a new result on space-time variation of alpha – part c
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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. Two interesting internal consistencies: - PowerPoint PPT PresentationTRANSCRIPT
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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
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What are the key points which collectively suggest this result might be cosmological and not due to systematics?
Two interesting internal consistencies:
1 Keck and VLT dipole positions agree (although errors fairly large). Independent samples, different data reduction procedures, different instruments and telescopes.
2 High 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 a.
And an interesting robustness indication:
3 Rather than increasing the statistical error bars to force c2n = 1, we can
instead iteratively trim the individual Da/a points relative to the dipole model. How much do we have to trim to destroy the result?
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aaaHighly exaggerated illustration of how transitions shift in different directions by different amounts – unique pattern
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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?
• c2n = 1 reached when ~10%
clipped• Dipole significance ~7s at c2
n = 1 • Dipole significance stays above 4s until ~50% of data discarded
• c2n = 1 reached when ~10% clipped
• Linear time fit significance ~5s at c2n =
1 • Linear time fit significance stays above 4s until ~40% of data discarded
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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
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Alignment of quasar image on the spectrograph slit
Good Ok Not nice Nasty
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l[Å]3000 5500 8000
Time
l[Å]3000 5500 8000
Time
HIRES: Single arm, single chip (pre-Aug.'04)
UVES: Dual-arm, 3 chips
VLT/UVES vs. Keck/HIRES:
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l[Å]3000 5500 8000
Time
l[Å]3000 5500 8000
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
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l[Å]3000 5500 8000
Time
l[Å]3000 5500 8000
Time
HIRES: Visitor mode, follow object with ThAr
UVES: Service mode, ThAr at end of night
Obj.Obj.
Cal.
Obj.Cal.
Cal.
Obj.Cal.
VLT/UVES vs. Keck/HIRES:
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To Earth
Quasar
Keck
VLT
7 quasars observed on both Keck and VLT – a direct test of combined systematics
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Comparing Da/a for 14 absorption systems observed with both Keck and VLT
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Pairs of quasar observations on both Keck and VLT
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~800 measurements, 7 quasar spectra observed on both Keck and VLT
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Same as previous plot but binned
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Implementing the dv test
€
€
fz,rest = f0 + Qα z
2
α 02 −1
⎛ ⎝ ⎜
⎞ ⎠ ⎟
fz,observed =f z,rest
1+ z( )
€
′ f z,rest = f0 +δf( ) + Qα z
2
α 02 −1
⎛ ⎝ ⎜
⎞ ⎠ ⎟
where
δf = −dvλ 0
⎛ ⎝ ⎜
⎞ ⎠ ⎟
then
′ f z,observed =f z,rest
1+ z( )
No systematic – standard laboratory frequencies:
Worst-case systematic – Modify the laboratory frequency:
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With
dv
syst
emati
cN
o dv
syst
emati
c ap
plie
dBefore and after
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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 Da/a 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.
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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 H2 absorption systems. • Other combinations of lines, e.g. HI 21cm + neutral.