Universe Is More Transparent To High-energy Radiation Than Previously Assumed

Astronomers assume that our universe is approximately 14 billion years old. During this time it has changed greatly, for example through the formation of new galaxies that emit electromagnetic waves including light that can be perceived visually. A small fraction of the latter is absorbed, but the majority is retained in the universe. In this respect the universe can also be imagined as a background of invisible electromagnetic radiation. Since high-energy rays interact with low-energy ones, astronomers assumed for a long time that because of this enormous universal background radiation, only high-energy rays from very close active galaxies would be detectable on Earth – the remainder would be absorbed.

However, a publication that appeared recently in Science by scientists belonging to the MAGIC Collaboration reaches a different conclusion. The team was the first to succeed in detecting on Earth high-energy gamma radiation originating from the quasar 3C 279, which is more than 5 billion light-years distant. A quasar is an active galaxy in which an enormous black hole more than a billion times the mass of the sun radiates gigantic amounts of energy in various wavelength regions. According to Adrian Biland from the Institute for Particle Physics (IPP) of ETH Zurich: “The results show that the radiation background in the universe is distinctly less dense than expected.” Together with seven other ETH Zurich physicists he is a member of the MAGIC Collaboration who are the authors of the publication.

According to calculations by the scientists, the density of the background radiation in the universe corresponds approximately to the expected radiation intensity of all the heavenly bodies known from astrophysics. This finding could disprove the theory that the universe also contains light from objects that are still entirely unknown and which could have existed in the early universe. According to Biland: “This means that if heavenly bodies of types at present unknown to us really did once exist, they must have been of a quite different nature to the stars we know today. Otherwise their light would still be detectable in today’s radiation background.”

High-energy physics with potential

Biland and the MAGIC Collaboration work in the relatively young field of astro-particle physics and study “very high-energy” (VHE) gamma radiation. The original energy of this radiation lies in the range between 50 giga-electron volts and several tera-electron volts, i.e. radiation with a wavelength more than a billion times shorter than sunlight. (VHE) gamma radiation is measured by the MAGIC telescope on the island of La Palma in the Canaries. This telescope makes the measurements by using the Cherenkov effect, which put simply is the optical equivalent of a supersonic bang. However the velocity of the particles causes a flash of light instead of a bang.

In other words charged particles can pass through the atmosphere faster than the associated electromagnetic radiation can propagate. As a result the polarisation of the atoms, which is normally symmetrical, can no longer balance out, and this leads to the emission of what is known as Cherenkov radiation. When a VHE photon penetrates into the atmosphere it generates a so-called Air Shower of thousands of electrons and positrons, all of which emit Cherenkov radiation.

These mini-lightning flashes are collected by the 934 aluminium mirrors of the MAGIC telescope positioned in the cone of this “shower” and are recorded by a high-performance camera taking two billion images per second, of which about 1000 are ultimately selected. The main problem is that in addition to the very rare VHE photons, similar air showers are also generated by cosmic ray particles, which are more than 10,000 times as frequent, and the actual measured signal needs to be found and interpreted against this background.

VHE gamma radiation is not an exceptional phenomenon

According to Biland, measurements in the VHE gamma radiation region provide astronomers with a unique opportunity to verify the laws of astronomy in new areas of energy. Thus the publication in Science was able to disprove the Blazar Sequence model, which says that the most energetic objects are also the brightest. Another opinion prevalent among astronomers until now was that VHE gamma rays are exceptional astronomical phenomena. As the data set grew steadily, it became possible within a few years to identify a whole series of different classes of active galaxies as well as various supernova remnants in the Milky Way as sources of VHE gamma rays. Biland says “We even observed sources that had not been discovered by conventional measurements in any other wavelength region. This means that objects emitting radiation only in the VHE gamma region also seem to exist – a phenomenon whose cause is still unknown at the present time.”

The potential of astro-particle physics, especially in the (VHE) gamma region, is recognized by scientists and research institutions all over the world. A second MAGIC telescope that will enable stereoscopic observations and thus more exact measurements will be commissioned on La Palma in September this year. Other similar installations exist in Namibia (H.E.S.S.), Australia (CANGAROO) and the USA (VERITAS). In addition the planning procedure is currently underway for the Cherenkov Telescope Array (CTA) project, in which more than 40 European research institutes are collaborating and which plans to bring into operation about 100 Cherenkov telescopes. Biland is convinced that “An installation of this kind allows us an entirely new window into the universe.”

Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC)

MAGIC is the world’s biggest gamma radiation telescope and is operated by a major collaboration involving more than 150 researchers from the whole of Europe. The telescope was commissioned in 2003 on the Roque de los Muchachos (“Rock of the Boys”), the highest mountain on La Palmas (2500 metres). The telescope, which has a diameter of 17 metres and a mirror surface area of 236 square metres, makes measurements only at night but is looked after round the clock by a team of scientists on the spot.