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The observed red shift of objects is assumed to indicate that those objects are moving away from Earth but is it possible that there could be another explanation for the observed red shift that does not require the expansion of the Universe ?

Qmechanic
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    Astronomy.se might be better. Here a notable claim is required. Reword it to be the inverse, ie find a Hubble quote and say "Is it true?" Or find someone espousing some other theory. –  May 09 '17 at 21:01
  • See https://physics.stackexchange.com/q/156618/ – ProfRob May 10 '17 at 06:14

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No theories that have held up or are currently seen as in any way a competitive explanation of the observations.

Yes, redshift as we've measured it, and theory, indicate an expanding universe.

The idea that it's something else has a long history, and not a single success. The latest one (maybe there's more recent alternative theories, but let's say) is Wetterich as @Brian Z discusses in his post. it hasn't taken hold, either untestable or no observations that support it. The long history you can see in https://www.plasma-universe.com/Redshift, up until about 2004. There's been more, some along the lines of Wetterich that some other constants change.

But there is too much supporting the cosmological redshift due to the expansion. First, it was measured and identified first by Hubble by comparing distances of galaxies not too far away but enough to recede from us, with velocity and redshift. And then as we expanded distance measurements and still found it to be true. We've found supernova that allows us to expand the distance at which we can check whether true. Then we saw the cosmic microwave background (CMB) and and measured lots of details, and the fact that it's everywhere, and that verifies that there was a much smaller universe way back and that CMB is a relic from 380,000 years after the Big Bang, and another proof of the expansion. And we keep getting more and more agreement with the standard model of cosmology, with more accurate measurements. It's also completely consistent and predicted by General Relativity's cosmological FLRW solution. So both theory and measurements agree on it all. See more on redshift, that is a mainstream consensus, at https://en.m.wikipedia.org/wiki/Redshift

Clearly @KAI's comment is right also, none of those other known possibilities to get redshifts hold for the cosmological redshift.

There's still physics to explain, like dark matter and energy, but the expansion is pretty hard to argue with.

Bob Bee
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  • IIRC Lemâitre postulated expansion given his solution before Hubble discovered it, but I'm no historian of science. – Selene Routley May 10 '17 at 02:51
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    I think you're right, but Hubble proved it. But if I remember right I think the Big Bang was not completely accepted till the CMB, at least by some of the Steady State people, they really fought for it. – Bob Bee May 10 '17 at 02:56
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There are 3 types of red shift:

  1. Gravitational red shift

  2. Red shift due to proper motion (peculiar velocity).

  3. Red shift due to expansion of the Universe.

For more information see: http://einstein.stanford.edu/content/relativity/a11859.html

DavePhD
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    How does this answer the question? – jwodder May 09 '17 at 21:31
  • @jwodder It explains that a given object observed to have a red-shift, does not necessarily have the red-shift due to expansion of the universe. There are two other possibilities. – DavePhD May 09 '17 at 21:37
  • When talking about cosmology, the other two are not realistic possibilities. For example, the fact that many (local) objects have peculiar velocities that cause a redshift doesn't have much to say about the observation that essentially all non local galaxies are redshifted. – KAI May 09 '17 at 21:44
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Christof Wetterich has proposed that the universe may not actually be expanding, and that the observed patterns of redshift could instead be caused by increasing mass. His paper on this topic is available online. It was covered in a news story from Nature and a question on Physics StackExchange even before it was formally published. To be clear however, the expanding universe model does remain the scientific consensus.

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    I'm with the answer on physics.SE. That paper looks a lot more like Poincare's philosophy games than a genuinely novel theory. A very simplified version is that given theory X that matches observations, you can come up with theory X' that also matches observations, but assumes a different geometry and slightly different physics to make the observations fit. The classic example I saw in school actually has your distance metric shrinking over time, which looks a lot like what is going on here. – KAI May 09 '17 at 21:39