18 research outputs found
Non-Gaussianity from isocurvature perturbations
We develop a formalism to study non-Gaussianity in both curvature and
isocurvature perturbations. It is shown that non-Gaussianity in the
isocurvature perturbation between dark matter and photons leaves distinct
signatures in the CMB temperature fluctuations, which may be confirmed in
future experiments, or possibly, even in the currently available observational
data. As an explicit example, we consider the QCD axion and show that it can
actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} =
O(10^9 - 10^{11})GeV.Comment: 24 pages, 6 figures; references added; version to appear in JCA
Constraint on the Effective Number of Neutrino Species from the WMAP and SDSS LRG Power Spectra
We derive constraint on the effective number of neutrino species N_nu from
the cosmic microwave background power spectrum of the WMAP and galaxy
clustering power spectrum of the SDSS luminous red galaxies (LRGs). Using these
two latest data sets of CMB and galaxy clustering alone, we obtain the limit
0.9 < N_nu < 8.2 (95% C.L.) for the power-law LambdaCDM flat universe, with no
external prior. The lower limit corresponds to the lower bound on the reheating
temperature of the universe T_R > 2 MeV.Comment: 16 pages, 3 figure. (v2) More explanation and discussion on our
analysis. A section and figures are adde
Non-Gaussianity from Symmetry
We point out that a light scalar field fluctuating around a symmetry-enhaced
point can generate large non-Gaussianity in density fluctuations. We name such
a particle as an "ungaussiton", a scalar field dominantly produced by the
quantum fluctuations,generating sizable non-Gaussianity in the density
fluctuations. We derive a consistency relation between the bispectrum and the
trispectrum, tau_NL = 10^3 f_NL^(4/3), which can be extended to arbitrary high
order correlation functions. If such a relation is confirmed by future
observations, it will strongly support this mechanism.Comment: 26 pages, 1 figure;v2 discussion and references added. To appear in
JCA
Light Higgsino from Axion Dark Radiation
The recent observations imply that there is an extra relativistic degree of
freedom coined dark radiation. We argue that the QCD axion is a plausible
candidate for the dark radiation, not only because of its extremely small mass,
but also because in the supersymmetric extension of the Peccei-Quinn mechanism
the saxion tends to dominate the Universe and decays into axions with a sizable
branching fraction. We show that the Higgsino mixing parameter mu is bounded
from above when the axions produced at the saxion decays constitute the dark
radiation: mu \lesssim 300 GeV for a saxion lighter than 2m_W, and mu less than
the saxion mass otherwise. Interestingly, the Higgsino can be light enough to
be within the reach of LHC and/or ILC even when the other superparticles are
heavy with mass about 1 TeV or higher. We also estimate the abundance of axino
produced by the decays of Higgsino and saxion.Comment: 18 pages, 1 figure; published in JHE
Present bounds on the relativistic energy density in the Universe from cosmological observables
We discuss the present bounds on the relativistic energy density in the
Universe parameterized in terms of the effective number of neutrinos N using
the most recent cosmological data on Cosmic Microwave Background (CMB)
temperature anisotropies and polarization, Large Scale galaxy clustering from
the Sloan Digital Sky Survey (SDSS) and 2dF, luminosity distances of type Ia
Supernovae, Lyman-alpha absorption clouds (Ly-alpha), the Baryonic Acoustic
Oscillations (BAO) detected in the Luminous Red Galaxies of the SDSS and
finally, Big Bang Nucleosynthesis (BBN) predictions for 4He and Deuterium
abundances. We find N= 5.2+2.7-2.2 from CMB and Large Scale Structure data,
while adding Ly-alpha and BAO we obtain N= 4.6+1.6-1.5 at 95 % c.l.. These
results show some tension with the standard value N=3.046 as well as with the
BBN range N= 3.1+1.4-1.2 at 95 % c.l., though the discrepancy is slightly below
the 2-sigma level. In general, considering a smaller set of data weakens the
constraints on N. We emphasize the impact of an improved upper limit (or
measurement) of the primordial value of 3He abundance in clarifying the issue
of whether the value of N at early (BBN) and more recent epochs coincide
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Solving the discrepancy among the light elements abundances and WMAP
Within the standard big bang nucleosynthesis (BBN) and cosmic microwave background (CMB) framework, the baryon density measured by the Wilkinson Microwave Anisotropy Probe (WMAP) or the primordial D abundance is much higher than the one measured by the He or Li abundances. To solve the discrepancy, we propose a scenario in which additional baryons appear after BBN. We show that simply adding the baryons can not be a solution but the existence of a large lepton asymmetry before BBN makes the scenario successful. These extra baryons and leptons, in addition to the initial baryons which exist before the BBN, can be all produced from -balls