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The observations of quasar absorption spectra show that the fine-structure constant varies on large scales. I know that $U(1)_{EM}$ coupling constant $\alpha$ depends upon the coupling constants of $SU(2)_L\times U(1)_Y$, which vary with energy scale. But at cosmological scales what causes the variation in $\alpha$.

Qmechanic
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Rahul
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    There is no conclusive evidence that the fine structure constant has ever varied. There is some experimental evidence that it might have been different in the past but there is too much uncertainty in the measurements to say this for sure. It would be very exciting if $\alpha$ is definitively proven to have changed because that would be our first real evidence for physics beyond the Standard Model. Sadly we'll have to wait. – John Rennie May 20 '17 at 07:44
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    Despite, as John Rennie says, the lack of conclusive evidence, there are several proposed mechanisms for a possible cause. See e.g. Sec 3. in Bronnikov & Kononogov (2006). – pela May 20 '17 at 08:05
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    Possible duplicates: https://physics.stackexchange.com/q/2725/2451 , https://physics.stackexchange.com/q/3607/2451 and links therein. – Qmechanic May 20 '17 at 08:08
  • @JohnRennie I thought our first evidence of physics beyond the standard model was that things fall down. – Emilio Pisanty May 20 '17 at 13:27
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    Isn't the existence of massive neutrinos (as in neutrinos possessing a non-zero mass) also evidence of physics beyond the standard model? – Bob May 20 '17 at 14:51

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