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Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

Why this mattered

Riess et al. mattered because it turned Type Ia supernovae into evidence for a startling conclusion: the expansion of the universe was not slowing under gravity, as the standard matter-dominated expectation had long suggested, but accelerating. By comparing light-curve-corrected luminosity distances of high-redshift supernovae with nearby samples, the paper found that distant SNe Ia were dimmer than expected in a decelerating universe. Interpreted within Friedmann cosmology, this favored a positive cosmological constant, with the paper reporting approximately Ω_M ≈ 0.24 and Ω_Λ ≈ 0.72 for a flat universe.

The paradigm shift was not merely a new parameter estimate; it reopened the cosmological constant as an observational necessity rather than a theoretical embarrassment. After this result, and the near-simultaneous Supernova Cosmology Project result, precision cosmology could treat dark energy as an empirical target: measuring its density, testing whether it behaved like Einstein’s cosmological constant, and combining supernova distances with cosmic microwave background and large-scale-structure data. The paper therefore helped move cosmology from debating whether Λ was allowed to asking what physical mechanism could explain cosmic acceleration.

Its downstream significance was enormous. The accelerating-universe result became a cornerstone of the ΛCDM model, shaped the interpretation of later CMB measurements such as WMAP and Planck, motivated dedicated dark-energy surveys, and led to the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess. It also created one of modern physics’ central open problems: if the cosmological constant is real, its observed energy scale is extraordinarily small compared with naive quantum-field-theory expectations, making cosmic acceleration both an astronomical discovery and a deep theoretical challenge.

Abstract

We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 z 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift M \ 1) methods. We estimate the dynamical age of the universe to be 14.2 ^1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely e ect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these e ects appear to reconcile the data with and ) " \ 0 q 0 0.

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