185 research outputs found
Friedmann cosmology with decaying vacuum density
Among the several proposals to solve the incompatibility between the observed
small value of the cosmological constant and the huge value obtained by quantum
field theories, we can find the idea of a decaying vacuum energy density,
leading from high values at early times of universe evolution to the small
value observed nowadays. In this paper we consider a variation law for the
vacuum density recently proposed by Schutzhold on the basis of quantum field
estimations in the curved, expanding background, characterized by a vacuum
density proportional to the Hubble parameter. We show that, in the context of
an isotropic and homogeneous, spatially flat model, the corresponding solutions
retain the well established features of the standard cosmology, and, in
addition, are in accordance with the observed cosmological parameters. Our
scenario presents an initial phase dominated by radiation, followed by a dust
era long enough to permit structure formation, and by an epoch dominated by the
cosmological term, which tends asymptotically to a de Sitter universe. Taking
the matter density equals to half of the vacuum energy density, as suggested by
observation, we obtain a universe age given by Ht = 1.1, and a decelerating
parameter equals to -1/2.Comment: Accepted for publication in General Relativity and Gravitatio
Problems and possibilities in fine-tuning of the Cepheid P-L relationship
Factors contributing to the scatter around the ridge-line period-luminosity
relationship are listed, followed by a discussion how to eliminate the adverse
effects of these factors (mode of pulsation, crossing number, temperature
range, reddening, binarity, metallicity, non-linearity of the relationship,
blending), in order to reduce the dispersion of the P-L relationship.Comment: 7 pages, 8 figures; accepted for publication in Astrophysics & Space
Scienc
Testing quantised inertia on galactic scales
Galaxies and galaxy clusters have rotational velocities apparently too fast
to allow them to be gravitationally bound by their visible matter. This has
been attributed to the presence of invisible (dark) matter, but so far this has
not been directly detected. Here, it is shown that a new model that modifies
inertial mass by assuming it is caused by Unruh radiation, which is subject to
a Hubble-scale (Theta) Casimir effect predicts the rotational velocity (v) to
be: v^4=2GMc^2/Theta (the Tully-Fisher relation) where G is the gravitational
constant, M is the baryonic mass and c is the speed of light. The model
predicts the outer rotational velocity of dwarf and disk galaxies, and galaxy
clusters, within error bars, without dark matter or adjustable parameters, and
makes a prediction that local accelerations should remain above 2c^2/Theta at a
galaxy's edge.Comment: 7 pages, 1 figure. Accepted for publication in Astrophysics and Space
Science on 27/7/201
Deformation of the Planetary Orbits Caused by the Time Dependent Gravitational Potential in the Universe
In the paper are studied the deformations of the planetary orbits caused by
the time dependent gravitational potential in the universe. It is shown that
the orbits are not axially symmetric and the time dependent potential does not
cause perihelion precession. It is found a simple formula for the change of the
orbit period caused by the time dependent gravitational potential and it is
tested for two binary pulsars.Comment: 7 page
The Large Magellanic Cloud: A power spectral analysis of Spitzer images
We present a power spectral analysis of Spitzer images of the Large
Magellanic Cloud. The power spectra of the FIR emission show two different
power laws. At larger scales (kpc) the slope is ~ -1.6, while at smaller ones
(tens to few hundreds of parsecs) the slope is steeper, with a value ~ -2.9.
The break occurs at a scale around 100-200 pc. We interpret this break as the
scale height of the dust disk of the LMC. We perform high resolution
simulations with and without stellar feedback. Our AMR hydrodynamic simulations
of model galaxies using the LMC mass and rotation curve, confirm that they have
similar two-component power-laws for projected density and that the break does
indeed occur at the disk thickness. Power spectral analysis of velocities
betrays a single power law for in-plane components. The vertical component of
the velocity shows a flat behavior for large structures and a power law similar
to the in-plane velocities at small scales. The motions are highly anisotropic
at large scales, with in-plane velocities being much more important than
vertical ones. In contrast, at small scales, the motions become more isotropic.Comment: 8 pages, 4 figures, talk presented at "Galaxies and their Masks",
celebrating Ken Freeman's 70-th birthday, Sossusvlei, Namibia, April 2010. To
be published by Springer, New York, editors D.L. Block, K.C. Freeman, & I.
Puerar
Reconstructing the primordial power spectrum from the CMB
We propose a straightforward and model independent methodology for
characterizing the sensitivity of CMB and other experiments to wiggles,
irregularities, and features in the primordial power spectrum. Assuming that
the primordial cosmological perturbations are adiabatic, we present a function
space generalization of the usual Fisher matrix formalism, applied to a CMB
experiment resembling Planck with and without ancillary data. This work is
closely related to other work on recovering the inflationary potential and
exploring specific models of non-minimal, or perhaps baroque, primordial power
spectra. The approach adopted here, however, most directly expresses what the
data is really telling us. We explore in detail the structure of the available
information and quantify exactly what features can be reconstructed and at what
statistical significance.Comment: 43 pages Revtex, 23 figure
The Local Velocity Anomaly
There is a velocity discontinuity at about 7 Mpc between the galaxies of the
Local Sheet that are moving together with low internal velocity dispersion and
the adjacent structures. The Local Sheet bounds the Local Void. The Local Sheet
is determined to have a peculiar velocity of 260 km/s away from the center of
the void. In order for this large velocity to be generated by an absence of
gravity, the Local Void must be at least 45 Mpc in diameter and be very empty.Comment: Invited review, "Galaxies in the Local Volume", Sydney, 8-13 July,
2007. eds. B. Koribalski & H. Jerjen, Astrophys. & Space Sci. Proceed. 10
pages with 7 figure
An HST/NICMOS view of the prototypical giant HII region NGC604 in M33
We present the first high-spatial resolution near-infrared (NIR) imaging of
NGC 604, obtained with the NICMOS camera aboard the Hubble Space Telescope
(HST). These NICMOS broadband images reveal new NIR point sources, clusters,
and diffuse structures. We found an excellent spatial correlation between the
8.4 GHz radio continuum and the 2.2mu-m nebular emission. Moreover, massive
young stellar object candidates appear aligned with these radio peaks,
reinforcing the idea that those areas are star-forming regions. Three different
scaled OB associations are recognized in the NICMOS images. The brightest NIR
sources in our images have properties that suggest that they are red supergiant
stars, of which one of them was previously known. This preliminary analysis of
the NICMOS images shows the complexity of the stellar content of the NGC 604
nebula.Comment: Paper presented in the Workshop "Young massive star clusters: initial
conditions and environments" (Granada, Spain - Sept 2007). Astrophysics &
Space Science in press, 7 pages, 4 figure
Cosmic microwave background and large scale structure limits on the interaction between dark matter and baryons
We study the effect on the cosmic microwave background (CMB) anisotropy and
large scale structure (LSS) power spectrum of a scattering interaction between
cold dark matter and baryons. This scattering alters the CMB anisotropy and LSS
spectrum through momentum transfer between the cold dark matter particles and
the baryons. We find that current CMB observations can put an upper limit on
the scattering cross section which is comparable with or slightly stronger than
previous disk heating constraints at masses greater than 1 GeV, and much
stronger at smaller masses. When large-scale structure constraints are added to
the CMB limits, our constraint is more stringent than this previous limit at
all masses. In particular, a dark matter-baryon scattering cross section
comparable to the ``Spergel-Steinhardt'' cross section is ruled out for dark
matter mass greater than 1 GeV.Comment: 8 pages, 2 figures, use RevTeX4, submitted to PRD replaced with
revised versio
Ultra Long Period Cepheids: a primary standard candle out to the Hubble flow
The cosmological distance ladder crucially depends on classical Cepheids
(with P=3-80 days), which are primary distance indicators up to 33 Mpc. Within
this volume, very few SNe Ia have been calibrated through classical Cepheids,
with uncertainty related to the non-linearity and the metallicity dependence of
their period-luminosity (PL) relation. Although a general consensus on these
effects is still not achieved, classical Cepheids remain the most used primary
distance indicators. A possible extension of these standard candles to further
distances would be important. In this context, a very promising new tool is
represented by the ultra-long period (ULP) Cepheids (P \geq 80 days), recently
identified in star-forming galaxies. Only a small number of ULP Cepheids have
been discovered so far. Here we present and analyse the properties of an
updated sample of 37 ULP Cepheids observed in galaxies within a very large
metallicity range of 12+log(O/H) from ~7.2 to 9.2 dex. We find that their
location in the colour(V-I)-magnitude diagram as well as their Wesenheit (V-I)
index-period (WP) relation suggests that they are the counterparts at high
luminosity of the shorter-period (P \leq 80 days) classical Cepheids. However,
a complete pulsation and evolutionary theoretical scenario is needed to
properly interpret the true nature of these objects. We do not confirm the
flattening in the studied WP relation suggested by Bird et al. (2009). Using
the whole sample, we find that ULP Cepheids lie around a relation similar to
that of the LMC, although with a large spread (~0.4 mag).Comment: 8 pages, 4 figures, accepted for publication in Astrophysics & Space
Scienc
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