Accelerating Universe and Type Ia Supernovae
By using exploding white dwarfs as standard candles, two teams discovered in the late 1990s that the expansion of the universe is accelerating, overturning the expectation that gravity should slow it down.
Definition
The accelerating universe is the finding, established with Type Ia supernovae, that the rate of cosmic expansion is increasing with time; Type Ia supernovae are thermonuclear explosions of white dwarfs whose standardized peak luminosity makes them precise distance indicators.
Scope
This topic covers Type Ia supernovae as standardizable candles, the measurement of their distances and redshifts to map the expansion history, the discovery that distant supernovae are dimmer than a decelerating universe predicts, and the inference that a dark-energy component drives accelerated expansion.
Core questions
- Why are Type Ia supernovae good standard candles?
- How did supernova observations reveal cosmic acceleration?
- What does the dimming of distant supernovae imply?
Key concepts
- Type Ia supernova
- Standardizable candle
- Light-curve standardization
- Luminosity distance
- Hubble diagram
- Deceleration parameter
- Cosmic acceleration
Key theories
- Standardizable candles
- Type Ia supernovae have a tight relation between light-curve shape and peak brightness, so after standardization their luminosities are nearly uniform, enabling accurate distance measurements to high redshift.
- Evidence for acceleration
- Distant supernovae appear fainter, and thus farther, than expected in a decelerating universe, indicating that expansion has been speeding up and requiring a component with negative pressure.
Mechanisms
A white dwarf reaching a critical mass undergoes a thermonuclear explosion of reproducible luminosity; correcting for light-curve width and color yields a standard candle, and plotting brightness against redshift produces a Hubble diagram whose curvature at high redshift reveals the accelerating expansion.
Clinical relevance
The supernova discovery is one of the most consequential results in modern cosmology: it established the existence of dark energy and the accelerating, flat universe, reshaped the cosmic energy budget, and launched a major observational program to characterize the cause of acceleration.
History
The Supernova Cosmology Project and the High-z Supernova Search Team independently found in 1998 and 1999 that high-redshift supernovae were dimmer than expected, implying acceleration; the result was rapidly corroborated and recognized with the 2011 Nobel Prize in Physics.
Debates
- Systematics in supernova cosmology
- Possible systematic effects such as evolution of supernova properties, dust extinction, and selection biases have been scrutinized as alternatives to acceleration, but the result has held up and been reinforced by independent probes.
Key figures
- Saul Perlmutter
- Brian Schmidt
- Adam Riess
- Robert Kirshner
Related topics
Seminal works
- riess1998
- perlmutter1999
Frequently asked questions
- Why are Type Ia supernovae so useful for cosmology?
- They are extremely bright, visible across much of the observable universe, and after standardizing for light-curve shape and color their peak luminosities are nearly identical, so their apparent brightness gives reliable distances to map the expansion history.
- How does dimming of supernovae imply acceleration?
- If distant supernovae are fainter than expected for their redshift, they must be farther away than a decelerating universe would place them, which means the expansion sped up over time, the signature of dark energy.