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Basic physics in remote universe (proton/electron mass ratio)

  1. Jun 24, 2008 #1


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    One of the basic numbers in fundamental physics is 1836.15
    the ratio of proton mass to electron mass.

    This ratio has been measured in a quasar-type galaxy some 8 billion lightyears from us, and it turns out to be the same as here in Milky.

    You guessed it would be? OK. But still it's good to check.

    Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe
    Michael T. Murphy (1), Victor V. Flambaum (2), Sebastien Muller (3), Christian Henkel (4) ((1) Swinburne Univ., Melbourne, Australia, (2) Univ. New South Wales, Sydney, Australia, (3) ASIAA, Taipei, Taiwan, (4) MPIfR, Bonn, Germany)
    published in Science 320th June 2008, 22 pages, 5 figures (12 EPS files), 2 tables, including Supporting Online Material;
    (Submitted on 18 Jun 2008)

    "The Standard Model of particle physics assumes that the so-called fundamental constants are universal and unchanging. Absorption lines arising in molecular clouds along quasar sightlines offer a precise test for variations in the proton-to-electron mass ratio, mu, over cosmological time and distance scales. The inversion transitions of ammonia are particularly sensitive to mu compared to molecular rotational transitions. Comparing the available ammonia spectra observed towards the quasar B0218+357 with new, high-quality rotational spectra, we present the first detailed measurement of mu with this technique, limiting relative deviations from the laboratory value to |dmu/mu| < 1.8x10^{-6} (95% confidence level) at approximately half the Universe's current age - the strongest astrophysical constraint to date. Higher-quality ammonia observations will reduce both the statistical and systematic uncertainties in these measurements."

    What does this mean for cosmology?
    Among other things, it is further confirmation that physical law has not been changing, which means being able to extrapolate models of conditions in the universe back in time with greater confidence. The light we are getting from this galaxy was emitted around 6.2 billion years ago---when the expansion of the universe was some 7.5 billion years old. So it gives a substantial look-back in time.

    Ten years ago or so there was a time when people were often wondering if basic constants like the speed of light, or more exactly a related one, the fine structure constant alpha (approx 1/137), might have changed over the course of time. A lot of measurements were made, mostly showing no change. So that was settled at least for the time being. We hear very little about that now.

    This helps to fill in the picture---the same kind of result but for a different fundamental constant.

    Here is a popularization piece about it in Astronomy Now:
    Last edited: Jun 24, 2008
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  3. Jun 24, 2008 #2


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    The quasar in question is at redshift 0.685
    Using Ned Wright's calculator with the usual default parameters gives a light travel time of 6.2 billion years, and a current distance of 8 billion lightyears.

    I should say that the earlier studies don't totally rule out alpha changing over time, there is still a margin of uncertainty. They just constrain the possibility---narrow the credible change down to a few parts per million.

    This is the first time a similar study has been done for the proton/electron mass ratio.
    Last edited: Jun 24, 2008
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