The current understanding of the Universe as a whole is summarized in the so-called standard cosmological model, or Lambda-CDM. According to this model, only 5% of the total energy content is represented by ordinary matter, i.e. the particle content we are all familiar with in our every-day life. The rest of the energy density is given by exotic components whose effects we can observe, although we are still missing a clear understanding of their nature. In particular, 25% of the energy density of the Universe is in the form of cold dark matter (the CDM in LCDM), a bizarre matter component that talks with the rest of the particle content only through gravitational effects, and eventually new kind of interactions yet to be observed. Even more intriguing, scientists believe that the remaining 70% of the energy density is in the form of a very elusive component, called dark energy, which is responsible for the observed accelerated expansion of the Universe at late times. The simplest form of dark energy is the cosmological constant, the Lambda in LCDM. It is defined as a constant because its energy density does not change with time, as opposed to the other matter and radiation components, and as opposed to other types of dark energy. Understanding the properties of this component is of paramount importance, as it governs the evolution of the Universe in the current epoch. Different types of dark energy can induce a different rate of expansion of the Universe, and a different growth of cosmological structures.
Data from The Simons Observatory complements observations from galaxy surveys by measuring the growth of matter structures at different epochs (redshifts). In addition, The Simons Observatory will provide very precise measurements of the current expansion rate (the Hubble constant H0). Discrepancies between cosmological measurements of H0 and measurements from experiments observing the local (short distances, or low redshift) Universe currently exist. If they persist even after the very tight measurements from SO, they could be an indication that the cosmological constant model needs to be revised.