The exploration of the Universe and the recently improved detection capabilities of cosmic messengers (photons, neutrinos, cosmic rays) brought us into a new era of knowledge. While these discoveries have set another milestone for studying new-found energetic cosmic events, they allow working on the prediction and phenomenological models to test our physical theories at energy regimes which are not directly accessible in accelerators. This is where the Quantum Gravity Phenomenology makes its entrance.
Introducing QG-MM
As a recent field of physics, the Quantum Gravity Phenomenology can provide a bottom-up approach to the study of quantum gravity problems. But to give this field the possibility to deploy its full potential, a strong collaboration between multidisciplinary experts is necessary. This is exactly the approach the COST Action ‘Quantum gravity phenomenology in the multi-messenger approach’ (QG-MM) took when initiating a brand-new network in 2019 to tackle this topic.
By making the most of COST Actions’ networking possibilities, QG-MM has already sparked specific achievements that will be crucial for advancing this research field.
Understanding the quantum gravity problem
While the two traditional physics’ theories are known as the quantum theories (small scale universe) and Einstein’s general relativity theory (gravity and the large-scale structure of the universe), they offer apparently incompatible ideas of space and time. Although they were mostly used in isolation of one another to study the different natural phenomenon, in some specific situations, such as studying the interior of black holes and the beginning of the universe, both gravity and quantum effects are equally relevant.
In these specific cases, the yet unknown field of ‘quantum gravity’ can offer a way out, unifying both descriptions into a specific knowledge that could cure the infinities. “Such a theory, searched very actively by theoretical physicists with different approaches during the last fifty years at least, would revolutionise our most basic concepts, starting with the very notions of space and time.” explains Prof Jose Manuel Carmona, Chair of QG-MM.
From quantum gravity to Quantum Gravity Phenomenology
Until recently there was no hope for experimental guidance for a theory of quantum gravity. But in the last decades, a new idea emerged from the observation of photons coming from very distant galaxies as they could reveal some footprints of quantum gravity. “The main observation was that even if the propagation of a high-energy photon (the more energetic, the better) may be very slightly affected by a ‘quantum space-time’, appearing in many quantum gravity formalisms, the extraordinary large distance between its source, where it is emitted, and the Earth, where we detect it, may serve as an amplifier of this effect” adds Prof Carmona.
Many proposals suggested that these photons should arrive with a delay depending on the difference of their energies: the larger this difference and the larger the distance they have moved along, the larger the amplification of the quantum space-time effects. This idea changed the perception about the apparent impossibility to have an experimental guidance of a theory of quantum gravity and started the field of quantum gravity phenomenology.
Several sources were additionally detected as sensitive to these ‘quantum gravity effects’ such as protons, but also very high-energy cosmic neutrinos (2013) and gravitational waves, whose first observation happened in 2015. These sources are called ‘messengers’ of the universe and make the case for what we call the ‘multi-messenger paradigm’.
Taking Quantum Gravity Phenomenology to the next level with QG-MM
The needs of knowledge transfer between experimentalists and theorists gave the initial impetus to propose a COST Action on this topic.
“I had learnt about the existence of COST Actions and saw clearly that the idea of joining communities fitted perfectly with the COST philosophy and that the network tools (in particular, STSMs, Working Group meetings and Training Schools) were really adapted to what we needed. We discussed this within my research group in the University of Zaragoza and we contacted two people that we knew well: Giovanni Amelino-Camelia, a theorist from the University of Naples and one of the founders of the field of quantum gravity phenomenology, who became the Action Vice-Chair of our COST Action, and Manel Martinez, an experimental physicist working on the detection of high-energy photons from IFAE, in Barcelona, who became the Grant Holder Scientific Representative of our Action.”
Prof Jose Manuel Carmona, Chair of QG-MM
Setting-up the COST Action in 2018 turned to be especially timely, as it closely followed the period of the birth of the multi-messenger astronomy (2017).
By fostering networking amongst multidisciplinary experts, the Action became a new collaborative platform where models could be tested beyond the usual practices, and subtler possible effects in the experimental data of the cosmic messengers could be examined. Thanks to the Action’s activities, innovative ideas on how to carry out these analyses in more effective ways or how to consider possible correlations between messengers emerged. There have also been discussions on data sharing and combination of experiments. Prof Carmona adds: “all of this puts us in a better path to be able to eventually detect signatures of new physics coming from quantum gravity.”
Amplifying the benefits of networking
Training Schools have been an essential tool for the COST Action QG-MM. One of its objectives was to train young scientists in the interdisciplinary expertise which will be needed to foster advances in the search of quantum gravity effects in the physics of the cosmic messengers.
The lectures and notes produced in the frame of these Training Schools are recorded and kept for the future purposes, as a legacy of this COST Action. QG-MM involves many trainees in this task, which will provide them with a publication and then will have an immediate effect on their curriculum.
This Action also allocates a strong importance to diversity within its network. It has set a Committee for outreach and gender activities and has made a full report on gender balance within the Action. In February 2022, it conducted a social media campaign paying a tribute to the female scientists who were part of the network.
The outputs of QG-MM
Although still running (until September 2023), this COST Action has already produced two main achievements essential for the field of quantum gravity phenomenology in the multi-messenger paradigm: a big review paper of the state of the art in their field and a database on “what to expect for which cosmic messenger in which experiment.”
The review paper, “Quantum gravity phenomenology at the dawn of the multi-messenger era – A review” published in Particle and Nuclear Physics will allow researchers from inside and outside the field to understand the approaches followed in quantum gravity phenomenology, both from the theoretical and the experimental sides, and the status of the field in a comprehensive way.
“Such a review, which joins many different expertise (covering quantum gravity theoretical and phenomenological models and techniques of experimental detection) would not have been produced without our COST Action“ explains Prof Jose Manuel Carmona
As for the database, it is a collection of the present status of the different experimental constraints, which offers an overall view of the different physical sectors where these models have been tested. Such a global view will be again useful for researchers in the field. QG-MM intends to keep maintaining it even after the end of the COST Action.
“Some of these collaborations would have been impossible without our Action, since it combines scientists (physicists) of different expertise, including theoreticians and experimentalists experts in different detection techniques that would not normally interact, since they belong to different communities. The common objective of the Action, however, has made this interaction possible. This has benefited specifically PhD students and young postdoc researchers, because these interactions will guide their research activity. They are also receiving transversal formation in our Training Schools, which was one of the Action’s objectives.”
Prof Jose Manuel Carmona, Chair of QG-MM
Further information
View the Action webpage
View the Network website
First ever COST Connect on advancing gender equality in research and innovation discusses how to accelerate progress ft. QG-MM