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ableman t1_jammpqd wrote

Reply to comment by nivlark in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47

> Special relativity says that velocity measured in an inertial frame will never exceed the speed of light, but cosmologically distant galaxies are not inertial from our perspective.

We are in an inertial reference frame, so we are measuring from an inertial reference frame, it shouldn't matter what frame they're in, SR works just fine on accelerating objects, as long as the observer is inertial. If you meant to say we're not in an inertial reference frame, it's a very poor explanation, because that's just a No True Scotsman fallacy, but also just seems wrong? The whole point of inertial reference frames is that you can tell whether you're on one or not with a local experiment.

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nivlark t1_jaogkfl wrote

Our reference frame is only locally inertial, where "local" means "close enough that the global geometry of spacetime can be ignored". In special relativity spacetime is flat everywhere and there is no such distinction, but in GR the same is not generally true.

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zutnoq t1_janifi1 wrote

Indeed, the inertial-ness of a reference frame is only a local property (local here meaning infinitesimal displacements).

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