3 research outputs found
Accelerating Universe and Cosmological Perturbation in the Ghost Condensate
In the simplest Higgs phase of gravity called ghost condensation, an
accelerating universe with a phantom era (w<-1) can be realized without ghost
or any other instabilities. In this paper we show how to reconstruct the
potential in the Higgs sector Lagrangian from a given cosmological history
(H(t), \rho(t)). This in principle allows us to constrain the potential by
geometrical information of the universe such as supernova distance-redshift
relation. We also derive the evolution equation for cosmological perturbations
in the Higgs phase of gravity by employing a systematic low energy expansion.
This formalism is expected to be useful to test the theory by dynamical
information of large scale structure in the universe such as cosmic microwave
background anisotropy, weak gravitational lensing and galaxy clustering.Comment: 30 pages; typos corrected; version accepted for publication in JCA
Modified-Source Gravity and Cosmological Structure Formation
One way to account for the acceleration of the universe is to modify general
relativity, rather than introducing dark energy. Typically, such modifications
introduce new degrees of freedom. It is interesting to consider models with no
new degrees of freedom, but with a modified dependence on the conventional
energy-momentum tensor; the Palatini formulation of theories is one
example. Such theories offer an interesting testing ground for investigations
of cosmological modified gravity. In this paper we study the evolution of
structure in these ``modified-source gravity'' theories. In the linear regime,
density perturbations exhibit scale dependent runaway growth at late times and,
in particular, a mode of a given wavenumber goes nonlinear at a higher redshift
than in the standard CDM model. We discuss the implications of this
behavior and why there are reasons to expect that the growth will be cut off in
the nonlinear regime. Assuming that this holds in a full nonlinear analysis, we
briefly describe how upcoming measurements may probe the differences between
the modified theory and the standard CDM model.Comment: 22 pages, 6 figures, uses iopart styl