Presentation on theme: "A new result on space-time variation of alpha – Part C John Webb, School of Physics, University of New South Wales, Australia Group members as per Michael."— Presentation transcript:
A new result on space-time variation of alpha – Part C John Webb, School of Physics, University of New South Wales, Australia Group members as per Michael Murphy’s first slide
What are the key points which collectively suggest this result might be cosmological and not due to systematics? Two interesting internal consistencies: 1Keck and VLT dipole positions agree (although errors fairly large). Independent samples, different data reduction procedures, different instruments and telescopes. 2High and low redshift dipole sky positions (using combined dataset) also agree - perhaps even more compelling because different species are used at low and high redshift – and different transitions respond differently to the same change in And an interesting robustness indication: 3Rather than increasing the statistical error bars to force = 1, we can instead iteratively trim the individual / points relative to the dipole model. How much do we have to trim to destroy the result?
Highly exaggerated illustration of how transitions shift in different directions by different amounts – unique pattern
Are a few high S/N outliers responsible for the signal, by chance? Alternative to growing error bars Robustness check – iterative trimming Adopt statistical-only errors and iteratively clip most deviant point How much data do we need to discard to remove the dipole and time dependence? reached when ~10% clipped Dipole significance ~7 at Dipole significance stays above 4 until ~50% of data discarded reached when ~10% clipped Linear time fit significance ~5 at Linear time fit significance stays above 4 until ~40% of data discarded
Can we nevertheless find a systematic which can reproduce these? Two approaches: 1)Identify all the systematics one can possibly think of and quantify them one by one 2)Find a purely empirical approach which in principle measures both known and unknown simultaneously
Alignment of quasar image on the spectrograph slit GoodOkNot niceNasty
[Å] Time [Å] Time HIRES: Single arm, single chip (pre-Aug.'04) UVES: Dual-arm, 3 chips VLT/UVES vs. Keck/HIRES:
[Å] Time [Å] Time HIRES: Single arm, single chip (pre-Aug.'04) UVES: Dual-arm, 3 chips VLT/UVES vs. Keck/HIRES: Molaro et al. (A&A, 2008): UVES slits well aligned
[Å] Time [Å] Time HIRES: Visitor mode, follow object with ThAr UVES: Service mode, ThAr at end of night Obj. Cal. Obj. Cal. Obj. Cal. VLT/UVES vs. Keck/HIRES:
To Earth Quasar Keck VLT 7 quasars observed on both Keck and VLT – a direct test of combined systematics
Comparing / for 14 absorption systems observed with both Keck and VLT
Pairs of quasar observations on both Keck and VLT
~800 measurements, 7 quasar spectra observed on both Keck and VLT
Same as previous plot but binned
Implementing the dv test No systematic – standard laboratory frequencies: Worst-case systematic – Modify the laboratory frequency:
With dv systematic No dv systematic applied Before and after
Conclusions The “raw” data might now be indicating both time and space variation of alpha. The quasar pair test results represent an upper limit on any effect. They do not provide a "correction” to the raw results. We do not expect this in the sample as a whole. Nevertheless, if we do impose such an extreme effect on the whole raw dataset, it does not generate a solution in which is constant. Further, imposing the effect slightly diminishes the previous internal consistency in the data, as would be expected if the dipole is real. Specifically, the significance of the dipole reduces (although still remains fairly significant) and we then also require a more significant monopole term (which may seem "unphysical"). Finally, we have so far been unable to find a way of explaining the results in terms of any known (or even unknown) systematics.
Further work Need completely independent check. Meanwhile, continue with quasar spectroscopy – will double the existing sample within ~3 years Also targeted observations: Need more duplicate observations on both Keck and VLT Also really need more H 2 absorption systems. Other combinations of lines, e.g. HI 21cm + neutral.