We report in Science:
"Molecular matter has not changed in the past 7 billion years"
From radio-astronomical observations of a far distant galaxy we deduce that the proton-to-electron mass ratio µ has not changed over cosmic history. Put more precisely, this fundamental constant of nature has changed by less than 10^-7, or less than one hundred thousands of a percent, in the past 7 billion years. The result was obtained from accurate measurement of the frequencies of methanol lines that caused absorption of microwave radiation at several characteristic frequencies. The extragalactic methanol, the simplest form of the family of alcohol molecules, was observed in a galaxy at redshift z=0.89 toward the quasar system PKS1830-211. The stringent limit on the fundamental constant can also be phrased as: molecules and molecular matter is the same now as it was 7 billion years ago. The observations were carried out at the Effelsberg radio telescope by our team consisting of Wim Ubachs (PI), Julija Bagdonaite and Paul Jansen (PhD students) and Rick Bethlem; the work was performed in collaboration with Christian Henkel and Karl Menten of the Max Planck Institute in Bonn (Germany). This work was published online in
Science Express on 13 Dec. 2012.
Photograph of the
Effelsberg Radiotelescope by night (Photo Paul Jansen -
Methanol; extragalactic alcohol
Previously the Amsterdam team had used optical observations of the hydrogen molecule (with the Very Large Telescope in Chile, and the Keck telescope at Hawaii) to search for a drifting proton-electron mass ratio (see link).
The methanol molecule is a much more sensitive probe for detecting a drift of the proton-electron mass ratio.
Some lines in the microwave spectrum of this molecule undergo a rather large shift upon a variation
of the proton-to-electron mass ratio, while other lines are not affected (so-called "anchor lines").
Recently the Amsterdam group found that the hindered internal rotational motion in molecules such as methanol
(in fact also a quantum tunneling process) can give rise to very high sensitivity coefficients. The sensitivity of each spectral line can be expressed in a value K, which may be calculated.
A figure of the methanol molecule is shown here:
Picture of the methanol molecule (CH3OH);
the black ball represents a C-atom, red an O-atom and grey H-atoms;
the yellow arrow indicates the internal rotational motion,
which is hindered, giving rise to quantum tunneling.
This idea, published in the Methanol paper, makes the methanol molecule an ideal probe to detect a possible temporal variation in the proton-electron mass ratio. For this reason the team proposed to search for methanol molecules in the far-distant universe, to compare the structure of the molecules by the ones observed in the present epoch in laboratory experiments. They performed an investigation of a galaxy known as a "molecular factory" in the line-of-sight of a strong radio source (PKS-1830-211), which is known to be at a distance of 7 billion light years from Earth. The search was for four lines in the radio spectrum of the methanol molecule. In observations carried out between December 2011 and April 2012 the lines were detected and the spectra are shown below. Note the the absorptions of the radio waves have occurred 7 billion years ago, and the radio waves traveling to Earth carry the fingerprint of the methanol molecules in the distant object to Earth.
Radio absorption spectra of 4 lines in methanol, indicated by the colors
Analysis of a drifting constant
From an analysis of the quantum level structure of the methanol molecule it can be derived that
two of the lines in the spectrum, both observed near 25 GHz, behave like anchor lines;
these lines are indicated with red and orange color in the spectrum. Their sensitivity to
a drifting µ is small; K=-1 for these lines. The other two lines are sensitive to a drifting constant;
the green line, observed at 32 GHz has K=-7.4, while the blue line, observed at 6.5 GHz is most sensitive
with K=-32.8. Note that the observed frequencies are not the true frequencies at which the radiation was
observed; in the mean time (of 7 billion years) the Universe has expanded and therefore all the line
have been redshifted by a factor of (1+z), where z=0.88582 for the absorbing galaxy.
The relation between the redshift parameter z and the look-back time in the Universe follows
from cosmological models on
While the anchor lines determine very accurately the redshift of the absorbing galaxy, the other lines
indicate possible changes of the proton-electron mass ratio. Below is shown a plot of the four line positions
with respect to a chosen local reference frame (at z=0.88582) where the points are plotted in terms of their
Doppler shift (V/c) with respect to this frame; the plot as function of sensitivity factors K reveals
an upper limit to a varying µ. The statistical outcome is included in the graph.
If systematic effects of the observations are included in
these data a final result is obtained from the study:
the fundamental constant of nature, the proton-electron mass ratio, has changed by less
than 10^-7 over the past 7 billion years. This result can be interpreted as: the structure of molecular matter,
as probed from its spectrum, is the same as it was 7 billion years ago; it has changed by less than one
hundred thousand's of a percent.
The research was carried out by four members of the Department of Physics and Astronomy,
and of the research institute LaserLaB at VU University, in collaboration with two German
colleagues (Henkel and Menten) of the
Max Planck Institute for Radio Astronomy at Bonn.
Currently the research on radio observations is extended to other radio observatories in the world. Recently, in summer 2012, observations have been carried out with the
IRAM-30 telescope at the Pico Veleta near Granada (Spain). These observations should further verify the present findings and provide even more stringent constraints on possibly drifting constants. Also at Effelsberg extended observations are planned, some of which have been carried out already in Nov. 2012, while more are planned for Jan. 2013.
Ubachs and Bagdonaite at the Pico Veleta observatory
Reference to the publication in
Science Express; Free download from Publications Website.
Bericht Afdeling Natuurkunde VU
Bericht Volkskrant 14 Dec. 2012
Bericht NRC 15 Dec. 2012
der Max Planck Gesellschaft
Blog Scientific America
Alles over Sterrenkunde
Welt der Physik
Alpha Galileo Foundation
EU Science News
Weird Things; what alcohol can tell us about the fate of the Universe
Newsbom.com; Alcohol, quasar en zeven miljard jaar geholpen wetenschappers bevestigen de juistheid van Einstein
Worldtoday.org.ua; Alcohol helped astronomers to "save" the theory of relativity
Sterne und Weltraum
Science News Online
World Socialists Website
El blog de Anatares; Noticias de astronomia y Fysica
Lecture Notes on variation of fundamental constants.
Poster on the hindered rotation mechanism in methanol