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| ©ETH Zurich |
| Diagrammatic representation of the measurement method for radiation in the (VHE) gamma region. |
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.








