Dark energy and the accelerating expansion

A surprising acceleration

For much of the twentieth century, cosmologists expected gravity to slow the expansion of the universe. Matter attracts matter, so expansion should decelerate. In the late 1990s, observations of distant Type Ia supernovae suggested the opposite: the expansion is accelerating.

The cause of this acceleration is called dark energy.

Supernova evidence

Type Ia supernovae can be standardized as distance indicators. Astronomers compare their apparent brightness with redshift. Distant supernovae were dimmer than expected in a decelerating universe, implying they were farther away than expected.

This indicated that expansion has accelerated during recent cosmic history.

Cosmological constant

The simplest model of dark energy is Einstein's cosmological constant, written $Lambda$. It acts like a constant energy density of empty space. Unlike matter, which dilutes as the universe expands, a cosmological constant remains constant per unit volume.

In Friedmann cosmology, accelerated expansion occurs when pressure is sufficiently negative. For a cosmological constant,

ho c^2$$ where $ ho$ is the equivalent mass density. ## Equation of state Dark energy is often described by an equation-of-state parameter $$w=rac{p}{ ho c^2}$$ For a cosmological constant, $$w=-1$$ Current observations are broadly consistent with $w=-1$, though measurements continue to test whether dark energy evolves over time. ## Energy densities Matter density decreases as the universe expands because matter spreads out:

ho_mpropto a^{-3}$$

Radiation density decreases faster:

ho_rpropto a^{-4}$$ because photon wavelengths are also stretched. A cosmological constant has

ho_Lambda=constant$$

Thus dark energy becomes more dominant at late times.

Cosmic acceleration

A simplified acceleration equation shows the role of pressure:

ho+rac{3p}{c^2} ight)$$ If pressure is negative enough, the quantity in parentheses can become negative, making $ddot{a}>0$. ## The cosmological constant problem Quantum field theory suggests that empty space may have vacuum energy, but naive estimates are enormously larger than the observed dark energy density. This mismatch is called the cosmological constant problem. It is one of the deepest unsolved problems in theoretical physics. ## Alternatives Alternatives to a cosmological constant include evolving scalar fields, modified gravity, or more complex cosmic fluids. Observations of supernovae, baryon acoustic oscillations, gravitational lensing, galaxy clustering, and the CMB test these possibilities. ## The big idea Dark energy is the name for whatever causes the observed acceleration of cosmic expansion. The simplest explanation is a cosmological constant with $w=-1$, but its tiny observed value is theoretically mysterious. Understanding dark energy is central to the future of cosmology.