Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-10-08
Phys.Rev. D71 (2005) 084018
Astronomy and Astrophysics
Astrophysics
Latex, 20 pages, 4 figures. Important new results include an appendix and the observation that the redshift of reionization in
Scientific paper
10.1103/PhysRevD.71.084018
We show that loitering at high redshifts ($z \gsim 6$) can easily arise in braneworld models of dark energy which, in addition to being spatially flat, also accelerate at late times. Loitering is characterized by the fact that the Hubble parameter dips in value over a narrow redshift range which we shall refer to as the `loitering epoch'. During loitering, density perturbations are expected to grow rapidly. In addition, since the expansion of the universe slows down, its age near loitering dramatically increases. An early epoch of loitering is expected to boost the formation of high redshift gravitationally bound systems such as $10^9 M_\odot$ black holes at $z \sim 6$ and lower-mass black holes and/or Population III stars at $z > 10$, whose existence could be problematic within the LCDM scenario. Loitering models also help to reduce the redshift of reionization from its currently (high) value of $z_{\rm reion} \simeq 17$ in LCDM cosmology, thus alleviating a significant source of tension between observations of the high-redshift universe and theoretical model building. Currently a loitering universe accelerates with an effective equation of state $w < -1$ thus mimicking phantom dark energy. Unlike phantom, however, the late-time expansion of the universe in our model is singularity free, and a universe that loitered in the past will approach a LCDM model asymptotically in the distant future.
Sahni Varun
Shtanov Yuri
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