118697834
Can the conformal factor of the gravitational field be a physical degree of freedom
The role of the conformal factor was analysed in two gauge-invariant perturbative formulations. Using the classical and quantum linearized perturbation approach given by Mukhanov et al (1992 Phys. Rep. 215 203), the non-physical behaviour of the conformal perturbation as an isolated degree of freedom is shown. In the quantum gravity context, the restriction to conformal perturbations gives us an incompatibility with the dynamical equations.
119465918
Anisotropic k-essence cosmologies
We investigate a Bianchi type-I cosmology with k-essence and find the set of models which dissipate the initial anisotropy. There are cosmological models with extended tachyon fields and k-essence having a constant barotropic index. We obtain the conditions leading to a regular bounce of the average geometry and the residual anisotropy on the bounce. For constant potential, we develop purely kinetic k-essence models which are dust dominated in their early stages, dissipate the initial anisotropy, and end in a stable de Sitter accelerated expansion scenario. We show that linear k-field and polynomial kinetic function models evolve asymptotically to Friedmann-Robertson-Walker cosmologies. The linear case is compatible with an asymptotic potential interpolating between V{sub l}{proportional_to}{phi}{sup -{gamma}{sub l}}, in the shear dominated regime, and V{sub l}{proportional_to}{phi}{sup -2} at late time. In the polynomial case, the general solution contains cosmological models with an oscillatory average geometry. For linear k-essence, we find the general solution in the Bianchi type-I cosmology when the k field is driven by an inverse square potential. This model shares the same geometry as a quintessence field driven by an exponential potential.
119208307
CROSSING THE PHANTOM DIVIDE WITH k-ESSENCE IN BRANEWORLDS
We study a flat 3-brane in presence of a linear k field with nonzero cosmological constant �4. In this model the crossing of the phantom divide (PD) occurs when the k-essence energy density becomes negative. We show that in the high energy regime the effective equation of state has a resemblance of a modified Chaplygin gas while in the low energy regime it becomes linear. We find a scale factor that begins from a singularity and evolves to a de Sitter stable stage while other solutions have a super-accelerated regime and end with a big rip. We use the energy conditions to show when the effective equation of state of the brane-universe crosses the PD.
119112732
Comments on Unified Dark Energy and Dark Matter from a Scalar Field Different from Quintessence
In a recent paper by C. Gao, M. Kunz, A. Liddle and D. Parkinson [arXiv:0912.0949], the unification of dark matter and dark energy was explored within a theory containing a scalar field of non-Lagrangian type. This scalar field, different from the classic quintessence, can be obtained from the scalar field representation of an interacting two-fluid mixture described in the paper by L.P. Chimento and M. Forte [arXiv:0706.4142]
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.
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.
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.
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.