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New Evidence for Dark Energy
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.
Gravitational Lensing and the CLASS Survey
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 here.
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 here.
Dark Matter, Dark Energy and the Flatness of Space
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 can be found here.
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 can be found here.
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 here.
The Importance of the New Gravitational Lensing Test
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
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 for lensing 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).
Text of Paper - Physical Review Letters
Jodrell Bank Observatory
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