We show that loitering at high redshifts ( z \lower - 1.5 pt \vbox { \hbox { \hbox to 0.0 pt { $ > $ } \lower 5.3 pt \vbox { \hbox { $ \sim$ } } % } } 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 _ { 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 .