# Atomic clock in expanding space

Is there any paper analyzing the validity of Relativity in expanding space?

The Great Atomic Clock of Kansas

photons in expanding space: how is energy conserved?

Is an atomic clock itself affected by gravity?

https://en.wikipedia.org/wiki/Speed_of_light

https://en.wikipedia.org/wiki/Second

The definition of c depends on the second.

The definition of second depends on the atomic clock.

So the local definition of speed of light depends on the atomic clock.

Now we know that the gravitational field has an effect on the atomic clock's speed, relatively when viewed from a far away observer, it will tick differently (Shapiro delay).

But none of these answers were talking about the effects of expanding space on an atomic clock.

The vacuum in the voids of the inter galaxy cluster space, where gravity has no effect, and dark energy is dominant, and space is expanding, in those regions, we do not know how atomic clocks tick.

I do not know whether atomic clocks would tick differently in those regions of space.

Question:

1. Do we know of atomic clock tick differently in expanding space (relatively), has there been any experiment on this?

2. will this affect the speed of light there (relatively)?

• $c$ is a defined constant (since the early 1980s), so it does not depend on the second. Commented Jul 4, 2018 at 18:21
• what is its value? it is m/s. the meter is defined in c so it depends on the second. If the atomic clocks would change ticking, it would change. How else would you measure it? I understand what you are saying, but that precludes the constancy of atomic clocks (gravitational field). Relativity says that c is different in different gravitational fields (Shapiro). So that exactly means what I say, that the speed of light is relative depending on the gravitational field, that defines the ticking of atomic clocks. Atomic clocks tick differently near the Sun. Commented Jul 4, 2018 at 18:29
• So they might tick differently in expanding space. Commented Jul 4, 2018 at 18:29
• The questions can be "How does expanding space change time dilation?" Commented Jul 4, 2018 at 18:29
• As I've mentioned earlier and Kyle Kanos confirms here, the local speed of light does not depend on the definition of the second. The speed of light is not a measured value, but a predefined number c = 299,792,458 m/s = 1 ls/s (whole numbers with no fractions). A remote speed of light depends on the difference in the atomic clocks locally and remotely (and also on the length contraction). The space expansion does indeed cause a time dilation and affects the remote speed of light: en.wikipedia.org/wiki/Redshift#Expansion_of_space (the time dilation and redshift are closely related). Commented Jul 5, 2018 at 3:37