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ESO Press Release 05/04

31 March 2004

For immediate release

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

[Preview
- JPEG: 496 x 400 pix - 36k]
[Normal
- JPEG: 991 x 800 pix - 320k]

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).

Contacts

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

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: [email-masked]

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last modified: 2004/03/31

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& Public Relations
Department
Karl-Schwarzschild-Strasse 2, D-85748
Garching, Germany

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