I Galaxy recession and Universe expansion

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The discussion centers on the complexities of measuring galaxy recession and universe expansion within the framework of general relativity. It highlights that in curved spacetime, unlike flat spacetime, defining relative velocities between distant objects becomes ambiguous due to the lack of global inertial frames. The concept of "recession speed" is clarified as a calculated value based on redshift and other observational data, rather than a direct measurement. Comoving coordinates are identified as a useful framework for understanding the universe's expansion, allowing for a defined expansion rate. Ultimately, the conversation emphasizes the nuanced differences between ordinary velocity and the metric expansion of space.
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