196 research outputs found
Gamma rays from active galactic nuclei
The general properties of Active Galactic Nuclei (AGN) and quasars are reviewed with emphasis on their continuum spectral emission. Two general classes of models for the continuum are outlined and critically reviewed in view of the impending GRO (Gamma Ray Observatory) launch and observations. The importance of GRO in distinguishing between these models and in general in furthering the understanding of AGN is discussed. The very broad terms the status of the current understanding of AGN are discussed
Binary black hole growth by gas accretion in stellar clusters
We show that binaries of stellar-mass black holes formed inside a young
protoglobular cluster, can grow rapidly inside the cluster's core by accretion
of the intracluster gas, before the gas may be depleted from the core. A black
hole with mass of the order of eight solar masses can grow to values of the
order of thirty five solar masses in accordance with recent gravitational waves
signals observed by LIGO. Due to the black hole mass increase, a binary may
also harden. The growth of binary black holes in a dense protoglobular cluster
through mass accretion indicates a potentially important formation and
hardening channel
Binary Black Hole Growth by Gas Accretion in Stellar Clusters
We show that binaries of stellar-mass black holes formed inside a young protoglobular cluster, can grow rapidly inside the clusters core by accretion of the intracluster gas, before the gas may be depleted from the core. A black hole with mass of the order of eight solar masses can grow to values of the order of thirty five solar masses in accordance with recent gravitational waves signals observed by LIGO. Due to the black hole mass increase, a binary may also harden. The growth of binary black holes in a dense protoglobular cluster through mass accretion indicates a potentially important formation and hardening channel
Cosmological Inflation: A Personal Perspective
We present a brief review of Cosmological Inflation from the personal
perspective of the author who almost 30 years ago proposed a way of resolving
the problem of Cosmological Horizon by employing certain notions and
developments from the field of High Energy Physics. Along with a brief
introduction of the Horizon and Flatness problems of standard cosmology, this
lecture concentrates on personal reminiscing of the notions and ideas that
prevailed and influenced the author's thinking at the time. The lecture then
touches upon some more recent developments related to the subject and concludes
with some personal views concerning the direction that the cosmology field has
taken in the past couple of decades and certain speculations some notions that
may indicate future directions of research.Comment: 13 pages, 3 figures. Text based on a public lecture presented at the
Academy of Athens within the activities of the Symposium "Chaos in Astronomy
2007", held in Athens Greece in September 200
The Problem of Inertia in a Friedmann Universe
In this talk I will discuss the origin of inertia in a curved spacetime, particularly the spatially flat, open and closed Friedmann universes. This is done using Sciama's law of inertial induction, which is based on Mach's principle, and expresses the analogy between the retarded far fields of electrodynamics and those of gravitation. After obtaining covariant expressions for electromagnetic fields due to an accelerating point charge in Friedmann models, we adopt Sciama's law to obtain the inertial force on an accelerating mass by integrating over the contributions from all the matter in the universe. The resulting inertial force has the form where the constant depends on the choice of the cosmological parameters such as and . The values of obtained suggest that inertial contribution from dark matter can be the source for the missing part of the inertial force
A Physical Interpretation of the Titius-Bode Rule and its Connection to the Closed Orbits of Bertrand's Theorem
We consider the geometric Titius-Bode rule for the semimajor axes of
planetary orbits. We derive an equivalent rule for the midpoints of the
segments between consecutive orbits along the radial direction and we interpret
it physically in terms of the work done in the gravitational field of the Sun
by particles whose orbits are perturbed around each planetary orbit. On such
energetic grounds, it is not surprising that some exoplanets in multiple-planet
extrasolar systems obey the same relation. But it is surprising that this
simple interpretation of the Titius-Bode rule also reveals new properties of
the bound closed orbits predicted by Bertrand's theorem and known since 1873.Comment: Final version, issues resolved, submitted to RA
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