18719800
Phantom cosmologies and fermions
Form invariance transformations can be used for constructing phantom cosmologies starting with conventional cosmological models. In this work we reconsider the scalar field case and extend the discussion to fermionic fields, where the 'phantomization' process exhibits a new class of possible accelerated regimes. As an application we analyze the cosmological constant group for a fermionic seed fluid.
118675874
Cosmological model with interactions in the dark sector
A cosmological model for the present Universe is analyzed whose constituents are a non-interacting baryonic matter field and interacting dark matter and dark energy fields. The dark energy and dark matter are coupled through their effective barotropic indexes, which are considered as functions of the ratio of their energy densities. Two asymptotically stable cases are investigated for the ratio of the dark energy densities which have their parameters adjusted by considering best fits to Hubble function data. It is shown that the deceleration parameter, the density parameters, and the luminosity distance have the correct behavior which is expected for a viable present scenario of the Universe.
119226844
Modified holographic Ricci dark energy coupled to interacting dark matter and a non-interacting baryonic component
We examine a Friedmann–Robertson–Walker universe filled with interacting dark matter, modified holographic Ricci dark energy (MHRDE), and a decoupled baryonic component. The estimations of the cosmic parameters with Hubble data lead to an age of the universe of 13.17 Gyr and show that the MHRDE is free from the cosmic-age problem at low redshift (0≤z≤2) in contrast to holographic Ricci dark energy (HRDE) case. We constrain the parameters with the Union2 data set and contrast with the Hubble data. We also study the behavior of dark energy at early times by taking into account the severe bounds found at recombination era and/or at big bang nucleosynthesis. The inclusion of a non-interacting baryonic matter forces that the amount of dark energy at $z_{t} \sim\mathcal{O}(1)$ changes abruptly implying that Ωx(z≃1100)=0.03, so the bounds reported by the forecast of Planck and CMBPol experiments are more favored for the MHRDE model than in the case of HRDE cutoff. For the former model, we also find that at high redshift the fraction of dark energy varies from 0.006 to 0.002, then the amount of Ωx at the big bang nucleosynthesis era does not disturb the observed helium abundance in the universe provided that the bound Ωx(z≃1010)<0.21 is hold.
119269872
SELF-INTERACTING HOLOGRAPHIC DARK ENERGY
We investigate a spatially flat Friedmann–Robertson–Walker (FRW) universe where dark matter exchanges energy with a self-interacting holographic dark energy (SIHDE). Using the χ2-statistical method on the Hubble function, we obtain a critical redshift that seems to be consistent with both BAO and CMB data. We calculate the theoretical distance modulus for confronting with the observational data of SNe Ia for small redshift z ≤ 0.1 and large redshift 0.1 ≤ z ≤ 1.5. The model gets accelerated faster than the ΛCDM one and it can be a good candidate to alleviate the coincidence problem. We also examine the age crisis at high redshift associated with the old quasar APM 08279+5255.
119208212
Tachyon and Quintessence in Brane-Worlds
Using tachyon or quintessence fields along with a barotropic fluid on the brane we examine the different cosmological stages in a Friedmann-Robertson-Walker universe, from the first radiation scenario to the later era dominated by cosmic string networks. We introduce a new algorithm to generalize previous works on exact solutions and apply it to study tachyon and quintessence fields localized on the brane. We also explore the low and high energy regimes of the solutions. Besides, we show that the tachyon and quintessence fields are driven by an inverse power law potential. Finally, we find several simple exacts solutions for tachyon and/or quintessence fields.
119134009
Unified model of baryonic matter and dark components
Abstract We investigate an interacting two-fluid cosmological model and introduce a scalar field representation by means of a linear combination of the individual energy densities. Applying the integrability condition to the scalar field equation we show that this “exotic quintessence” is driven by an exponential potential and the two-fluid mixture can be considered as a model of three components. These components are associated with baryonic matter, dark matter and dark energy respectively. We use the Simon, Verde and Jimenez [J. Simon, L. Verde, R. Jimenez, Phys. Rev. D 71 (2005) 123001] determination of the redshift dependence of the Hubble parameter to constrain the current density parameters of this model. With the best fit density parameters we obtain the transition redshift between non-accelerated and accelerated regimes z acc = 0.66 and the time elapsed since the initial singularity t 0 = 19.8 Gyr . We study the perturbation evolution of this model and find that the energy density perturbation decreases with the cosmological time.
121404062
First Measurement of Parity-Nonconserving Neutron-Spin Rotation: The Tin Isotopes
A parity-nonconserving (PNC) neutron-spin rotation, ${\ensuremath{\phi}}_{\mathrm{PNC}}$, for neutrons passing through matter due, presumably, to the weak interaction is observed for the first time. For tin isotopes, the following values for ${\ensuremath{\phi}}_{\mathrm{PNC}}$ in ${10}^{\ensuremath{-}6}$ rad/cm are found: $^{124}\mathrm{Sn}$, +0.48\ifmmode\pm\else\textpm\fi{}1.49; $^{117}\mathrm{Sn}$, +36.7\ifmmode\pm\else\textpm\fi{}2.7; and natural Sn, +4.95\ifmmode\pm\else\textpm\fi{}0.93, with the positive sign corresponding to a right-handed rotation about the momentum. Evidence in $^{117}\mathrm{Sn}$ for a larger total cross section for positive-helicity neutrons than for negative-helicity ones is also reported.
122896950
Measurement of parity nonconserving neutron spin rotation in lanthanum
The rotation, phi/sub pnc/, of a neutron beam polarization vector due to parity nonconserving forces is measured in a lanthanum target. It is found that phi/sub pnc//l = -(21.9 +- 2.9) x 10- /sup 5/ rad/cm, where the negative sign corresponds to a left-handed rotation of the neutron spin about its momentum.
122190148
A mass-selecting apparatus to measure the energy loss of fission fragments in gases
Abstract We describe an apparatus combining solid-state and gas ionization chamber techniques to measure — for both fragments released in a fission event — the energy lost in a given gaseous range and the residual energy. This device allows to measure the energy loss in gases of fission fragments selecting their masses The performances of the apparatus are discussed referring to the data obtained by slowing down fission fragments from 252 Cf in gaseous argon.