672 research outputs found
Low red-shift effects of local structure on the Hubble parameter in presence of a cosmological constant
In order to estimate the effects of local structure on the Hubble parameter
we calculate the low-redshift expansion for and for
an observer at the center of a spherically symmetric matter distribution in
presence of a cosmological constant. We then test the accuracy of the formulae
comparing them with fully relativistic non perturbative numerical calculations
for different cases for the density profile. The low red-shift expansion we
obtain gives results more precise than perturbation theory since is based on
the use of an exact solution of Einstein's field equations. For larger density
contrasts the low red-shift formulae accuracy improves respect to the
perturbation theory accuracy because the latter is based on the assumption of a
small density contrast, while the former does not rely on such assumption.
The formulae can be used to take into account the effects on the Hubble
expansion parameter due to the monopole component of the local structure. If
the observations will show deviations from the prediction
compatible with the formulae we have derived, this could be considered an
independent evidence of the existence of a local inhomogeneity, and the
formulae could be used to determine the characteristics of this local
structure.Comment: 14 pages, 4 figure
Reconstructing the metric of the local Universe from number counts observations
Number counts observations available with new surveys such as the Euclid
mission will be an important source of information about the metric of the
Universe. We compute the low red-shift expansion for the energy density and the
density contrast using an exact spherically symmetric solution in presence of a
cosmological constant. At low red-shift the expansion is more precise than
linear perturbation theory prediction. We then use the local expansion to
reconstruct the metric from the monopole of the density contrast. We test the
inversion method using numerical calculations and find a good agreement within
the regime of validity of the red-shift expansion. The method could be applied
to observational data to reconstruct the metric of the local Universe with a
level of precision higher than the one achievable using perturbation theory.Comment: 18 pages,5 figure
Are primordial black holes produced by entropy perturbations in single field inflationary models?
We show that in single field inflationary models the super-horizon evolution
of curvature perturbations on comoving slices , which can cause
the production of primordial black holes (PBH), is not due to entropy
perturbations, but to the background evolution effect on the conversion between
entropy and curvature perturbations. We derive a general relation between the
time derivative of comoving curvature perturbations and entropy perturbations,
in terms of a conversion factor depending on the background evolution. Contrary
to previous results derived in the uniform density gauge assuming the gradient
term can be neglected on super-horizon scales, the relation is valid on any
scale for any minimally coupled single scalar field model, also on sub-horizon
scales where gradient terms are large.
We apply it to the case of quasi-inflection inflation, showing that while
entropy perturbations are decreasing, can grow on super-horizon
scales, due to a large increase of the conversion factor. This happens in the
time interval during which a sufficiently fast decrease of the equation of
state transforms into a growing mode that in slow-roll models would be a
decaying mode. The same mechanism also explains the super-horizon evolution of
in globally adiabatic systems, for which entropy perturbations
vanish on any scale, such as ultra slow-roll inflation and its generalizations
The MESS of cosmological perturbations
We introduce two new effective quantities for the study of comoving curvature
perturbations : the space dependent effective sound speed (SESS) and the
momentum dependent effective sound speed (MESS) . We use the SESS and the MESS
to derive a new set of equations which can be applied to any system described
by an effective stress-energy-momentum tensor (EST), including multi-fields
systems, supergravity and modified gravity theories. We show that this approach
is completely equivalent to the standard one and it has the advantage of
requiring to solve only one differential equation for instead of a
system, without the need of explicitly computing the evolution of entropy
perturbations. The equations are valid for perturbations respect to any
arbitrary flat spatially homogeneous background, including any inflationary and
bounce model.
As an application we derive the equation for for multi-fields
models and show that observed features of the primordial curvature perturbation
spectrum are compatible with the effects of an appropriate local variation of
the MESS in momentum space. The MESS is the natural quantity to parametrize in
a model independent way the effects produced on curvature perturbations by
multi-fields systems, particle production and modified gravity theories and
could be conveniently used in the analysis of LSS observations, such as the
ones from the upcoming EUCLID mission or CMB radiation measurements.Comment: We study the MESS of cosmological perturbations, version accepted in
Physics Letters
Directional dependence of the local estimation of and the non perturbative effects of primordial curvature perturbations
Recent measurements of the cosmic microwave background (CMB) radiation have
shown an apparent tension with the present value of the Hubble parameter
inferred from local observations of supernovae, which look closer, i.e.
brighter, than what is expected in a homogeneous model with a value of
equal to the one estimated from CMB observations. We examine the possibility
that such a discrepancy is the consequence of the presence of a local
inhomogeneity seeded by primordial curvature perturbations, finding that a
negative peak of the order of less than two standard deviations could allow to
fit low red-shift supernovae observations without the need of using a value of
the Hubble parameter different from . The type of inhomogeneity we
consider does not modify the distance to the last scattering, making it
compatible with the constraints of the PLANCK mission data. Our calculation is
fully relativistic and non perturbative, and for this reason shows important
effects which were missed in the previous investigations using relativistic
perturbations or Newtonian approximations, because the structures seeded by
primordial curvature perturbations can be today highly non linear, and
relativist Doppler terms cannot be neglected.
Comparison with studies of local structure based on galaxy surveys and
luminosity density analysis reveals that the density profile we find could in
fact be compatible with the one obtained for the same region of sky where is
located most of the Cepheids used to calibrate the luminosity distance of the
supernovae employed for the local estimation, suggesting a possible
directional dependence and calibration bias which could be partially attributed
to the presence of the Sloan Great Wall.Comment: New updated references have been added. Some more information has
been included about the region of sky in which the density profile predicted
by our model is in agreement with luminosity density and galaxy surveys
observations. Our model predicts the consequent overall supernovae luminosity
distance calibration bias effect due to the presence in that region of the
Cepheid
Revisión del género Hermodice Kinberg, 1857 (Polychaeta: Amphinomidae)
The genus Hermodice Kinberg, 1857 was established with the species Aphrodita carunculata Pallas, 1766, based on the shape and development of the caruncle. Several species were later described within the genus; however, it is currently regarded as monotypical, with H. carunculata as a widespread species. An evaluation of available type and non-type specimens together with observations of living specimens has allowed the diagnostic features to be refined, and two new features have been included: the number of branchial filaments and the anal lobe. Consequently, in this study we have been able to confirm the differences between Hermodice and Pherecardia Horst, 1886. The type species, H. carunculata, has been redescribed, and H. nigrolineata Baird, 1870 has been re-established. Further, H. formosa (Quatrefages, 1866) has been transferred to Pherecardia, together with H. pennata Treadwell, 1906 and H. distincta Hoagland, 1920.El género Hermodice, Kinberg, 1857 se estableció con la especie Aphrodita carunculata Pallas, 1766, debido a la forma y desarrollo de la carúncula. Posteriormente, se describieron varias especies dentro del género. Sin embargo, actualmente se considera que el género es monotípico y que H. carunculata es una especie cosmopolita. La revisión de material tipo y especímenes adicionales, incluyendo ejemplares vivos, nos ha permitido redefinir las características diagnósticas e incorporar dos nuevos atributos: el número de filamentos branquiales y el lóbulo anal. Como consecuencia, ha sido posible confirmar las diferencias entre Hermodice y Pherecardia Horst, 1886. Así mismo, se redescribe la especie tipo H. carunculata y se reestablece H. nigrolineata Baird, 1870. Además, H. formosa (Quatrefages, 1866) se transfiere a Pherecardia, junto H. pennata Treadwell, 1906 y H. distincta Hoagland, 1920
Dodecaceria carolinae n. sp. (Polychaeta: Cirratulidae), una especie de aguas someras del mar Caribe noroccidental
The cirratulid genus Dodecaceria Örsted, 1843 is well characterized by having palps inserted dorsally or laterally and branchial filaments restricted to some anterior chaetigers. The genus has species recorded from many marine environments worldwide and species are differentiated mainly by the number of branchial filaments. In this contribution, D. carolinae n. sp. is described based upon extensive materials collected in the northwestern Caribbean Sea. This species is distinguished by having 12 pairs of branchiae in two distinctive sizes, the first three four times longer than the remaining ones, and by having spoon-shaped hooks from notopodia 14-19 and from neuropodia 13-18. An analysis of the intra-specific variability, together with a table of diagnostic features and a key to all known species, is also included.El género Dodecaceria Örsted, 1843 de la familia Cirratulidae, se caracteriza por la presencia de palpos insertados dorsal o lateralmente, y filamentos branquiales restringidos a lo largo de pocos setígeros anteriores. El género tiene especies registradas en muchos ambientes marinos mundiales y las especies se diferencian usando el número de filamentos branquiales principalmente. En esta contribución, D. carolinae n. sp., se describe en base a abundante material recolectado en el mar Caribe noroccidental. Esta especie se distingue por tener 12 pares de branquias en dos tamaños distintivos,
las tres primeras cuatro veces más largas que las restantes, y porque sus ganchos con forma de cuchara están presentes desde el notopodio 14-19 y el neuropodio 13-18. Se incluye también un análisis de la variabilidad intraespecífica, junto con una tabla comparativa de los atributos diagnósticos y una clave para todas las especies conocidas
- …