Physics in Universe's Youth (Forwarded)

ESO Education and Public Relations Dept.

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Contact

Cédric Ledoux
ESO, Chile
Phone: +56 2 463 30 56 or +56 55 43 53 11

Patrick Petitjean
Institut d'Astrophysique de Paris, France
Phone: +33 1 44 32 81 50

Raghunathan Srianand
Inter University Centre for Astronomy and Astrophysics,
Pune, India
Phone: +91 20 569 1414 (ext 320)

For immediate release: 8 May 2006

ESO Science release 16/06

Physics in Universe's Youth

With ESO's VLT, Astronomers Find Molecular Hydrogen at Edge of Universe

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 [1].

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"
[2].

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."
[3]

A similar set of accurate measurements of molecular hydrogen lines
was made by the astronomers [4] 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 [5]. 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.

Notes

[1]: The results are described in a paper accepted for publication
in the Astrophysical Journal Letters ("Molecular Hydrogen in a
Damped Lyman-? system at zabs=4.224", by C. Ledoux, P. Petitjean,
and R. Srianand).

[2]: 200 to 500 million years is indeed the time necessary for a
star with a mass between 4 and 8 solar masses to synthesise and
expel into the Interstellar Medium the nitrogen that is observed.

[3]: 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.

[4]: 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 ESO PR 05/04 on results
about the possible variation of the fine-structure constant over
cosmological time by the same team.

[5]: 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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