The evolution of organizational niches : U.S. automobile manufacturers, 1885-1981.

Although the niche figures prominently in contemporary theories of organization, analysts often fail to tie micro processes within the niche to long-term changes in the broader environment. In this paper, we advance arguments about the relationship between an organization's niche and evolution in the structure of its organizational population over time. We focus on the technological niche and processes of positioning and crowding among firms in the niche space, relating them to the level of concentration among all firms in the market. Building on previous empirical studies in organizational ecology, we study the evolution of concentration in the American automobile industry from 1885 to 1981 and estimate models of the hazard of exit of individual producers from the market. The findings show that niche and concentration interact in complex ways, yielding a more unified depiction of organizational evolution than typically described or reported

New Models of f(R) Theories of Gravity

We introduce new models of f(R) theories of gravity that are generalization of Horava-Lifshitz gravity.Comment: 16 pages, typos corrected, v2:minor changes, references adde

Gott time machines in the Anti-de Sitter space

In 1991 Gott presented a solution of Einstein's field equations in 2+1 dimensions with $\Lambda = 0$ that contained closed timelike curves (CTC's). This solution was remarkable because at first it did not seem to be unphysical in any other respect. Later, however, it was shown that Gott's solution is tachyonic in a certain sense. Here the case $\Lambda < 0$ is discussed. We show that it is possible to construct CTC's also in this case, in a way analogous to that used by Gott. We also show that this construction still is tachyonic. $\Lambda < 0$ means that we are dealing with Anti-de Sitter space, and since the CTC-construction necessitates some understanding of its structure, a few pages are devoted to this subject.Comment: 11 page

A symmetry for vanishing cosmological constant

Two different realizations of a symmetry principle that impose a zero cosmological constant in an extra-dimensional set-up are studied. The symmetry is identified by multiplication of the metric by minus one. In the first realization of the symmetry this is provided by a symmetry transformation that multiplies the coordinates by the imaginary number i. In the second realization this is accomplished by a symmetry transformation that multiplies the metric tensor by minus one. In both realizations of the symmetry the requirement of the invariance of the gravitational action under the symmetry selects out the dimensions given by D = 2(2n+1), n=0,1,2,... and forbids a bulk cosmological constant. Another attractive aspect of the symmetry is that it seems to be more promising for quantization when compared to the usual scale symmetry. The second realization of the symmetry is more attractive in that it is posible to make a possible brane cosmological constant zero in a simple way by using the same symmetry, and the symmetry may be identified by reflection symmetry in extra dimensions.Comment: Talk in the conference IRGAC 2006, 2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology, Barcelon

Two Component Model of Dark Energy

We consider the possibility that the dark energy is made up of two or more independent components, each having a different equation of state. We fit the model with supernova and gamma-ray burst (GRB) data from resent observations, and use the Markov Chain Monte Carlo (MCMC) technique to estimate the allowed parameter regions. We also use various model selection criteria to compare the two component model with the LCDM, one component dark energy model with static or variable w(XCDM), and with other multi-component models. We find that the two component models can give reasonably good fit to the current data. For some data sets, and depending somewhat on the model selection criteria, the two component model can give better fit to the data than XCDM with static w and XCDM with variable w parameterized by w = w_0 + w_az/(1+z).Comment: 10 pages, 8 figures, 3 tables; Version accepted by PR

Graviton localization and Newton's law for brane models with a non-minimally coupled bulk scalar field

Brane world models with a non-minimally coupled bulk scalar field have been studied recently. In this paper we consider metric fluctuations around an arbitrary gravity-scalar background solution, and we show that the corresponding spectrum includes a localized zero mode which strongly depends on the profile of the background scalar field. For a special class of solutions, with a warp factor of the RS form, we solve the linearized Einstein equations, for a point-like mass source on the brane, by using the brane bending formalism. We see that general relativity on the brane is recovered only if we impose restrictions on the parameter space of the models under consideration.Comment: 17 pages, revised versio

A supersymmetric model of gamma ray bursts

We propose a model for gamma ray bursts in which a star subject to a high level of fermion degeneracy undergoes a phase transition to a supersymmetric state. The burst is initiated by the transition of fermion pairs to sfermion pairs which, uninhibited by the Pauli exclusion principle, can drop to the ground state of minimum momentum through photon emission. The jet structure is attributed to the Bose statistics of sfermions whereby subsequent sfermion pairs are preferentially emitted into the same state (sfermion amplification by stimulated emission). Bremsstrahlung gamma rays tend to preserve the directional information of the sfermion momenta and are themselves enhanced by stimulated emission.Comment: published versio

Topological Quintessence

A global monopole (or other topological defect) formed during a recent phase transition with core size comparable to the present Hubble scale, could induce the observed accelerating expansion of the universe. In such a model, topological considerations trap the scalar field close to a local maximum of its potential in a cosmologically large region of space. We perform detailed numerical simulations of such an inhomogeneous dark energy system (topological quintessence) minimally coupled to gravity, in a flat background of initially homogeneous matter. We find that when the energy density of the field in the monopole core starts dominating the background density, the spacetime in the core starts to accelerate its expansion in accordance to a \Lambda CDM model with an effective inhomogeneous spherical dark energy density parameter \Omega_\Lambda(r). The matter density profile is found to respond to the global monopole profile via an anti-correlation (matter underdensity in the monopole core). Away from the monopole core, the spacetime is effectively Einstein-deSitter (\Omega_\Lambda(r_{out}) -> 0) while at the center \Omega_\Lambda(r ~ 0) is maximum. We fit the numerically obtained expansion rate at the monopole core to the Union2 data and show that the quality of fit is almost identical to that of \Lambda CDM. Finally, we discuss potential observational signatures of this class of inhomogeneous dark energy models.Comment: Accepted in Phys. Rev. D (to appear). Added observational bounds on parameters. 10 pages (two column revtex), 6 figures. The Mathematica files used to produce the figures of this study may be downloaded from http://leandros.physics.uoi.gr/topquin

Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae

We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in spectral line profiles are inverted to a stellar surface temperature map using our $i$Map code. The long-term photometric record is employed to compare our map with previously published maps. Four cool spots were reconstructed, but no polar spot was seen. The temperature difference to the photosphere of the spots is between 990 and 280K. Two spots are reconstructed with a typical solar morphology with an umbra and a penumbra. For the one isolated and relatively round spot (A), we determine an umbral temperature of 990K and a penumbral temperature of 180K below photospheric temperature. The umbra to photosphere intensity ratio of EK Dra is approximately only half of that of a comparison sunspot. A test inversion from degraded line profiles showed that the higher spectral resolution of PEPSI reconstructs the surface with a temperature difference that is on average 10% higher than before and with smaller surface areas by 10-20%. PEPSI is therefore better suited to detecting and characterising temperature inhomogeneities. With ten more years of photometry, we also refine the spot cycle period of EK Dra to 8.9$\pm$0.2 years with a continuing long-term fading trend. The temperature morphology of spot A so far appears to show the best evidence for the existence of a solar-like penumbra for a starspot. We emphasise that it is more the non-capture of the true umbral contrast rather than the detection of the weak penumbra that is the limiting factor. The relatively small line broadening of EK Dra, together with the only moderately high spectral resolutions previously available, appear to be the main contributors to the lower-than-expected spot contrasts when comparing to the Sun.Comment: Accepted for A&

Geometry and Destiny

The recognition that the cosmological constant may be non-zero forces us to re-evaluate standard notions about the connection between geometry and the fate of our Universe. An open Universe can recollapse, and a closed Universe can expand forever. As a corollary, we point out that there is no set of cosmological observations we can perform that will unambiguously allow us to determine what the ultimate destiny of the Universe will be.Comment: 7 pages, Gravity Research Foundation Essa