Browsing by Subject "Cosmologie"

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    Constraining scalar-tensor quintessence by cosmic clocks
    (2007) Capozziello, S; Dunsby, P K S; Piedipalumbo, E; Rubano, C
    Aims. We study scalar tensor theories of gravity with power law scalar field potentials as cosmological models for accelerating universe, using cosmic clocks. Methods. Scalar-tensor quintessence models can be constrained by identifying suitable cosmic clocks which allow one to select confidence regions for cosmological parameters. In particular, we constrain the characterizing parameters of non-minimally coupled scalar-tensor cosmological models which admit exact solutions of the Einstein field equations. Lookback time to galaxy clusters at low intermediate, and high redshifts is considered. The high redshift time-scale problem is also discussed in order to select other cosmic clocks such as quasars. Results. The presented models seem to work in all the regimes considered: the main feature of this approach is the fact that cosmic clocks are completely independent of each other, so that, in principle, it is possible to avoid bias due to primary, secondary and so on indicators in the cosmic distance ladder. Conclusions. We have used different methods to test cosmological models at low, intermediate and high redshift by different indicators: this seems to confirm independently these proposed dark energy models.
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    QPOs in cataclysmic variables and in X-ray binaries
    (2005) Kluźniak, W; Lasota, J-P; Abramowicz, M A; Warner, B
    Recent observations, reported by Warner and Woudt, of Dwarf Nova Oscillations (DNOs) exhibiting frequency drift, period doubling, and 1:2:3 harmonic structure, can be understood as disc oscillations that are excited by perturbations at the spin frequency of the white dwarf or of its equatorial layers. Similar quasi-periodic disc oscillations in black hole low-mass X-ray binary (LMXB) transients in a 2:3 frequency ratio show no evidence of frequency drift and correspond to two separate modes of disc oscillation excited by an internal resonance. Just as no effects of general relativity play a role in white dwarf DNOs, no stellar surface or magnetic field effects need be invoked to explain the black hole QPOs.