Please help in explaining 'we are not the center of the Universe'

  • #1
When I discuss about the Big Bang, the expansion of the Universe and the fact that on average every galaxy is receeding from us, I get "oh, so then we are at the 'center' of the Universe."
I know that's not the case, we are not in a special place, etc. But, is this a proven fact, or is just a philosophical and historical conclusion after being wrong so many times?
Can we scientifically say we are not? Please help me answer that question that come up so often and for which I have not a strong convincing answer.
 

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  • #2
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If you go at any other place in our universe, you will always conclude the same thing as all the galaxies are receding away from you. This proves that there is no unique centre.
 
  • #3
If you go at any other place in our universe, you will always conclude the same thing as all the galaxies are receding away from you. This proves that there is no unique centre.
Thanks for the reply! I agree and use the expanding balloon example as an explanation of that. But my question is whether that fact has been scientifically proven. We never experienced being in other place of the universe so I'm not sure I can say is proven
 
  • #4
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scientifically proven.
No such thing in science.

If I want to show "reindeer can't fly" and start pushing reindeer off the roof to their deaths, after a dozen reindeer, all I have really proved is that that dozen reindeer couldn't - or at least didn't - fly.
 
  • #5
berkeman
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No such thing in science.

If I want to show "reindeer can't fly" and start pushing reindeer off the roof to their deaths, after a dozen reindeer, all I have really proved is that that dozen reindeer couldn't - or at least didn't - fly.
You didn't push Santa too, did you? :oops:
 
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  • #6
PeroK
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Thanks for the reply! I agree and use the expanding balloon example as an explanation of that. But my question is whether that fact has been scientifically proven. We never experienced being in other place of the universe so I'm not sure I can say is proven
When Einstein developed General Relativity (GR), it naturally predicted an expanding universe. That was the first theoretical basis on which we might expect an expanding universe.

There's no model arising from GR where one point would be at the centre. So, there is no theoretical support for that idea.

This is a case where theory and observation agree.

You can't ask more than that.
 
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  • #7
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You didn't push Santa too, did you?
Don't believe in Santa. Anyone who can get in and out of so many houses without getting seen is going to be taking stuff, not leaving stuff.
 
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  • #8
Orodruin
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Don't believe in Santa. Anyone who can get in and out of so many houses without getting seen is going to be taking stuff, not leaving stuff.
Ho ho ho! Merrrry chrrristmas!
 
  • #9
anorlunda
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If you look at distant galaxies and you see that the universe appears isotropic, (Looks the same no matter which direction you look.) what possible models fit that observation? There are only two.

  1. The universe is a series of concentric shells, with your eye at the exact center.
    1629654493084.png
    https://en.wikipedia.org/wiki/Celestial_spheres#Early_ideas_of_spheres_and_circles
  2. The universe is actually homogenous and isotopic. No matter where you are in the universe or which direction you look, it is isotropic. In that model, there can not be a center. If there was a center and it was not here, things would look different when looking in the direction of the center.
Number 1 is just hopelessly anthropic, or even creationist. With #1, not only is there a center, but the center of the entire universe is exactly here where your eye is. Science rejects it as absurd.

Number 2 wins by default, it is the only other possibility.
 
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  • #10
atyy
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Thanks for the reply! I agree and use the expanding balloon example as an explanation of that. But my question is whether that fact has been scientifically proven. We never experienced being in other place of the universe so I'm not sure I can say is proven
It's a model-based statement as @PeroK said. However, we have observations that match the model. The issue is discussed in https://arxiv.org/abs/1111.3794.

The isotropic blackbody CMB as evidence for a homogeneous universe
Timothy Clifton, Chris Clarkson, Philip Bull
The question of whether the Universe is spatially homogeneous and isotropic on the largest scales is of fundamental importance to cosmology, but has not yet been answered decisively. Surprisingly, neither an isotropic primary CMB nor combined observations of luminosity distances and galaxy number counts are sufficient to establish such a result. The inclusion of the Sunyaev-Zel'dovich effect in CMB observations, however, dramatically improves this situation. We show that even a solitary observer who sees an isotropic blackbody CMB can conclude that the universe is homogeneous and isotropic in their causal past when the Sunyaev-Zel'dovich effect is present. Critically, however, the CMB must either be viewed for an extended period of time, or CMB photons that have scattered more than once must be detected. This result provides a theoretical underpinning for testing the Cosmological Principle with observations of the CMB alone.
 
  • #11
timmdeeg
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This is a case where theory and observation agree.
Is it scientifically proven that what we conclude from the observable universe is transferable to the whole universe?
 
  • #12
If you look at distant galaxies and you see that the universe appears isotropic, (Looks the same no matter which direction you loo.) what possible models fit that observation? There are only two.

  1. The universe is a series of concentric shells, with your eye at the exact center.View attachment 287913 https://en.wikipedia.org/wiki/Celestial_spheres#Early_ideas_of_spheres_and_circles
  2. The universe is actually homogenous and isotopic. No matter where you are in the universe or which direction you look, it is isotropic. In that model, there can not be a center. If there was a center and it was not here, things would look different when looking in the direction of the center.
Number 1 is just hopelessly anthropic, or even creationist. With #1, not only is there a center, but the center of the entire universe is exactly here where your eye is. Science rejects it as absurd.

Number 2 wins by default, it is the only other possibility.
Hi anorlunda, thank you for taking the time.
You said: 'Number 1 is just hopelessly anthropic, or even creationist. With #1, not only is there a center, but the center of the entire universe is exactly here where your eye is. Science rejects it as absurd.'
Of course I don't believe in 1, but let me play devil's advocate here (because that's what I get :)
On what grounds science rejects it as absurd? Seems a philosophical reason rather than evidence based, like when elliptical orbits were not considered because they had to be circular
 
  • #13
No such thing in science.

If I want to show "reindeer can't fly" and start pushing reindeer off the roof to their deaths, after a dozen reindeer, all I have really proved is that that dozen reindeer couldn't - or at least didn't - fly.
I agree, you are right. Please disregard my expression "scientifically proven". What I ment is: there actually is a line of evidence that suggests we 'are' the center of the universe, since we see everything else moving away from us on average. Are there other lines of evidence suggesting otherwise, besides us not like anthropic/creationist reasons?
Some others responded about universe uniformity and CMB. It could be, I have to think about it. Is that all we have or am I missing something?
 
  • #14
It's a model-based statement as @PeroK said. However, we have observations that match the model. The issue is discussed in https://arxiv.org/abs/1111.3794.

The isotropic blackbody CMB as evidence for a homogeneous universe
Timothy Clifton, Chris Clarkson, Philip Bull
The question of whether the Universe is spatially homogeneous and isotropic on the largest scales is of fundamental importance to cosmology, but has not yet been answered decisively. Surprisingly, neither an isotropic primary CMB nor combined observations of luminosity distances and galaxy number counts are sufficient to establish such a result. The inclusion of the Sunyaev-Zel'dovich effect in CMB observations, however, dramatically improves this situation. We show that even a solitary observer who sees an isotropic blackbody CMB can conclude that the universe is homogeneous and isotropic in their causal past when the Sunyaev-Zel'dovich effect is present. Critically, however, the CMB must either be viewed for an extended period of time, or CMB photons that have scattered more than once must be detected. This result provides a theoretical underpinning for testing the Cosmological Principle with observations of the CMB alone.
Thanks for the answer atyy. I'll take a look at the link and do some research (I don't know that the Snyaev-Zel'dovich effect is, but will definitely look into it).
 
  • #15
anorlunda
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Others gave you more complete answers in this thread, including observations.

But this thread is marked B level, whereas some of those answers are understandable only by PhD researchers in the field.

But at the B level, the best evidence comes from models as @PeroK said.
  1. Assume the universe is homogeneous and isotropic.
  2. Make a mathematical model based on that assumption.
  3. The model provides more detailed predictions about the properties of the universe.
  4. Compare predictions to observations. If they give good agreement, that gives us more confidence in the model including the assumptions. If not, we have to tweak the theory and the models. That's how much of science works.
  5. But there will never be the case where we prove cosmological theories in the same sense as proof of mathematical theorems.
But by definition, we can never make observations outside the observable limit. So models for the non-observable (possibly infinite) rest of the universe can never be confirmed by observation. That doesn't make the models wrong, it means we have less confidence in applying them. If someone claims that the non-observable universe is made of green cheese, that can not be disproved.
 
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  • #16
I think this explanation will serve my purposes.
Thanks a lot anorlunda
 
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  • #17
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Is it scientifically proven that what we conclude from the observable universe is transferable to the whole universe?
First, nothing can be "scientifically proven". The best you can do is to have a model for which no discrepancies are currently know between its predictions and observations.

Second, since we obviously can't observe anything that's outside our observable universe, we obviously have no way of comparing the predictions of our models with observations of things outside our observable universe. We have to adopt some kind of inductive principle to extend our models beyond what is observable to us, and the obvious inductive principle to adopt is Occam's Razor: since the simplest way to extend our current model of our observable universe, which is homogeneous and isotropic, is to assume that the rest of the universe is the same (since that is how "homogeneous and isotropic" extends), that is the assumption we should make. Scientists make such assumptions all the time; without them we could not do science at all.

So logically speaking, if you want to be either a pedant or an annoyance, you can always point out that we have no firm logical basis for believing a model that predicts things about the entire universe, not just the observable universe. And then scientists will simply say "so what", and continue to use the inductive principle science has always used.
 
  • #18
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So logically speaking, if you want to be either a pedant or an annoyance, you can always point out that we have no firm logical basis for believing a model that predicts things about the entire universe, not just the observable universe. And then scientists will simply say "so what", and continue to use the inductive principle science has always used.
Relativity makes this easier. No one can disprove that everything outside our past light cone is dancing pink elephants ?:)
 
  • #19
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I agree, you are right. Please disregard my expression "scientifically proven". What I ment is: there actually is a line of evidence that suggests we 'are' the center of the universe, since we see everything else moving away from us on average. Are there other lines of evidence suggesting otherwise, besides us not like anthropic/creationist reasons?

If one claims that there is a center of the universe AND that we are at the center of it, then there are some big, big problems with that claim. First, where exactly is the center? Is it at the core of the Earth? Is two feet to my left? Is it down the street at my local grocery store? Perhaps it is near the Marianas trench?

Now, if there is a center, is the Earth stationary with respect to it? That's a huge problem, as you'd have to explain why it appears that we rotate and move around the Sun, the galaxy, etc. Why does the raw CMB data show exactly the change in redshift and blueshift that our orbit would make? The same goes for other red/blueshift observations, spacecraft communications, and more. What could possibly cause everything in the universe to move around us in such a way as to exactly replicate our orbital motion, galactic motion, and rotation?

There's a center but the Earth is moving with respect to it? Okay. But now your predicted observations are potentially identical to what we already see. So why even put a center in your model? It's just an added complication that doesn't appear to have any observable differences at this time. You'd do just as well to leave it out and it would be trivial to add it in in the future when observations no longer match what we'd expect in a centerless universe.

At the end of the day, you either wind up with a model that has a large number of serious and obvious flaws in it, or you wind up with a model that matches our current observations just as well as a centerless model, but has an irrelevant addition to it. Since we already have models which can explain the universe just fine without a center there's no reason to try to shoehorn one in.
 
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  • #20
Keith_McClary
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When I discuss about the Big Bang, the expansion of the Universe and the fact that on average every galaxy is receeding from us, I get "oh, so then we are at the 'center' of the Universe."
Explain the "raisin bread" model: in a rising loaf, each raisin is receding from every other raisin with velocity proportional to distance. Then challenge them to invent a model in which only one raisin has this property. :smile:
 
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  • #21
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So, the idea that we might actually be very close to the center of a large void was put forward sometime ago as an alternative explanation for dark energy. The reasoning for this is basically that while it's easy to see that our universe is approximately isotropic, it's really really hard to measure if it changes with distance. But, apparently enough data was collected some time ago to do just that, showing that we can't explain away dark energy by claiming that we're near the center of a void:
https://arxiv.org/abs/1007.3725

I think it's impossible to completely rule out the possibility that we are at the center of something, but the specific models that have been tried don't work.
 
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  • #22
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When I discuss about the Big Bang, the expansion of the Universe and the fact that on average every galaxy is receeding from us, I get "oh, so then we are at the 'center' of the Universe."
I know that's not the case, we are not in a special place, etc. But, is this a proven fact, or is just a philosophical and historical conclusion after being wrong so many times?
Can we scientifically say we are not? Please help me answer that question that come up so often and for which I have not a strong convincing answer.

We ARE in fact exactly at the centre of the OBSERVABLE universe. But the OBSERVABLE universe is precisely defined by where we are in the universe, because it depends on our position in space. Inhabitants of Andromeda will see a slightly different observable universe. So both Andromedands and inhabitants of the Milky Way can claim they are in the centre of THEIR observable universe, and they are both right. And that is the case for every observer in the universe, no matter where they are. But that can only be true if the universe itself has no centre.
 
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