3,233 research outputs found
A Primeval Magellanic Stream and Others
The Magellanic Stream might have grown out of tidal interactions at high
redshift, when the young galaxies were close together, rather than from later
interactions among the Magellanic Clouds and Milky Way. This is illustrated in
solutions for the orbits of Local Group galaxies under the cosmological
condition of growing peculiar velocities at high redshift. Massless test
particles initially near and moving with the Large Magellanic Cloud in these
solutions end up with distributions in angular position and redshift similar to
the Magellanic Stream, though with the usual overly prominent leading component
that the Milky Way corona might have suppressed. Another possible example of
the effect of conditions at high redshift is a model primeval stream around the
Local Group galaxy NGC 6822. Depending on the solution for Local Group dynamics
this primeval stream can end up with position angle similar to the HI around
this galaxy, and a redshift gradient in the observed direction. The gradient is
much smaller than observed, but might have been increased by dissipative
contraction. Presented also is an even more speculative illustration of the
possible effect of initial conditions, primeval stellar streams around M31.Comment: 23 pages, 7 figure
Holographic field theory models of dark energy in interaction with dark matter
We discuss two lagrangian interacting dark energy models in the context of
the holographic principle. The potentials of the interacting fields are
constructed. The models are compared with CMB distance information, baryonic
acoustic oscilations, lookback time and the Constitution supernovae sample. For
both models the results are consistent with a non vanishing interaction between
dark sectors - with more than three standard deviations of confidence for one
of them. Moreover, in both cases, the sign of coupling is consistent with dark
energy decaying into dark matter, alleviating the coincidence problem.Comment: arXiv admin note: substantial text overlap with arXiv:0912.399
Newtonian and Relativistic Cosmologies
Cosmological N-body simulations are now being performed using Newtonian
gravity on scales larger than the Hubble radius. It is well known that a
uniformly expanding, homogeneous ball of dust in Newtonian gravity satisfies
the same equations as arise in relativistic FLRW cosmology, and it also is
known that a correspondence between Newtonian and relativistic dust cosmologies
continues to hold in linearized perturbation theory in the marginally
bound/spatially flat case. Nevertheless, it is far from obvious that Newtonian
gravity can provide a good global description of an inhomogeneous cosmology
when there is significant nonlinear dynamical behavior at small scales. We
investigate this issue in the light of a perturbative framework that we have
recently developed, which allows for such nonlinearity at small scales. We
propose a relatively straightforward "dictionary"---which is exact at the
linearized level---that maps Newtonian dust cosmologies into general
relativistic dust cosmologies, and we use our "ordering scheme" to determine
the degree to which the resulting metric and matter distribution solve
Einstein's equation. We find that Einstein's equation fails to hold at "order
1" at small scales and at "order " at large scales. We then find the
additional corrections to the metric and matter distribution needed to satisfy
Einstein's equation to these orders. While these corrections are of some
interest in their own right, our main purpose in calculating them is that their
smallness should provide a criterion for the validity of the original
dictionary (as well as simplified versions of this dictionary). We expect that,
in realistic Newtonian cosmologies, these additional corrections will be very
small; if so, this should provide strong justification for the use of Newtonian
simulations to describe relativistic cosmologies, even on scales larger than
the Hubble radius.Comment: 35 pages; minor change
Empathic ability and perceptual disposition.
Thesis (Ph.D.)--Boston UniversityThe purpose of this study was to investigate the relationship between two variables: empathic ability, the ability to perceive another person as that person perceives himself; and perceptual disposition, the characteristic organization of the perceptual-cognitive response pattern. Theoretically, both variables are related to psychological adjustment. With respect to process, a relationship between the variables is assumed in terms of the observer's flexibility in selecting phenomena for interpretation and in forming and revising interpretative hypotheses in the direction of the subject's own frame of reference. [TRUNCATED]
It is concluded that there is a relationship between empathic ability and perceptual disposition as presently defined. In view of the homogeneity of the experimental population, no generalization beyond this group and its characteristics is actually warranted by the present findings. Also, the many factors which may be involved in actual interaction between observer and subject are absent in the present experiment, and must be considered in applying the present findings to the empathic process as it operates in everyday life.
Two lines of future research are suggested. One is the exploration of the content of predictive hypotheses made by successful and unsuccessful observers, and by the different perceptual disposition groups. The second is the investigation of the etiological correlates of the perceptual dispositions. For clinical psychology, the overall aim of such research would be to study learning readiness in interpersonal relationships as it relates to psychological health
Probing fundamental physics with pulsars
Pulsars provide a wealth of information about General Relativity, the
equation of state of superdense matter, relativistic particle acceleration in
high magnetic fields, the Galaxy's interstellar medium and magnetic field,
stellar and binary evolution, celestial mechanics, planetary physics and even
cosmology. The wide variety of physical applications currently being
investigated through studies of radio pulsars rely on: (i) finding interesting
objects to study via large-scale and targeted surveys; (ii) high-precision
timing measurements which exploit their remarkable clock-like stability. We
review current surveys and the principles of pulsar timing and highlight
progress made in the rotating radio transients, intermittent pulsars, tests of
relativity, understanding pulsar evolution, measuring neutron star masses and
the pulsar timing array.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU XXVII GA - JD3
- Neutron Stars: Timing in Extreme Environments XXVII IAU General Assembly,
Rio de Janeiro, Brazil, 3-14 August 200
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
Effects of P-wave Annihilation on the Angular Power Spectrum of Extragalactic Gamma-rays from Dark Matter Annihilation
We present a formalism for estimating the angular power spectrum of
extragalactic gamma-rays produced by dark matter annihilating with any general
velocity-dependent cross section. The relevant density and velocity
distribution of dark matter is modeled as an ensemble of smooth, universal,
rigid, disjoint, spherical halos with distribution and universal properties
constrained by simulation data. We apply this formalism to theories of dark
matter with p-wave annihilation, for which the relative-velocity-weighted
annihilation cross section is \sigma v=a+bv^2. We determine that this
significantly increases the gamma-ray power if b/a >> 10^6. The effect of
p-wave annihilation on the angular power spectrum is very similar for the
sample of particle physics models we explored, suggesting that the important
effect for a given b/a is largely determined by the cosmic dark matter
distribution. If the dark matter relic from strong p-wave theories is thermally
produced, the intensities of annihilation gamma-rays are strongly p-wave
suppressed, making them difficult to observe. If an angular power spectrum
consistent with a strong p-wave were to be observed, it would likely indicate
non-thermal production of dark matter in the early Universe.Comment: 20 pages, 3 figure
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