# QuestionIs the expansion of the universe constant in all reference frames?

#### ASTROSTONER

Is the expansion rate of the universe constant in all reference frames? Like the speed of light is constant in all reference frames.

is space/time expansion absolute or relative?

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#### billslugg

No, rate of expansion of the universe varies by how fast the observer is moving. Example is the Cosmic Microwave Background which is blue shifted in the direction the Earth is moving relative to it and red shifted behind us. This is known as CMB Dipole Isotropy. Earth is moving 371 km/s in the direction of the constellation Leo.

#### ASTROSTONER

No, rate of expansion of the universe varies by how fast the observer is moving. Example is the Cosmic Microwave Background which is blue shifted in the direction the Earth is moving relative to it and red shifted behind us. This is known as CMB Dipole Isotropy. Earth is moving 371 km/s in the direction of the constellation Leo.
the Hubble constant is only isotropic if your are not moving? is it different depending on which direction you look due to our movement?

what is the expansion rate in the forward direction?
what is the expansion rate in the backward direction?

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#### billslugg

The expansion rate of the Universe is the same everywhere. Our motion through the universe is additive to it thus the observed rate varies.

#### ASTROSTONER

No, rate of expansion of the universe varies by how fast the observer is moving

The expansion rate of the Universe is the same everywhere. Our motion through the universe is additive to it thus the observed rate varies.

The observed rate varies by how much?

if speed affects the expansion rate, then does gravity affect the expansion rate as well?

Does the observed rate on the moon differ from the observed rate on earth?

#### billslugg

When we observe the speed something is receding from us there are three different numbers that get added together. The absolute rate at which the Earth is moving through space, the rate at which the observed object is moving and the rate at which space is expanding between the two.

The Earth is moving 371 km/s towards Leo.
The Universe expands by 70 km/s per megaparsec.
Each observed object has its own local velocity which gets added.

1)The observed rate varies from hundreds of km/s approaching Earth to c moving away from Earth. For example, gravity overwhelms other considerations and the Andromeda galaxy, 2 million light years away is moving towards the Earth at 700 km/s.

2)Gravity affects the relative motion between us and any observed body but is not connected to the Hubble constant.

3) Since the Moon moves in a different reference frame than Earth, velocities observed from the Moon will be slightly different.

#### Helio

“Is the expansion rate of the universe constant in all reference frames?”

It effectively serves as a constant, but the rate may have been faster in the early periods. Also, we now see an accelerating universe, thus the rate is slightly increasing. That’s my understanding.

#### ASTROSTONER

2)Gravity affects the relative motion between us and any observed body but is not connected to the Hubble constant.

So gravitational time dilation doesn't affect the s in the 70 km/s per megaparsec?

so the rate is the same in all reference frames?

even moving it sounds like it is the same but with an offset.

does gravitational length contraction also add an offset of some sort?

#### billslugg

Gravitational time dilation is insignificant except in the near vicinity of extremely massive objects. Here on the surface of the Earth, which is not an "extremely massive object", gravitational time dilation is meaured in microseconds per day.

The Hubble constant is the same regardless of reference frame. Your local motion simply has to be added to it. Yes, that would be an "offset".

#### Helio

It's a little hard not to state that the Hubble Flow is indeed a preferred frame. All other frames must reference it to establish the expansion, directly or indirectly. I don't see how this changes anything, but it's one of those itchy things that won't go away, at least for me. #### DrRaviSharma

The expansion rate would be an acceleration; does your definition of 70 km/s for megaparsec include acceleration?

#### billslugg

Yes, acceleration is the first derivative of velocity. Since velocity increases 70 km/s/Mpc, the number "70 kn/s/Mpc" is the acceleration.

#### ASTROSTONER

Yes, acceleration is the first derivative of velocity. Since velocity increases 70 km/s/Mpc, the number "70 kn/s/Mpc" is the acceleration.
Peculiar that mass is irrelevant in this acceleration equation.
when Force = Mass x Acceleration seems that something is missing.

#### billslugg

Peculiar that mass is irrelevant in this acceleration equation.
when Force = Mass x Acceleration seems that something is missing.
In a gravitational field, the acceleration of a mass is independent of its mass. This is the classic Galileo experiment with two balls of different mass dropped off the Leaning Tower of Pisa.

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