>[!abstract] >Hourglass cosmology is an informal term that refers to cosmological models where time (or entropy) evolves symmetrically away from a central "neck", much like the narrowest part of an hourglass. **Core ideas:** - The universe begins in a low-entropy state (the "neck" of the hourglass). - Time flows outward in both directions from that low-entropy point, increasing entropy on both sides. - This implies **two arrows of time**, one in each direction from the central point. - To an observer on either side, the universe looks like it has a "beginning" and time only moves forward—despite being symmetric globally. **Key motivation:** - Addresses the **low-entropy problem**: Why did the universe start in such a special, low-entropy state? - Aims to embed this condition naturally in a broader spacetime where it is not the "beginning" but a **minimal entropy surface**. **Notable proponents:** - Sean Carroll and Jennifer Chen (2004) proposed a model where baby universes nucleate from de Sitter space and evolve with bidirectional time arrows. - Barbour, Koslowski, and Mercati proposed a "Janus point" model based on shape dynamics, where time emerges from relational configurations, with entropy increasing symmetrically away from this point. >[!related] >- **North** (upstream): — >- **West** (similar): — >- **East** (different): — >- **South** (downstream): —