1802.03403
Gauge-invariant formalism of cosmological weak lensing
Yoo, Grimm, Mitsou, Amara, Refregier
Present the gauge-invariant formalism of cosmo WL, accounting for all the relativistic effects due to the scalar, vector, and tensor perturbations at the linear order. While the light propagation is fully described by the geodesic equation, the relation of the photon wave vector to the physical quantities requires the specification of the frames, where they are defined. By constructing the local tetrad bases at the observer and the source positions, clarify the relation of the WL observables such as convergence, the shear, and the rotation to the physical size and shape defined in the source rest-frame and the observed angle and redshift measured in the observer rest-frame. Compared to the standard lensing formalism, additional relativistic effects contribute to all the lensing observables. Explicitly verify the gauge-invariance of the lensing observables and compare the results to previous work. In particular, demonstrate that even in the presence of the vector and tensor perturbations, the physical rotation of the lensing observables vanishes at the linear order, while the tetrad basis rotates along the light propagation compared to a FRW coordinate. Though the latter is often used as a probe of primordial gravitational waves, the rotation of the tetrad basis is indeed not a physical observable. Further clarify its relation to the E-B decomposition in WL. The formalism provides a transparent and comprehensive perspective of cosmological WL.
1802.03474
Skyglow changes over Tucson, Arizona, resulting from a municipal LED street lighting conversion
Barentine, et al
The transition from earlier lighting technologies to white light-emitting diodes (LEDs) is a significant change in the use of artificial light at night. LEDs emit considerably more short-wavelength light into the environment than earlier technologies on a per-lumen basis. Radiative transfer models predict increased skyglow over cities transitioning to LED unless the total lumen output of a new lighting systems is reduced. The City of Tucson, AZ (U.S.), recently converted its municipal street lighting system from a mixture of fully shielded high- and low-pressure sodium (HPS/LPS) luminaries to fully shielded 3000K white LED luminaries. The lighting design intended to minimize increases to sky glow in order to protect the sites of nearby astronomical observatories without compromising public safety. This involved the migration of over 445 million fully shielded HPS/LPS lumens to roughly 142 million fully shielded 3000K white LED lumens and an expected concomitant reduction in the amount of visual skyglow over Tucson. SkyGlow Simulator models predict skyglow decreases in the order of 10-20% depending on whether fully shielded or partly shielded lights are in use. Tested this prediction using visual night sky brightness estimates and luminance-calibrated, panchromatic all-sky imagery at 15 locations in and near the city. Data were obtained in 2014, before the LED conversion began, and in mid-2017 after approximately 95% of ~18,000 luminaries was converted. Skyglow differed marginally, and in all cases with valid data changed by <±20%. Over the same period, the city's upward-directed optical radiance detected from Earth orbit decreased by approximately 7%. While these results are not conclusive, they suggest that LED conversions paired with dimming can reduce skyglow over cities.
1802.03609
The strong gravitational lens finding challenge
Metcalf, et al
Large scale imaging surveys will increase the number of galaxy-scale strong lensing candidates by maybe 3 orders of magnitudes beyond the number known today. Finding these rare objects will require picking them out of at least tens of millions of images and deriving scientific results from them will require quantifying the efficiency and bias of any search method. To achieve these objectives automated methods must be developed. Because gravitational lenses are rare objects, reducing false positive will be particularly important. Present a description and results of an open gravitational lens finding challenge. Participants were asked to classify 100,000 candidate objects as to whether they were gravitational lenses or not with the goal of developing better automated methods for finding lenses in large data sets. A variety of methods were used including visual inspection, arc and ring finders, support vector machines (SVM) and convolutional neural networks (CNN). Find that many of the methods will be easily fast enough to analyse the anticipated data flow. In test data, several methods are able to identify upwards of half the lenses after applying some thresholds on the lens characteristics such as lensed image brightness, size or contrast with the lens galaxy without making a single cals-positive identification. This is significantly better than direct inspection by humans was able to do.
1802.04271
Machine learning cosmological structure formation
Lucie-Smith, Peiris, Pnotzen, Lochner
Train a machine learning algorithm to learn cosmological structure formation from N-body simulations. The algorithm infers the relationship between the initial conditions and the final dark matter haloes, without the need to introduce approximate halo collapse models. Gain insights into the physics driving halo formation by evaluating the predictive performance of the algorithm when provided when different types of information about the local environment around DM particles. The algorithm learns to predict whether or not DM particles will end up in haloes of a given mass range, based on spherical over densities. Show that the resulting predictions match those of spherical collapse approximations such as extended Press-Schechter theory. Additional information on the shape of the local gravitational potential is not able to improve halo collapse predictions; the linear density field contains sufficient information for the algorithm to also reproduce ellipsoidal collapse predictions based on the Sheth-Tormen model. Investigate the algorithm's performance in terms of halo mass and radial position and perform blind analyses on independent initial conditions realizations to demonstrate the generality of the results.
1802.04379
Search for Neutrinos in Super-Kamiokande associated with the GW170817 neutron-star merger
Abe, et al
Calculated 90% CL upper limits on the neutrino fluency for GW170817. From the upward-going-muon events in the energy region above 1.6 GeV, the neutrino fluency limit is 16.0±0.7 (21.3±1.1) cm^{-2} for muon neutrinos (muon antineutrinos), with an error range of ±5deg around the zenith angle of NGC4993, and the energy spectrum is under the assumption of an index of -2. The fluency limit for neutrino energies less than 100 MeV, for which the emission mechanism would be different than for higher-energy neutrinos, is also calculated. It is 6.6e7 cm^-2 for anti-electron neutrinos under the assumption of a Fermi-Direc spectrum with average energy of 20 MeV.
1802.04462
Robustness of the covariance matrix for galaxy clustering measurements
Baumgarten, Chuang
Present a study on the robustness of the covariance matrix estimation for galaxy clustering measurements depending on the cosmological parameters and galaxy bias. To this end, produced 9000 galaxy mock catalogues relying on the effective Zel'dovich approximation implemented in the EZmocks computer code, using different input cosmological models and bias parameters. The reference catalogue has also been produced with this code making the study insensitive to the approximation at least on a relative-qualitative level. The findings indicate that the covariance matrix is insensitive to the input power spectrum (including sigma8), as long as the 2- and 3-point galaxy clustering measurements agree with the given data. In fact, the covariance matrix shows a bias at small scales (r<~40 Mpc/h) when the chosen galaxy bias parameters yield a 3-pt statistics, which is not compatible with the reference one within the error bars, even though the 2-pt statistics agree within 1%. Nevertheless, the error becomes negligible at large scales making the covariance matrix still reliable for data analysis using only measurements in that regime (e.g., measuring baryons acoustic oscillations). High precision in cosmological parameter estimation is expected for covariance matrices extracted from mock galaxy catalogues which take accurately into account both the 2- and the 3-pt statistics. This is independent of whether this is achieved by using the right cosmology and galaxy bias (which are not a priori known) or just any combination of both fitting the net observed galaxy clustering.
1802.04718
The random walk of cars and their collision probabilities with planets
Rein, Tamayo, Vokrouhlicky
On Feb. 6th, 2018, SpaceX launched a Tesla Roadster on a Mars crossing orbit. Perform N-body simulations to determine the fate of the object over the next several million years, und the relevant perturbations acting on the orbit. The orbital evolution is initially dominated by close encounters with the Earth. The first close encounter with the Earth will occur in 2091. The repeated encounters lead to a random walk that eventually causes close encounters with other terrestrial planets and the Sun. Long-term interactions become highly sensitive to the initial conditions after several such close encounters. By running a large ensemble of simulations with slightly perturbed initial conditions, estimate the probability of a collision with Earth and Venus over the next one million years to be 6% and 2.5%, respectively. Estimate the dynamical lifetime of the Tesla to be a few tens of millions of years.
1802.04891
Modified Gravity (MOG) and its test on galaxy clusters
Nieuwenhuizen, Morandi, Limousin
The MOdified Gravity (MOG) theory of J. Moffat assumes a massive vector particle which causes a repulsive contribution to the tensor gravitation. For the galaxy cluster A1689 new data for the X-ray gas and the strong lensing properties are presented. Fits to MOG are possible by adjusting the galaxy density profile. However, this appears to work as an effective DM component, posing a serious problem for MOG. New gas and strong lensing data for the cluster A1835 support these conclusions and point at a tendency of the gas-alone to overestimate the lensing effects in MOG theory.
1802.04816
Cosmic curvature tested directly from observations
Denissenya, Linder, Shafieloo
Cosmic spatial curvature is a fundamental geometric quantity of the Universe. Investigate a model independent, geometric approach to measure spatial curvature directly from observations, without any derivatives of data. This employs strong lensing time delays and supernova distance measurements to measure the curvature itself, rather than just testing consistency with flatness. Define 2 curvature estimators, with differing error propagation characteristics, that can crosscheck each other, and also show how they can be used to map the curvature in z slices, to test constancy of curvature as required by the Robertson-Walker metric. Simulating realizations of redshift distributions and distance measurements of lenses and sources, estimate uncertainties on the curvature enabled by next generation measurements. The results indicate that the model independent methods, using only geometry without assuming forms for the energy density constituents, can determine the curvature at the ~6e-3 level.
1802.05089
Gravitational lensing of gravitational waves: a statistical perspective
Li, Mao, Zhao, Lu
In this paper, study the strong GL of gravitational waves (GWs) from a statistical perspective, with particular focus on the high frequency GWs from stellar binary BH coalescences. These are most promising targets for ground-based detectors such as aLIGO and the proposed Einstein Telescope (ET) and can be safely treated under the geometrical optics limit for GW propagation. Performa a thorough calculation of the lensing rate, by taking account of effects caused by the ellipticity of lensing galaxies, lens environments, and magnification bars. Find that in certain GW source rate scenarios, should be able to observed strongly lensed GW events once per year (~1/yr) in the aLIGO survey at its design sensitivity; for the proposed ET survey, the rate could be as high as ~80/yr. These results depend on the estimate of GW source abundance, and hence can be correspondingly modified with an improvement in the understanding of the merger rate of stellar binary BHs. Also compute the fraction of 4-image lens systems in each survey, predicting it to be ~30% for the aLIGO survey and ~6% for the ET survey. Finally, evaluate the possibility of missing some images due to the finite survey duration, by presenting the probability distribution of lensing time delays. Predict that this selection bias will be insignificant in future GW surveys, as most of the lens systems (~90%) will have time delays less than ~1 month which will be far shorter than survey durations.
1802.05257
Dark Energy Survey Year 1 Results: methodology and projection for joint analysis of galaxy clustering, galaxy lensing, and CMB lensing two-point functions
Baxter, et al
Optical imaging surveys measure both the galaxy density and the gravitational lensing-induced shear fields cross the sky. Recently, the DES collaboration used a joint fit to 2-pt correlations between these observables to place tight constraints on cosmology (DES+2017). In this work, develop the methodology to extend the DES collaboration+2017 analysis to include cross-correlations of the optical survey observables with gravitational lensing of the CMB as measured by the SPT and Planck. Using simulated analyses, show how the resulting set of five 2pt functions increases the robustness of the cosmo constraints to systematic errors in galaxy lensing shear calibration. Additionally, show that contamination of the SPT+Planck CMB lensing map by the thermal SZ effect is a potentially large source of systematic error for 2-pt function analyses, but show that it can be reduced to acceptable levels in the analysis by masking clusters of galaxies and imposing angular scale cuts on the 2-pt functions. The methodology developed here will be applied to the analysis of data from the DES, the SPT, and Planck in a companion work.
1802.05273
Reinterpreting low frequency LIGO/Virgo events as magnified stellar-mass black holes at cosmological distances
Broadhurst, Diego, Smoot
GWs can be focussed by the gravity of an intervening galaxy, just like light, thereby magnifying binary merging events in the far Universe. High magnification by galaxies is found to be responsible for the brightest sources detected in sky surveys, but the low angular resolution of LIGO/Virgo is insufficient to check this lensing possibility directly. Here, find that the first 6 binary black hole (BBH) merging events reported by LIGO/Virgo show clear evidence for lensing in the plane of observed mass and source distance. The four lowest frequency events follow an apparent locus in this plane, which can be reproduced by galaxy lensing, where the higher the magnification, the generally more distant the source, so the wave train is stretched more by the Universal expansion, by factors of 2-4. This revises the reported BBH distances upwards by an order of magnitude, equal to the square root of the magnification. Furthermore, the reported BH masses must be deceased by 2-4 to counter the larger stretch factor, since the orbital frequency is used to derive the BH masses. This lowers the masses to 5-15 solar, well below the puzzlingly high values of 20-35 solar masses otherwise estimated, with the attraction of finding agreement in mass with BHs orbiting stars in our own Galaxy, thereby implying a stellar origin for the low frequency events in the far Universe. Also show that the other 2 BBH events of higher frequency detected by LIGO/VIRGO, lie well below the lensing locus, consistent with being nearby and unlensed. If this apparent division between local and distant lensed events is reinforced by new detections then the spins and masses of stellar BHs can be compared over a timespan of 10B years by LIGO/Virgo.
1802.05706
Weak lensing of intensity mapping: the cosmic infrared background
Schaan, Ferraro, Spergel
GL deflects the paths of cosmic infrared background (CIB) photons, leaving a measurable imprint on CIB maps. The resulting statistical anisotropy can be used to reconstruct the matter distribution out to the redshifts of CIB sources. To this end, generalize the CMB lensing quadratic estimator to any weakly non-Gaussian source field, by deriving the optimal lensing weights. Point out the additional noise and bias caused by the non-Gaussianity and the 'self-lensing' of the source field. Propose methods to reduce, subtract or model these non-Gaussianiities. Show that CIB lensing should be detectable with Planck data, and detectable at high significance for future CMB experiments like CCAT-Prime. The CIB thus constitutes a new source image for lensing studies, providing constraints on the amplitude of structure at intermediate z's between galaxies and the CMB. CIB lensing measurements will also give valuable information on the SFH in the universe, constraining CIB halo models beyond the CIB power spectrum. By laying out a detailed treatment of lens reconstruction from a weakly non-Gaussian source field, this work constitutes a stepping stone towards lens reconstruction from continuum or line intensity mapping data, such as the Lyman-alpha emission, absorption, and the 21cm radiation.
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