12 research outputs found
Inflation and Dark Matter Primordial Black Holes
In this thesis a broad range of single field models of inflation are analyzed in light of all relevant recent cosmological data, checking whether they can lead to the formation of long--lived Primordial Black Holes (PBHs) to serve as candidates for Dark Matter. To that end we calculate the spectral index of the power spectrum of primordial perturbations as well as its first and second derivatives. PBH formation is possible only if the spectral index increases significantly at small scales, {\it i.e.~}large wave number . Since current data indicate that the first derivative of the spectral index is negative at the pivot scale , PBH formation is only possible in the presence of a sizable and positive second derivative (``running of the running'') . Among the three small--field and five large--field inflation models we analyze, only one small--field model, the "running--mass" model, allows PBH formation, for a narrow range of parameters. We also note that none of the models we analyze can accord for a large and negative value of , which is weakly preferred by current data. Similarly, proving conclusively that the second derivative of the spectral index is positive would exclude all the large--field models we investigated
Hierarchical Merger of Primordial Black Holes in Dwarf Galaxies
We study the merger history of primordial black holes (PBHs) in a scenario
where they represent the dominant dark matter component of a typical dwarf
galaxies' core. We investigate the possibility of a sequence of collisions
resulting in a hierarchical merger of black holes and look at the final mass
spectrum in such {\it clusters}, which initially present a monochromatic
(single-mass) PBH population. Our study shows that the merging process results
in the transfer of about of the total mass of the core to the merger
products regardless of the initial mass of PBHs, with about of energy
radiated out in the form of gravitational waves. We find that, in the lighter
mass limit, black holes up to eight times more massive than the original
population can be formed within a Hubble time.Comment: 17 pages, 3 figures, updated reference
Running-Mass Inflation Model and Primordial Black Holes
We revisit the question whether the running-mass inflation model allows the
formation of Primordial Black Holes (PBHs) that are sufficiently long-lived to
serve as candidates for Dark Matter. We incorporate recent cosmological data,
including the WMAP 7-year results. Moreover, we include "the running of the
running" of the spectral index of the power spectrum, as well as the
renormalization group "running of the running" of the inflaton mass term. Our
analysis indicates that formation of sufficiently heavy, and hence long-lived,
PBHs still remains possible in this scenario. As a by-product, we show that the
additional term in the inflaton potential still does not allow significant
negative running of the spectral index.Comment: 22 pages, 6 figures, Refs. added, Published in JCA
Running Spectral Index and Formation of Primordial Black Hole in Single Field Inflation Models
A broad range of single field models of inflation are analyzed in light of
all relevant recent cosmological data, checking whether they can lead to the
formation of long-lived Primordial Black Holes (PBHs). To that end we calculate
the spectral index of the power spectrum of primordial perturbations as well as
its first and second derivatives. PBH formation is possible only if the
spectral index increases significantly at small scales, i.e. large wave number
. Since current data indicate that the first derivative of the
spectral index is negative at the pivot scale , PBH formation
is only possible in the presence of a sizable and positive second derivative
("running of the running") . Among the three small-field and five
large-field models we analyze, only one small-field model, the "running mass"
model, allows PBH formation, for a narrow range of parameters. We also note
that none of the models we analyze can accord for a large and negative value of
, which is weakly preferred by current data.Comment: 26 pages, 5 figures, Refs. added, Minor textual change; version to
appear in JCA
Dilaton stabilization by massive fermion matter
The study started in a former work about the Dilaton mean field stabilization
thanks to the effective potential generated by the existence of massive
fermions, is here extended. Three loop corrections are evaluated in addition to
the previously calculated two loop terms. The results indicate that the Dilaton
vacuum field tend to be fixed at a high value close to the Planck scale, in
accordance with the need for predicting Einstein gravity from string theory.
The mass of the Dilaton is evaluated to be also a high value close to the
Planck mass, which implies the absence of Dilaton scalar signals in modern
cosmological observations. These properties arise when the fermion mass is
chosen to be either at a lower bound corresponding to the top quark mass, or
alternatively, at a very much higher value assumed to be in the grand
unification energy range. One of the three 3-loop terms is exactly evaluated in
terms of Master integrals. The other two graphs are however evaluated in their
leading logarithm correction in the perturbative expansion. The calculation of
the non leading logarithmic contribution and the inclusion of higher loops
terms could made more precise the numerical estimates of the vacuum field value
and masses, but seemingly are expected not to change the qualitative behavior
obtained. The validity of the here employed Yukawa model approximation is
argued for small value of the fermion masses with respect to the Planck one. A
correction to the two loop calculation done in the previous work is here
underlined.Comment: 18 pages, 5 figures, the study was extended and corrections on the
former calculations and redaction were done. The paper had been accepted for
publication in "Astrophysics and Space Science
Axiology
Axions might play a crucial role for the solution of the strong CP problem and explanation of cold dark matter in the universe. In addition they may find applications in the formulation of inflationary models for the early universe and can serve as candidates for quintessence. We show that all these phenomena can be described within a single framework exhibiting a specific pattern of mass scales: the axionic see-saw. We also discuss the role of supersymmetry (susy) in this axionic system in two specific examples: weak scale susy in the (multi) TeV range and tele-susy with a breakdown scale coinciding with the decay constant of the QCD axion: f(a) similar to 10(11)-10(12) GeV