The discussion centers on the nature of simultaneity in General Relativity (GR) versus Special Relativity (SR), particularly in the context of the Friedmann-Lemaître-Robertson-Walker (FLRW) universe. Participants debate whether the expansion of the universe and concepts like the Cosmic Microwave Background Radiation (CMBR) can be understood through a global simultaneity framework. Key points include the assertion that simultaneity is observer-dependent and that different families of observers can define their own congruences without affecting the underlying physics. The conversation highlights the complexities of defining simultaneity in curved spacetimes and the implications for understanding cosmic expansion. Ultimately, the relativity of simultaneity remains a critical aspect of both GR and SR, emphasizing that there is no absolute simultaneity in either framework.
