Accretion

Ruzmaǐkina, T. V.; Wuchter, G.

doi:10.1007/1-4020-4520-4_2

Cited by 5 publications

(6 citation statements)

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“…More precisely, every higherorder term always appears as a contribution of order two in the equations of motion. For example, a term like R 2 gives fourth order equations [39], R R gives sixth order equations [38,40], R 2 R gives eighth order equations [41] and so on. By a conformal transformation, any 2nd -order of derivation corresponds to a scalar field: for example, fourth -order gravity gives Einstein plus one scalar field, sixth order gravity gives Einstein plus two scalar fields and so on [38,42].…”

Section: Introductionmentioning

confidence: 99%

Reconciling dark energy models withf(R)theories

2005

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Higher order theories of gravity have recently attracted a lot of interest as alternative candidates to explain the observed cosmic acceleration without the need of introducing any scalar field. A critical ingredient is the choice of the function f (R) of the Ricci scalar curvature entering the gravity Lagrangian and determining the dynamics of the universe. We describe an efficient procedure to reconstruct f (R) from the Hubble parameter H depending on the redshift z. Using the metric formulation of f (R) theories, we derive a third order linear differential equation for f (R(z)) which can be numerically solved after setting the boundary conditions on the basis of physical considerations. Since H(z) can be reconstructed from the astrophysical data, the method we present makes it possible to determine, in principle, what is the f (R) theory which best reproduces the observed cosmological dynamics. Moreover, the method allows to reconcile dark energy models with f (R) theories finding out what is the expression of f (R) which leads to the same H(z) of the given quintessence model. As interesting examples, we consider "quiessence" (dark energy with constant equation of state) and the Chaplygin gas. PACS numbers: 98.80.-k, 04.50+h, 98.80.Jk, 98.80.Es * Corresponding author, email: winny@na.infn.it dubbed quintessence [ 8,9]. These models typically involve scalar fields with a particular class of potentials, allowing the vacuum energy to become dominant only recently (see [10] for comprehensive reviews). Although quintessence by a scalar field is the most studied candidate for dark energy, it generally does not avoid ad hoc fine tuning to solve the coincidence problem.

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Section: Introductionmentioning

confidence: 99%

Reconciling dark energy models withf(R)theories

2005

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“…The most promising explanation for the large-scale galactic fields has been the dynamo mechanism, with the required seeds coming either from local astrophysical processes, such as battery effects, or from primordial magnetogenesis (for recent reviews, see Grasso & Rubinstein 2001;Widrow 2002) The linear evolution of large scale magnetic fields and their implications for structure formation have been studied by several authors (see, e.g. Ruzmaikina & Ruzmaikin 1971;Wasserman 1978;Papadopoulos & Esposito 1982;Evans & Fennelly 1985;Tsagas & Barrow 1997;Barrow & Subramanian 1998;Tsagas & Maartens 2000). Certain aspects of the mildly non-linear clustering can be analysed in spherical symmetry, but this approximation inevitably breaks down as the collapse proceeds and any initially small anisotropies take over.…”

Section: Introductionmentioning

confidence: 99%

Magnetic field amplification in cold dark matter anisotropic collapse

Bruni

Maartens

Tsagas

2003

Monthly Notices of the Royal Astronomical Society

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We use the Zel'dovich approximation to analyse the amplification of magnetic fields in gravitational collapse of cold dark matter during the mildly non‐linear regime, and identify two key features. First, the anisotropy of collapse effectively eliminates one of the magnetic components, confining the field in the pancake plane. Secondly, in agreement with recent numerical simulations, we find that the shear anisotropy can amplify the magnetic field well beyond the isotropic case. Our results suggest that the magnetic strengths observed in spiral and disc galaxies today might have originated from seeds considerably weaker than previous estimates.

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“…Including nonlinear terms of the various curvature tensors (Riemann, Ricci, Weyl) and nonminimally coupled terms in the effective action of gravity has become, recently, a very common trend from quantum field theory side and cosmology [1]. The basic motivation for studying such theories comes from the fact that they provide a possible approach to quantum gravity from a perturbative point of view.…”

Section: Introductionmentioning

confidence: 99%