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Spacetime expands at an accelerated rate and the particles with movement associated to this expansion are said to be coupled to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and therefore kinetic energy associated with it.

However, can there be any situations where this kinetic energy coming from the Hubble flow can be transformed into some other type of energy (thermal, electromagnetic, electric, gravitational, mass...etc)?

I've been told that, for example, if an object that is coupled to the Hubble flow collided with another object, the kinetic energy of the motion relative to the expansion of space could be transformed into other forms of energy, such as thermal energy or the kinetic energy of the other object. Similarly, if an object coupled to the Hubble flow was subjected to a force or underwent some other process that did work on it, the kinetic energy associated with its motion relative to the expansion of space could be transformed into other forms of energy.

However, I have not found any examples of papers or studies talking about this. If anyone could find an example of this, it would be appreciated.

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  • $\begingroup$ The concept of "spacetime expanding" is incoherent. "Space", "time", and "spacetime" are just virtual coordinate systems, not something real you can manipulate or that can "expand". Physics is the study of objects, maybe this question belongs to philosophy. $\endgroup$ Dec 31, 2022 at 20:53
  • $\begingroup$ On p.3 of her book titled "Existential Physics", Hossenfelder makes it clear that trajectories through space differ from intervals of time, due to the fact that the duration of passages through time is reduced, not increased, by increases in the lengh of travel through those curving trajectories that, in a cosmos whose material objects are basically spherical, are those usually followed by particles and by those assemblages of matter that particles comprise: A tiny clock would register passage thru time in cycles shorter than those registered by a big clock with identically-shaped components. $\endgroup$
    – Edouard
    Jan 19, 2023 at 13:00

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In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and therefore kinetic energy associated with it.

No, they don't have kinetic energy in the sense you seem to think. Objekts which follow the Hubble flow are at rest relativ to the microwave background. You can also think of them as dots on a balloon which is blown up: The dots don't move, its just that the distances between them are increasing.

However please note that kinetic energy is not invariant. It is frame dependent. If you e.g. follow a car with high speed such that you are at rest relative to that car then the kinetic energy of this car is zero in your frame.

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