[JC] What does it take to have N_{eff} < 3 at CMB times?

by Yi Chung

Thursday 6 Aug 2026, 15:00 → 16:00 Asia/Seoul

CTPU seminar room (IBS)

In this talk, I will discuss the paper arXiv:2603.22391, in which the authors investigate various scenarios that can lead to N_eff < 3 . I will start from the basics and include other relevant works to present the topic as comprehensively as possible. 

 

Authors: Miguel Escudero, Maksym Ovchynnikov, Neal Weiner

 

Abstract of the paper: The vast majority of extensions of the Standard Model affecting the number of effective relativistic neutrino species (N_{eff}) do so additively, namely, they enhance this quantity with some light state contributing to dark radiation. In this work, we consider precisely the opposite case: new physics scenarios that can lead to N_{eff}< 3 that are consistent with all known cosmological, astrophysical, and laboratory data. We are motivated by three main reasons: 
1) a recent measurement from ACT and SPT in combination with Planck that leads to $N_{eff} = 2.81 +/- 0.12,
2) by a new and powerful measurement of the primordial helium abundance, which anchors N_{eff} to be very close to the Standard Model value one second after the Big Bang,
3) by the deployment of the Simons Observatory which will provide precise tests of the radiation content in the Universe and which may detect with a high significance cosmologies with N_{eff}<3.
We survey the main theoretical possibilities and find that only a few simple scenarios can consistently give N_{eff}=2.81 +/- 0.12. One class consists of thermal electrophilic relics with masses m ~ 8--13MeV. Another consists of out-of-equilibrium particles decaying to e⁺e^- or ​γγ , with a rather particular lifetime 0.05s < τ < 3min, mass 250MeV < m < 600MeV, and abundance ρ/ρ_γ ~ 0.1 at decay. Thermal electrophilic particles are especially interesting because they can account for the dark matter in the Universe and can be tested in experiments such as SENSEI, DAMIC-M, and Oscura, and their portals to the visible sector at experiments such as NA64 and LDMX. We conclude that if the Simons Observatory confirms that N_{eff} ~ 2.8, it will point to very specific extensions of the Standard Model.