Probing a drifting proton-electron mass ratio from H2 spectra

Constant of nature not constant?

Initially in 2006 a team of scientists of the Department of Physics and Astronomy, and of LaserLaB, at VU University Amsterdam, and of the European Southern Observatory in Chile have found indication for a small variation of one of the constants of nature: the ratio between the mass of the proton and the mass of the electron. They had reported this finding in the April 21 issue (2006) of Physical Review Letters, in a paper entitled:
Indication of a cosmological variation of the proton-electron mass ratio based on laboratory measurement and reanalysis of H2 spectra (Click to download a free reprint). Previously in an intial report, published in 2004, our group had already made an attempt of determining a constraint on mu-varaiation in the paper: Highly accurate H2 Lyman and Werner band laboratory measurements and an improved constraint on a cosmological variation of the Proton-to-electron mass-ratio.

The scientists at LaserLaB VU and at ESO (in Paranal Chile) discovered indication for such a variation by comparing the proton-electron mass ratio in molecular hydrogen as it is now and how it was 12 billion years ago. They performed extremely accurate measurements on spectral lines of hydrogen molecules in the laboratory and compared the results with the same lines observed in spectra of quasars. Quasars are astronomical objects that emitted light way back in the past (twelve Billion years ago). They are so far distant, that their light reaches the Earth only now. In the 'old light' a fingerprint of molecules of hydrogen or H2, as they were then, is carried to Earthbound telescopes, in this case to the Very Large Telescope in Chile.

the VLT at Paranal, Chile, at 2.5 km altitude in the Andes

The laboratory measurements were performed with a special laser, developed at LaserLaB VU Amsterdam, operating at the specific wavelengths absorbed by hydrogen molecules. Those wavelengths are in the extreme ultraviolet (XUV) between 90 en 110 nanometer. The beam of XUV-radiation is crossed with a beam of H2 molecules in otherwise vacuum conditions (Click for picture of Vacuum chamber) Some essential parts of the laser setup are displayed below:

On the left the Pulsed-Dye Amplifier; on the right the ring dyelaser.
(Click on picture for higher pixel size)

In the expanding universe the quasars move away from us (at Earth). As a result of the cosmological redshift all spectral lines are shifted to the visible wavelength domain. Hence the light can penetrate through the Earth's atmosphere, and the spectra can be recorded accurately by means of a spectrometer connected to the telescope. For these observations use was made of the UVES spectrograph:

The "Ultraviolet and Visible Echelle Spectrograph".

H2 molecules and the proton-electron mass ratio

The hydrogen molecule, built from two protons and two electrons, is the most abundant molecule in the universe. From a comparison of data obtained via laser spectroscopic measurements and from quasar observations, in combination with calculations of the structure of the H2 molecule, it follows that the mass ratio of the proton and the electron may have changed in the mean time. In the 2006-study indication was found for a mass ratio becoming smaller by 0.002% in the past twelve Billion years. That seems just a little, but according to the laws of physics, this ratio should not vary at all. But it was just a study on a single object. This finding of 2006 was highly cited and had a broad Press coverage.

The proton-electron mass ratio is an important fundamental constant of nature. This constant is dimensionless, so independent of any system of units. It is a number that can be measured and the current status is: Mp/me = 1836,1526726. See also the NIST website for the constants. Standard physics does not have an explanation as to why Mp/me has this value, nor can it provide an explanation as to why it would vary. Note that also the possibility of a variation in alpha, the fine structure constant, is a subject of current research. See: A Constant Worthy of the Name.

It is interesting to note in this respect that the German scientist F. Lenz, in the possibly the shortest paper ever to appear in Physical Review argued, that the value for Mp/me, as it was known then in 1951 (1836.12) exactly coincides with 6 times the number pi to the 5th power. The modern value of Mp/me no longer matches such an algebraic relation.

Further observations and results after the initial 2006 result

After the 2006 result on the two quasar systems Q0347 and Q0405 a number of additional high-redhsift H2 absorption systems was investigated, in particular with involvement of the Amsterdam team:

J2123-005 at z=2.05 (Keck)

J2123-005 at z=2.05 (VLT)

Q2348-011 at z=2.43

    This system, exhibiting 7 (!) velocity components, was investigated in a visitors mode observation at the VLT in 2007 and analyzed since. Even though 19.5 hours of observation time were spent the resulting signal-to-noise was rather limited; this is due to the weakness of the quasar source. An additional problem encountered was that there are in fact two Dampled Lyman-alpha systems along the line-of sight. This led to a constraint on a drift of the proton-electron mass ratio of only: Dmu/mu = (0.68 +/- 2.78) x 10^-5. The work was published in: Constraint on a variation of the proton-to-electron mass ratio from H2 absorption towards quasar Q2348-011.

Ubachs at observations at Paranal in 2007

Q0528-250 at z=2.81

    This system was previously investigated by a number of authors. We reobserved this system at VLT in service mode (program 082.A-0087) on nights 23 and 25 Nov. 2008, 23 Dec. 2008 and 25 and 26 Jan. 2009 with attached Th-Ar calibration files for optimum wavelength accuracy. This study led to the tight constraint: Dmu/mu = (2.3 +/- 2.2) x 10^-6. The work was published in: New constraint on cosmological variation of the proton-to-electron mass ratio from Q0528-250.

Status on mu-variation

Combining the avaliable information on highly redshifted H2 absorption systems this leads to the following situation, as shown in the graph.

Status of mu-variation from highly redshifted H2 absorbers

The red points show the VLT studies performed by our group, the green point our investigation of J2123 with Keck-Hires. The blue points derive from a study of King et al.: Stringent Null Constraint on Cosmological Evolution of the Proton-to-Electron Mass Ratio. The grey point is from a study by Wendt and Molaro.
This compilation of results may be interpreted as follows: The proton electron mass ratio has varied by less than 10^-5 in the redshift range z=2-3; this corresponds to lookback times of 10-12 billion years.

Future work

In coming spring our team will observe the system Q1441 at redshift z=4.2 to analyze H2 absorption at the highest redshift so far. This is indicated by the yellow bar in the figure. We are granted observation time at the Very Large Telescope in Paranal (Chile) on nights 27-30 March 2013.

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