CTA: multi messenger astronomy and a potential probe of extreme particle processes

An image to illustrate multi messenger astronomy
©iStock/Dronandy

Professor John Ellis FRS spoke to SciTech Europa about how the Cherenkov Telescope Array stands to impact on the fields of theoretical and particle physics.

The Cherenkov Telescope Array’s (CTA) First Science Symposium was held in Bologna, Italy, on 6-9 May 2019. The symposium focused on the novel investigations CTA will bring to efforts exploring the high energy Universe, as well as its synergies with other wavebands
and messengers.

Ahead of the event, SciTech Europa spoke with Professor John Ellis FRS – a theoretical physicist at CERN who also holds the post of Clerk Maxwell Professor of Theoretical Physics at King’s College London, UK – about how the CTA stands to also benefit theoretical and particle physics.

In a general sense, what do you feel the CTA will bring to the field of theoretical physics?

CTA is going to observe some of the most energetic events taking place in the Universe which produce high energy photons, gamma rays, and these are potentially great probes of extreme particle processes in the Universe.

There is the possibility that these high energy photons are produced by the annihilation of dark matter particles. There is also the possibility that these gamma rays originate in very distant locations in the Universe and so might provide useful probes for special relativity (which states that light always travels at the speed of light, and which we believe to essentially be true, apart from some very small corrections). The explosions that are taking place in the distant parts of the Universe could help us to better explore these theories, and CTA observations will hopefully provide more stringent limits on the possibilities.

How long will it take to generate enough data to actually be able to start making those calculations and to properly explore these areas?

Once the overall sensitivity of CTA takes over from that of the current generation of experiments, I think we will start to see some exciting developments. Current experiments, such as the High Energy Stereoscopic System (HESS) and others, have been generating great results, but CTA is billed as being the next generation, generating perhaps an order of magnitude more data. Of course, there are a number of different ways that CTA will improve on the current experiments, and this means that after a year or so of operations with the fully-operational CTA, the sensitivity will be better than that of the previous experiments and so there is huge potential there.

CTA is expected to expand the number of known gamma-ray-emitting celestial objects tenfold, detecting more than 1,000 new objects. How important is this from the point of view of a fundamental physicist?

The more energetic sources there are, and the more distant those sources, the better. One thing I am particularly interested in is boosting the search for energetic sources at a larger distances due to their potential as probes to test special relativity.
Furthermore, there is currently a debate going on as to whether there might be particles of dark matter annihilating close to the centre of our galaxy which are resulting in energetic photons and gamma rays, with the debate also focusing on whether that comes from fundamental physics processes or is due to a population of unresolved forces. I hope that CTA will be able to help resolve that argument as it will have the capability to pick out individual sources that other experiments are unable to identify.

CTA will be linking up with other infrastructures. Is there scope for the CTA to work in collaboration with organisations such as CERN in order to bring the particle physics community more into the area of multi messenger astronomy?

Multi messenger astronomy is a burgeoning activity and is set to revolutionise the way people think about astronomy. One of the components of multi messenger astronomy stems from the advent of gravitational wave astronomy, with the first gravitational waves being detected by the LIGO observatory when two neutron stars merged together.

Such mergers will, of course, continue to happen, as will other events such as the merging of black holes and neutron stars, which will be a very intense area of multi messenger astronomy and where I expect CTA to play a key role.

When LIGO observed the neutron star/neutron star merger, an ultra-high energy neutrino was simultaneously detected by the IceCube collaboration, which was associated with a blazar. Looking back through their data, the IceCube team found a cluster of events and other evidence of neutrinos which also seemed to be associated with the same blazar.

This is something that I foresee as a growth area once CTA is able to provide significantly more information on high energy gamma rays. Alongside this, there are also planned upgrades for IceCube, which could allow the team there to detect many more neutrinos than they have been able to detect so far. There are also plans for another high energy astrophysical neutrino observatory in the Mediterranean, KM3NeT.

Multi messenger astronomy will look at neutrinos combined with gamma rays as well as gravitational waves combined with gamma rays, and the former will give us new insights into fundamental physics processes taking place in energetic astrophysical forces such as blazars.

With regard to possible connections before between CTA and CERN, dark matter, of course, is a field of research that the two have in common, although they approach this in different ways, with CERN looking at the production of dark matter originating in particle/particle collisions, and CTA looking for the production of ordinary particles in the annihilation of dark matter particles.

These are complementary ways of addressing the dark matter question, and if we are to actually see dark matter particles then there is a strong sense that we will need to combine the results from the two classes of experiments.

Another connection between CERN and CTA is that in order to understand what processes are taking place in energetic astrophysical forces such as blazars, we need to understand high energy particle collisions.

Regarding multi messenger astronomy, the future will see the Large Scale Synoptic Telescope (LSST) come online. This will scan the whole sky on an almost continuous basis, looking for things that flash. Currently, no systematic survey of cosmic flashes has been conducted, so this could be a real game changer.

In the coming years, once the LSST comes up to speed, it is hoped that it will be able to observe a whole new classes of variable objects in optical wavelengths, and we can then correlate that data with observations at other wavelengths, in particular gamma rays, and that is where CTA would come in.
This is therefore multi messenger in the sense of different species of particles, gamma rays and gravitational waves and neutrinos, and it would be a potential new way of trying to understand things that flash on and off in the Universe.

Will CTA also have a significant impact on theoretical physics, then?

It is highly likely that CTA will go on to discover dark matter, which would be the biggest possible discovery in particle physics in the coming years. Observing energetic gamma rays coming from dark matter annihilation would also be truly revolutionary.
While I would also like to see CTA’s capabilities employed to probe special relativity and look for variations in the velocity of light, this is more speculative and less likely to be achieved.

What do you think should be prioritised when it comes to the search for annihilating dark matter particles and deviations from Einstein’s theory of relativity?

As previously mentioned, one possible place where we might find dark matter particles is in the centre of our galaxy, and there has been a lot of discussions between observers and theorists about how existing data can be interpreted, and CTA could potentially resolve this.
The centre of our galaxy is a complicated place, however, and so it is also important to investigate other, less complicated locations, and observations have been made, in particular by the FERMI satellite, of gamma ray emissions from dwarf spheroidal galaxies (small, low-luminosity galaxies with very little dust and an older stellar population). These galaxies are distant, simple to model, and provide if not exactly a background free source of gamma rays then at least a location where there is a hope of understanding what is happening.

Professor John Ellis FRS
Clerk Maxwell Professor of Theoretical Physics
King’s College London
Theoretical physicist
CERN
+44 (0)20 7848 2470
john.ellis@cern.ch
Tweet @KingsCollegeLon
www.kcl.ac.uk/nms/depts/physics/people/academicstaff/ellis

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