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How precise is cosmology?

  1. Oct 27, 2013 #1
    I am quite a stranger to cosmology, so please go easy on me if what I say below is actually very stupid/ignorant!

    From the little exposure I've had to cosmology, it seems to me that in cosmology, a lot of approximations are taken, as well as a lot of huge assumptions made. For example, I've also read due to how crude the approximations are, sometimes the estimated age of a star seem to be even older than the estimated age of the universe. Also, the assumption of isotropy and homogeneity, seem to be argued in a hand-wavingly by the statement "when viewed on large scale".

    From what little I am exposed to so far, it seems that cosmology is not a very precise, and often times arguments are made in hand-waving ways and huge approximations are taken. The whole field just seems really crude。 Is that really so? Or am I dead wrong?
  2. jcsd
  3. Oct 27, 2013 #2


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    You are dead wrong. No reputable scientist will estimate the age of a star as older than the universe and isotropy and homogeneity are not hand waving.

    As you study the topic more, you will realize all this.

    EDIT: isotropy and homogeneity ARE stated (correctly) to be only applicable on large scales but that is not hand-waving.
  4. Oct 27, 2013 #3


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    How precise does this look to you?
  5. Oct 27, 2013 #4


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    Actually, modern cosmology is amazingly accurate given all the assumptions and approximations used. We can estimate the masses of galaxies, clusters of galaxies, predict the spin down rate of distant stars, the temperature of intergalactic space, and a vast array of other stuff to within a matter of percentage points. Its really quite amazing to see some of these predictions validated by observation.
  6. Oct 28, 2013 #5


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    Please provide a citation to support one's assertion, "sometimes the estimated age of a star seem to be even older than the estimated age of the universe."

    There have reports of a star in the Milky Way that appeared to be older than the current estimated age of the universe.

    We make observations, and those observations have greatly improved with improvements in technology. We make assumptions about the universe based on our local experience; that's all we can go on. Certainly there may be inconsistencies, but that is the challenge of investigating the universe and trying to understand what we observe.
  7. Oct 28, 2013 #6


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    The primary thing you need to recognize to make sense of the various claims of discrepancy is this:

    Cosmology is easy. Astrophysics is hard.

    In a bit more detail, why I mean by the above two sentences is that it's comparatively easy to make measurements of things like the ratio of normal matter to dark matter, for the reason that the Cosmic Microwave Background provides an astonishingly-good measurement of that ratio (this was what the plot bapowell posted above refers to). That it, it is easy to understand this sort of thing compared to the difficulty in understanding the behavior of galaxies, and stars are more difficult to understand still.

    You might think it somewhat odd that we can understand well how the whole behaves without understanding the details of the pieces that make up that whole, but consider this: it is comparatively easy to understand how a baseball behaves, but extraordinarily difficult to understand how the atoms of the baseball interact with one another. It is pretty generally the case that you don't need to understand the details of the behavior of the component pieces to get a very good understanding of the whole. And this is true with cosmology as well.

    Stars are hard, because we can only ever see their surfaces and the stars themselves comprise less than a single pixel in most of our images of them (aside from our own Sun). There's a lot of complex physics that goes on within stars that is very difficult to model on a computer.

    By contrast, cosmology is easy because we essentially got lucky with the behavior of physics: the CMB itself is astonishingly simple, as it is an almost perfect black body with small (1/100000) deviations in temperature from place to place acting in a regime where we can use the approximation that structure formation is linear.
  8. Nov 1, 2013 #7

    Vanadium 50

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    A number of off-topic posts have been deleted. Please stick to the OP's question.
  9. Nov 4, 2013 #8
    Please specify precise. The mathematics are pretty much as precise as they can be, the interpretations of what they tell us is open to question.
  10. Nov 4, 2013 #9


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    You mean precise as in big bang nucleosynthesis, which predicts the primordial universe should just under 25% helium and the current observed value is 24.6%?
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