New Evidence for Dark Energy in the Universe

An international team of astronomers, led by scientists at
the University of Manchester have produced new evidence that
most of the energy in the Universe is in the form of the
mysterious “Dark Energy”. The new evidence comes from a
10-year census of the sky for examples of gravitational
lenses, which are seen when a galaxy bends the light from a
distant quasar to form several images of the same quasar.
Linking the number of lenses they found with the latest
information on the numbers of galaxies, the scientists have
been able to infer that most of the energy in the Universe
is likely to be in an invisible, and presently unknown, form.

Dark Energy is closely related to the idea of a Cosmological
Constant introduced by Einstein over 80 years ago, but
most astronomers, including Einstein himself, have always
strongly doubted its reality. However, in the past 5 years
several independent groups of astronomers have amassed
evidence suggesting that Dark Energy exists and could well
dominate the total energy of the Universe.

Dark Energy only affects the properties of the Universe
over very large distances and the observations which are
sensitive to its presence, in particular studies of
exploding stars in distant galaxies, are all close to the
limit of current capabilities. Astronomers have therefore
been keen to exploit many different tests and Dr. Ian
Browne makes the point that “the new gravitational lens
test is based on completely different physical arguments
to the previous ones and so provides independent evidence
in support of Dark Energy”.

When a quasar is gravitationally lensed by an intervening
galaxy two or more images of the quasar are produced but
they are hard to recognise as the images are less than one
thousandth of a degree apart. The team therefore employed
several of the world’s most powerful radio telescope arrays
to make radio pictures of thousands of distant quasars.
Professor Peter Wilkinson points out that “we chose to use
radio telescopes for our survey since they can pick out
details many times finer than optical ones, even the Hubble
Space Telescope”. The census showed that about one out of
every 700 distant quasars is lensed by a foreground galaxy.

To calculate the fraction of the energy in the Universe
which is Dark Energy Manchester’s Dr. Kyu-Hyun Chae combined
the gravitational lens statistics with the latest results
on the numbers and types of galaxies in the Universe made
with optical telescopes. The result which emerged is that
around two thirds of the Universe’s energy appears to be
Dark Energy. The remaining third is made up of Dark Matter,
whose form is presently unknown, and “ordinary” matter
which makes up the stars and planets. For both of these
forms of matter gravity acts as normal and attracts. In
contrast Dark Energy has long-range anti-gravity properties
and now appears to be causing the expansion of the universe
to accelerate, rather than slow down as would be expected
if gravity was the dominant force. While astronomers have
no idea about what Dark Energy might be, these new results
add to their growing confidence that it is real.

Further Information:

Gravitational Lensing and the CLASS survey

Galaxies, bend the paths of light (or radio waves) and can
act as distorting lenses focussing the light from a more
distant object e.g. a quasar, lying behind the lens. The
first such gravitational lens was discovered by a team led
by a Jodrell Bank astronomer Dennis Walsh in 1979. A brief
illustrated introduction to gravitational lensing can be
found at:
http://www.jb.man.ac.uk/booklet/GravitationalLenses.html

The gravitational lens survey referred to in the main text
is called CLASS, which is an acronym for the Cosmic Lens
All Sky Survey. A description of the survey and a montage
of the radio images of all the CLASS gravitational lenses
can be found at
http://www.jb.man.ac.uk/research/gravlens/class/class.html

The idea underlying CLASS was to make radio maps of very
many distant radio sources looking for evidence of the
splittings and distortions characteristic of gravitational
lensing. Three major radio telescopes which were used in
turn to make the census are:

• The Very Large Array (NRAO) in Socorro NM USA
• The UK National Radio Astronomy Facility MERLIN (JBO)
• The Very Long Baseline Array (NRAO)

with each offering a unique combination of resolution and
observing speed. The VLA (lowest resolution, highest speed)
made the initial survey; MERLIN (higher resolution —
similar to that of the Hubble Space Telescope) followed
up promising candidates: the VLBA (highest resolution)
followed up the candidates not ruled out by MERLIN. After
this systematic sifting process the identification of the
survivors as gravitational lenses was almost certain.
However we then observed each of the survivors in the
optical and infra-red bands with the Hubble Space
Telescope; this invariably revealed the lensing galaxy
and hence confirmed that we had indeed found a lens.

By adopting such a rigorous protocol, which took many
years follow through, the observing team is confident that
the likelihood of any lenses being overlooked is small.
Eventually CLASS found 22 cases of lensing, about 1 for
every 700 radio sources examined. Full details of the
CLASS survey are about to be published in the Monthly
Notices of the Royal Astronomical Society and have just
become publicly available on:

• http://xxx.soton.ac.uk/abs/astro-ph/?0211073
• http://xxx.soton.ac.uk/abs/astro-ph/?0211069

Dark Matter, Dark Energy and the Flatness of Space

Dark Matter: is matter with normal gravitational properties
but which does not emit sufficient electromagnetic
radiation to be observed directly in any type of telescope.
Large amounts normal matter (in the form of stars or
hydrogen gas) in galaxies and clusters of galaxies, are
seen to be moving so fast that they would escape, unless
there is up to ten times more gravity than that of the
normal matter itself. This additional gravity is ascribed
to Dark Matter but what it consists of is currently
unknown. Astronomers now favour the idea that the Dark
Matter must be in the form of sub-atomic particles which
do not interact strongly with normal matter. Searches for
such particles are underway at many laboratories throughout
the world.

More about Dark Matter:
http://astron.berkeley.edu/~mwhite/darkmatter/dm.html

Dark Energy: Einstein’s General Theory of Relativity also
allows for the existence of Dark Energy (also called the
Cosmological Constant). This is a property of empty space
that causes the universe to expand more and more rapidly.
Unlike Dark Matter, whose effects can be seen within a
single galaxy, Dark Energy only shows up in observations
which probe significant fractions of the observable
Universe. The accelerating expansion of space was
discovered in the last few years by observations of
distant supernovae but the observations are difficult
and open to other interpretations.

More about Dark Energy and the searches for it:

• http://supernova.lbl.gov/
• http://cfa-www.harvard.edu/cfa/oir/Research/supernova/HighZ.html
• http://supernova.lbl.gov/~evlinder/sci.html

The Flatness of Space: The General Theory of Relativity is
based on the idea that matter and energy cause space to
become curved. In curved space geometry works differently
to normal flat (Euclidean) geometry: the angles of a
triangle don’t add up to 180 degrees. Einstein showed that
the curvature of the entire universe depends on the amount
of matter and energy in it. If the amount of matter/energy
is just right, space is flat, and traditional school
geometry does apply. Recent observations of the Cosmic
Microwave Background Radiation (CMBR), which effectively
measure the angles of a triangle, are showing that space
is indeed flat. Both Dark Matter and Dark Energy
contribute to the flatness of the universe but there is
not enough Dark Matter to make the universe flat, so the
CMBR results provide additional evidence that there must
be a contribution from Dark Energy.

Up-to-date information (“A New Picture of the Early
Universe”) on the results from a UK telescope (the Very
Small Array) studying the CMBR can be found at:
http://www.jb.man.ac.uk/news/vsa

The Importance of the New Gravitational Lensing Test

Claims for new physical phenomena, such as Dark Energy,
require very strong evidence to back them up. Since all
the previously reported observations are close to the
limit of current observational capabilities and depend
on various assumptions about the properties of the
Universe, it is vital to find new and independent ways
to look for the effects of Dark Energy. The statistics
of gravitational lensing provides such a test.

The basis of the calculation is that the probability of a
distant radio source being lensed by an intervening galaxy
depends on the volume of the observable Universe and hence
on the amount of Dark Energy. The lensing probability
increases rapidly as the fraction of Dark Energy in
the Universe increases. While additional results and
assumptions are needed to infer the Dark Energy content,
these are different and independent from those required
by the other methods.

The New Lensing Calculation

Dr Kyu-Hyun Chae made a detailed analysis of lens statistics
based on the final results from the 10-year CLASS census for
gravitational lenses and the latest results on the numbers
of galaxies in the Universe made with optical telescopes.
In particular Dr. Chae noticed that the lensing cross-
sections of galaxies (or, effective lens sizes) measured
by the image splittings were smaller than previously
thought, and consequently required a large amount of Dark
Energy in the Universe for the observed rate of multiple
image splittings to be compatible with the measured numbers
and types of galaxies in the nearby universe.

The new calculations now agree with the other methods, as a result of including much more extensive data: i.e.

a) many more lenses have been found and their red shifts and the redshift distribution of the distant quasars have been measured.

b) the angular splitting in each lens image has been determined, which tells us the cross-section forlensing directly (see comment above)

c) the latest results from two large recent galaxy surveys: the Anglo-Australian 2-degree field survey the Sloan Digital Sky Survey which have counted the number of potential lens galaxies in the local universe.

Dr. Chae’s calculations assume that the average number of
distant galaxies per unit volume of space is the same as
that found locally. It is possible that the number of
galaxies is less at high redshift but this would only serve
to increase the amount of Dark Energy implied by the new
results. It is also possible that the lens survey has
missed some cases of lensing — but more lenses would again
only increase the implied Dark Energy content. Our results
therefore add strong, and completely independent support
for a Universe dominated by Dark Energy (constituting about
70% of the energy in the Universe).

The text of the paper which has just appeared in Physical
Review Letters can be downloaded from:
http://www.jb.man.ac.uk/research/gravlens/class/PRL51301.pdf

The CLASS team members

University of Manchester Jodrell Bank Observatory

A. Biggs, I. Browne, K.Chae, P. Helbig, N. Jackson, S. Mao,
M. Norbury, J. McKean, P.Phillips, C. Sykes, P. Wilkinson, E.
Xanthopoulos, T. York

California Institute of Technology

R. Blandford, L, Koopmans, T.Pearson, A. Readhead

National Radio Astronomy Observatory

S. Myers

Netherlands Foundation for Radio Astronomy

A. de Bruyn

Space Telescope Science Institute

C. Fassnacht

University of Bonn

L. King

University of Pennsylvania

D. Rusin, D. Marlow