After a century of study, researchers still struggle to understand the origin of cosmic rays, particles with extreme energies that fill the Universe and bombard the Earth from all directions... On Tuesday at KIPAC@10, we asked: Where Did That Come From? and spent the morning talking about particle acceleration in the Universe. Afterwards, Luigi Tibaldo talked to Angela Olinto (KICP) and Neil Gehrels (NASA Goddard)

After a century of study, researchers still struggle to understand the origin of cosmic rays, particles with extreme energies that fill the Universe and bombard the Earth from all directions. Their very existence tells us that there are objects in the Universe capable of accelerating them to energies up to hundred million times larger than those we reach in the most powerful accelerator built on Earth, the Large Hadron Collider at CERN in Geneva, Switzerland. Cosmic rays are largely screened by the atmosphere, making life on Earth possible, but they limit our possibilities to spend long times far above its surface in long-distance flights, or out in space.

However, cosmic rays are not just curious high-energy monsters populating the Universe or a threat for humans aspiring to explore space. We believe that they play a key role in the evolution of galactic ecosystems, influencing the physics and chemistry of the tenuous gas from which stars and planets are formed, and of life on Earth, occasionally causing mutations in the DNA.

We think of different origins for cosmic rays according to their energies, that we express in units of GeV, or gigaelectronvolts (for comparison, the energy of visible light is a few billionths of a GeV, X-rays are about a thousandth of a GeV). For fifty years we've been stuck with a few usual suspects, objects large and powerful enough to accelerate particles to the incredible energies of cosmic rays.

Within our own galaxy, the Milky Way, we managed to pin down the probable sources of cosmic rays with energies lower than a million GeV to shock fronts produced by the dramatic supernova explosions that occur at the end of the lives of massive stars.

Theoreticians think that supernova shock fronts can accelerate particles up to ten million GeV. Above these energies, particles may be accelerated by other objects, such as neutron stars or giant black holes at the center of galaxies spewing out matter in jets at almost the speed of light (known as AGN, or active galactic nuclei).

Cosmic rays with the most extreme energies, higher than ten trillion GeV, most probably coming from outside the Milky Way, are still a deep mystery. Our largest cosmic-ray detectors on Earth, like the Pierre Auger Observatory in Argentina, have only caught a handful of them over many years.

We can also try to identify cosmic accelerators by looking at the gamma rays that energetic particles can produce when they interact with matter, radiation, or magnetic fields. Current gamma-ray telescopes like Fermi, AGILE, Integral and Swift are revealing a wealth of powerful accelerators in the Universe. They're also revealing lots of surprises, such as the unexpected bright flares of the Crab Nebula that demonstrate it can accelerate electrons up to a million GeV. This increases the need for instruments like the future Cherenkov Telescope Array or HAWC that have collection areas large enough to track the evolution of gamma-ray brightness with time and can observe a large fraction of the sky at any given time.

The exact mechanisms by which these objects accelerate particles remains a major open question. Fermi acceleration (named after its discoverer, physicist Enrico Fermi) was successful to explain cosmic-ray acceleration in supernova remnants. In this case, particles are trapped close to shock fronts by magnetic fields and are bounced back and forth, gaining energy each time. Fermi acceleration can also occur in systems where two magnetic fields crash with each other head-on, forming a reconnection layer that accelerates particles in the same way as a shock front does.

However, other cosmic particle acceleration mechanisms still seem to be needed, especially to explain what accelerates cosmic rays to the most extreme energies. We have detected particles with energies that are off the charts, but a definitive answer to what is energizing them continues to puzzle us. As Roger Blandford, former KIPAC director reminded us during our discussion, "Nature is a lot cleverer than we are."

You can watch all the talks in this session on the KIPAC youtube channel.