Imagine time running backwards. People would grow younger instead of older and, after a long life of gradual rejuvenation – unlearning everything they know – they would end as a twinkle in their parents’ eyes. That’s time as represented in a novel by science fiction writer Philip K Dick but, surprisingly, time’s direction is also an issue that cosmologists are grappling with.
Cosmic rays are rays of particles and radiation emitted from various astrophysical environments, for instance shocks, Active Galactic Nuclei (AGN), Puslar Wind Nebulae and Supernova Remnants. We can observe these cosmic rays from the Earth, and their spectrum takes on a distinctive power-law shape with a peak at a few GeV (billions of electron volts, eV), and ‘knee’ features around 4 and 400 PeV (1 PeV = 1015 eV = 1 000 000 000 000 000 eV), with an ‘ankle’ at 1 EeV (1 EeV = 1018 eV). The lower energy Cosmic Rays are thought to originate within our galaxy, while higher energy ones come from further afield, providing astronomers with a different way to probe the cosmos, without using conventional observations of electromagnetic radiation.
We live in interesting times. For thousands of years, we have thought we knew what the universe – and everything in it – was made of: normal matter, the kind that make up the elements of the periodic table.
However, the discovery in the 1990s of a completely unknown force dubbed dark energy that makes up 70% of the cosmos – causing it to expand at an accelerated rate – has taught us to be humble. Since then, astronomers have begun investing billions of pounds in experiments which aim to find out what this mysterious phenomenon is. What they discover is guaranteed to change physics forever.