Tuning Gravitationally Lensed Standard Sirens

Gravitational waves emitted by chirping supermassive black hole binaries could in principle be used to obtain very accurate distance determinations. Provided they have an electromagnetic counterpart from which the redshift can be determined, these standard sirens could be used to build a high redshift Hubble diagram. Errors in the distance measurements will most likely be dominated by gravitational lensing. We show that the (de)magnification due to inhomogeneous foreground matter will increase the scatter in the measured distances by a factor ~10. We propose to use optical and IR data of the foreground galaxies to minimize the degradation from weak lensing. We find that the net effect of correcting the estimated distances for lensing is comparable to increasing the sample size by a factor of three when using the data to constrain cosmological parameters.Comment: 21 pages, 4 figures, accepted for publication in Ap

The effect of inhomogeneities on dark energy constraints

Constraints on models of the late time acceleration of the universe assume the cosmological principle of homogeneity and isotropy on large scales. However, small scale inhomogeneities can alter observational and dynamical relations, affecting the inferred cosmological parameters. For precision constraints on the properties of dark energy, it is important to assess the potential systematic effects arising from these inhomogeneities. In this study, we use the Type Ia supernova magnitude-redshift relation to constrain the inhomogeneities as described by the Dyer-Roeder distance relation and the effect they have on the dark energy equation of state ($w$), together with priors derived from the most recent results of the measurements of the power spectrum of the Cosmic Microwave Background and Baryon Acoustic Oscillations. We find that the parameter describing the inhomogeneities ($\eta$) is correlated with $w$. The best fit values $w = -0.933 \pm 0.065$ and $\eta = 0.61 \pm 0.37$ are consistent with homogeneity at $< 2 \sigma$ level. Assuming homogeneity ($\eta =1$), we find $w = -0.961 \pm 0.055$, indicating only a small change in $w$. For a time-dependent dark energy equation of state, $w_0 = -0.951 \pm 0.112$ and $w_a = 0.059 \pm 0.418$, to be compared with $w_0 = -0.983 \pm 0.127$ and $w_a = 0.07 \pm 0.432$ in the homogeneous case, which is also a very small change. Current data are not sufficient to constrain the fraction of dark matter (DM) in compact objects, $f_p$ at the 95$\%$ C.L., however at 68$\%$ C.L. $f_p < 0.73$. Future supernova surveys will improve the constraints on $\eta$, and $f_p$, by a factor of $\sim$ 10.Comment: 16 pages, 4 figures, accepted by JCA

Rates and Properties of Strongly Gravitationally Lensed Supernovae and their Host Galaxies in Time-Domain Imaging Surveys

Supernovae that are strongly gravitationally lensed (gLSNe) by galaxies are powerful probes of astrophysics and cosmology that will be discovered systematically by next-generation wide-field, high-cadence imaging surveys such as the Zwicky Transient Facility (ZTF) and the Large Synoptic Survey Telescope (LSST). Here we use pixel-level simulations that include dust, observing strategy, and multiple supernova subtypes to forecast the rates and properties of gLSNe that ZTF and LSST will find. Applying the resolution-insensitive discovery strategy of Goldstein et al. (2018), we forecast that ZTF (LSST) can discover 0.02 (0.79) 91bg-like, 0.17 (5.92) 91T-like, 1.22 (47.84) Type Ia, 2.76 (88.51) Type IIP, 0.31 (12.78) Type IIL, and 0.36 (15.43) Type Ib/c gLSNe per year. We also forecast that the surveys can discover at least 3.75 (209.32) Type IIn gLSNe per year, for a total of at least 8.60 (380.60) gLSNe per year under fiducial observing strategies. ZTF gLSNe have a median $z_s=0.9$, $z_l=0.35$, $\mu_\mathrm{tot}=30$, $\Delta t_\mathrm{max}= 10$ days, $\min(\theta)= 0.25^{\prime\prime}$, and $N_\mathrm{img} = 4$. LSST gLSNe are less compact and less magnified, with a median $z_s=1.0$, $z_l=0.4$, $\mu_\mathrm{tot}\approx6$, $\Delta t_\mathrm{max} = 25$ days, $\min(\theta)=0.6^{\prime\prime}$, and $N_\mathrm{img} = 2$. As the properties of lensed host galaxy arcs provide critical information for lens mass modeling, we develop a model of the supernova--host galaxy connection and use it to simulate realistic images of the supernova--host--lens systems. We find that the vast majority of gLSN host galaxies will be multiply imaged, enabling detailed constraints on lens models with sufficiently deep high-resolution imaging taken after the supernova has faded. We release the results of our simulations to the public as catalogs at this URL: http://portal.nersc.gov/project/astro250/glsne/.Comment: 57 pages, 66 equations, 36 figures, 4 tables, Submitted to ApJS, comments welcome, v2 replaced some figures with rasterized versions to reduce load on PDF viewer

Narrowing down the possible explanations of cosmic acceleration with geometric probes

Recent re-calibration of the Type Ia supernova (SNe~Ia) magnitude-redshift relation combined with cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data have provided excellent constraints on the standard cosmological model. Here, we examine particular classes of alternative cosmologies, motivated by various physical mechanisms, e.g. scalar fields, modified gravity and phase transitions to test their consistency with observations of SNe~Ia and the ratio of the angular diameter distances from the CMB and BAO. Using a model selection criterion for a relative comparison of the models (the Bayes Factor), we find moderate to strong evidence that the data prefer flat $\Lambda$ CDM over models invoking a thawing behaviour of the quintessence scalar field. However, some exotic models like the growing neutrino mass cosmology and vacuum metamorphosis still present acceptable evidence values. The bimetric gravity model with only the linear interaction term can be ruled out by the combination of SNe~Ia and CMB/BAO datasets whereas the model with linear and quadratic interaction terms has a comparable evidence value to standard $\Lambda$ CDM. Thawing models are found to have significantly poorer evidence compared to flat $\Lambda$ CDM cosmology under the assumption that the CMB compressed likelihood provides an adequate description for these non-standard cosmologies. We also present estimates for constraints from future data and find that geometric probes from oncoming surveys can put severe limits on non-standard cosmological models.Comment: 31 pages, 9 figures, to be submitted to JCAP. Comments welcom

Selection of high-z supernovae candidates

Deep, ground based, optical wide-field supernova searches are capable of detecting a large number of supernovae over a broad redshift range up to z~1.5. While it is practically unfeasible to obtain spectroscopic redshifts of all the supernova candidates right after the discovery, we show that the magnitudes and colors of the host galaxies, as well as the supernovae, can be used to select high-z supernova candidates, for subsequent spectroscopic and photometric follow-up. Using Monte-Carlo simulations we construct criteria for selecting galaxies in well-defined redshift bands. For example, with a selection criteria using B-R and R-I colors we are able to pick out potential host galaxies for which z>0.85 with 80% confidence level and with a selection efficiency of 64-86%. The method was successfully tested using real observations from the HDF. Similarly, we show that that the magnitude and colors of the supernova discovery data can be used to constrain the redshift. With a set of cuts based on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a redshift interval sigma_z <0.15.Comment: 33 pages, 13 figures, accepted for publication in PASP (March 2002 issue

Supernova cosmology: legacy and future

The discovery of dark energy by the first generation of high-redshift supernova surveys has generated enormous interest beyond cosmology and has dramatic implications for fundamental physics. Distance measurements using supernova explosions are the most direct probes of the expansion history of the Universe, making them extremely useful tools to study the cosmic fabric and the properties of gravity at the largest scales. The past decade has seen the confirmation of the original results. Type Ia supernovae are among the leading techniques to obtain high-precision measurements of the dark energy equation of state parameter, and in the near future, its time dependence. The success of these efforts depends on our ability to understand a large number of effects, mostly of astrophysical nature, influencing the observed flux at Earth. The frontier now lies in understanding if the observed phenomenon is due to vacuum energy, albeit its unnatural density, or some exotic new physics. Future surveys will address the systematic effects with improved calibration procedures and provide thousands of supernovae for detailed studies.Comment: Invited review, Annual Review of Nuclear and Particle Science (submitted version