Background: In the last decade, our current understanding of the Universe we inhabit has been challenged as different measurements of its current expansion rate appear in disagreement with each other. Indeed, local measurements of the expansion rate based on inferring distances to relatively nearby celestial objects return a higher value of this expansion rate (called H₀) than the value based on studying the relic glow from the Big Bang. This glow, the oldest known light in the Universe, is usually referred to at the cosmic microwave background (CMB) since it is currently brightest in the microwave waveband. This discrepancy between the local measurements and those inferred from CMB observations could be pointing to missing ingredients in our current cosmological model. Exploring which physics could be responsible for this was the major goal of this award.

Intellectual Merit: While searching for possible physical scenarios that could explain the expansion rate discrepancy, we discovered a previously unknown symmetry of cosmological observables, which could be exploited to address the current tension between different measurements of the expansion rate. Importantly, the existence of this symmetry suggests that our Universe could be split into two different worlds that mirror each other. On the one side, we would have our familiar world with the Earth, the Sun, and all the normal matter and particles surrounding us. On the other side, we could have a dark mirror world containing all the same particles and interactions that are familiar to us, except that this matter is invisible to us except for its gravitational interaction with matter we can see. While seemingly baroque, this idea of a mirror world has been studied for decades within particle physics circles as it could address important problems in that field of physics. Our work potentially connects this well-studied idea to the current expansion rate discrepancy currently observed in cosmology.

Another avenue of research we have pursued is to study whether neutrinos, elusive particles that barely interact with the matter that surrounds us, could have something to do with the above-mentioned H₀ tension. Since they interact so weakly with us, neutrinos are the least constrained particle in our current Standard Model of particle physics. The question is then whether new physics present exclusively in the neutrino sector could be responsible for the challenges that we are currently experiencing in cosmology. While our work has shown that such new physics is unlikely to explain the current discrepancy, cosmological observations suggests that yet-unknown neutrino interactions could be present in earlier epochs of the Universe.

Broader Impacts: This award was the main source of support for a graduate student, contributing to their professional and intellectual development, as well as to the STEM workforce. The codes developed as part of this award are publicly available for anyone to use.

 As part of this award, several local Albuquerque schools were visited to share with the pupils the excitement of astronomy and of the mysteries of the Universe. Six public astronomy nights were organized to highlight the night sky with the public. Several public talks about the mysteries of the Universe were given by the PI and his team.

Last Modified: 03/15/2024
Modified by: Francis-Yan Cyr-Racine