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For distant galaxies to be accelerating away at equal speed in every direction, Earth would have to be at the centre of the universe. Since it is not, some region of the distant galaxies should appear to be retreating faster and this increased speed would be a function of how far the Earth is away from the centre. Assuming light travels at the same speed in all directions, earth is at the center of the observable universe. Since we are part of the expanding universe, how far has "earth" travelled since the big bang?

The distant galaxies around the observable universe periphery provide the references to access the relative motion of the earth.

Clifford Lingwood
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    There is no centre. Please see (possible duplicate) https://astronomy.stackexchange.com/q/669/16685 – PM 2Ring Mar 14 '23 at 17:35
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    Your first sentence is simply wrong. – Chenmunka Mar 14 '23 at 21:49
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    I'm not sure the downvotes are entirely fair and would ask members to reconsider them. The OP seems to simply have a genuine misconception of how the expansion of the universe works (and why). In fairness it's quite a difficult concept to grasp. – StephenG - Help Ukraine Mar 15 '23 at 01:09
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    Everywhere you go, there you are... – Jon Custer Mar 15 '23 at 12:54
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    How far has Earth traveled with respect to which frame? That is the basic problem. Every comoving observer (that is, observer moving only due to the expansion) has an equally good reference frame and is at rest with respect to their own frame. There's no universal reference frame with respect to which we can say how far an object has traveled due to the expansion of the universe. – Sten Mar 17 '23 at 23:35
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    As Sten says, there is no universal reference frame. The closest thing we have to such a frame is the frame in which the CMB is isotropic, see https://physics.stackexchange.com/q/411073/123208 However, points in that frame are not at rest with respect to each other, they are moving apart due to the Hubble flow. With respect to that frame, the Solar System is currently moving at roughly 600 km/s. – PM 2Ring Mar 18 '23 at 04:47
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    Indeed, if you want to ask how far we've traveled with respect to the comoving observers, that is answerable. That's a question of how far we've moved compared to the expansion, not due to the expansion. – Sten Mar 18 '23 at 08:57
  • I think you are wrong. There is a universal reference , in fact millions of them- not at the centre of the universe (wherever or if that exists) but at the periphery. Galaxies accelerating away from us in the same direction we are travelling, will seem to be accelerating more slowly while those travelling in the opposite direction will appear to be moving faster. Calculation of these relative speeds vs direction would allow determination of earth's movement and direction – Clifford Lingwood Mar 23 '23 at 13:53
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    What is the center of the surface of a ball? – planetmaker Mar 28 '23 at 17:08
  • There is no way to know and your question is nonsenseical – Starship - On Strike Mar 28 '23 at 17:52
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    @Clifford The CMB frame is similar to your shell of distant galaxies, except that the shell which emitted the CMB we're currently receiving (aka the surface of last scattering) is further than the galaxies, and more uniform. – PM 2Ring Mar 28 '23 at 17:55
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    Note that cosmologically distant objects don't supply a well defined reference frame for us; see "What does general relativity say about the relative velocities of objects that are far away from one another?". But the light reaching us from such objects does, as @PM2Ring notes. If that is the aim, then you are essentially asking about our motion with respect to the CMB frame (discussed in earlier comments). – Sten Mar 28 '23 at 17:58
  • You may enjoy this popular article on expansion (PDF) by Tamara Davis & Charles Lineweaver. They have a formal paper on the topic here, and there are numerous other resources on Professor Davis's personal site. – PM 2Ring Mar 28 '23 at 18:14

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