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Physics in Universe's Youth
With ESO's VLT, Astronomers Find Molecular Hydrogen at Edge of Universe
8 mai 2006
Using a quasar located 12.3 billion light-years away as a beacon, a team of astronomers detected the presence of molecular hydrogen in the farthest system ever, an otherwise invisible galaxy that we observe when the Universe was less than 1.5 billion years old, that is, about 10% of its present age. The astronomers find that there is about one hydrogen molecule for 250 hydrogen atoms. A similar set of observations for two other quasars, together with the most precise laboratory measurements, allows scientists to infer that the ratio of the proton to electron masses may have changed with time. If confirmed, this would have important consequences on our understanding of physics.
"Detecting molecular hydrogen and measuring its properties in the most remote parts of the Universe is important to understand the gas environment and determine the rate of star formation in the early Universe", said Cédric Ledoux, lead-author of the paper presenting the results.
Although molecular hydrogen is the most abundant molecule in the Universe, it is very difficult to detect directly. For the time being, the only way to detect it directly in the far Universe is to search for its telltale signatures in the spectra of quasars or gamma-ray burst afterglows. This requires high spectral resolution and large telescopes to reach the necessary precision.
A team of astronomers, comprised of Cédric Ledoux (ESO), Patrick Petitjean (IAP, Paris, France) and Raghunathan Srianand (IUCAA, Pune, India), is conducting a survey for molecular hydrogen at high redshift using the Ultraviolet and Visible Echelle Spectrograph (UVES) at ESO's Very Large Telescope. Out of the 75 systems observed up to now, 14 have firm detection of molecular hydrogen. Among these, one is found having a redshift of 4.224.
While using the 12.3 billion light-years distant quasar PSS J 1443+2724 as a beacon, the astronomers detected several features belonging to an unseen galaxy having a redshift of 4.224. In particular, many lines from molecular hydrogen were found, breaking the record for the detection of this element in the farthest object in the Universe. This also implies that the gas in this galaxy must be rather cold, about -90 to -180 degrees Celsius.
In addition, several lines from 'metals' are also seen, allowing the researchers to deduce the amount of various chemical elements.
"From the abundance of Nitrogen observed, we argue that it had to be produced in the late stage of the life of 4 to 8 solar mass stars," said Patrick Petitjean. "Thus, star-formation activity must have formed at least 200 to 500 million years before we are observing the galaxy, that is, when the Universe was about one billion years old".
If the galaxy went through a phase of intense star-formation activity, it is now, at the time of the observations, in a rather quiescent state.
"These observations demonstrate the possibility to perform these studies at the highest redshift with ESO's VLT", said Raghunathan Srianand. "In particular, the possibility to observe the interstellar medium of distant galaxies revealed by using gamma-ray bursts as beacons will boost this field in the near future."
A similar set of accurate measurements of molecular hydrogen lines was made by the astronomers  with UVES on the VLT towards two others quasars, Q 0405-443 and Q 0347-383.
This set of data allowed the scientists to compare the ratio of the mass of a proton to that of an electron in molecular hydrogen as it is now and how it was about 12 billion years ago . To this aim, they performed extremely accurate measurements of spectral lines of hydrogen molecules in the laboratory and compared the results with the same lines observed in the spectra of these quasars.
These measurements show that the mass ratio of the proton and the electron may have changed, becoming 0.002% smaller in the past twelve billion years. Albeit such a change may look tiny, it would have important consequences on our understanding of physics. The scientists stress however that their result is just an 'indication', not yet a 'proof' and that it should be confirmed by further measurements, both astronomical and in the laboratory.
: The results are described in a paper accepted for publication in the Astrophysical Journal Letters ("Molecular Hydrogen in a Damped Lyman-α system at zabs=4.224", by C. Ledoux, P. Petitjean, and R. Srianand).
: Using these observations, but looking at Carbon instead of Hydrogen, the authors were able to derive the temperature of the Microwave Background at this epoch. This fossil radiation has been emitted as a direct consequence of the Big Bang, when the Universe was only 300 000 years old and had, at that time, a temperature of 3 000 K. As the Universe expands, it gets cooler and this temperature has dropped to only 3 K (-270 degrees Celsius) nowadays. The astronomers measure that when it was 1.5 billion years old, the Universe had a temperature of 14 K (-259 degrees Celsius), in agreement with the Big Bang theory.
: This study appeared in 2005 in Astronomy and Astrophysics, vol. 440, p. 45 ("A new constraint on the time dependence of the proton-to-electron mass ratio. Analysis of the Q 0347-383 and Q 0405-443 spectra", by A. Ivanchik et al.). See also eso0407 on results about the possible variation of the fine-structure constant over cosmological time by the same team.
: This finding is reported in the April 21 issue of Physical Review Letters ("Indication of a cosmological variation of the proton-to-electron mass ratio based on laboratory measurement and reanalysis of H2 spectra", by E. Reinhold et al.). The laboratory measurements were performed with a special laser, developed in the Laser Centre VU Amsterdam, operating at the specific wavelengths absorbed by hydrogen molecules. Those wavelengths are in the extreme ultraviolet (XUV) between 90 and 110 nanometres. The beam of XUV-radiation is crossed with a beam of H2 molecules in otherwise vacuum conditions. The laboratory measurements, the calculations on the hydrogen molecule, and the statistical analysis of the data were carried out by a team at the Vrije Universiteit Amsterdam (The Netherlands) led by Wim Ubachs, further consisting of Elmar Reinhold (now associated with the European Space Agency, ESA, in Noordwijk, the Netherlands), Urs Hollenstein (now at the ETH in Zürich, Switzerland) and Ruth Buning. The observations of the quasars with UVES on ESO's VLT were carried out by a team headed by Patrick Petitjean (Institut d'Astrophysique de Paris, France) and Alexander Ivanchik (Ioffe Institute, St. Petersburg, Russia). See also the web page of Wim Ubachs at http://www.nat.vu.nl/~wimu/NatCont-Eng.html.
The proton-to-electron mass ratio is an important fundamental constant of Nature. This constant is dimensionless, that is, independent of any system of units. Its current value is Mp/me = 1836.1526726.
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