11,566 research outputs found
Seeing patterns in noise: Gigaparsec-scale `structures' that do not violate homogeneity
Clowes et al. (2013) have recently reported the discovery of a Large Quasar
Group (LQG), dubbed the Huge-LQG, at redshift z~1.3 in the DR7 quasar catalogue
of the Sloan Digital Sky Survey. On the basis of its characteristic size ~500
Mpc and longest dimension >1 Gpc, it is claimed that this structure is
incompatible with large-scale homogeneity and the cosmological principle. If
true, this would represent a serious challenge to the standard cosmological
model. However, the homogeneity scale is an average property which is not
necessarily affected by the discovery of a single large structure. I clarify
this point and provide the first fractal dimension analysis of the DR7 quasar
catalogue to demonstrate that it is in fact homogeneous above scales of at most
130 Mpc/h, which is much less than the upper limit for \Lambda CDM. In
addition, I show that the algorithm used to identify the Huge-LQG regularly
finds even larger clusters of points, extending over Gpc scales, in explicitly
homogeneous simulations of a Poisson point process with the same density as the
quasar catalogue. This provides a simple null test to be applied to any cluster
thus found in a real catalogue, and suggests that the interpretation of LQGs as
`structures' is misleading.Comment: 9 pages, 6 figures. MNRAS published online. v2: minor typo corrected,
added one missing referenc
Weyl curvature and the Euler characteristic in dimension four
We give lower bounds, in terms of the Euler characteristic, for the
-norm of the Weyl curvature of closed Riemannian 4-manifolds. The same
bounds were obtained by Gursky, in the case of positive scalar curvature
metrics.Comment: 6 page
Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars
Magnetic fields in galaxies are produced via the amplification of seed
magnetic fields of unknown nature. The seed fields, which might exist in their
initial form in the intergalactic medium, were never detected. We report a
lower bound ~gauss on the strength of intergalactic
magnetic fields, which stems from the nonobservation of GeV gamma-ray emission
from electromagnetic cascade initiated by tera-electron volt gamma-ray in
intergalactic medium. The bound improves as if magnetic
field correlation length, , is much smaller than a megaparsec. This
lower bound constrains models for the origin of cosmic magnetic fields.Comment: 14 pages, 3 figure
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