If the Universe is infinite, and there are infinitely many stars in it, why is the sky dark at night — shouldn't there be stars everywhere? This old question is known as Olber's paradox; its answer relates to the fact that the visible Universe is not infinite, that it has a finite age, and that there is only a finite amount of energy available to be converted into starlight. The question how much — or how little — starlight there exactly is, is a very interesting issue since it tells scientists about the history of star formation in the Universe. After the Big Bang, the Universe expanded and cooled rapidly; then, matter slowly started to condense under the pull of gravity. Some 100 million years after the big bang, stars formed, ignited and to began fill the then dark Universe with their light. The total intensity of starlight visible today relates to the density of stars in the Universe, and to time over which stars shine; the intensity of starlight is thus a key observable for cosmologists trying to decipher the history and evolution of the Cosmos.
However, there's a catch: the amount of cosmic light one measures on earth or with instruments on satellites is mostly “foreground” light, emitted by dust in the solar system and by “nearby” objects in our own Galaxy; this intensity is not at all representative of the typical intensity of light in intergalactic space and in the vast volume of the Universe. It's a bit like attempting to watch stars on a sunny day — the starlight is certainly there, but it's all hidden in the scattered sunlight of the blue sky. One approach to solve this problem is to estimate the amount of foreground light and to subtract it — a formidable task given that at least in some wavebands the foreground light amounts to more than 99% of the total measured intensity.
What one should really do is measure starlight in the space between galaxies, away from all sources of foreground light. This may sound impossible, but gamma ray astronomers have actually found a trick to perform this measurement. The idea is to use distant “active galaxies” which send a beam of high-energy gamma rays towards earth, across billions of light years of intergalactic space. The gamma rays are produced at the cores of such galaxies, where black holes with a mass a billion times the mass of the sun gobble up nearby stars and convert part of the accreted mass into beamed high-energy radiation. The energy of these gamma rays — some 100 000 000 000 times more energetic than normal light — is so high that sometimes when they “hit” a quantum of starlight, the energy is sufficient to create an electron-positron pair, converting energy into matter according to Einstein's E = mc2. By this process, the beam of gamma rays is attenuated. The attenuation is the stronger the higher the energy of the gamma rays is — because then it is easier to produce a particle pair — and the more intense the “target” starlight is. By measuring the spectrum of high-energy gamma rays from such distant active galaxies — known as “Blazars“ — one can identify the absorption features imprinted on their gamma-ray spectrum, and conclude from there on the intensity of starlight in intergalactic space.
The high-energy gamma rays are detected with detectors called Cherenkov telescopes, which image the cascades of secondary particles created when high-energy gamma rays are absorbed high up in the Earth's atmosphere. Among the most advanced of such instruments is the High Energy Stereoscopic System (H.E.S.S.) in the Khomas Highland of Namibia, built and operated by a group of over 100 scientists from Germany, France, the United Kingdom, Ireland, the Czech Republic, Armenia, South Africa, and Namibia. In data recorded since 2004, H.E.S.S. scientists have now discovered gamma-rays from two distant Blazars, a few billion light years from Earth, as announced in an article in the April 20 issue of Nature. The objects known under their catalogue names H 2356-309 and 1ES 1101-232 were initially discovered in radio and X-ray surveys of the sky. The gamma-ray spectra of these two Blazars were expected to show strong absorption features, effectively cutting off all gamma rays above a certain energy. Yet the measured spectra show no such effect, indicating that intergalactic absorption and hence the intensity of near-infrared starlight in intergalactic space must be significantly lower than previously assumed. Galaxies visible in deep exposures with the Hubble space telescopes essentially account for the maximal permissible amount of starlight, meaning that in these images we have essentially seen all relevant sources of starlight, back to the Big Bang! The H.E.S.S. data clearly exclude hypotheses according to which the very first stars — referred to as population III stars — generate an intensity peak detected in the near-infrared starlight near Earth, which is probably a remainder of not perfectly subtracted foreground light.
H.E.S.S. has in the last years achieved a number of important discoveries concerning high-energy gamma-ray sources in our own Galaxy and had revolutionized high-energy gamma-ray astronomy. The new H.E.S.S. results illustrate the power of the instrument for extragalactic astronomy and cosmology. The discovery of low levels of intergalactic near-infrared starlight has the interesting side effect that the Universe becomes more transparent to gamma rays and that the telescopes can look deeper into the Cosmos, increasing their reach for further discoveries!