Dark energy and the fate of the universe
In 1998, astronomers discovered that the universe's expansion is accelerating, driven by a mysterious dark energy that dominates the cosmos.
An accelerating universe
Astronomers expected gravity to slow cosmic expansion. In 1998, two teams studying distant Type Ia supernovae found they were fainter than expected, meaning the expansion is speeding up. Saul Perlmutter, Brian Schmidt, and Adam Riess shared the 2011 Nobel Prize in Physics for this discovery.
Dark energy
The cause of acceleration is called dark energy. Its nature is unknown. The simplest explanation is a cosmological constant, an energy of empty space itself, which Einstein had once added to his equations. Dark energy appears to have a nearly constant density, so as space expands it comes to dominate.
The cosmic inventory
According to precise measurements such as Planck's, the universe today is about 68% dark energy, 27% dark matter, and only about 5% ordinary matter, which makes up all stars, planets, gas, and people. Everything we can see is a small fraction of the whole.
The standard model of cosmology
The Lambda-CDM model, combining a cosmological constant (Lambda) with cold dark matter, explains the CMB, the large-scale distribution of galaxies, light-element abundances, and accelerating expansion with just a few parameters. Despite its success, its two main ingredients, dark energy and dark matter, remain unexplained.
The geometry of space
The overall density of the universe determines its geometry. Measurements of the CMB show the universe is very nearly flat, meaning parallel lines stay parallel on cosmic scales, consistent with predictions of inflation.
The fate of the universe
If dark energy stays constant, the universe will expand forever, galaxies beyond the Local Group will eventually recede out of sight, stars will burn out, and the universe will grow cold and dark: a 'big freeze' or heat death. If dark energy strengthened, a 'big rip' could tear structures apart; if it weakened, expansion might slow. Recent results from the Dark Energy Spectroscopic Instrument (DESI, 2024-2025) hint that dark energy may be evolving, which is being tested.
The observable universe
Because the universe has a finite age and light has finite speed, we can only see a finite region: the observable universe. Due to expansion, its radius is about 46 billion light-years, larger than 13.8 billion. The whole universe may be much larger, possibly infinite.
Worked example
If the universe is about 68% dark energy, 27% dark matter, and 5% ordinary matter, then for every kilogram of ordinary matter there are about 5.4 kilograms of dark matter, and dark energy accounts for more than twice the total of all matter combined. Atoms, everything in the periodic table, make up only about one-twentieth of the cosmos.
Key takeaways
In 1998, distant supernovae revealed that cosmic expansion is accelerating (Nobel Prize 2011). Dark energy, possibly a cosmological constant, drives it. The universe is about 68% dark energy, 27% dark matter, and 5% ordinary matter. The Lambda-CDM model explains many observations but its main ingredients are unexplained. The universe is nearly flat. If dark energy stays constant, the universe faces a big freeze; DESI hints dark energy may evolve. The observable universe has a radius of about 46 billion light-years.
Flashcard
Discovered 1998
Check your understanding
What fraction of the universe is made of ordinary matter, such as atoms in stars and people?