Discussion Overview
The discussion centers on the amount of "gravitating" matter required in Loop Quantum Gravity (LQG) and String Theory for the universe to maintain its current state, specifically focusing on the concept of critical density. The conversation explores theoretical implications and calculations related to cosmological constants and the relevance of density in these frameworks.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
Main Points Raised
- Some participants express skepticism about LQG and String Theory's ability to provide a definitive figure for critical density, suggesting that these theories are not yet sufficiently developed.
- One participant cites the Friedmann equation from General Relativity as a basis for calculating critical density, estimating it to be about 0.83 joules per cubic kilometer, with a breakdown of 73% dark energy and 27% matter.
- It is noted that the actual density of matter (baryonic and dark combined) is approximately 0.22 joules per cubic kilometer, which is considered sparse by human standards.
- Participants discuss the significance of density in determining the curvature of space, emphasizing that density at each point is more relevant than total volume.
- There is a sentiment of disappointment regarding the inability of LQG and String Theory to predict cosmological constants, raising questions about the purpose of pursuing these theories if they cannot provide such predictions.
- One participant mentions that LQG is focused on demonstrating the existence of four-dimensional spacetime from first principles, while String Theory is similarly trying to align its predictions with General Relativity.
Areas of Agreement / Disagreement
Participants generally express a lack of consensus on the ability of LQG and String Theory to provide concrete predictions regarding critical density and cosmological constants. Multiple perspectives on the relevance and development of these theories are presented, indicating ongoing debate.
Contextual Notes
The discussion highlights limitations in the current understanding of LQG and String Theory, particularly regarding their predictive power and the definitions of critical density. There are unresolved questions about the implications of density measurements and the nature of the universe's curvature.