Discussion Overview
The discussion revolves around the interpretation of the distance to the Sagittarius A* black hole from Earth within the framework of General Relativity (GR). Participants explore the implications of using the Friedmann-Lemaître-Robertson-Walker (FLRW) model versus the weak field approximation at the galactic scale, as well as the nature of the galactic rest frame.
Discussion Character
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- One participant suggests that the distance to Sgr A* can be understood as the proper distance at a given cosmological time using the FLRW model.
- Another participant counters that the FLRW model is inadequate for galactic scales, proposing that the weak field approximation is more appropriate for describing spacetime in this context.
- There is a question about whether the weak field approximation leads to a metric tensor on spacetime as a manifold.
- Clarifications are made regarding the definition of the galactic rest frame, with some participants indicating it is the irrotational frame where the galaxy's center of mass is at rest.
- Participants discuss the implications of the irrotational frame, questioning whether the galaxy's constituents rotate within this frame and how their motion is characterized.
- It is noted that while the constituents of the galaxy move in approximately circular orbits, there are local variations to this motion.
Areas of Agreement / Disagreement
Participants express differing views on the applicability of the FLRW model versus the weak field approximation for galactic distances. The discussion remains unresolved regarding the precise implications of the irrotational frame and the nature of motion within it.
Contextual Notes
The discussion highlights limitations in the application of cosmological models at galactic scales, as well as the need for clarity on the definitions and implications of the galactic rest frame and irrotational motion.