Probing variations in the fundamental constants with quasar absorption lines

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Copyright: Murphy, Michael T.
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Abstract
Precision cosmology challenges many aspects of fundamental physics. In particular, quasar absorption lines test the assumed constancy of fundamental constants over cosmological time-scales and distances. Until recently, the most reliable technique was the alkali doublet (AD) method where the measured doublet separation probes variations in the fine-structure constant, αΞ e2/ħc. However, the recently introduced many-multiplet (MM) method provides several advantages, including a demonstrated ≈10-fold precision gain. This thesis presents detailed MM analyses of 3 independent Keck/HIRES samples containing 128 absorption systems with 0.2 > zabs > 3.7. We find 5.6 σ statistical evidence for a smaller α in the absorption clouds: Δα/α = (-0.574 ± 0.102) x 10-5. All three samples separately yield consistent, significant Δα/α. The data marginally prefer constant dα/dt rather than constant Δα/α. The two-point correlation function for α and the angular distribution of Δα/α give no evidence for spatial variations. We also analyse 21 Keck/HIRES Si iv doublets, obtaining a 3-fold relative precision gain over previous AD studies: Δα/α = (-0.5 ± 1.3) x 10-5 for 2.0 > zabs > 3.1. Our statistical evidence for varying α requires careful consideration of systematic errors. Modelling demonstrates that atmospheric dispersion is potentially important. However, the quasar spectra suggest a negligible effect on Δα/α. Cosmological variation in Mg isotopic abundances may affect Δα/α at zabs > 1.8. Galactic observations and theory suggest diminished 25;26Mg abundances in the low metallicity quasar absorbers. Removing 25;26Mg isotopes yields more negative Δα/α values. Overall, known systematic errors can not explain our results. We also constrain variations in y Ξ α 2gp, comparing H i 21-cm and millimetrewave molecular absorption in 2 systems. Fitting both the H i and molecular lines yields the tightest, most reliable current constraints: Δy/y = (-0.20±0.44)x10-5 and (-0.16±0.54)x10-5 at zabs = 0.2467 and 0.6847 respectively. Possible line-ofsight velocity differences between the H i and molecular absorbing regions dominate these 1 σ errors. A larger sample of mm/H i comparisons is required to reliably quantify this uncertainty and provide a potentially crucial check on the MM result.
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Murphy, Michael T.
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Publication Year
2002
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Thesis
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PhD Doctorate
UNSW Faculty
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