106 research outputs found
Towards understanding the ultraviolet behavior of quantum loops in infinite-derivative theories of gravity
In this paper we will consider quantum aspects of a non-local,
infinite-derivative scalar field theory - a depiction of a
covariant infinite-derivative, non-local extension of Einstein's general
relativity which has previously been shown to be free from ghosts around the
Minkowski background. The graviton propagator in this theory gets an
exponential suppression making it , thus providing
strong prospects of resolving various classical and quantum divergences. In
particular, we will find that at -loop, the -point function is still
divergent, but once this amplitude is renormalized by adding appropriate
counter terms, the ultraviolet (UV) behavior of all other -loop diagrams as
well as the -loop, -point function remains well under control. We will go
on to discuss how one may be able to generalize our computations and arguments
to arbitrary loops.Comment: 52 pages, 8 figures; v3: Published in Classical and Quantum Gravity;
v4: minor revision
Cosmological perturbations from statistical thermal fluctuations
Cosmological perturbations due to statistical thermal fluctuations in a
single fluid characterized by an arbitrary equation of state are computed.
Formulas to predict the scalar and tensor perturbation spectra and
nongaussianity parameters at a given temperature are derived. These results are
relevant to any cosmological scenario where cosmic structures may have been
seeded thermally instead of originating purely from quantum vacuum
fluctuations.Comment: 13 pages, no figures. v2: references adde
Can we have a stringy origin behind ?
Inspired by the current observations that the ratio of the abundance of dark
energy , and the matter density, , is such that
, we provide a string inspired
phenomenological model where we explain this order one ratio, the smallness of
the cosmological constant, and also the recent cosmic acceleration. We observe
that any effective theory motivated by a higher dimensional physics provides
radion/dilaton couplings to the standard model and the dark matter component
with different strengths. Provided radion/dilaton is a dynamical field we show
that tracks and dominates very recently.Comment: 4 pages, discussions added on recent acceleration, typos corrected,
references adde
Atick-Witten Hagedorn Conjecture, near scale-invariant matter and blue-tilted gravity power spectrum
We will provide an interesting new mechanism to generate almost scale invariant seed density perturbations with a red spectrum, while keeping the gravitational wave spectrum blue-tilted in a stringy thermal contracting phase at temperatures beyond the Hagedorn temperature. This phase is often referred to as the Hagedorn phase where the free energy has been conjectured by Atick and Witten to grow more slowly than ordinary radiation. The primordial fluctuations are created by the statistical thermal fluctuations determined by the partition function, rather than quantum vacuum driven fluid dynamical fluctuations. Our mechanism assumes a non-singular bounce
Current Acceleration from Dilaton and Stringy Cold Dark Matter
We argue that string theory has all the ingredients to provide us with
candidates for the cold dark matter and explain the current acceleration of our
Universe. In any generic string compactification the dilaton plays an important
role as it couples to the Standard Model and other heavy non-relativistic
degrees of freedom such as the string winding modes and wrapped branes, we
collectively call them stringy cold dark matter. These couplings are
non-universal which results in an interesting dynamics for a rolling dilaton.
Initially, its potential can track radiation and matter while beginning to
dominate the dynamics recently, triggering a phase of acceleration. This
scenario can be realized as long as the dilaton also couples strongly to some
heavy modes. We furnish examples of such modes. We provide analytical and
numerical results and compare them with the current supernovae result. This
favors certain stringy candidates.Comment: 16 pages, 4 figures (colour
Wiggles in the cosmic microwave background radiation: echoes from non-singular cyclic-inflation
In this paper we consider a unique model of inflation where the universe
undergoes rapid asymmetric oscillations, each cycle lasting millions of Planck
time. Over many-many cycles the space-time expands to mimic the standard
inflationary scenario. Moreover, these rapid oscillations leave a distinctive
periodic signature in ln(k) in the primordial power spectrum, where k denotes
the comoving scale. The best fit parameters of the cyclic-inflation model
provides a very good fit to the 7-year WMAP data.Comment: Computational details and a figure adde
Introductory Review of Swarm Intelligence Techniques
With the rapid upliftment of technology, there has emerged a dire need to
fine-tune or optimize certain processes, software, models or structures, with
utmost accuracy and efficiency. Optimization algorithms are preferred over
other methods of optimization through experimentation or simulation, for their
generic problem-solving abilities and promising efficacy with the least human
intervention. In recent times, the inducement of natural phenomena into
algorithm design has immensely triggered the efficiency of optimization process
for even complex multi-dimensional, non-continuous, non-differentiable and
noisy problem search spaces. This chapter deals with the Swarm intelligence
(SI) based algorithms or Swarm Optimization Algorithms, which are a subset of
the greater Nature Inspired Optimization Algorithms (NIOAs). Swarm intelligence
involves the collective study of individuals and their mutual interactions
leading to intelligent behavior of the swarm. The chapter presents various
population-based SI algorithms, their fundamental structures along with their
mathematical models.Comment: Submitted to Springe
- …