New Quasar Studies Keep Fundamental Physical Constant Constant (Forwarded)

ESO Education and Public Relations Dept.

Contacts:

Patrick Petitjean
Institut d'Astrophysique de Paris
98bis, Boulevard Arago
75014 Paris, France
Phone: +33-1-44328150
Fax: +33-1-44328001
Email:

Raghunathan Srianand
Inter University Center for Astronomy and Astrophysics
Post bag 4, Ganeshkind
Pune University Campus, Pune
India
Fax: 091-20-569 0760 411 007
Phone: 091-20-569 1414 (ext 320)
Email:

----------------------------------------------------------------------------
Text with all links and the photos are available on the ESO Website at URL:
http://www.eso.org/outreach/press-re.../pr-05-04.html
----------------------------------------------------------------------------

For immediate release: 31 March 2004

ESO Press Release 05/04

New Quasar Studies Keep Fundamental Physical Constant Constant

Very Large Telescope sets stringent limit on possible variation of the
fine-structure constant over cosmological time

Summary

Detecting or constraining the possible time variations of fundamental
physical constants is an important step toward a complete
understanding of basic physics and hence the world in which we
live. A step in which astrophysics proves most useful.

Previous astronomical measurements of the fine structure constant --
the dimensionless number that determines the strength of interactions
between charged particles and electromagnetic fields -- suggested
that this particular constant is increasing very slightly with time.
If confirmed, this would have very profound implications for our
understanding of fundamental physics.

New studies, conducted using the UVES spectrograph on Kueyen, one of
the 8.2-m telescopes of ESO's Very Large Telescope array at Paranal
(Chile), secured new data with unprecedented quality. These data,
combined with a very careful analysis, have provided the strongest
astronomical constraints to date on the possible variation of the
fine structure constant. They show that, contrary to previous claims,
no evidence exist for assuming a time variation of this fundamental
constant.

PR Photo 07/04: Relative Changes with Redshift of the Fine Structure
Constant (VLT/UVES)

A fine constant

To explain the Universe and to represent it mathematically, scientists rely on
so-called fundamental constants or fixed numbers. The fundamental laws of
physics, as we presently understand them, depend on about 25 such constants.
Well-known examples are the gravitational constant, which defines the strength
of the force acting between two bodies, such as the Earth and the Moon, and the
speed of light.

One of these constants is the so-called "fine structure constant", alpha =
1/137.03599958, a combination of electrical charge of the electron, the Planck
constant and the speed of light. The fine structure constant describes how
electromagnetic forces hold atoms together and the way light interacts with atoms.

But are these fundamental physical constants really constant? Are those numbers
always the same, everywhere in the Universe and at all times? This is not as
naive a question as it may seem. Contemporary theories of fundamental
interactions, such as the Grand Unification Theory or super-string theories that
treat gravity and quantum mechanics in a consistent way, not only predict a
dependence of fundamental physical constants with energy -- particle physics
experiments have shown the fine structure constant to grow to a value of about
1/128 at high collision energies -- but allow for their cosmological time and
space variations. A time dependence of the fundamental constants could also
easily arise if, besides the three space dimensions, there exist more hidden
dimensions.

Already in 1955, the Russian physicist Lev Landau considered the possibility of
a time dependence of alpha. In the late 1960s, George Gamow in the United States
suggested that the charge of the electron, and therefore also alpha, may vary.
It is clear however that such changes, if any, cannot be large or they would
already have been detected in comparatively simple experiments. Tracking these
possible changes thus requires the most sophisticated and precise techniques.

Looking back in time

In fact, quite strong constraints are already known to exist for the possible
variation of the fine structure constant alpha. One such constraint is of
geological nature. It is based on measures taken in the ancient natural fission
reactor located near Oklo (Gabon, West Africa) and which was active roughly
2,000 million years ago. By studying the distribution of a given set of elements
-- isotopes of the rare earths, for example of samarium -- which were produced
by the fission of uranium, one can estimate whether the physical process
happened at a faster or slower pace than we would expect it nowadays. Thus we
can measure a possible change of the value of the fundamental constant at play
here, alpha. However, the observed distribution of the elements is consistent
with calculations assuming that the value of alpha at that time was precisely
the same as the value today. Over the 2 billion years, the change of alpha has
therefore to be smaller than about 2 parts per 100 millions. If present at all,
this is a rather small change indeed.

But what about changes much earlier in the history of the Universe?

To measure this we must find means to probe still further into the past. And
this is where astronomy can help. Because, even though astronomers can't
generally do experiments, the Universe itself is a huge atomic physics
laboratory. By studying very remote objects, astronomers can look back over a
long time span. In this way it becomes possible to test the values of the
physical constants when the Universe had only 25% of is present age, that is,
about 10,000 million years ago.

Very far beacons

To do so, astronomers rely on spectroscopy -- the measurement of the properties
of light emitted or absorbed by matter. When the light from a flame is observed
through a prism, a rainbow is visible. When sprinkling salt on the flame,
distinct yellow lines are superimposed on the usual colours of the rainbow,
so-called emission lines. Putting a gas cell between the flame and the prism,
one sees however dark lines onto the rainbow: these are absorption lines. The
wavelength of these emission and absorption lines is directly related to the
energy levels of the atoms in the salt or in the gas. Spectroscopy thus allows
us to study atomic structure.

The fine structure of atoms can be observed spectroscopically as the splitting
of certain energy levels in those atoms. So if alpha were to change over time,
the emission and absorption spectra of these atoms would change as well. One way
to look for any changes in the value of alpha over the history of the Universe
is therefore to measure the spectra of distant quasars, and compare the
wavelengths of certain spectral lines with present-day values.

Quasars are here only used as a beacon -- the flame -- in the very distant
Universe. Interstellar clouds of gas in galaxies, located between the quasars
and us on the same line of sight and at distances varying from six to eleven
thousand of million light years, absorb parts of the light emitted by the
quasars. The resulting spectrum consequently presents dark "valleys" that can be
attributed to well-known elements.

If the fine-structure constant happens to change over the duration of the
light's journey, the energy levels in the atoms would be affected and the
wavelengths of the absorption lines would be shifted by different amounts. By
comparing the relative gaps between the valleys with the laboratory values, it
is possible to calculate alpha as a function of distance from us, that is, as a
function of the age of the Universe.

These measures are however extremely delicate and require a very good modelling
of the absorption lines. They also put exceedingly strong requirements on the
quality of the astronomical spectra. They must have enough resolution to allow
very precise measurement of minuscule shifts in the spectra. And a sufficient
number of photons must be captured in order to provide a statistically
unambiguous result.

For this, astronomers have to turn to the most advanced spectral instruments on
the largest telescopes. This is where the Ultra-violet and Visible Echelle
Spectrograph (UVES) and ESO's Kueyen 8.2-m telescope at the Paranal Observatory
is unbeatable, thanks to the unequalled spectral quality and large collecting
mirror area of this combination.

Constant or not?

ESO PR Photo 07/04
Relative Changes with Redshift of the Fine Structure Constant

Captions: ESO PR Photo 07/04 shows measured values of the relative
change of alpha from the sample of absorption systems studied by
Hum Chand and his colleagues, plotted as a function of the redshift
and the corresponding look-back time. The open circle is the
measurement from the Oklo natural reactor. The horizontal long
dashed lines show the area of the previous claim of variation of the
fine structure constant. Clearly, the new UVES data are inconsistent
with this range.

A team of astronomers [1], led by Patrick Petitjean (Institut d'Astrophysique de
Paris and Observatoire de Paris, France) and Raghunathan Srianand (IUCAA Pune,
India) very carefully studied a homogeneous sample of 50 absorption systems
observed with UVES and Kueyen along 18 distant quasars lines of sight. They
recorded the spectra of quasars over a total of 34 nights to achieve the highest
possible spectral resolution and the best signal-to-noise ratio. Sophisticated
automatic procedures specially designed for this programme were applied.

In addition, the astronomers used extensive simulations to show that they can
correctly model the line profiles to recover a possible variation of alpha.

The result of this extensive study is that over the last 10,000 million years,
the relative variation of alpha must be less than 0.6 part per million. This is
the strongest constraint from quasar absorption lines studies to date. More
importantly, this new result does not support previous claims of a statistically
significant change of alpha with time.

Interestingly, this result is supported by another -- less extensive --
analysis, also conducted with the UVES spectrometer on the VLT [2]. Even though
those observations were only concerned with one of the brightest known quasar HE
0515-4414, this independent study lends further support to the hypothesis of no
variation of alpha.

Even though these new results represent a significant improvement in our
knowledge of the possible (non-) variation of one of the fundamental physical
constants, the present set of data would in principle still allow variations
that are comparatively large compared to those resulting from the measurements
from the Oklo natural reactor. Nevertheless, further progress in this field is
expected with the new very-high-accuracy radial velocity spectrometer HARPS on
ESO's 3.6-m telescope at the La Silla Observatory (Chile). This spectrograph
works at the limit of modern technology and is mostly used to detect new planets
around stars other than the Sun -- it may provide an order of magnitude
improvement on the determination of the variation of alpha.

Other fundamental constants can be probed using quasars. In particular, by
studying the wavelengths of molecular hydrogen in the remote Universe, one can
probe the variations of the ratio between the masses of the proton and the
electron. The same team is now engaged in such a large survey with the Very
Large Telescope that should lead to unprecedented constraints on this ratio.

More Information

The research presented in this Press Release is based on papers published in
Physical Review Letters ("Limits on the time variation of the electromagnetic
fine-structure constant in the low energy limit from absorption lines in the
spectra of distant quasars" by Raghunathan Srianand, Hum Chand, Patrick
Petitjean, and Bastien Aracil) and in the leading European astronomy journal
Astronomy & Astrophysics ("Probing the cosmological variation of the
fine-structure constant: Results based on VLT-UVES sample" by Hum Chand,
Raghunathan Srianand, Patrick Petitjean, and Bastien Aracil).

Notes

[1] The team is composed of Hum Chand and Raghunathan Srianand (IUCAA Pune,
India), Patrick Petitjean (Institut d'Astrophysique de Paris, CNRS, and LERMA,
Observatoire de Paris, France) and Bastien Aracil (Institut d'Astrophysique de
Paris, CNRS, France).

[2] This result has been published in Astronomy & Astrophysics Letters :
"Probing the variability of the fine-structure constant with the VLT/UVES" by
Ralf Quast, Dieter Reimers (Hamburg Observatory, Germany), and Sergei A.
Levshakov (Department of Theoretical Astrophysics, St. Petersburg, Russia).

----------------------------------------------------------------------------
ESO Press Information is available on Receive email notification
the WWW at about important news from ESO -
http://www.eso.org/outreach/press-rel/. subscribe to the
----------------------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
----------------------------------------------------------------------------