The use of Type Ia supernovae (SNe Ia) as distance indicators is essential for studying the expansion history of the Universe and for exploring the nature of dark energy. However, a lack of understanding of the progenitor systems and the empirically derived colour-brightness corrections represent severe limitations for SNe Ia as cosmological probes. In this thesis, we study how dust along the line of sight towards SNe Ia affects the observed light over a wide range of wavelengths; from X-rays to infrared.

Unless properly corrected for, the existence of intergalactic dust will introduce a redshift dependent magnitude offset to standard candle sources and bias the cosmological parameter estimates as derived from observations of SNe Ia. We model the optical extinction and X-ray scattering properties of intergalactic dust grains to constrain the intergalactic opacity using a combined analysis of observed quasar colours and measurements of the soft X-ray background. We place upper limits on the extinction A_B(z = 1) < 0.10 - 0.25 mag, and the dust density parameter Ω_dust < 10⁻⁵ − 10⁻⁴ (ρ_grain/3 g cm⁻³), for models with R_V < 12 − ∞, respectively.

Dust in the host galaxies, and dust that may reside in the circumstellar (CS) environment, have important implications for the observed colours of SNe Ia. Using the Hubble Space Telescope and several ground based telescopes, we measure the extinction law, from UV to NIR, for a sample of six nearby SNe Ia. The SNe span a range of E(B − V ) ≈ 0.1 − 1.4 mag and R_V  ≈ 1.5 − 2.7, showing a diversity of dust extinction parameters. We present mid- and far-infrared (IR) observations for a number of SNe Ia, obtained with the Herschel Space Observatory and Spitzer Space Telescope, addressing CS dust as an explanation for “peculiar” extinction towards some SNe Ia. No excess IR emission is detected, limiting CS dust masses, M_dust < 10⁻⁵ solar masses. In particular, the timely appearance of SN 2014J in M82 - the closest SN Ia in several decades - allows for detailed studies, across an unprecedented wavelength range, of its lightcurve and spectral evolution along with the host galaxy and CS environment.