7,106 research outputs found
Cosmological CPT Violation, Baryon/Leptogenesis and CMB Polarization
In this paper we study the cosmological CPT-violation and its implications in
baryo/leptogenesis and CMB polarization. We propose specifically a variant of
the models of gravitational leptogenesis. By performing a global analysis with
the Markov Chain Monte Carlo (MCMC) method, we find the current CMB
polarization observations from the three-year WMAP (WMAP3) and the 2003 flight
of BOOMERANG (B03) data provide a weak evidence for our model. However to
verify and especially exclude this type of mechanism for baryo/leptogenesis
with cosmological CPT-violation, the future measurements on CMB polarization
from PLANCK and CMBpol are necessary.Comment: The version appears in PL
Information Cascades on Arbitrary Topologies
In this paper, we study information cascades on graphs. In this setting, each
node in the graph represents a person. One after another, each person has to
take a decision based on a private signal as well as the decisions made by
earlier neighboring nodes. Such information cascades commonly occur in practice
and have been studied in complete graphs where everyone can overhear the
decisions of every other player. It is known that information cascades can be
fragile and based on very little information, and that they have a high
likelihood of being wrong.
Generalizing the problem to arbitrary graphs reveals interesting insights. In
particular, we show that in a random graph , for the right value of
, the number of nodes making a wrong decision is logarithmic in . That
is, in the limit for large , the fraction of players that make a wrong
decision tends to zero. This is intriguing because it contrasts to the two
natural corner cases: empty graph (everyone decides independently based on his
private signal) and complete graph (all decisions are heard by all nodes). In
both of these cases a constant fraction of nodes make a wrong decision in
expectation. Thus, our result shows that while both too little and too much
information sharing causes nodes to take wrong decisions, for exactly the right
amount of information sharing, asymptotically everyone can be right. We further
show that this result in random graphs is asymptotically optimal for any
topology, even if nodes follow a globally optimal algorithmic strategy. Based
on the analysis of random graphs, we explore how topology impacts global
performance and construct an optimal deterministic topology among layer graphs
Primordial Gravitational Waves Measurements and Anisotropies of CMB Polarization Rotation
Searching for the signal of primordial gravitational waves in the B-modes
(BB) power spectrum is one of the key scientific aims of the cosmic microwave
background (CMB) polarization experiments. However, this could be easily
contaminated by several foreground issues, such as the thermal dust emission.
In this paper we study another mechanism, the cosmic birefringence, which can
be introduced by a CPT-violating interaction between CMB photons and an
external scalar field. Such kind of interaction could give rise to the rotation
of the linear polarization state of CMB photons, and consequently induce the
CMB BB power spectrum, which could mimic the signal of primordial gravitational
waves at large scales. With the recent polarization data of BICEP2 and the
joint analysis data of BICEP2/Keck Array and Planck, we perform a global
fitting analysis on constraining the tensor-to-scalar ratio by considering
the polarization rotation angle which can be separated into a background
isotropic part and a small anisotropic part. Since the data of BICEP2 and Keck
Array experiments have already been corrected by using the "self-calibration"
method, here we mainly focus on the effects from the anisotropies of CMB
polarization rotation angle. We find that including the anisotropies in the
analysis could slightly weaken the constraints on , when using current CMB
polarization measurements. We also simulate the mock CMB data with the
BICEP3-like sensitivity. Very interestingly, we find that if the effects of the
anisotropic polarization rotation angle can not be taken into account properly
in the analysis, the constraints on will be dramatically biased. This
implies that we need to break the degeneracy between the anisotropies of the
CMB polarization rotation angle and the CMB primordial tensor perturbations, in
order to measure the signal of primordial gravitational waves accurately.Comment: 7 pages, 5 figure
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