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Theory of expanding universe

  1. Oct 9, 2012 #1
    I know that the Hubble constant proves this and scientist have done plenty of research on this topic.

    Today in my philosophy class we came about the topic of the universe expanding and the philosophy teacher and I were going back and forth on the subject of "How do you really know the universe is expanding."

    I gave her a pretty good lesson on the scientific facts behind it, but she kept pushing me saying how do you know, how do you know the science experiments are right.

    My question is, is it 100% fact that the universe is expanding, is it possible that there is a 0.0000001 percent chance that it could not be expanding. Or is it a FACT?
     
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  3. Oct 9, 2012 #2

    Ryan_m_b

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    Nothing is 100% but for things like this we have a lot of strong, independent evidence pointing towards it being true. At this stage it's beyond doubt and any revision is just adding details to the edges of our understanding rather than radically changing it. And even if any radical change in our understanding did happen it would still explain our current observations.

    Also if you have a philosophy teacher who doesn't understand epistemology and philosophy of science then you have a bad teacher. Unless she knew you were right and was trying to push you and your fellow students to provide the right answer.
     
  4. Oct 9, 2012 #3
    I don't know if she knows about philosophy of science, but the conversations were about epistemology and the if there is such a thing as "true knowledge."
     
  5. Oct 10, 2012 #4

    Chalnoth

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    Right. At a fundamental level, it's impossible to be absolutely, positively certain. I think the way to respond to this sort of thing is to pull back the question a bit, and ask if we can know well enough that we don't realistically need to worry about the conclusion being wrong. The answer to that is that we merely need to collect enough observational evidence that it is so overwhelmingly likely that the interpretation is accurate that we don't need to worry about it being wrong.

    But if you want to always have absolute certainty, then that isn't possible.
     
  6. Oct 10, 2012 #5
    The knowledge of the expansion of the universe is derived from a set of axioms (namely, the universe on large scales has the properties to be homogeneous and isotropic). When you perform your observations, you interpret them within this set of axioms.

    What we do observe is not an expanding universe; it is, for instance, a fainter reception of light from supernovae. Now, this fainter reception of light corresponds, within our set of axioms, to an expanding universe.

    Change the set of axioms, and you will have to reinterpret the data. You might end up with different conclusions. For instance, in some other models (isotropic but inhomogeneous), there is no more an expanding universe.

    Of course, the expansion of the universe seems very likely to be true because there are several interpretations of data/observations that, together, converge towards a consistent picture.

    However, some problem remains within these homogeneous and isotropic models. These problems are not present in other models, but these latter have other problems too. The larger part of cosmologists/astrophysicists agree on the expansion of the universe because of my previous remark.

    Now, it seems to me (at least I have this feeling) that your philosophy teacher was underlining the "how do you know" rather than the "expanding universe". But I might be wrong :)

    E.g., how do I know that cells divide? If I had never observed it, I would have simply referred to the widest and spread knowledge of biologists. In this case, of course, if I would like to know it for sure, I would have to make the experience/observe it myself.

    Here, I would simply quote Chalnoth:
     
  7. Oct 10, 2012 #6

    Chalnoth

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    There are far, far more measures of distance than just supernova brightness, however. The most direct observations of the universe's expansion stem from the redshift-distance relationship. So it fundamentally comes down to how much you trust the redshift and distance measures.

    Redshift is on extremely solid ground, and there really isn't any good alternative explanation of it (as long as you're relying upon redshifts derived from spectra instead of simply the relative brightness in broad color bands).

    Distance is a bit shakier, but can be bolstered by pointing out that there are a great many distance measures, and they tend to agree rather strongly.
     
  8. Oct 10, 2012 #7
    Yes, I agree! This was the reason of
     
  9. Oct 10, 2012 #8

    Chalnoth

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    Right, but that to me stems more from the fact that there are other things we can measure that are consistent with the expansion but aren't directly related to a redshift-distance measure, such as the CMB. The consistency of such extremely different measures with the overall expansion is very powerful evidence indeed that this is the correct interpretation.
     
  10. Oct 10, 2012 #9
    I would quote again my previous sentence :p I completely see your point! Thanks for your answer.
     
  11. Oct 10, 2012 #10

    Ryan_m_b

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