the result of a questionable regression protocol to infer a measurement of primordial helium abundance.Comment: 5 pages, 1 table, 1 figure. Minor improvements and extensions in the analysis, clarifications and reference added, conclusions slightly strengthened. Matches version published in Phys. Lett.

Strongest model-independent bound on the lifetime of Dark Matter

Dark Matter is essential for structure formation in the late Universe so it must be stable on cosmological time scales. But how stable exactly? Only assuming decays into relativistic particles, we report an otherwise model independent bound on the lifetime of Dark Matter using current cosmological data. Since these decays affect only the low-$\ell$ multipoles of the CMB, the Dark Matter lifetime is expected to correlate with the tensor-to-scalar ratio $r$ as well as curvature $\Omega_k$. We consider two models, including $r$ and $r+\Omega_k$ respectively, versus data from Planck, WMAP, WiggleZ and Baryon Acoustic Oscillations, with or without the BICEP2 data (if interpreted in terms of primordial gravitational waves). This results in a lower bound on the lifetime of CDM given by 160Gyr (without BICEP2) or 200Gyr (with BICEP2) at 95% confidence level.Comment: 15 pages, 5 figures. Prepared for submission to JCA

Multi-momentum and multi-flavour active-sterile neutrino oscillations in the early universe: role of neutrino asymmetries and effects on nucleosynthesis

We perform a study of the flavour evolution in the early universe of a multi-flavour active-sterile neutrino system with parameters inspired by the short-baseline neutrino anomalies. In a neutrino-symmetric bath a "thermal" population of the sterile state would quickly grow, but in the presence of primordial neutrino asymmetries a self-suppression as well as a resonant sterile neutrino production can take place, depending on temperature and chosen parameters. In order to characterize these effects, we go beyond the usual average momentum and single mixing approximations and consider a multi-momentum and multi-flavour treatment of the kinetic equations. We find that the enhancement obtained in this case with respect to the average momentum approximation is significant, up to \sim 20 % of a degree of freedom. Such detailed and computationally demanding treatment further raises the asymmetry values required to significantly suppress the sterile neutrino production, up to large and preferentially net asymmetries |L_{\nu}| > O(10^{-2}). For such asymmetries, however, the active-sterile flavour conversions happen so late that significant distortions are produced in the electron (anti)neutrino spectra. The larger |L_{\nu}|, the more the impact of these distortions takes over as dominant cosmological effect, notably increasing the 4 He abundance in primordial nucleosynthesis (BBN). The standard expression of the primordial yields in terms of the effective number of neutrinos and asymmetries is also greatly altered. We numerically estimate the magnitude of such effects for a few representative cases and comment on possible implications for forthcoming cosmological measurements.Comment: v2 (12 pages, 4 eps figures) revised version. Comments added, references updated. Matches the version published in PR

Effects of non-standard neutrino-electron interactions on relic neutrino decoupling

We consider the decoupling of neutrinos in the early Universe in presence of non-standard neutral current neutrino-electron interactions (NSI). We first discuss a semi-analytical approach to solve the relevant kinetic equations and then present the results of fully numerical and momentum-dependent calculations, including flavor neutrino oscillations. We present our results in terms of both the effective number of neutrino species (N_eff) and the impact on the abundance of He-4 produced during Big Bang Nucleosynthesis. We find that, for NSI parameters within the ranges allowed by present laboratory data, non-standard neutrino-electron interactions do not essentially modify the density of relic neutrinos nor the bounds on neutrino properties from cosmological observables, such as their mass. Nonetheless, the presence of neutrino-electron NSI may enhance the entropy transfer from electron-positron pairs into neutrinos instead of photons, up to a value of N_eff=3.12. This is almost three times the correction to N_eff=3 that appears for standard weak interactions.Comment: 23 pages, 5 figures. To be published in NP

Future constraints on neutrino isocurvature perturbations in the curvaton scenario

In the curvaton scenario, residual isocurvature perturbations can be imprinted in the cosmic neutrino component after the decay of the curvaton field, implying in turn a non-zero chemical potential in the neutrino distribution. We study the constraints that future experiments like Planck, SPIDER or CMBPol will be able to put on the amplitude of isocurvature perturbations in the neutrino component. We express our results in terms of the square root \gamma of the non-adiabaticity parameter \alpha and of the extra relativistic degrees of freedom \Delta N_eff. Assuming a fiducial model with purely adiabatic fluctuations, we find that Planck (SPIDER) will be able to put the following upper limits at the 1sigma level: \gamma < 5.3x10^-3 (1.2x10^-2) and \Delta N_eff < 0.16 (0.40) . CMBPol will further improve these constraints to \gamma < 1.5x10^-3 and \Delta N_eff < 0.043. Finally, we recast these bounds in terms of the background neutrino degeneracy parameter \xi\ and the corresponding perturbation amplitude \sigma_\xi, and compare with the bounds on \xi\ that can be derived from Big Bang Nucleosynthesis.Comment: 6 pages, 2 figures. References added. Matches version accepted for publication in Phys. Rev.

Prospects for detection of very high-energy emission from GRB in the context of the external shock model

The detection of the 100 GeV-TeV emission by a gamma-ray burst (GRB) will provide an unprecedented opportunity to study the nature of the central engine and the interaction between the relativistic flow and the environment of the burst's progenitor. In this paper we show that there are exciting prospects of detecting from the burst by MAGIC high-energy (HE) emission during the early X-ray flaring activity and, later, during the normal afterglow phase. We also identify the best observational strategy, trigger conditions and time period of observation. We determine the expected HE emission from the flaring and afterglow phases of GRBs in the context of the external shock scenario and compare them with the MAGIC threshold. We find that an X-ray flare with the average properties of the class can be detected in the 100 GeV range by MAGIC, provided that z<0.7. The requested observational window with MAGIC should then start from 10-20 s after the burst and cover about 1000-2000 s. Furthermore, we demonstrate that there are solid prospects of detecting the late afterglow emission in the same energy range for most of the bursts with z<0.5 if the density of the external medium is n> a few cm^-3. In this case, the MAGIC observation shall extend to about 10-20 ks. We provide recipes for tailoring this prediction to the observational properties of each burst,in particular the fluence in the prompt emission and the redshift, thus allowing an almost real time decision procedure to decide whether to continue the follow-up observation of a burst at late times.Comment: 6 pages, 2 color figures, accepted for the pubblication in A&

Relic neutrino decoupling including flavour oscillations

In the early universe, neutrinos are slightly coupled when electron-positron pairs annihilate transferring their entropy to photons. This process originates non-thermal distortions on the neutrino spectra which depend on neutrino flavour, larger for nu_e than for nu_mu or nu_tau. We study the effect of three-neutrino flavour oscillations on the process of neutrino decoupling by solving the momentum-dependent kinetic equations for the neutrino spectra. We find that oscillations do not essentially modify the total change in the neutrino energy density, giving N_eff=3.046 in terms of the effective number of neutrinos, while the small effect over the production of primordial 4He is increased by O(20%), up to 2.1 x 10^{-4}. These results are stable within the presently favoured region of neutrino mixing parameters.Comment: 18 pages, 2 figure

Well Design Challenges in Geothermal Energy Applications

Geothermal resources represent precious energy sources to ensure sustainable power generation. As proposed in the majority of the future sustainable energy scenarios, geothermal energy exploitation is going to play a significant role in the energy mix to meet carbon neutrality target. Upon the different technologies involved, geothermal wells constitute the core and turning point for proper fluid/heat mining. Indeed, the number of suitable candidates for geothermal applications could be significantly enhanced by overcoming a series of wells related technological issues. Therefore, the object of this work is to provide a general overview of the principal challenges that characterized well design and construction in geothermal applications which are mainly related to the type of geological system and its relative temperature level. As a matter of fact, reservoir temperature guides most of the choices referring to geothermal systems not only in the selection of the final energy application purpose (direct use, power generation, combined heat and power) but also in well design definition. Based on temperature range, geothermal fields are usually grouped in enthalpy classes (low, medium and high) referring to fields characterized by similar energy potential. From a well design and construction perspective, the low and medium enthalpy classes, in the range of temperature lower than 150 °C, do not present specific criticalities. On the contrary, high enthalpy scenarios, for temperatures higher than 170 °C, present many challenges for most of the current drilling and completion technologies. Even though some field applications exist in high/ultra-high enthalpy scenarios, they still present an elevated risk of failure. Therefore, dedicated studies shall be conducted for all the elements involved in the well construction process such as: drilling fluids, cement slurry, metallurgy, drilling and completion equipment to properly account for their specific technical limitations. In this framework, a clear picture of the actual technical gaps constitutes the starting point for current and next research activities. In the close future, the growing interest in geothermal applications will surely boost the born and development of dedicated tools to unlock the enormous potential of geothermal energy

Swift-UVOT detection of GRB 050318

We present observations of GRB 050318 by the Ultra-Violet and Optical Telescope (UVOT) on-board the Swift observatory. The data are the first detections of a Gamma Ray Burst (GRB) afterglow decay by the UVOT instrument, launched specifically to open a new window on these transient sources. We showcase UVOTs ability to provide multi-color photometry and the advantages of combining UVOT data with simultaneous and contemporaneous observations from the high-energy detectors on the Swift spacecraft. Multiple filters covering 1,800-6,000 Angstroms reveal a red source with spectral slope steeper than the simultaneous X-ray continuum. Spectral fits indicate that the UVOT colors are consistent with dust extinction by systems at z = 1.2037 and z = 1.4436, redshifts where absorption systems have been pre-identified. However, the data can be most-easily reproduced with models containing a foreground system of neutral gas redshifted by z = 2.8 +/- 0.3. For both of the above scenarios, spectral and decay slopes are, for the most part, consistent with fireball expansion into a uniform medium, provided a cooling break occurs between the energy ranges of the UVOT and Swifts X-ray instrumentation.Comment: 15 pages, 4 figures, ApJ Letters, in pres

Testing a new view of Gamma Ray Burst Afterglows

The optical and X-ray light-curves of long Gamma Ray Bursts (GRBs) often show a complex evolution and in most cases do not track each other. This behaviour can not be easily explained by the simplest standard afterglow models. A possible interpretation is to consider the observed optical and X-ray light-curves as the sum of two separate components. This scenario requires the presence of a spectral break between these bands. One of the aims of this work is to test whether such a break is present within the observed Swift XRT energy range. We analyse the X-ray afterglow spectra of a sample of 33 long GRBs with known redshift, good optical photometry and published estimate of the host galaxy dust absorption A_V(host). We find that indeed in 7 bright events a broken power-law provides a fit to the data that is better than a single power-law model. For 8 events, instead, the X-ray spectrum is better fitted by a single power-law. We discuss the role of these breaks in connection to the relation between the host hydrogen column density N_H(host) and A_V(host) and check the consistency of the X-ray spectral breaks with the optical bands photometry. We analyse the optical to X-ray spectral energy distributions at different times and find again consistency with two components interpretation.Comment: 13 pages, 10 figures, accepted for publication in MNRA