The early Universe was a very extreme and rich environment. It was hot, dense, very energetic. Lots of different particle species populated those stages: photons, neutrinos, electrons, positrons, and much more! At some point, even quarks were freely going around instead of being trapped in protons as they usually are. And scientists believe it is possible that even more, exotic, unknown particles were around at those times. The theory of special relativity tells us that the mass of a particle contributes to the total energy possessed by that particle (mass-energy equivalence). However, in the early Universe, the energy densities of particles were so high that in most cases whether or not those particles had a certain mass was a negligible property. In fact, many of those particles were relativistic, and the only important property we care about in this case is their energy, or temperature as cosmologists like to label it.
Scientists consider the contribution of relativistic particles to the energy density of the Universe as a radiation contribution, in units of the contribution provided by the cosmic microwave background photons. Therefore, if we are interested in computing the energy density in radiation in the early Universe, we just need to know the CMB temperature at those times and the relative temperature of any additional relativistic species with respect to the CMB. General relativity tells us that how the universe expands depends on the total energy density at the time. As a result, different values of the radiation density in the early Universe can alter the rate at which the Universe expands, an effect that we can clearly observe in the distribution of CMB hot and cold spots in the sky.
The Simons Observatory will be sensitive enough to measure the energy contribution from the interaction of particles in the early Universe. Identifying these heretofore unmeasured contributions is very important to better understand the building blocks of our Universe. A completely new window into the so-called ‘beyond the standard model’ of particle physics may be opened, should the presence of these exotic particles be spotted.