 Accelerating expansion of the universe

An accelerating universe has $\ddot{a}>0$, equivalently $q<0$ while expansion continues.

Here $a$ is the scale factor and $q=-a\ddot{a}/{\dot{a}}^2$ the deceleration parameter. Accelerating expansion means $\dot{a}>0$ and $\ddot{a}>0$; the Hubble parameter $H=\dot{a}/a$ can still decrease. The observational challenge is to reconstruct that history from a past light cone, rather than watch a distant galaxy speed up over the observing campaign.

The accelerating expansion of the universe refers to the observation that the rate at which the universe is expanding is increasing over time. This discovery is one of the most significant findings in modern cosmology and has profound implications for our understanding of the universe. ### Key Points: 1. **Observed Expansion**: The universe has been expanding since the Big Bang, which occurred approximately 13.8 billion years ago.