A recent experiment at CERN has shed new light on the mysterious absence of blazar radiation, a phenomenon that has puzzled scientists for years. Led by Charles Arrowsmith from the University of Oxford, the team's findings suggest that the missing gamma rays may be the result of an unexplained phenomenon in the early universe, rather than the presence of intergalactic magnetic fields.
Blazars, incredibly bright objects with supermassive black holes at their cores, emit intense beams of radiation. When a blazar points towards Earth, we observe a bright source of light, including terahertz gamma rays. However, during their journey across intergalactic space, these gamma-ray photons collide with background starlight, creating cascades of electrons and positrons. These particles can then scatter off photons to produce gamma rays in the gigaelectronvolt energy range, which should travel in the direction of the original jet. Yet, this secondary radiation has never been detected.
One potential explanation for this phenomenon involves magnetic fields. Arrowsmith explains that electrons and positrons in the pair cascade would be deflected by an intergalactic magnetic field, steering them away from our line of sight. However, the existence and origin of such fields remain unclear.
Another theory focuses on the sparse plasma that permeates intergalactic space. The beam of electron-positron pairs could interact with this plasma, generating magnetic fields that separate the pairs. Over time, this process could lead to beam-plasma instabilities, reducing the beam's ability to create gigaelectronvolt gamma rays directed towards Earth.
To test these theories, the team created an experimental platform at the HiRadMat facility at CERN, mimicking the interaction of pair cascades from blazars with the intergalactic medium. They found that the beams remained far more tightly focused than expected, suggesting that beam-plasma instabilities are not strong enough to explain the absence of gigaelectronvolt gamma rays. This implies that the pair beam must be perfectly collimated or composed of pairs with equal energies to suppress instabilities in the plasma.
While the experiment suggests that intergalactic magnetic fields remain the best explanation for the missing gamma rays, the mystery is far from solved. As Gianluca Gregori from Oxford explains, the early universe's uniformity and the need for electric currents, gradients, and inhomogeneities in the primordial plasma make the existence of such fields a complex and intriguing possibility. Confirming their existence could point to new physics beyond the Standard Model, which may have dominated the early universe.
The Cherenkov Telescope Array Observatory, a ground-based gamma-ray detector network, is set to provide more insights. With improved resolutions, it will help scientists better understand the nature of blazar radiation and the role of magnetic fields in the early universe.
