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:

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?

aaaHighly 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?

• 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

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

Good Ok Not nice Nasty

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:

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

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:

To Earth

Quasar

Keck

VLT

7 quasars observed on both Keck and VLT – a direct test of combined systematics

Comparing Da/a 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

€

€

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:

With

dv

syst

emati

cN

o dv

syst

emati

c ap

plie

dBefore 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 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.

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.