Discussion Overview
The discussion revolves around the concept of the universe being modeled as a "homogeneous perfect fluid" as described in Schutz's textbook. Participants explore the implications of this model, its relationship to the ether concept, and the characteristics of different spacetime solutions in general relativity (GR), including the Friedmann-Lemaître-Robertson-Walker (FLRW) model and alternatives like Minkowski spacetime.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants suggest that the "homogeneous perfect fluid" serves as a medium filling the universe, contrasting it with the historical concept of ether.
- Others clarify that a homogeneous perfect fluid represents a uniform distribution of matter, dark matter, radiation, and dark energy, leading to the FLRW solution, but does not imply the existence of ether.
- There is a discussion about the implications of not having a uniform density fluid in GR, with some stating that it would not yield the perfect FLRW solution.
- Participants mention various spacetime solutions that could arise instead of the FLRW model, including Minkowski, Schwarzschild, Kerr, and Godel spacetimes, depending on the stress-energy distribution.
- One participant raises a question regarding the equation of state for highly relativistic particles, specifically why it takes the form ##p=\frac{1}{3}\rho##, seeking derivation or explanation.
- There is a mention of the cosmological principle leading to the Friedmann equations and the FLRW universe, which is noted to match observations well on large scales but fails on smaller scales due to structure formation.
- Some participants discuss the nature of flatness in the universe, distinguishing between spatial flatness and curvature in spacetime.
- Questions are raised about the possibility of the universe transitioning between different spacetimes over time, suggesting a dynamic nature of the metric.
Areas of Agreement / Disagreement
Participants express multiple competing views regarding the relationship between the homogeneous perfect fluid and ether, the implications of different spacetime models, and the nature of the universe's flatness. The discussion remains unresolved on several points, particularly regarding the equation of state for relativistic particles and the potential for changing spacetimes.
Contextual Notes
Participants note that the cosmological principle and the assumptions it entails lead to specific models, but local deviations from homogeneity and isotropy must be accounted for in more refined models. There is also acknowledgment of the limitations of the FLRW model on smaller scales.