evidence for cosmological evolution of the fine structure constant?
DESCRIPTION
Evidence For Cosmological Evolution of the Fine Structure Constant?. Chris Churchill (Penn State). a = e 2 /hc. Da = ( a z - a 0 )/ a 0. John Webb (UNSW) - Analysis; Fearless Leader Steve Curran (UNSW)- QSO (mm and radio) obs. - PowerPoint PPT PresentationTRANSCRIPT
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Evidence For Cosmological Evolution of the
Fine Structure Constant?
Chris Churchill(Penn State)
= (z-0)/0
= e2/hc
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John Webb (UNSW) - Analysis; Fearless LeaderSteve Curran (UNSW) - QSO (mm and radio) obs.Vladimir Dzuba (UNSW) - Computing atomic parametersVictor Flambaum (UNSW) - Atomic theoryMichael Murphy (UNSW) - Spectral analysisJohn Barrow (Cambridge) - InterpretationsFredrik T Rantakyrö (ESO) - QSO (mm) observationsChris Churchill (Penn State) - QSO (optical) observations Jason Prochaska (Carnegie Obs.) - QSO (optical) observationsArthur Wolfe (UC San Diego) - QSO optical observationsWal Sargent (CalTech) - QSO (optical) observationsRob Simcoe (CalTech) - QSO (optical) observationsJuliet Pickering (Imperial) - FT spectroscopyAnne Thorne (Imperial) - FT spectroscopyUlf Greismann (NIST) - FT spectroscopyRainer Kling (NIST) - FT spectroscopy
Webb etal. 2001 (Phys Rev Lett 87, 091391)
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QSO Spectra
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Intrinisic QSO Emission/Absorption Lines
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H I (Lyman-) 1215.67
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C IV 1548, 1550 & Mg II 2796, 2803
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And, of course…
Keck Twins10-meter Mirrors
The Beam Collector.
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The High Resolution Echelle Spectrograph (HIRES)
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2-Dimensional Echelle Image of the Sun
Dark features are absorption lines
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We require high resolution spectra…
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Interpreting cloud-cloud velocity splittings….
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Parameters describing ONE absorption line
b (km/s)
1+z)rest
N (atoms/cm2)
3 Cloud parameters: b, N, z
“Known” physics parameters: rest, f,
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Cloud parameters describing TWO (or more) absorption lines from the same species… (eg. MgII 2796 + MgII 2803 A)
z
b
bN
3 cloud parameters (no assumptions),
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We decompose the complex profiles as multiple clouds, usingVoigt profile fitting
natural line broadening + Gaussian broadeningGaussian is line of sight thermal broadening gives “b”
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The “alkali doublet method”
Resonance absorption lines such as CIV, SiIV, MgII are commonly
seen at high redshift in intervening gas clouds. Bethe & Salpeter 1977
showed that the of alkali-like doublets, i.e transitions of the
sort
are related to by
which leads to
:
:
2
1
221
2
)(
Note, measured relative to same ground state
2/12
2/12
2/32
2/12
PS
PS
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But there is more than justThe doublets… there are
other transitions too!
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Cloud parameters describing TWO absorption lines from different species (eg. MgII 2796 + FeII 2383 A)
b(FeII)b(MgII)
z(FeII)
z(MgII)
N(FeII)N(MgII)
maximum of 6 cloud parameters, without assumptions
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We reduce the number of cloud parameters describing TWO absorption lines from different species:
bKb
z
N(FeII)N(MgII)
4 cloud parameters, with assumptions:
no spatial or velocity segregation for different species
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In addition to alkali-like doublets, many other more complex species are seen in quasar spectra. Now we measure relative to different ground states
Ec
Ei
Represents differentFeII multiplets
The “Many-Multiplet method” - using different multiplets and different species simultaneously -
Low mass nucleusElectron feels small potential and moves slowly: small relativistic correction
High mass nucleusElectron feels large potential and moves quickly: large relativistic correction
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Relativistic shift of the central line in the multiplet
Procedure1. Compare heavy (Z~30) and light (Z<10) atoms, OR
2. Compare s p and d p transitions in heavy atoms.
Shifts can be of opposite sign.
Illustrative formula:
1qEE2
0
z0zz
Ez=0 is the laboratory frequency. 2nd term is non-zero only if has changed. q is derived from relativistic many-body calculations.
)S.L(KQq K is the spin-orbit splitting parameter.
Numerical examples:
Z=26 (s p) FeII 2383A: = 38458.987(2) + 1449x
Z=12 (s p) MgII 2796A: = 35669.298(2) + 120x
Z=24 (d p) CrII 2066A: = 48398.666(2) - 1267xwhere x = z02 - 1 MgII “anchor”
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High-z (1.8 – 3.5) Low-z (0.5 – 1.8)
FeII
MgI, MgII
ZnII
CrII
FeIIPositiveMediocre
Anchor
MediocreNegative
SiIV
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Low-z vs. High-z constraints:
/ = -5×10-5High-z Low-z
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Current results:
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Possible Systematic Errors
1. Laboratory wavelength errors2. Heliocentric velocity variation3. Differential isotopic saturation4. Isotopic abundance variation (Mg and Si)5. Hyperfine structure effects (Al II and Al III)6. Magnetic fields7. Kinematic Effects8. Wavelength mis-calibration9. Air-vacuum wavelength conversion (high-z sample)10.Temperature changes during observations11.Line blending12.Atmospheric dispersion effects13. Instrumental profile variations
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2-Dimensional Echelle Image of the Sun
Dark features are absorption lines
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ThAr lines
Quasar spectrum
Using the ThAr calibration spectrum to see if wavelength
calibration errors could mimic a change in
Modify equations used on quasar data:quasar line: = (quasar) + q1x
ThAr line: = (ThAr) + q1x
(ThAr) is known to high precision (better than 0.002 cm-1)
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ThAr calibration results:
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Atmospheric dispersion effects:
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Rotator
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Isotopic ratio evolution:
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Isotopic ratio evolution results:
Isotope
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Correcting for both systematics:
Rotator + Isotope
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Uncorrected: Quoted Results
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Conclusions and the next step
~100 Keck nights; QSO optical results are “clean”, i.e. constrain a directly, and give ~6s result. Undiscovered systematics? If interpreted as due to , was smaller in the past.
3 independent samples from Keck telescope. Observations and data reduction carried out by different people. Analysis based on a RANGE of species which respond differently to a change in :
Work for the immediate future: (a) 21cm/mm/optical analyses. (b) UVES/VLT, SUBARU data, to see if same effect is seen in
independent instruments; (c) new experiments at Imperial College to verify/strengthen laboratory wavelengths;
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Last scattering vs. z CMB spectrum vs. l
CMB Behavior and ConstraintsSmaller a delays epoch of last scattering and results in first peak at larger scales (smaller l) and suppressed second peak due to larger baryon to photon density ratio.
Solid (=0); Dashed (=-0.05); dotted (=+0.05)
(Battye etal 2000)
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BBN Behavior and Constraints
D, 3He, 4He, 7Li abundances depend upon baryon fraction, b.
Changing changes b by changing p-n mass difference and Coulomb barrier.
Avelino etal claim no statistical significance for a changed a from neither the CMB nor BBN data.
They refute the “cosmic concordance” results of Battye etal, who claim that da=-0.05 is favored by CMB data.
(Avelino etal 2001)
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49 Systems ; 0.5 < z < 3.5 ; 28 QSOs
= -0.72 +/- 0.18 x 10-5 (4.1)
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Numerical procedure: Use minimum no. of free parameters to fit the data
Unconstrained optimisation (Gauss-Newton) non-linear least-squares method (modified version of VPFIT, explicitly included as a free parameter);
Uses 1st and 2nd derivates of with respect to each free parameter ( natural weighting for estimating ;
All parameter errors (including those for derived from diagonal terms of covariance matrix (assumes uncorrelated variables but Monte Carlo verifies this works well)
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However…
bobserved2 b b
kT
mcons tthermal bulk
2 2 2tan
T is the cloud temperature, m is the atomic mass
So we understand the relation between (eg.) b(MgII) and b(FeII). The extremes are:
A: totally thermal broadening, bulk motions negligible,
B: thermal broadening negligible compared to bulk motions,
b MgIIm Fe
m Mgb FeII Kb FeII( )
( )
( )( ) ( )
b MgII b FeII( ) ( )
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How reasonable is the previous assumption?
FeII
MgII
Line of sight to Earth
Cloud rotation or outflow or inflow clearly results in a systematic bias for a given cloud. However, this is a random effect over and ensemble of clouds.
The reduction in the number of free parameters introduces no bias in the results
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We model the complex profiles as multiple clouds, usingVoigt profile fitting (Lorentzian + Gaussian convolved)
Free parameters are redshift, z, and
Lorentzian is natural line broadening Gaussian is thermal line broadening (line of sight)
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1. Zero Approximation – calculate transition frequencies using complete set of Hartree-Fock energies and wave functions;
2. Calculate all 2nd order corrections in the residual electron-electron interactions using many-body perturbation theory to calculate effective Hamiltonian for valence electrons including self-energy operator and screening; perturbation V = H-HHF.
This procedure reproduces the MgII energy levels to 0.2% accuracy (Dzuba, Flambaum, Webb, Phys. Rev. Lett., 82, 888, 1999)
Dependence of atomic transition frequencies on
Important points: (1) size of corrections are proportional to Z2, so effect is small in light atoms;(2) greatest precision will be achieved when considering all relativistic effects (ie. including ground state)
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Wavelength precision and q values
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Line removal checks:
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Removing MgII2796: Post-removal Pre-removal
Line Removal
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Removing MgII2796: Post-removal Pre-removal
Line Removal
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Number of systems where transition(s) can be removed
Transition(s) removed
Pre-removalPost-removal
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The position of a potential interloper “X”
Suppose some unidentified weak contaminant is present, mimicking a change in alpha. Parameterise its position and effect by d:
MgII line generated withN = 1012 atoms/cm2
b = 3 km/s
Interloper strength can vary
Position of fitted profile is measured
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2-Dimensional Echelle Image
Dark features are absorption lines