Small Scale Anisotropies of UHECRs from Super-Heavy Halo Dark Matter

The decay of very heavy metastable relics of the Early Universe can produce ultra-high energy cosmic rays (UHECRs) in the halo of our own Galaxy. In this model, no Greisen-Zatsepin-Kuzmin cutoff is expected because of the short propagation distances. We show here that, as a consequence of the hierarchical build up of the halo, this scenario predicts the existence of small scale anisotropies in the arrival directions of UHECRs, in addition to a large scale anisotropy, known from previous studies. We also suggest some other observable consequences of this scenario which will be testable with upcoming experiments, as Auger, EUSO and OWL.Comment: Contribution given at ICRC 2001 - August 7-15, 2001 - Hambur

Lagrangian bias in the local bias model

It is often assumed that the halo-patch fluctuation field can be written as a Taylor series in the initial Lagrangian dark matter density fluctuation field. We show that if this Lagrangian bias is local, and the initial conditions are Gaussian, then the two-point cross-correlation between halos and mass should be linearly proportional to the mass-mass auto-correlation function. This statement is exact and valid on all scales; there are no higher order contributions, e.g., from terms proportional to products or convolutions of two-point functions, which one might have thought would appear upon truncating the Taylor series of the halo bias function. In addition, the auto-correlation function of locally biased tracers can be written as a Taylor series in the auto-correlation function of the mass; there are no terms involving, e.g., derivatives or convolutions. Moreover, although the leading order coefficient, the linear bias factor of the auto-correlation function is just the square of that for the cross-correlation, it is the same as that obtained from expanding the mean number of halos as a function of the local density only in the large-scale limit. In principle, these relations allow simple tests of whether or not halo bias is indeed local in Lagrangian space. We discuss why things are more complicated in practice. We also discuss our results in light of recent work on the renormalizability of halo bias, demonstrating that it is better to renormalize than not. We use the Lognormal model to illustrate many of our findings.Comment: 14 pages, published on JCA

The Mass Function of Dark Halos in Superclusters and Voids

A modification of the Press-Schechter theory allowing for presence of a background large-scale structure (LSS) - a supercluster or a void, is proposed. The LSS is accounted as the statistical constraints in form of linear functionals of the random overdensity field. The deviation of the background density within the LSS is interpreted in a pseudo-cosmological sense. Using the constraints formalism may help us to probe non-trivial spatial statistics of haloes, e.g. edge and shape effects on boundaries of the superclusters and voids. Parameters of the constraints are connected to features of the LSS: its mean overdensity, a spatial scale and a shape, and spatial momenta of higher orders. It is shown that presence of a non-virialized LSS can lead to an observable deviation of the mass function. This effect is exploited to build a procedure to recover parameters of the background perturbation from the observationally estimated mass function.Comment: 23 pages, 6 figures; to be appeared in Astronomy Reports, 2014, Vol. 58, No. 6, pp. 386-39

Self-consistency of the Excursion Set Approach

The excursion set approach provides a framework for predicting how the abundance of dark matter halos depends on the initial conditions. A key ingredient of this formalism comes from the physics of halo formation: the specification of a critical overdensity threshold (barrier) which protohalos must exceed if they are to form bound virialized halos at a later time. Another ingredient is statistical, as it requires the specification of the appropriate statistical ensemble over which to average when making predictions. The excursion set approach explicitly averages over all initial positions, thus implicitly assuming that the appropriate ensemble is that associated with randomly chosen positions in space, rather than special positions such as peaks of the initial density field. Since halos are known to collapse around special positions, it is not clear that the physical and statistical assumptions which underlie the excursion set approach are self-consistent. We argue that they are at least for low mass halos, and illustrate by comparing our excursion set predictions with numerical data from the DEUS simulations.Comment: 5 pages, 2 figure

Challenges of Creating a Knowledge-Based Society: Education & Research for India & Gujarat

Presented at the World Gujarat Conference, Edison, NJ, August 30, 2008

CS 875: Semantic Web

World Wide Web (Web 1.0, or the Web, as we now know it) centers on documents and semistructured data in html, rss, and xml. The next generation Web, also called Web 2.0 and Web 3.0, has already started to emerge. Web 2.0 is about user-generated content, user participation such as through tagging, and social networking. Web 3.0, also called Semantic Web, is about labeling content such that machines can process it more intelligently and humans can exploit it more effectively. These labels or metadata add semantics (meaning) to data, and their formal representation enables powerful reasoning that leads not only to better (semantic) search but also to analysis, discovery, and decision making. Semantic Web is already a rapidly emerging field, with standards, technologies, products, and applications-as well as to excellent job prospects (for MS students) and research opportunities (for PhD students)

CS 475/675: Web Information Systems

This course covers advanced topics in managing W eh-based resources, with a focus on building applications involving heterogeneous data. It will expose students to the following concept, topics, architectures, techniques, and technologies: • data, metadata, information, knowledge, and ontologies• unstructured, semi-structured, structured, multimodal, multimedia, and sensor data syntax,structural/representational, and semantic aspects of data• architectures: federated databases, mediator, information brokering• integration and analysis of Web-based information• automatic information/metadata extraction (entity identification/recognition, disambiguation)• Web search engines, social networks, Web 2.0• Semantic Web and Web 3.0• relevant Web standards and technologies• real-world examples that have major research projects and commercial product

Semantic Web in Action: Ontology-driven Information Search, Integration and Analysis

Keynote at the Net Object Days and MATES, Erfurt, Germany, September 23, 2003