2011 Nobel Prize in Physics
Reason for Award
for the discovery of the accelerating expansion of the Universe through observations of distant supernovae (Astrophys. J.: 517 (1999) 565-586; Astrophys. J.: 507 (1998) 46-63; Astron. J.: 116 (1998) 1009-1038)
Laureates
United States of America
Australia, United States of America
United States of America
Explanation
The Universe spreads out like an inflating balloon, but this research showed the spreading is getting faster and faster. Scientists used huge star explosions called “supernovae” as measuring sticks. By checking how dim these distant supernovae look, they could tell how far away they are. They found the explosions were farther than expected, meaning the expansion is speeding up. Something called “dark energy” seems to push this acceleration, but nobody knows what it is yet. Imagine a ball rolling down a hill that suddenly starts speeding up on its own—that is what the whole Universe is doing.
Related Keywords
supernova
A supernova is a stellar explosion that can outshine an entire galaxy for a short time. Type Ia supernovae occur when a white dwarf approaches the Chandrasekhar mass, giving them nearly uniform peak luminosities. This uniformity allows them to serve as standard candles even at cosmological distances. Correcting the light-curve shape reduces distance uncertainties to roughly 10 %. Supernova observations are therefore a powerful probe of the expansion history of the Universe.
accelerating expansion
Observations show that the Universe’s expansion rate is not merely positive but increasing with time. In Friedmann equations this implies a component with negative pressure. Baryon acoustic oscillations and cosmic microwave background measurements independently support the result inferred from supernovae. Accelerating expansion leads to a “Big Freeze” scenario in which distant galaxies eventually slip beyond our horizon. For physicists it poses deep questions about gravity and quantum vacuum energy.
dark energy
Dark energy is an unknown component estimated to constitute about 70 % of the cosmic energy budget. The simplest model treats it as a cosmological constant Λ—vacuum energy with negative pressure and time-independent density. Alternatives include scalar-field quintessence and modified-gravity scenarios. Observationally the key quantity is the equation-of-state parameter w and how close it is to −1; current measurements reach ±0.05 precision. Revealing the nature of dark energy is one of the foremost goals of 21st-century cosmology.
standard candle
A standard candle is an astronomical object with known absolute luminosity that can serve as a distance indicator. Cepheid variables are used for local scales, while type Ia supernovae extend measurements to cosmological distances. Calibration is crucial: light-curve shape, color indices, and host-galaxy properties are used to correct object-to-object variations. Combining multiple standard candles builds a “cosmic distance ladder.” The ladder’s accuracy underpins precise determinations of the Hubble constant and the age of the Universe.
observational cosmology
Observational cosmology seeks to unveil the origin, structure, and evolution of the Universe using empirical data. Techniques range from supernova photometry and CMB mapping to galaxy redshift surveys and gravitational-lens measurements. Rapid advances have solidified the ΛCDM model as the concordance framework, yet mysteries such as dark energy and inflation remain. Statistical inference, machine learning, and big-data pipelines are central to current research. Future wide-field telescopes and space missions aim to reduce cosmological-parameter uncertainties to the percent level.
Hubble constant
The Hubble constant H₀ quantifies the current expansion rate of the Universe, expressed in km s⁻¹ Mpc⁻¹. A >10 % discrepancy between local measurements (Cepheid + SNe Ia ladder) and indirect estimates from Planck CMB data is known as the “Hubble tension.” The value of H₀ ties directly to the cosmic age and dark-energy properties. Independent probes such as time-delay gravitational lenses and baryon acoustic oscillations are key to resolving the tension. The discovery of accelerating expansion has spurred ever more precise techniques for determining H₀.