Detecting signatures of neutrinos with the Universe's Anthem

You might have heard sound doesn’t travel through space. You would be right, because space is extremely empty, so there is no stuff for sound waves (vibrations of particles) to be able to move through. But if we wound back time, back to the early Universe, billions of years ago, the Universe was a hot dense plasma – sound waves travelled in the early Universe via the dense fluid of particles, in a symphony, distorting the dense fluid with an array of notes and chords.

Eventually, the Universe cooled and became less dense by expanding, so the sound waves stopped; the cosmic chords froze in place. However, they were not lost. From this moment onwards, galaxies started forming in accordance with the notes and chords imprinted by the sound waves in the fluid. The anthem of the Universe is written in the distribution of galaxies in the Universe we can see today. At present, this distribution has been measured most precisely by fresh data I am working on, provided by the Dark Energy Spectroscopic Instrument (DESI) collaboration.

Up until this point, most of this exciting data has been able to allow us to measure large distances extremely precisely. However, if we inspect the musical notes more carefully, they are influenced by particles that didn’t always interact with the sound waves strongly and moved past the waves at a supersonic speed. With the highly precise data now available, we can pick out the notes and chords that tell us about these particles. Physicists consider that these are most likely a kind of particle called neutrinos – a mysterious particle, that weakly interacts with normal matter. Physicists understand how normal matter interacts with other normal matter well and we encode this in something called the Standard Model. But we don’t understand neutrinos well; understanding them better will improve our model. We know neutrinos shift the relative timing of the chords in the early sound waves; we call a phase-shift. By measuring it, we can understand how many kinds of neutrino species (or types) may exist.

This figure shows what the phase-shift signal is expected to look like, which different colours showing how the shift when there is more or less neutrinos (increasing or decreasing the effective number of neutrino species - darker means less neutrinos).

In my project, I am extending the software used by the DESI collaboration that allow us to measure the Universe’s anthem imprinted in the galaxy distribution we see today. Compared to previous analyses, we are extending the usual analysis to include the neutrino-induced phase-shift. By measuring it, we hope to provide an important test of the Standard Model, which expects to find just 3 species of neutrinos – some physicists expect, based on experiments in the lab, there may be more than just 3. We can report on the presence or lack thereof of new species based on the phase-shift we see in the data. This research demonstrates the ability of cosmological data to contribute to our understanding of particle physics for experiments and the fundamental particles that took part in the orchestra for the Anthem of the Universe.

By Abbé Whitford