Is matter visible

VHF radiation makes dark matter visible

Max Planck researchers from Garching show that giant radio telescopes could deliver high-resolution images of the cosmic mass distribution

Galaxies make up only a small part of the mass in the universe, the large rest consists of a foreign matter that has so far stubbornly eluded direct observation. Now researchers at the Max Planck Institute for Astrophysics have calculated that a very large radio telescope could create an extremely detailed picture of the cosmic distribution of this invisible dark matter. It would have a much higher resolution than the best images obtained with optical telescopes. This new finding should encourage the construction of larger VHF telescopes. Because such images would bring a huge amount of new knowledge about how the universe and its galaxies were formed (online: November 28, 2006).

Light from distant sources is diverted by gravity from objects closer to us on its way to us. This gravitational deflection of light distorts the images from these sources like a distant landscape that can be seen through a crooked window pane or on a rippled pond surface. The visible and invisible mass in the foreground can be calculated from the distortion. So far, researchers have only worked with the distortion of light from distant galaxies. The scientists R. Benton Metcalf and Simon White of the Max Planck Institute for Astrophysics in Garching have now determined that the gravitational distortion of radio images of pregalactic gas would provide much more detailed maps of the cosmic matter distribution. The previously only blurry, almost unusable images from optical telescopes could be resolved by radio telescopes up to 20 times higher.

The corresponding radio waves come from the first half billion years after the Big Bang, before the first galaxies appeared. During this period normal matter consisted of an almost evenly distributed gas mixture of hydrogen and helium with slight fluctuations in density. However, these weak structures influence the cosmic background radiation at a wavelength of 21 centimeters, which is characteristic of hydrogen, and are therefore visible. As the universe expands, this wavelength has now grown to two to 20 meters, which corresponds to the VHF range.

The wavelength changes depending on the distance from the radio source. A radio telescope can therefore differentiate these structures from one another - up to a thousand in each direction. This gives you many very distant radiation sources: the ideal prerequisite for precisely calculating the mass of the objects in front of them from their gravitational distortion. Unlike in the past, researchers could also use the radio waves to capture structures that are far away from the galaxies and whose optical image distortion can be measured. Also, you could get a picture of the early universe, when there were no galaxies.

"Corresponding investigations with very large radio telescopes would usher in a new era in high-precision cosmology and allow us to understand more precisely how galaxies are formed," says Simon White. Very high-resolution images, however, require gigantic radio telescopes: for example, an area densely equipped with radio antennas about 100 kilometers in diameter, ideally located on the back of the moon, where the antennas could work without the disruptive influences of the earth's atmosphere.

"In order to achieve new results with radio waves, we don't have to wait for such a giant telescope," says Simon White. In addition to dark matter, there is another dark riddle in space: the mysterious dark energy, which accelerates the expansion of the universe. The scientists have shown that even with a less precise mass map from smaller radio telescopes, the properties of this dark energy can be determined more precisely than with any of the previously planned methods.

In any case, the results increase expectations of radio telescopes that are currently under construction or in planning. One of the most advanced projects is the Low Frequency Array (LOFAR) in the Netherlands, which will consist of thousands of small radio antennas connected via a network. However, it only covers a ten-thousandth of the area of ​​the ideal giant telescope. The Max Planck Institute for Astrophysics, together with other German institutes, wants to play an important role in the LOFAR project.