# Browsing by Author "Popovic, Brodie"

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Item Open Access Cosmology Using Photometric Samples of Type Ia Supernovae: The First Joint Photometric Light Curve Analysis(2023) Popovic, BrodieOver the last twenty five years, type Ia supernova (SNIa) have been a crucial cosmological probe, responsible for the discovery of the accelerating expansion of the universe. But with the dawn of the next generation of telescopes and SNIa samples, new methods and techniques are needed to increase precision and handle ever-smaller systematics.The Dark Energy Survey (DES) Supernova program has recently completed, and will be the first program whose success relies on the analysis of a sample in which we do not know the typing of the supernova. These `photometric samples' are a significant switch from spectroscopic samples, in which every SNIa used in the analysis is spectroscopically identified. The motivation for this switch is the opportunity to have $\sim 10$ times larger samples than what would be feasible with spectroscopic identification; however, these analyses are replete with new and untested sources of systematic uncertainty. One less obvious, but equally significant, challenge is understanding the selection effects of this new survey strategy. This thesis serves to demonstrate the efficacy of these photometric samples. Simultaneously, as our statistical constraints improve, a higher burden is placed on other systematic uncertainties, like a better understanding of the environment that SNe occur in. Dust attenuates and reddens SNIa light curves, obscuring the true properties and astrophysical origins of the SNIa explosion. Here I combine the results of my four first-author papers in non-chronological order, laying out the steps I took to perform a cosmology analysis from start to finish.

In Figure \ref{fig:roadmap}, I provide a general overview and outline of the course of this thesis, and how my papers aid in answering these questions. My first question is \textit{how do we observe the universe?} I go over the surveys and experiments that have been used to peer back billions of years into cosmic history. I also discuss how our imperfect instruments and selection effects all impact our observations, alongside how non-cosmological effects make these observations even more challenging, and how we try to mitigate those issues.

To aid our methods of observing the universe, I ask - \textit{how do we simulate the universe?} These simulations are able to create extremely realistic, catalogue level samples that can be tuned to mirror our data in every measurable metric. This question leads to the first published work in this thesis: \textit{The Pantheon+ Analysis: Forward-Modeling the Dust and Intrinsic Colour Distributions of Type Ia Supernovae, and Quantifying their Impact on Cosmological Inferences}. The specific characteristics and modeling of dust distributions are drawn from the data using a multi-dimensional Markov Chain Monte Carlo method to infer and separate the intrinsic SNIa properties from those caused by external dust effects, informing us as to \textit{how our measurements are biased}.

Given our improved simulations, I ask \textit{how do we fix these biases?}. In the next paper, \textit{Improved Treatment of Host-Galaxy Correlations in Cosmological Analyses With Type Ia Supernovae}, I introduce a method to fix these biases using simulations by providing the first phenomenological model of correlating SNIa properties with that of their host galaxy. This framework is used as the basis for a novel set of bias corrections that are able to account for realistic correlations between SNIa properties, as well as separately introducing the first bias corrections methodology to correct dust models of SNIa scatter.

In the process of building up to cosmology measurements, I ask \textit{what biases are unique to photometric surveys?}. In \textit{Assessment of the Systematic Uncertainties in the Cosmological Analysis of the SDSS Supernovae Photometric Sample}, I investigate a collection of potential biases and uncertainties that are unique to photometric samples: assessing the impact of mis-associating the host galaxy, modeling of non-Ia contamination, and changing the modeled efficiency of detecting the host galaxies.

My last paper puts all of these papers and methods together to answer the question: \textit{What cosmological result do we find?} I create the first-ever joint analysis of two photometric SNIa samples, making the largest SNIa analysis to-date. I test for consistency between the samples with the first comparison of statistically-independent SNIa samples, and show that the results of this joint photometric analysis are competitive with the best spectroscopic SNIa analysis.

Item Open Access The Pantheon+ Analysis: Evaluating Peculiar Velocity Corrections in Cosmological Analyses with Nearby Type Ia SupernovaePeterson, Erik R; Kenworthy, W D'Arcy; Scolnic, Daniel; Riess, Adam G; Brout, Dillon; Carr, Anthony; Courtois, Helene; Davis, Tamara; Dwomoh, Arianna; Jones, David O; Popovic, Brodie; Rose, Benjamin M; Said, KhaledSeparating the components of redshift due to expansion and motion in the nearby universe ($z<0.1$) is critical for using Type Ia Supernovae (SNe Ia) to measure the Hubble constant ($H_0$) and the equation-of-state parameter of dark energy ($w$). Here, we study the two dominant 'motions' contributing to nearby peculiar redshifts: large-scale, coherent-flow (CF) motions and small-scale motions due to gravitationally-associated galaxies deemed to be in a galaxy group. We use a set of 585 low-$z$ SNe from the Pantheon+ sample, and evaluate the efficacy of corrections to these motions by measuring the improvement of SN distance residuals. We study multiple methods for modeling the large and small-scale motions and show that while group assignments and CF corrections individually contribute to small improvements in Hubble residual scatter, the greatest improvement comes from the combination of the two (relative standard deviation of the Hubble residuals RSD improves from 0.167 mag to 0.157 mag). We find the optimal flow corrections derived from various local density maps significantly reduce Hubble residuals while raising $H_0$ by $\sim0.4$ km s$^{-1}$ Mpc$^{-1}$ as compared to using CMB redshifts, disfavoring the hypothesis that unrecognized local structure could resolve the Hubble tension. We estimate that the systematic uncertainties in cosmological parameters after optimally correcting redshifts are 0.08-0.17 km s$^{-1}$ Mpc$^{-1}$ in $H_0$ and 0.02-0.03 in $w$ which are smaller than the statistical uncertainties for these measurements: 1.5 km s$^{-1}$ Mpc$^{-1}$ for $H_0$ and 0.04 for $w$.Item Open Access The Pantheon+ Analysis: Forward-Modeling the Dust and Intrinsic Colour Distributions of Type Ia Supernovae, and Quantifying their Impact on Cosmological InferencesPopovic, Brodie; Brout, Dillon; Kessler, Richard; Scolnic, DanielRecent studies have shown that the observed colour distributions of Type Ia SNe (SNIa) are well-described by a combination of distributions from dust and intrinsic colour. Here we present a new forward-modeling fitting method (Dust2Dust) to measure the parent dust and colour distributions, including their dependence on host-galaxy mass. At each fit step, the SNIa selection efficiency is determined from a large simulated sample that is re-weighted to reflect the proposed distributions. We use five separate metrics to constrain the Dust2Dust parameters: distribution of fitted light-curve colour $c$, cosmological residual trends with $c$, cosmological residual scatter with $c$, fitted colour-luminosity relationship $\beta_{\rm SALT2}$, and intrinsic scatter $\sigma_{\rm int}$. Using the Pantheon+ data sample, we present results for a Dust2Dust fit that includes 4 parameters describing intrinsic colour variations and 8 parameters describing dust. Furthermore, we propagate the Dust2Dust parameter uncertainties and covariance to the dark energy equation-of-state $w$ and Hubble constant H$_0$: we find $\sigma_w = 0.005$ and $\sigma_{\textrm{H}_0} = 0.145~$km/s/Mpc. The Dust2Dust code is publically available.Item Open Access The Pantheon+ Type Ia Supernova Sample: The Full Dataset and Light-Curve ReleaseScolnic, Dan; Brout, Dillon; Carr, Anthony; Riess, Adam G; Davis, Tamara M; Dwomoh, Arianna; Jones, David O; Ali, Noor; Charvu, Pranav; Chen, Rebecca; Peterson, Erik R; Popovic, Brodie; Rose, Benjamin M; Wood, Charlotte; Brown, Peter J; Coulter, David A; Dettman, Kyle G; Dimitriadis, Georgios; Filippenko, Alexei V; Foley, Ryan J; Jha, Saurabh W; Kilpatrick, Charles D; Kirshner, Robert P; Pan, Yen-Chen; Rest, Armin; Rojas-Bravo, Cesar; Siebert, Matthew R; Stahl, Benjamin E; Zheng, WeiKangHere we present 1701 light curves of spectroscopically confirmed Type Ia supernovae (SNe Ia) that will be used to infer cosmological parameters as part of the Pantheon+ SN analysis and the SH0ES (Supernovae and H0 for the Equation of State of dark energy) distance-ladder analysis. This effort is one part of a series of works that perform an extensive review of redshifts, peculiar velocities, photometric calibration, and intrinsic-scatter models of SNe Ia. The total number of light curves, which are compiled across 18 different surveys, is a significant increase from the first Pantheon analysis (1048 SNe), particularly at low redshift ($z$). Furthermore, unlike in the Pantheon analysis, we include light curves for SNe with $z<0.01$ such that SN systematic covariance can be included in a joint measurement of the Hubble constant (H$_0$) and the dark energy equation-of-state parameter ($w$). We use the large sample to compare properties of 170 SNe Ia observed by multiple surveys and 12 pairs/triplets of "SN siblings" - SNe found in the same host galaxy. Distance measurements, application of bias corrections, and inference of cosmological parameters are discussed in the companion paper by Brout et al. (2022b), and the determination of H$_0$ is discussed by Riess et al. (2022). These analyses will measure w with $\sim3\%$ precision and H$_0$ with 1 km/s/Mpc precision.