Do neutrinos interACT? Looking for hints with the Atacama Cosmology Telescope (ACT)

Title: The Atacama Cosmology Telescope: The Persistence of Neutrino Self-Interaction in Cosmological Measurements

First author: Christina D. Kriesch 

First author institution: Department of Astrophysical Sciences, Princeton University 

Status: Published in Physical Review D [open access]

The early Universe consisted partially of a hot dense plasma of interacting photons and baryons (for cosmologists, ‘baryons’ basically means normal matter). Some hundreds of thousands of years after the Big Bang, the photons and baryons eventually stopped interacting due to the Universe expanding and cooling, and the photons began freely propagating without further interactions with other particles. The freely moving photons are what we call the Cosmic Microwave Background (CMB) radiation and we can detect in space today. When we measure the CMB, we see many tiny fluctuations in the temperature of the photons coming from different directions across the sky. These tiny fluctuations encode information about cosmological parameters such as the density of matter in the Universe, or the Hubble constant. The statistical information in the CMB is thus extremely informative and can be used to test models of cosmology (this bite on a classic paper discusses this in more detail).

The Atacama Cosmology Telescope (ACT) was a telescope in Chile that measured the CMB (see the telescope in Figure 1). ACT began observations in 2007 and ended mid-2022. Compared to other CMB telescopes like the Planck satellite, ACT was able to measure the angular fluctuations in the temperature and polarisation of the CMB photons to a higher resolution. Because of this, ACT allows us to do tests of cosmology using information from angular scales (think different angular sizes for the temperature/polarisation fluctuations across the sky) that can’t be accessed with other probes.

The authors of today’s featured paper use the high resolution data in ACT to specifically look for signatures of interacting neutrinos. But firstly, what are neutrinos? Why can we learn about them from cosmological data? And why is this interesting to cosmologists?

Neutrinos are weird

Neutrinos are a kind of particle that we know exist, and are included in the Standard Model, but we don’t really get them. They are hard to detect generally as they only interact via the weak nuclear force and have a low cross-section for interactions with other particles. However, due to these characteristics, we expect neutrinos to have stopped interacting with the hot plasma in the early Universe (before recombination happened) and started moving freely through the Universe (called free-streaming) – this refers to propagation without further interactions. Furthermore, while the Standard Model predicts them to be massless, we know from terrestrial particle physics experiments they actually have non-zero masses – but we don’t exactly know what the masses are (see more on that topic in this bite).

However, the fact that they 1) should free-stream AND 2) are not massless should impact the gravitational potential across the Universe. As it turns out, the effect of neutrinos on the gravitational potential caused a shift in the phase of the early Universe sound waves (AKA Baryon Acoustic Oscillations) that propagated through space prior to recombination. Additionally, their presence had some impact on how fast the Universe expanded and impacted the growth of matter structures (on small length scales at least). The phase-shift from neutrinos in the BAOs and the effect their masses have on the growth of matter structures have both been constrained with cosmological data, from the CMB and galaxy surveys. Overall, cosmologists are interested in understanding the properties of neutrinos in order to understand how neutrinos have impacted the Universe and include these impacts in cosmological models.

Despite all of this, our understanding of neutrinos is limited. There have been various anomalies (unexplained experimental results – such as the fact that neutrinos have mass at all) noted in terrestrial neutrino experiments that makes us wonder whether neutrinos may have interactions or cause phenomena that are not captured by the Standard Model. If neutrinos have self-interactions, they could be detectable in the CMB data measured from ACT because it can access information from smaller angular scales that can’t be accessed by other experiments. Self-interactions could delay the time we expect that neutrinos began to free-stream, and would impact the effects on the CMB or galaxy surveys that come from neutrino free-streaming – this is what allows us to test this idea in the data.

For this paper, the strength of self-interactions between neutrinos is described by a parameter called . Previous works to today’s paper have found two regions of the parameter space for that could potentially allow for neutrinos to have a non-zero self-interaction. These regions are referred to as the MI region (for moderate interactions) and SI region (for strong interactions) in this work.

Results for neutrino interACTions

The combination of the ACT data with other CMB datasets (and ACT alone) tends to again find the bimodal (having two maxima) probability distribution function (PDF) for , as seen in Figure 2.

While the ACT data seems to prefer the SI model, the Planck CMB data independently prefers it much less. The ACT polarisation data (polarisation referring to the orientation of the photon E-fields) most strongly contributes to the preference for the SI model. In particular, this is true for small angular scales seen in the data, which may be due to the fact that these modes are particularly sensitive scales to the effects of neutrino free-streaming. These modes also tend to have a higher amplitude in the ACT data compared to the Planck data, which tends to be favoured by the SI model. However, the ACT data still prefers the SI model if the data is fit by using only the smaller or only the larger angular scales. 

Adding galaxy data

The authors expand their results to include data from the Baryon Acoustic Oscillations (BAOs – the early Universe sound waves as measured by their imprint in galaxy surveys) with the ACT data. Interestingly, they find this tends to increase the significance of the SI model preference compared to the MI model. In contrast, the combination of Planck with the ACT data and BAOs prefers models without neutrino interactions.

Summary

Overall, we cannot conclusively state here that neutrinos do (or don’t) have self-interactions, but there is an interesting preference for strongly interacting neutrinos with the ACT data. The results certainly motivate us to continue to push the boundaries with cosmological data, to search for hints of neutrino physics beyond the current Standard Model.

Edited by Tori Bonidie 

Featured image credit (and Figure 1): The Atacama Cosmology Telescope viewed from the top of the outer ground screen, by M. Devlin, CCA by 4.0, https://commons.wikimedia.org/wiki/File:Atacama_cosmology_telescope_top_down.jpg via Wikimedia Commons 

• About the Author

About Abbé Whitford

I am a third year PhD student at the University of Queensland, studying Large Scale Structure cosmology with galaxy clustering and peculiar velocities, and using Large Scale Structure to measure the properties of neutrinos.

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