Weak gravitational lensing with supernovae
Student thesis: Doctoral Thesis
Supernovae are important probes of cosmology. In 1999, Type Ia Supernovae (SNeIa) provided the first evidence for the accelerating expansion of the Universe (Riesset al., 1998, Perlmutter et al., 1999), and since then there have been many widefield SN surveys with the scope of increasing the number of observed SNe, thus improving the constraints on cosmological parameters. Among these SN surveys,the Dark Energy Survey (DES) and the planned Large Synoptic Survey Telescope(LSST) will increase the number of available SNe Ia respectively to ' 3000 and ∼10^{5} (possibly ∼ 10^{6}) in the coming decade. Weak gravitational lensing effects willthen become important for these new surveys.
Weak gravitational lensing have different effects on the distance modulus measurements of SNe. Firstly, it introduces a nonGaussian scatter on the distance moduli of SNe Ia, and this effect increases as a function of redshift. The nonGaussian weak lensing distribution can also introduce a bias on the cosmological parameter values recovered by fitting the Hubble diagram. Secondly, it introduces spatial correlations on the magnitudes of close SN pairs, with angular separation of the order of arcminutes. Weak lensing of SNe can also be used to probe the growth of structures along the lineofsight, giving further constraints on cosmological parameters like σ_{8} and Ω_{m}.
In Chapter 2, we present our results on the fit of the Hubble diagram from the
Jointed Lightcurve Analysis sample (JLA, Betoule et al. 2014) including weak lensing and peculiar velocities, the latter introducing an extra dispersion on the distance modulus measurements of low redshift SNe. We give constraints on the cosmological parameters when fitting for the the first four moments of the weak lensing distribution together with the variance induced by peculiar velocities. We test our method via numerical simulations and we find Ω_{m}=0.274±0.013 and σ_{8}=0.44^{+0.63}_{0.44} when fitting the JLA sample. We also apply the Kernel Density Estimation technique to reduce the problem of biased estimates of the moments measured on sparse data sample, and a bootstrap resampling method when computing the covariance between the moments.
In Chapter 3 we propose to measure the twopoint magnitude correlation function from SN data and compare such measurements to theoretical expectations. As available data sample appear to be insufficient to detect this weak correlation (we report a tentative detection with the JLA sample), we predict measurements with current (DES) and future (LSST) SN surveys, finding that the LSST should be able to detect such correlations at 6σ level of confidence (15,000 SNe over 70 deg^{2} and assuming an intrinsic scatter of 0.15 magnitudes). DES (deep field) is expected to detect a crosscorrelation between the Hubble residuals and the foreground galaxies at 12σ (integrated up to 9 arcminutes of separation and assuming an intrinsic scatter of 0.15 magnitudes), taking advantage of the higher galaxy density on the sky, while LSST should detect the same crosscorrelation with signaltonoise ⪆ 100. We also give forecasts on cosmological parameters when fitting Ωm and σ_{8} from the twopoint magnitude autocorrelation function, i.e. we can achieve a 25% measurement of σ_{8} from LSST (assuming 0.15 magnitudes of intrinsic scatter and applying a Gaussian prior on the matter density parameter).
In Chapter 4, we investigate Type Ic Superluminous Supernovae (SLSNe Ic) as a new class of potential standard candles, which appear to be standardisable in their peak magnitudes with a scatter of only 0.2 – 0.3 magnitudes. Moreover, their exceptional peak magnitude (up to 100 times brighter than SNe Ia) allows them to be discovered to redshift ∼ 3, shedding new light on the deceleration epoch of the Universe. We give predictions for SLSN Ic redshift distribution within present (DES and SUDSS, which are expected to find 15 and 75 SLSNe respectively) and future surveys (LSST and Euclid, which should increase the available SLSNe to 10; 000 and 300 respectively, the latter up to redshift 4). We construct simulated Hubble diagrams for SLSNe Ic, spanning the likely values of intrinsic scatter for these sources ( 0:15  0:25 magnitudes), and fit the Hubble diagrams to infer cosmological constraints. We find that the addition of 75 SLSNe from SUDSS to the 3800 SNe Ia from DES can improve the constraints on w (the dark energy state parameter) and Ω_{m} by 20% (assuming a flat wCDM universe). Moreover, the combination of DES SNe Ia and 10,000 LSST SLSNe can measure Ω_{m} and w to 2% and 4% respectively. When considering temporal variations in w(a), we find possible uncertainties of 2%, 5% and 0.14 on Ω_{m}, w_{0} and w_{a} respectively, from the combination of DES SNe Ia, LSST SLSNe and Planck Cosmic Microwave Background temperature power spectrum. We find that SLSNe from Euclid can constrain the matter density parameter to 10%, and can help constraining the equationofstate parameters w_{0} and w_{a}. All these surveys will also improve the knowledge about SLSN astrophysics, their progenitors and possible classification into subclasses.
Weak gravitational lensing have different effects on the distance modulus measurements of SNe. Firstly, it introduces a nonGaussian scatter on the distance moduli of SNe Ia, and this effect increases as a function of redshift. The nonGaussian weak lensing distribution can also introduce a bias on the cosmological parameter values recovered by fitting the Hubble diagram. Secondly, it introduces spatial correlations on the magnitudes of close SN pairs, with angular separation of the order of arcminutes. Weak lensing of SNe can also be used to probe the growth of structures along the lineofsight, giving further constraints on cosmological parameters like σ_{8} and Ω_{m}.
In Chapter 2, we present our results on the fit of the Hubble diagram from the
Jointed Lightcurve Analysis sample (JLA, Betoule et al. 2014) including weak lensing and peculiar velocities, the latter introducing an extra dispersion on the distance modulus measurements of low redshift SNe. We give constraints on the cosmological parameters when fitting for the the first four moments of the weak lensing distribution together with the variance induced by peculiar velocities. We test our method via numerical simulations and we find Ω_{m}=0.274±0.013 and σ_{8}=0.44^{+0.63}_{0.44} when fitting the JLA sample. We also apply the Kernel Density Estimation technique to reduce the problem of biased estimates of the moments measured on sparse data sample, and a bootstrap resampling method when computing the covariance between the moments.
In Chapter 3 we propose to measure the twopoint magnitude correlation function from SN data and compare such measurements to theoretical expectations. As available data sample appear to be insufficient to detect this weak correlation (we report a tentative detection with the JLA sample), we predict measurements with current (DES) and future (LSST) SN surveys, finding that the LSST should be able to detect such correlations at 6σ level of confidence (15,000 SNe over 70 deg^{2} and assuming an intrinsic scatter of 0.15 magnitudes). DES (deep field) is expected to detect a crosscorrelation between the Hubble residuals and the foreground galaxies at 12σ (integrated up to 9 arcminutes of separation and assuming an intrinsic scatter of 0.15 magnitudes), taking advantage of the higher galaxy density on the sky, while LSST should detect the same crosscorrelation with signaltonoise ⪆ 100. We also give forecasts on cosmological parameters when fitting Ωm and σ_{8} from the twopoint magnitude autocorrelation function, i.e. we can achieve a 25% measurement of σ_{8} from LSST (assuming 0.15 magnitudes of intrinsic scatter and applying a Gaussian prior on the matter density parameter).
In Chapter 4, we investigate Type Ic Superluminous Supernovae (SLSNe Ic) as a new class of potential standard candles, which appear to be standardisable in their peak magnitudes with a scatter of only 0.2 – 0.3 magnitudes. Moreover, their exceptional peak magnitude (up to 100 times brighter than SNe Ia) allows them to be discovered to redshift ∼ 3, shedding new light on the deceleration epoch of the Universe. We give predictions for SLSN Ic redshift distribution within present (DES and SUDSS, which are expected to find 15 and 75 SLSNe respectively) and future surveys (LSST and Euclid, which should increase the available SLSNe to 10; 000 and 300 respectively, the latter up to redshift 4). We construct simulated Hubble diagrams for SLSNe Ic, spanning the likely values of intrinsic scatter for these sources ( 0:15  0:25 magnitudes), and fit the Hubble diagrams to infer cosmological constraints. We find that the addition of 75 SLSNe from SUDSS to the 3800 SNe Ia from DES can improve the constraints on w (the dark energy state parameter) and Ω_{m} by 20% (assuming a flat wCDM universe). Moreover, the combination of DES SNe Ia and 10,000 LSST SLSNe can measure Ω_{m} and w to 2% and 4% respectively. When considering temporal variations in w(a), we find possible uncertainties of 2%, 5% and 0.14 on Ω_{m}, w_{0} and w_{a} respectively, from the combination of DES SNe Ia, LSST SLSNe and Planck Cosmic Microwave Background temperature power spectrum. We find that SLSNe from Euclid can constrain the matter density parameter to 10%, and can help constraining the equationofstate parameters w_{0} and w_{a}. All these surveys will also improve the knowledge about SLSN astrophysics, their progenitors and possible classification into subclasses.
Original language  English 

Supervisors/Advisors 

Award date  Feb 2017 
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5.89 MB, PDF document
ID: 7697208