Cosmology and astrophysics

Dark energy and accelerated expansion

Distant supernovae, the CMB, and large-scale structure indicate that cosmic expansion is accelerating. “Dark energy” names the component producing this effect in cosmological models.

Distant supernovae, the CMB, and large-scale structure indicate that cosmic expansion is accelerating. “Dark energy” names the component producing this effect in cosmological models.

Model and observations

In general relativity, sufficiently negative pressure can make the scale factor a(t) accelerate. A cosmological constant Λ is a simple description consistent with current data, while the deeper physical nature remains open.

The Friedmann acceleration equation shows that pressure also gravitates in general relativity: a component accelerates expansion when $\rho+3p/c^2<0$. For a constant equation-of-state parameter $w=p/(\rho c^2)$, its density scales as $\rho\propto a^{-3(1+w)}$; a cosmological constant has $w=-1$ and constant density. Type Ia supernovae measure luminosity distance versus redshift, while the CMB and baryon acoustic oscillations provide complementary standard scales. Their combination helps separate expansion history from the intrinsic brightness of any one class of objects.

The $\Lambda$CDM hypothesis fits current data with a cosmological constant but does not explain why vacuum energy is so small compared with theoretical expectations—the cosmological-constant problem. Dynamic fields such as quintessence allow $w$ to vary with time, yet observations constrain them tightly and degeneracies with curvature or systematics can arise. Thus “dark energy” names a component in the model, not a microscopic identification. Future tests must consistently measure distances, structure-growth rates, and weak lensing.

Quick check