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

Journal Article 3 Mentions

Adam G. Riess•1998

A. V. FilippenkoP. Challis

Top 5% · 95th percentile

18,214 citations · Astronomy and Astrophysics

Open Access

TLDR

By looking at distant exploding stars, this study shows that the universe is not just expanding, but speeding up, which means something mysterious is pushing everything apart.

Summary

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1 Study Aim

The main goal of this paper is to use observations of distant Type Ia supernovae (a kind of exploding star with predictable brightness) to measure how the universe is expanding. The authors aim to determine key properties of the universe, such as how much matter it contains, whether there is a cosmological constant (a form of energy that could cause acceleration), and how old the universe is. They want to find out if the universe's expansion is slowing down or speeding up, and what this means for its ultimate fate. Simply put: The study wants to find out if the universe is speeding up or slowing down by looking at faraway exploding stars.

2 Study Design

The researchers observed 10 new Type Ia supernovae at redshifts between 0.16 and 0.62, and combined these with previous data for a total of 16 distant and 34 nearby supernovae. They measured the distances to these supernovae using two methods that relate the brightness and shape of their light curves (how their brightness changes over time). The team carefully corrected for possible errors, such as dust, differences in star types, and measurement biases. They compared the observed distances to predictions from different models of the universe, testing for the effects of matter and a cosmological constant. Simply put: The researchers compared how bright faraway and nearby exploding stars look to figure out how fast the universe is growing.

3 Findings

The study reveals that distant Type Ia supernovae appear dimmer than expected if the universe contained only matter and no cosmological constant. This means they are farther away than predicted, suggesting the universe's expansion is accelerating. The results strongly favor a model with a positive cosmological constant (vacuum energy), ruling out a universe made only of matter. The data indicate the universe is about 14.2 billion years old and will keep expanding forever. The authors checked for possible errors, like dust or changes in supernova properties, but found none that could explain the results without a cosmological constant. They recommend further studies to better understand any remaining uncertainties. Simply put: The findings show the universe is speeding up, not slowing down, and something unknown is causing this push.

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 supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters: the Hubble constant (H_0), the mass density (Omega_M), the cosmological constant (i.e., the vacuum energy density, Omega_Lambda), the deceleration parameter (q_0), and the dynamical age of the universe (t_0). The distances of the high-redshift SNe Ia are, on average, 10%-15% farther than expected in a low mass density (Omega_M = 0.2) universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Omega_Lambda > 0) and a current acceleration of the expansion (i.e., q_0 < 0). With no prior constraint on mass density other than Omega_M >= 0, the spectroscopically confirmed SNe Ia are statistically consistent with q_0 < 0 at the 2.8 sigma and 3.9 sigma confidence levels, and with Omega_Lambda > 0 at the 3.0 sigma and 4.0 sigma confidence levels, for two different fitting methods, respectively. Fixing a ``minimal'' mass density, Omega_M = 0.2, results in the weakest detection, Omega_Lambda > 0 at the 3.0 sigma confidence level from one of the two methods. For a flat universe prior (Omega_M + Omega_Lambda = 1), the spectroscopically confirmed SNe Ia require Omega_Lambda > 0 at 7 sigma and 9 sigma formal statistical significance for the two different fitting methods. A universe closed by ordinary matter (i.e., Omega_M = 1) is formally ruled out at the 7 sigma to 8 sigma confidence level for the two different fitting 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 effect 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 effects appear to reconcile the data with Omega_Lambda = 0 and q_0 >= 0.

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