Has ESA's XMM-Newton cast doubt over dark energy? (Forwarded)

ESA News
http://www.esa.int

12 December 2003

Has ESA's XMM-Newton cast doubt over dark energy?

ESA's X-ray observatory, XMM-Newton, has returned tantalising new data about the
nature of the Universe. In a survey of distant clusters of galaxies, XMM-Newton
has found puzzling differences between today's clusters of galaxies and those
present in the Universe around seven thousand million years ago. Some scientists
claim that this can be interpreted to mean that the 'dark energy' which most
astronomers now believe dominates the Universe simply does not exist …

Observations of eight distant clusters of galaxies, the furthest of which is
around 10 thousand million light years away, were studied by an international
group of astronomers led by David Lumb of ESA's Space Research and Technology
Centre (ESTEC) in the Netherlands. They compared these clusters to those found
in the nearby Universe. This study was conducted as part of the larger
XMM-Newton Omega Project, which investigates the density of matter in the
Universe under the lead of Jim Bartlett of the College de France.

Clusters of galaxies are prodigious emitters of X-rays because they contain a
large quantity of high-temperature gas. This gas surrounds galaxies in the same
way as steam surrounds people in a sauna. By measuring the quantity and energy
of X-rays from a cluster, astronomers can work out both the temperature of the
cluster gas and also the mass of the cluster.

Theoretically, in a Universe where the density of matter is high, clusters of
galaxies would continue to grow with time and so, on average, should contain
more mass now than in the past.

Most astronomers believe that we live in a low-density Universe in which a
mysterious substance known as 'dark energy' accounts for 70% of the content of
the cosmos and, therefore, pervades everything. In this scenario, clusters of
galaxies should stop growing early in the history of the Universe and look
virtually indistinguishable from those of today.

In a paper soon to be published by the European journal Astronomy and
Astrophysics, astronomers from the XMM-Newton Omega Project present results
showing that clusters of galaxies in the distant Universe are not like those of
today. They seem to give out more X-rays than today. So clearly, clusters of
galaxies have changed their appearance with time.

In an accompanying paper, Alain Blanchard of the Laboratoire d'Astrophysique de
l'Observatoire Midi-Pyrénées and his team use the results to calculate how the
abundance of galaxy clusters changes with time. Blanchard says, "There were
fewer galaxy clusters in the past."

Such a result indicates that the Universe must be a high-density environment, in
clear contradiction to the 'concordance model,' which postulates a Universe with
up to 70% dark energy and a very low density of matter. Blanchard knows that
this conclusion will be highly controversial, saying, "To account for these
results you have to have a lot of matter in the Universe and that leaves little
room for dark energy."

To reconcile the new XMM-Newton observations with the concordance models,
astronomers would have to admit a fundamental gap in their knowledge about the
behaviour of the clusters and, possibly, of the galaxies within them. For
instance, galaxies in the faraway clusters would have to be injecting more
energy into their surrounding gas than is currently understood. That process
should then gradually taper off as the cluster and the galaxies within it grow
older.

No matter which way the results are interpreted, XMM-Newton has given
astronomers a new insight into the Universe and a new mystery to puzzle over. As
for the possibility that the XMM-Newton results are simply wrong, they are in
the process of being confirmed by other X-ray observations. Should these return
the same answer, we might have to rethink our understanding of the Universe.

Note to editors:

The two papers, "The XMM-Newton Omega Project: I. The X-ray
Luminosity-Temperature Relationship at z0.4" by D.H. Lumb et al. and "The
XMM-Newton Omega Project: II. Cosmological implications from the high redshift
L-T relation of X-ray clusters" by S.C. Vauclair, A. Blanchard et al. will be
published shortly in Astronomy and Astrophysics.

The contents of the Universe

The content of the Universe is widely thought to consist of three types of
substance: normal matter, dark matter and dark energy. Normal matter consists of
the atoms that make up stars, planets, human beings and every other visible
object in the Universe. As humbling as it sounds, normal matter almost certainly
accounts for a small proportion of the Universe, somewhere between 1% and 10%.

The more astronomers observed the Universe, the more matter they needed to find
to explain it all. This matter could not be made of normal atoms, however,
otherwise there would be more stars and galaxies to be seen. Instead, they
coined the term dark matter for this peculiar substance precisely because it
escapes our detection. At the same time, physicists trying to further the
understanding of the forces of nature were starting to believe that new and
exotic particles of matter must be abundant in the Universe. These would hardly
ever interact with normal matter and many now believe that these particles are
the dark matter. At the present time, even though many experiments are underway
to detect dark matter particles, none have been successful. Nevertheless,
astronomers still believe that somewhere between 30% and 99% of the Universe may
consist of dark matter.

Dark energy is the latest addition to the contents of the Universe. Originally,
Albert Einstein introduced the idea of an all-pervading 'cosmic energy' before
he knew that the Universe is expanding. The expanding Universe did not need a
'cosmological constant' as Einstein had called his energy. However, in the 1990s
observations of exploding stars in the distant Universe suggested that the
Universe was not just expanding but accelerating as well. The only way to
explain this was to reintroduce Einstein's cosmic energy in a slightly altered
form, called dark energy. No one knows what the dark energy might be.

In the currently popular 'concordance model' of the Universe, 70% of the cosmos
is thought to be dark energy, 25% dark matter and 5% normal matter.

XMM-Newton

XMM-Newton can detect more X-ray sources than any previous satellite and is
helping to solve many cosmic mysteries of the violent Universe, from black holes
to the formation of galaxies. It was launched on 10 December 1999, using an
Ariane-5 rocket from French Guiana. It is expected to return data for a decade.
XMM-Newton's high-tech design uses over 170 wafer-thin cylindrical mirrors
spread over three telescopes. Its orbit takes it almost a third of the way to
the Moon, so that astronomers can enjoy long, uninterrupted views of celestial
objects.

For further information, please contact:

Fred Jansen, ESA XMM Project scientist
ESA ESTEC, The Netherlands
Tel: + 31 71 565 4426
E-mail:

Alain Blanchard
Laboratoire d'Astrophysique
Observatoire Midi-Pyrénées, France
Tel: + 33 5 61 33 2842
E-mail:

ESA Media Relations Service
Tel: +33 1.53.69.71.55
Fax: +331.53.69.76.90

More on XMM-Newton

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/SEMR..._index_1.html]
The fuzzy object at the centre of the frame is one of the galaxy clusters
observed by XMM-Newton in its investigation of the distant Universe. The
cluster, designated RXJ0847.2+3449, is about 7000 million light years away, so
we see it here as it was 7000 million years ago, when the Universe was only
about half of its present age. This cluster is made up of several dozen galaxies

Credits: ESA

[Image 2:
http://www.esa.int/export/esaCP/SEMR...html#subhead1]
Artist's impression of XMM-Newton separating from its Ariane-5 stage.

Credits: ESA 1999

[Image 3:
http://www.esa.int/export/esaCP/SEMR...html#subhead2]
One of three XMM-Newton mirror modules in ESA's XMM-Newton Satellite, flight
model. Each one contains 58 mirrors with a total telescope optical surface of
more than 120 m2 – bigger than a tennis court.

Credits: ESA