27,721 research outputs found
Ultra-Light Scalar Fields and the Growth of Structure in the Universe
Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.Comment: 12 pages, 11 figure
Quintessence in a quandary: prior dependence in dark energy models
The archetypal theory of dark energy is quintessence: a minimally coupled
scalar field with a canonical kinetic energy and potential. By studying random
potentials we show that quintessence imposes a restricted set of priors on the
equation of state of dark energy. Focusing on the commonly-used
parametrisation, , we show that there is a natural
scale and direction in the plane that distinguishes quintessence
as a general framework. We calculate the expected information gain for a given
survey and show that, because of the non-trivial prior information, it is a
function of more than just the figure of merit. This allows us to make a
quantitative case for novel survey strategies. We show that the scale of the
prior sets target observational requirements for gaining significant
information. This corresponds to a figure of merit FOM, a
requirement that future galaxy redshift surveys will meet.Comment: 5 pages, 3 figures. For the busy reader, Fig. 1 is the money plot.
v2: Minor changes, matches published version. Code open source at
gitorious.org/random-quintessenc
Tensor Detection Severely Constrains Axion Dark Matter
The recent detection of B-modes by BICEP2 has non-trivial implications for
axion dark matter implied by combining the tensor interpretation with
isocurvature constraints from Planck. In this paper the measurement is taken as
fact, and its implications considered, though further experimental verification
is required. In the simplest inflation models implies . If the axion decay constant constraints on
the dark matter (DM) abundance alone rule out the QCD axion as DM for (where accounts for theoretical
uncertainty). If then vacuum fluctuations of the axion field
place conflicting demands on axion DM: isocurvature constraints require a DM
abundance which is too small to be reached when the back reaction of
fluctuations is included. High QCD axions are thus ruled out. Constraints
on axion-like particles, as a function of their mass and DM fraction, are also
considered. For heavy axions with we find
, with stronger constraints on heavier
axions. Lighter axions, however, are allowed and (inflationary)
model-independent constraints from the CMB temperature power spectrum and large
scale structure are stronger than those implied by tensor modes.Comment: 6 pages, 1 figure. v2: Some discussion and references added. v3
Update on QCD discussion. Version accepted for publication in Physical Review
Letter
CMB Likelihood Functions for Beginners and Experts
Although the broad outlines of the appropriate pipeline for cosmological
likelihood analysis with CMB data has been known for several years, only
recently have we had to contend with the full, large-scale, computationally
challenging problem involving both highly-correlated noise and extremely large
datasets (). In this talk we concentrate on the beginning and end of
this process. First, we discuss estimating the noise covariance from the data
itself in a rigorous and unbiased way; this is essentially an iterated
minimum-variance mapmaking approach. We also discuss the unbiased determination
of cosmological parameters from estimates of the power spectrum or experimental
bandpowers.Comment: Long-delayed submission. In AIP Conference Proceedings "3K Cosmology"
held in Rome, Oct 5-10, 1998, edited by Luciano Maiani, Francesco Melchiorri
and Nicola Vittorio, 343-347, New York, American Institute of Physics 199
Electric and magnetic fields effects on the excitonic properties of elliptic core-multishell quantum wires
The effect of eccentricity distortions of core-multishell quantum wires on
their electron, hole and exciton states is theoretically investigated. Within
the effective mass approximation, the Schrodinger equation is numerically
solved for electrons and holes in systems with single and double radial
heterostructures, and the exciton binding energy is calculated by means of a
variational approach. We show that the energy spectrum of a core-multishell
heterostructure with eccentricity distortions, as well as its magnetic field
dependence, are very sensitive to the direction of an externally applied
electric field, an effect that can be used to identify the eccentricity of the
system. For a double heterostructure, the eccentricities of the inner and outer
shells play an important role on the excitonic binding energy, specially in the
presence of external magnetic fields, and lead to drastic modifications in the
oscillator strength.Comment: 17 pages, 10 figure
Statistical isotropy of the Cosmic Microwave Background
The breakdown of statistical homogeneity and isotropy of cosmic perturbations
is a generic feature of ultra large scale structure of the cosmos, in
particular, of non trivial cosmic topology. The statistical isotropy (SI) of
the Cosmic Microwave Background temperature fluctuations (CMB anisotropy) is
sensitive to this breakdown on the largest scales comparable to, and even
beyond the cosmic horizon. We propose a set of measures,
() which for non-zero values indicate and quantify statistical
isotropy violations in a CMB map. We numerically compute the predicted
spectra for CMB anisotropy in flat torus universe models.
Characteristic signature of different models in the spectrum are
noted.Comment: Presented at PASCOS'03, January 3-8, 2003, in TIFR, Mumbai; to be
published in a special issue of 'Pramana' (4 pages, 1 figure, style files
included
Observational Characterization of the Downward Atmospheric Longwave Radiation at the Surface in the City of São Paulo
This work describes the seasonal and diurnal variations of downward longwave atmospheric irradiance (LW) at the surface in São Paulo, Brazil, using 5-min-averaged values of LW, air temperature, relative humidity, and solar radiation observed continuously and simultaneously from 1997 to 2006 on a micrometeorological platform, located at the top of a 4-story building. An objective procedure, including 2-step filtering and dome emission effect correction, was used to evaluate the quality of the 9-yr-long LW dataset. The comparison between LW values observed and yielded by the Surface Radiation Budget project shows spatial and temporal agreement, indicating that monthly and annual average values of LW observed in one point of São Paulo can be used as representative of the entire metropolitan region of São Paulo. The maximum monthly averaged value of the LW is observed during summer (389 ± 14 W m-2; January), and the minimum is observed during winter (332 ± 12 W m-2; July). The effective emissivity follows the LW and shows a maximum in summer (0.907 ± 0.032; January) and a minimum in winter (0.818 ± 0.029; June). The mean cloud effect, identified objectively by comparing the monthly averaged values of the LW during clear-sky days and all-sky conditions, intensified the monthly average LW by about 32.0 ± 3.5 W m-2 and the atmospheric effective emissivity by about 0.088 ± 0.024. In August, the driest month of the year in São Paulo, the diurnal evolution of the LW shows a minimum (325 ± 11 W m-2) at 0900 LT and a maximum (345 ± 12 W m-2) at 1800 LT, which lags behind (by 4 h) the maximum diurnal variation of the screen temperature. The diurnal evolution of effective emissivity shows a minimum (0.781 ± 0.027) during daytime and a maximum (0.842 ± 0.030) during nighttime. The diurnal evolution of all-sky condition and clear-sky day differences in the effective emissivity remain relatively constant (7% ± 1%), indicating that clouds do not change the emissivity diurnal pattern. The relationship between effective emissivity and screen air temperature and between effective emissivity and water vapor is complex. During the night, when the planetary boundary layer is shallower, the effective emissivity can be estimated by screen parameters. During the day, the relationship between effective emissivity and screen parameters varies from place to place and depends on the planetary boundary layer process. Because the empirical expressions do not contain enough information about the diurnal variation of the vertical stratification of air temperature and moisture in São Paulo, they are likely to fail in reproducing the diurnal variation of the surface emissivity. The most accurate way to estimate the LW for clear-sky conditions in São Paulo is to use an expression derived from a purely empirical approach
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