6,234 research outputs found
COBE Constraints on a Local group X-ray Halo
We investigate the effect of a putative X-ray emitting halo surrounding the
Local Group of galaxies, and specifically the possible temperature anisotropies
induced in the COBE-DMR four-year sky maps by an associated Sunyaev-Zel'dovich
effect. By fitting the isothermal spherical halo model proposed by Suto et.al.
(1996) to the coadded four-year COBE-DMR 53 and 90 GHz sky maps in Galactic
coordinates, we find no significant evidence of a contribution. We therefore
reject the claim that such a halo can affect the estimation of the primordial
spectral index and amplitude of density perturbations as inferred from the DMR
data. We find that correlation with the DMR data imposes constraints on the
plausible contribution of such an X-ray emitting halo to a distortion in the
CMB spectrum (as specified by the Compton-y parameter), up to a value for R --
the ratio of the core radius of the isothermal halo gas distribution to the
distance to the Local Group centroid -- of 0.68. For larger values of R, the
recent cosmological upper limit derived by COBE-FIRAS provides stronger
constraints on the model parameters. Over the entire parameter space for R, we
find an upper limit to the inferred sky-RMS anisotropy signal of 14 microKelvin
(95% c.l.), a negligible amount relative to the 35 microKelvin signal observed
in the COBE-DMR data.Comment: 4 pages, 3 figures; accepted for publication in MNRAS pink page
Evidence for non-Gaussianity in the CMB
In a recent Letter we have shown how COBE-DMR maps may be used to disprove
Gaussianity at a high confidence level. In this report we digress on a few
issues closely related to this Letter. We present the general formalism for
surveying non-Gaussianity employed. We present a few more tests for
systematics. We wonder about the theoretical implications of our result.Comment: Proceedings of the Planck meeting, Santender 9
Scale free effects in world currency exchange network
A large collection of daily time series for 60 world currencies' exchange
rates is considered. The correlation matrices are calculated and the
corresponding Minimal Spanning Tree (MST) graphs are constructed for each of
those currencies used as reference for the remaining ones. It is shown that
multiplicity of the MST graphs' nodes to a good approximation develops a power
like, scale free distribution with the scaling exponent similar as for several
other complex systems studied so far. Furthermore, quantitative arguments in
favor of the hierarchical organization of the world currency exchange network
are provided by relating the structure of the above MST graphs and their
scaling exponents to those that are derived from an exactly solvable
hierarchical network model. A special status of the USD during the period
considered can be attributed to some departures of the MST features, when this
currency (or some other tied to it) is used as reference, from characteristics
typical to such a hierarchical clustering of nodes towards those that
correspond to the random graphs. Even though in general the basic structure of
the MST is robust with respect to changing the reference currency some trace of
a systematic transition from somewhat dispersed -- like the USD case -- towards
more compact MST topology can be observed when correlations increase.Comment: Eur. Phys. J. B (2008) in pres
World currency exchange rate cross-correlations
World currency network constitutes one of the most complex structures that is
associated with the contemporary civilization. On a way towards quantifying its
characteristics we study the cross correlations in changes of the daily foreign
exchange rates within the basket of 60 currencies in the period December 1998
-- May 2005. Such a dynamics turns out to predominantly involve one outstanding
eigenvalue of the correlation matrix. The magnitude of this eigenvalue depends
however crucially on which currency is used as a base currency for the
remaining ones. Most prominent it looks from the perspective of a peripheral
currency. This largest eigenvalue is seen to systematically decrease and thus
the structure of correlations becomes more heterogeneous, when more significant
currencies are used as reference. An extreme case in this later respect is the
USD in the period considered. Besides providing further insight into subtle
nature of complexity, these observations point to a formal procedure that in
general can be used for practical purposes of measuring the relative currencies
significance on various time horizons.Comment: 4 pages, 3 figures, LaTe
Coevolution of Information Processing and Topology in Hierarchical Adaptive Random Boolean Networks
Random Boolean networks (RBNs) are frequently employed for modelling complex
systems driven by information processing, e.g. for gene regulatory networks
(GRNs). Here we propose a hierarchical adaptive RBN (HARBN) as a system
consisting of distinct adaptive RBNs - subnetworks - connected by a set of
permanent interlinks. Information measures and internal subnetworks topology of
HARBN coevolve and reach steady-states that are specific for a given network
structure. We investigate mean node information, mean edge information as well
as a mean node degree as functions of model parameters and demonstrate HARBN's
ability to describe complex hierarchical systems.Comment: 9 pages, 6 figure
The 4 Year COBE DMR data is non-Gaussian
I review our recent claim that there is evidence of non-Gaussianity in the 4
Year COBE DMR data. I describe the statistic we apply, the result we obtain and
make a detailed list of the systematics we have analysed. I finish with a
qualitative understanding of what it might be and its implications.Comment: Proceedings of Rome 3K conference, 5 pages, 3 figure
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