We present a systematic comparison of several existing and new void finding algorithms, focusing on their potential power to test a particular class of modified gravity modelschameleon f (R) gravity. These models deviate from standard General Relativity (GR) more strongly in low-density regions and thus voids are a promising venue to test them. We use Halo Occupation Distribution (HOD) prescriptions to populate haloes with galaxies, and tune the HOD parameters such that the galaxy two-point correlation functions are the same in both f (R) and GR models. We identify both 3D voids as well as 2D underdensities in the planeof-the-sky to find the same void abundance and void galaxy number density profiles across all models, which suggests that they do not contain much information beyond galaxy clustering. However, the underlying void dark matter density profiles are significantly different, with f (R) voids being more underdense than GR ones, which leads to f (R) voids having a larger tangential shear signal than their GR analogues. We investigate the potential of each void finder to test f (R) models with near-future lensing surveys such as EUCLID and LSST. The 2D voids have the largest power to probe f (R) gravity, with a LSST analysis of tunnel (which is a new type of 2D underdensity introduced here) lensing distinguishing at 80 and 11σ (statistical error) f (R) models with parameters, | f R0 | = 10 −5 and 10 −6 , from GR.

We study cosmic voids in the normal-branch Dvali-Gabadadze-Porrati (nDGP) braneworld models, which are representative of a class of modified gravity theories where deviations from General Relativity are usually hidden by the Vainshtein screening in high-density environments. This screening is less efficient away from these environments, which makes voids ideally suited for testing this class of models. We use N-body simulations of Λ-cold dark matter (ΛCDM) and nGDP universes, where dark matter haloes are populated with mock galaxies that mimic the clustering and number densities of the BOSS CMASS galaxy sample. We measure the force, density and weak lensing profiles around voids identified with six different algorithms. Compared to ΛCDM, voids in nDGP are more under-dense due to the action of the fifth force that arises in these models, which leads to a faster evacuation of matter from voids. This leaves an imprint on the weak lensing tangential shear profile around nDGP voids, an effect that is particularly strong for 2D underdensities that are identified in the plane-ofthe-sky. We make predictions for the feasibility of distinguishing between nDGP and ΛCDM using void lensing in upcoming large-scale surveys such as LSST and EUCLID. We compare with the analysis of voids in chameleon gravity theories and find that the weak lensing signal for 3D voids is similar to nDGP, whereas for 2D voids the differences with ΛCDM are much stronger for the chameleon gravity case, a direct consequence of the different screening mechanisms operating in these theories.

Observations spanning a large wavelength range, from X-ray to radio, of the Type IIb supernova 2011hs are presented, covering its evolution during the first year after explosion. The optical light curve presents a narrower shape and a fainter luminosity at peak than previously observed for Type IIb SNe. High expansion velocities are measured from the broad absorption H I and He I lines. From the comparison of the bolometric light curve and the time evolution of the photospheric velocities with hydrodynamical models, we found that SN 2011hs is consistent with the explosion of a 3-4 M ⊙ He-core progenitor star, corresponding to a main sequence mass of 12-15 M ⊙ , that ejected a mass of 56 Ni of about 0.04 M ⊙ , with an energy of E = 8.5 × 10 50 erg. Such a low-mass progenitor scenario is in full agreement with the modelling of the nebular spectrum taken at ∼215 days from maximum. From the modelling of the adiabatic cooling phase, we infer a progenitor radius of ≈500-600 R ⊙ , clearly pointing to an extended progenitor star. The radio light curve of SN 2011hs yields a peak luminosity similar to that of SN 1993J, but with a higher mass loss rate and a wind density possibly more similar to that of SN 2001ig. Although no significant deviations from a smooth decline have been found in the radio light curves, we cannot rule out the presence of a binary companion star.

We present ultraviolet, optical, near-infrared photometry and spectroscopy of SN 2009N in NGC 4487. This object is a type II-P supernova with spectra resembling those of subluminous II-P supernovae, while its bolometric luminosity is similar to that of the intermediate luminosity SN 2008in. We created synow models of the plateau phase spectra for line identification and to measure the expansion velocity. In the near-infrared spectra we find signs indicating possible weak interaction between the supernova ejecta and the pre-existing circumstellar material. These signs are also present in the previously unpublished near-infrared spectra of SN 2008in. The distance to SN 2009N is determined via the expanding photosphere method and the standard candle method as D = 21.6 ± 1.1 Mpc. The produced nickel-mass is estimated to be ∼ 0.020±0.004 M ⊙ . We infer the physical properties of the progenitor at the explosion through hydrodynamical modelling of the observables. We find the values of the total energy as ∼ 0.48 × 10 51 erg, the ejected mass as ∼ 11.5 M ⊙ , and the initial radius as ∼ 287 R ⊙ .

We present optical and near-infrared photometry and spectroscopy of SN 2009ib, a Type II-P supernova in NGC 1559. This object has moderate brightness, similar to those of the intermediate-luminosity SNe 2008in and 2009N. Its plateau phase is unusually long, lasting for about 130 days after explosion. The spectra are similar to those of the subluminous SN 2002gd, with moderate expansion velocities. We estimate the 56 Ni mass produced as 0.046 ± 0.015 M ⊙ . We determine the distance to SN 2009ib using both the expanding photosphere method (EPM) and the standard candle method. We also apply EPM to SN 1986L, a type II-P SN that exploded in the same galaxy. Combining the results of different methods, we conclude the distance to NGC 1559 as D = 19.8 ± 3.0 Mpc. We examine archival, pre-explosion images of the field taken with the Hubble Space Telescope, and find a faint source at the position of the SN, which has a yellow colour ((V − I) 0 = 0.85 mag). Assuming it is a single star, we estimate its initial mass as M ZAMS = 20 M ⊙ . We also examine the possibility, that instead of the yellow source the progenitor of SN 2009ib is a red supergiant star too faint to be detected. In this case we estimate the upper limit for the initial zero-age main sequence mass of the progenitor to be ∼ 14 − 17 M ⊙ . In addition, we infer the physical properties of the progenitor at the explosion via hydrodynamical modelling of the observables, and estimate the total energy as ∼ 0.55 × 10 51 erg, the pre-explosion radius as ∼ 400 R ⊙ , and the ejected envelope mass as ∼ 15 M ⊙ , which implies that the mass of the progenitor before explosion was ∼ 16.5 − 17 M ⊙ .