6,721 research outputs found
Tradeoff between Smoother and Sooner "Little Rip"
There exists dark energy models that predict the occurrence of "little rip".
At the point of little rip the Hubble rate and its cosmic time derivative
approach infinity, which is quite similar to the big rip singularity except
that the former happens at infinite future while the latter at a finite cosmic
time; both events happen in the future and at high energies. In the case of the
big rip, a combination of ultra-violet and infra-red effects can smooth its
doomsday. We therefore wonder if the little rip can also be smoothed in a
similar way. We address the ultra-violet and infra-red effects in general
relativity through a brane-world model with a Gauss-Bonnet term in the bulk and
an induced gravity term on the brane. We find that the little rip is
transformed in this case into a sudden singularity, or a "big brake". Even
though the big brake is smoother than the little rip in that the Hubble rate is
finite at the event, the trade-off is that it takes place sooner, at a finite
cosmic time. In our estimate, the big brake would happen at roughly 1300Gyr.Comment: 9 pages, 4 figures. RevTex4-1. Title modified and discussion
expanded. Version accepted in European Physical Journal
Constraints on single entity driven inflationary and radition eras
We present a model that attempts to fuse the inflationary era and the
subsequent radiation dominated era under a unified framework so as to provide a
smooth transition between the two. The model is based on a modification of the
generalized Chaplygin gas. We constrain the model observationally by mapping
the primordial power spectrum of the scalar perturbations to the latest data of
WMAP7. We compute as well the spectrum of the primordial gravitational waves as
would be measured today.Comment: 4 pages, 6 figures. Contribution to the Spanish Relativity Meeting
(ERE2011), Madrid, Spai
Cosmological Imprints of a Generalized Chaplygin Gas Model for the Early Universe
We propose a phenomenological model for the early universe where there is a
smooth transition between an early "quintessence" phase and a radiation
dominated era. The matter content is modelled by an appropriately modified
Chaplygin gas for the early universe. We constrain the model observationally by
mapping the primordial power spectrum of the scalar perturbations to the latest
data of WMAP7. We compute as well the spectrum of the primordial gravitational
waves as would be measured today. We show that the high frequencies region of
the spectrum depends on the free parameter of the model and most importantly
this region of the spectrum can be within the reach of future gravitational
waves detectors.Comment: 10 pages, 8 figures. RevTex
New Constructions of Zero-Correlation Zone Sequences
In this paper, we propose three classes of systematic approaches for
constructing zero correlation zone (ZCZ) sequence families. In most cases,
these approaches are capable of generating sequence families that achieve the
upper bounds on the family size () and the ZCZ width () for a given
sequence period ().
Our approaches can produce various binary and polyphase ZCZ families with
desired parameters and alphabet size. They also provide additional
tradeoffs amongst the above four system parameters and are less constrained by
the alphabet size. Furthermore, the constructed families have nested-like
property that can be either decomposed or combined to constitute smaller or
larger ZCZ sequence sets. We make detailed comparisons with related works and
present some extended properties. For each approach, we provide examples to
numerically illustrate the proposed construction procedure.Comment: 37 pages, submitted to IEEE Transactions on Information Theor
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