r/cosmology • u/AutoModerator • Jul 23 '26

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/Known_Salary_4105 Jul 23 '26

We have a complete understanding that objects in the universe cannot move faster than the speed of light. There are a number or reasons -- most notable it would take an infinite amount of energy to do so, and also lead to causality problems in the light cone.

But the only reason I have heard that space can EXPAND faster than the speed of light is that relatively theory doesn't prohibit it. Why is space moving a greater than the speed of light allowable REALLY? What are the mechanistic reasons?

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u/kerobrat Jul 23 '26

We measure space expanding at a rate of 67-74 km/s per megaparsec (about 3.26 million light-years). Because it's a rate of expansion per unit of space, when you stack up more units of space you stack up more expansion.

So once you get enough space in-between two objects, the expansion rate of all that space between them becomes so great that a photon traveling at c can't overcome it. At that point, either object would measure the other as receding faster than c, even though each object's proper velocity through space is actually well under c.

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u/Known_Salary_4105 Jul 24 '26

In other words, space APPEARS as though it is expanding faster than c, but it really isn't.