2,551 research outputs found
Compact strain-sensitive flexible photonic crystals for sensors
A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors
Cosmography beyond standard candles and rulers
We perform a cosmographic analysis using several cosmological observables
such as the luminosity distance moduli, the volume distance, the angular
diameter distance and the Hubble parameter. These quantities are determined
using different data sets: Supernovae type Ia and Gamma Ray Bursts, the
Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum
and the Hubble parameter as measured from surveys of galaxies. This data set
allows to put constraints on the cosmographic expansion with unprecedented
precision. We also present forecasts for the coefficients of the kinematic
expansion using future but realistic data sets: constraints on the coefficients
of the expansions are likely to improve by a factor ten with the upcoming large
scale structure probes. Finally, we derive the set of the cosmographic
parameters for several cosmological models (including CDM) and compare
them with our best fit set. While distance measurements are unable to
discriminate among these models, we show that the inclusion of the Hubble data
set leads to strong constraints on the lowest order coefficients and in
particular it is incompatible with CDM at 3- confidence level.
We discuss the reliability of this determination and suggest further
observations which might be of crucial importance for the viability of
cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR
Sources of H0 -tension in dark energy scenarios
By focusing on a simple extension of Lambda cold dark matter (ÎCDM) in which the dark energy equation of state is allowed to vary, we assess which epoch(s) possibly source the H0-tension. We consider cosmic microwave background (CMB) data in three possible ways: (i) complete CMB data; (ii) excluding the low-l+lowE (l<30 temperature and polarization) likelihoods; (iii) imposing early Universe priors, which allow us to disentangle early- and late-time physics. Through a joint analysis with low-redshift supernovae type-Ia and gravitationally lensed time delay datasets, and neglecting galaxy clustering baryonic acoustic oscillation (BAO) data, we find that the inclusion of early Universe CMB priors is consistent with local estimate of H0, while excluding the low-l+lowE likelihoods mildly relaxes the tension. This is in contrast to joint analyses with the complete CMB data. Our simple implementation of contrasting the effect of different CMB priors on the H0 estimate shows that the early Universe information from the CMB data when decoupled from late-time physics could be in agreement with a higher value of H0, even for ÎCDM model with no necessary modification. We also find no evidence for the early dark energy model using only the early Universe physics within the CMB data. Finally, using the BAO data in different redshift ranges to perform inverse distance ladder analysis, we find that the early Universe modifications, while perfectly capable of alleviating the H0-tension when including the BAO galaxy clustering data, would be at odds with the Ly-α BAO data due to the difference in rd vs H0 correlation between the two BAO datasets. We therefore infer and speculate that source for the H0-tension between CMB and local estimates could possibly originate in the modeling of late-time physics within the CMB analysis. This in turn recasts the H0-tension as an effect of late-time physics in CMB, instead of the current early-time CMB vs local late-time physics perspective
Probing 3-D matter distribution at z~2 with QSO multiple lines of sight
We investigate the 3-D matter distribution at z~2 with high resolution (R ~
40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at
ESO VLT. Our sample is unique for the number density of objects and the variety
of separations, between 0.5 and 7 proper Mpc. We compute the real space
cross-correlation function of the Lyman-alpha forest transmitted fluxes. There
is a significant clustering signal up to ~2 proper Mpc, which is still present
when absorption lines with high column density (log N > 13.8) are excluded.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing
Galaxies through Quasar Absorption Lines" held in Shanghai, China from March
14th to 18th, 200
Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs
We study the three-dimensional distribution of non virialised matter at z~2
using high resolution spectra of QSO pairs and simulated spectra drawn from
cosmological hydrodynamical simulations. We have collected the largest sample
of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations
between ~1 and 14 arcmin. The observed correlation functions of the transmitted
flux in the HI Lyman alpha forest along and transverse to the lines of sight
are in good agreement implying that the distortions in redshift space due to
peculiar velocities are small. The clustering signal is significant up to
velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions
at lower overdensity (rho/ < 6.5) are still clustered but on smaller
scales (Delta v < 100 km/s). The observed and simulated correlation functions
are compatible at the 3 sigma level. A better concordance is obtained when only
the low overdensity regions are selected for the analysis or when the effective
optical depth of the simulated spectra is increased artificially, suggesting a
deficiency of strong lines in the simulated spectra. We found that also a lower
value of the power-law index of the temperature-density relation for the Lyman
alpha forest gas improves the agreement between observed and simulated results.
If confirmed, this would be consistent with other observations favouring a late
HeII reionization epoch (at z~3). We remark the detection of a significant
clustering signal in the cross correlation coefficient at a transverse velocity
separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.Comment: Accepted for publication in MNRAS, revised version matching the
accepted on
Fast, large volume, GPU enabled simulations for the Ly-alpha forest: power spectrum forecasts for baryon acoustic oscillation experiments
High redshift measurements of the baryonic acoustic oscillation scale (BAO)
from large Ly-alpha forest surveys represent the next frontier of dark energy
studies. As part of this effort, efficient simulations of the BAO signature
from the Ly-alpha forest will be required. We construct a model for producing
fast, large volume simulations of the Ly-alpha forest for this purpose.
Utilising a calibrated semi-analytic approach, we are able to run very large
simulations in 1 Gpc^3 volumes which fully resolve the Jeans scale in less than
a day on a desktop PC using a GPU enabled version of our code. The Ly-alpha
forest spectra extracted from our semi-analytical simulations are in excellent
agreement with those obtained from a fully hydrodynamical reference simulation.
Furthermore, we find our simulated data are in broad agreement with
observational measurements of the flux probability distribution and 1D flux
power spectrum. We are able to correctly recover the input BAO scale from the
3D Ly-alpha flux power spectrum measured from our simulated data, and estimate
that a BOSS-like 10^4 deg^2 survey with ~15 background sources per square
degree and a signal-to-noise of ~5 per pixel should achieve a measurement of
the BAO scale to within ~1.4 per cent. We also use our simulations to provide
simple power-law expressions for estimating the fractional error on the BAO
scale on varying the signal-to-noise and the number density of background
sources. The speed and flexibility of our approach is well suited for exploring
parameter space and the impact of observational and astrophysical systematics
on the recovery of the BAO signature from forthcoming large scale spectroscopic
surveys.Comment: 16 pages, 11 figures, accepted to MNRA
Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates
We investigate the effect of non-evaporating primordial black holes (PBHs) on
the ionization and thermal history of the universe. X-rays emitted by gas
accretion onto PBHs modify the cosmic recombination history, producing
measurable effects on the spectrum and anisotropies of the Cosmic Microwave
Background (CMB). Using the third-year WMAP data and FIRAS data we improve
existing upper limits on the abundance of PBHs with masses >0.1 Msun by several
orders of magnitude. Fitting WMAP3 data with cosmological models that do not
allow for non-standard recombination histories, as produced by PBHs or other
early energy sources, may lead to an underestimate of the best-fit values of
the amplitude of linear density fluctuations (sigma_8) and the scalar spectral
index (n_s). Cosmological parameter estimates are affected because models with
PBHs allow for larger values of the Thomson scattering optical depth, whose
correlation with other parameters may not be correctly taken into account when
PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95%
CF. This result that may relieve recent tension between WMAP3 data and clusters
data on the value of sigma_8. PBHs may increase the primordial molecular
hydrogen abundance by up to two orders of magnitude, this promoting cooling and
star formation. The suppression of galaxy formation due to X-ray heating is
negligible for models consistent with the CMB data. Thus, the formation rate of
the first galaxies and stars would be enhanced by a population of PBHs.Comment: 17 pages (Apj style), 9 figures, submitted to Ap
Tracing the Warm Hot Intergalactic Medium in the local Universe
We present a simple method for tracing the spatial distribution and
predicting the physical properties of the Warm-Hot Intergalactic Medium (WHIM),
from the map of galaxy light in the local universe. Under the assumption that
biasing is local and monotonic we map the ~ 2 Mpc/h smoothed density field of
galaxy light into the mass density field from which we infer the spatial
distribution of the WHIM in the local supercluster. Taking into account the
scatter in the WHIM density-temperature and density-metallicity relation,
extracted from the z=0 outputs of high-resolution and large box size
hydro-dynamical cosmological simulations, we are able to quantify the
probability of detecting WHIM signatures in the form of absorption features in
the X-ray spectra, along arbitrary directions in the sky. To illustrate the
usefulness of this semi-analytical method we focus on the WHIM properties in
the Virgo Cluster region.Comment: 16 pages 11 Figures. Discussion clarified, alternative methods
proposed. Results unchanged. MNRAS in pres
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