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Allow me to preface this question by stating that I am not a Big Bang denier.

I am reading the book A Universe from Nothing by Lawrence M. Krauss. In his book, he presents the following card as one piece of evidence for the Big Bang:

enter image description here

Apparently, the boxes outline the predicted abundance of each element in the universe, whilst the shaded areas represent measured abundances.

This may just be because I'm a (student) mathematician, but this seems like extremely poor and unconvincing evidence on the part of physicists. The only prediction that is convincing is that of deuterium; the other predictions seem to be horribly inaccurate.

As I stated, this is obviously only one of many pieces of evidence for the Big Bang. However, without considering these other pieces of evidence, it seems that physicists regard this specific piece of evidence as being in itself extremely convincing. As such, I wonder what it is that I'm misunderstanding? Am I interpreting this incorrectly? I realise that absolute proofs exist only in mathematics, but these predictions (excluding deuterium) seem surprisingly bad.

EDIT:

Sean Lake commented on what seems to be a graph with much more accurate measurements: http://www.astro.ucla.edu/~wright/BBNS.html.

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  • $\begingroup$ With respect to the new graph link: Look at the numbers of the scale of the x axis. . It is a narrower graphically scale and looks better. The data is about the same. $\endgroup$
    – anna v
    Jan 12 '17 at 11:12
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First, you have it the wrong way around. The boxes show the measurements/estimates of what the initial primordial abundances are. These are measured in various, indirect, ways and are afflicted by both measurement uncertainties and known systematic uncertainties. The height of the boxes represents these uncertainties. This is an old graph. The size of the deuterium and lithium uncertainties have been reduced.

The very convincing evidence that you speak of, is that the shaded regions represent the raw predictions of the "vanilla", homogeneous big bang model as a function of the current density of baryons (which is tens of orders of magnitude less than it was the epoch at which these elements were made).

To me, it is extraordinary that these measured primordial abundances of D, He and Li, which differ by many orders of magnitude and for which we have no a priori expectations, are simultaneously predicted to a good accuracy with a single value of the baryon density (represented by the vertical shaded strip), and that this baryon density (about 4% of the critical density) is itself close to the measured value from the cosmic microwave background, which is entirely independent of the primordial abundances.

Lithium is currently problematic, both to predict and measure, because there are nucleosynthetic uncertainties and it is both created and destroyed in various ways other than primordial nucleosynthesis. The horizontal shaded region in this case represents the lithium abundance that is measured in very old, metal-poor stars (the so-called "Spite plateau"). There are various classes of explanation for this "lithium problem", the most likely of which (in my view) is that these stars have depleted lithium from the higher primordial value. See Discrepancy problem in lithium?

A slightly more modern (post-Planck results) version of this "Schramm plot" is shown below (from Coc et al. 2014). The green shaded boxes represent observational estimates of the primordial abundances (though note what I said above about Li), while the red dotted lines mark the range of uncertainty in the predictions (due to things like the lifetime of the neutron and nuclear reaction cross-sections). The vertical baryon density measurement from the CMB is now very precise. Note the spectacularly good agreement between the predicted and measured D abundance with the measured baryon to photon ratio from the CMB. There are no free parameters tuned to get this agreement.

enter image description here

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  • $\begingroup$ Thanks for the insightful response. Sean Lake provided an alternative graph that seems to have used much more accurate data: astro.ucla.edu/~wright/BBNS-sm.gif. But to be fair, the data used in the original graph (in the OP) is completely unconvincing: it is obvious that either our hypothesis surrounding elementary particles after the Big Bang is incorrect and/or we cannot accurately measure the phenomenon. The deuterium measurement is accurate, but the others are so inaccurate that It seems plausible that the original hypothesis is incorrect in some significant way. $\endgroup$ Jan 12 '17 at 8:54
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    $\begingroup$ @ThePointer I do not understand your language. The accuracy of the measurements is extremely high compared to the many orders of magnitude differences in the abundances of the various elements. The only measurement that may be out of kilter with the standard hot big bang model is lithium, by a factor of two or so. Given that these elements can be drastically produced or destroyed in many ways, the simultaneous close agreement between data and prediction is remarkable. $\endgroup$
    – ProfRob
    Jan 12 '17 at 9:04
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It is the difference between mathematics and physics.

Measurements have an error width, and the boxes represent, usually, the 1 sigma error in the measurement. So the measurements need more accuracy. Nevertheless a physicist considers this as good first order evidence that the model plays in the correct ball park: it agrees with the data within one sigma.

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  • $\begingroup$ But extraordinary claims require extraordinary evidence; this is not extraordinary evidence. The measurements (empirical evidence) need to support the hypothesis, before the hypothesis is regarded as correct. I was surprised because of all the predictions I have seen in other fields of science, this has to be one of the worst. Perhaps I'm just disappointed ... $\endgroup$ Jan 12 '17 at 7:21
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    $\begingroup$ For a physicist one sigma agreement with a model validates the model, for the present. It is not a matter of extraordinary evidence but of statistical estimates. If in the future the data fal four and more sigma away from the model , then it will be rejected. $\endgroup$
    – anna v
    Jan 12 '17 at 7:23
  • $\begingroup$ I understand. Perhaps I just expected too much. Thanks for taking the time to response; I appreciate it. $\endgroup$ Jan 12 '17 at 7:24
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    $\begingroup$ @ThePointer The graph here is much more clear on what the measured values are vs. the theory. Grey vertical = observed baryon density, horizontal colored bands = observed concentrations, lines = prediction as a function of baryon density. $\endgroup$ Jan 12 '17 at 7:32
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    $\begingroup$ @ThePointer No idea. Krauss's book is newer than that graph, so it should be more up to date, but that's no guarantee. Have a go through Prof. Wright's pages, there's lots of good stuff there (e.g. big bang evidence summary ). He was on the team that won the Nobel for COBE, and would have had it if they'd awarded 3 people instead of just 2. $\endgroup$ Jan 12 '17 at 7:43

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