Looking back in time observing the first light ever emitted, the Cosmic Microwave Background, we can investigate and understand how the Universe was in its first instants, how it evolved up to what we observe today and how structures as galaxies and galaxy clusters originated. The classical Big Bang theory states that the Universe was generated from a hot dense state and subsequently expanded and cooled allowing the formation of particles, matter and cosmic structures.
Thanks to cosmological observations, the classical theory has been integrated, and became the standard cosmological model, with the introduction of the dark sectors, like the dark energy and dark matter to explain the recent acceleration of the Universe expansion and the large-scale structures formation and dynamics. The addition of an inflationary phase allowed solving some remaining issues of the model.
Inflation is defined as a phase of accelerated expansion. During this period the energetic content of the Universe is dominated by a component which exerts a negative pressure (with an equation of state parameter smaller than -1/3) allowing a quasi-exponential expansion, the simplest candidate which satisfies this condition being a scalar field. In the simplest inflationary models, a scalar field, called inflaton, is the only responsible of inflation but there are also several models predicting the existence of multiple fields with different natures, like scalars, pseudo-scalars, vectors etc., involved in the inflationary phase. An inflationary phase taking place before the hot Big Bang would explain the size of our observable Universe (which must be at least the CMB largest observable scale) and its almost perfect Euclidean geometry (flatness) together with the abundances of topological relicts, which have been the three historical problems affecting the original Big Bang theory. Inflation also provides a natural mechanism to generate the primordial fluctuations (through quantum effects) which after inflation grew up through gravitational instability generating galaxies and galaxy clusters.
Inflation is supported both on theoretical and observational grounds (the spectral index of scalar perturbations being different from scale invariance in first row) but is still lacking the definitive proof: the presence of a B-mode polarization signal from primordial gravitational waves, an unavoidable prediction of inflation. This signal is the primary target of the LiteBIRD mission and its detection will provide the energetic scale at which primordial perturbations were created. The measurement of the primordial B-mode angular power spectrum, supported also by non-Gaussianity measures also provided by LiteBIRD, will disclose the dynamics of the early Universe and select which model of inflation among the several proposed occurred. On the other hand, it may even point towards inflation alternatives like bouncing models or Ekpyrotic Universes, carrying in all the cases profound consequences for the fundamental physics we know. In fact, the early Universe, and as a consequence its observables like the CMB, represents the best high energy laboratory that we have to test and investigate fundamental and particle physics allowing to test theories at energies not ever reachable by any Earth bound experiment.
LiteBIRD might also provide information on the relic neutrinos, that weak processes occurring in the early Universe brought in thermal equilibrium with the rest of the cosmological plasma. Flavour oscillation experiments have shown that neutrinos have a mass. However, oscillation experiments only measure mass differences between the three neutrino mass eigenstates. Thus, we do not know how massive they are – we only know that they are much lighter than the other known fermions in the Standard Model (SM) of particle physics, like the electron. In fact, laboratory experiments looking at the beta decay of 3H have shown that the electron neutrino should be lighter than ~2 eV, while future experiments using the same technique could improve this limit by one order of magnitude. Moreover, we do not know which of the two possibilities for the neutrino mass ordering, i.e the so-called normal (the two lighter neutrinos are closer in mass) or inverted (the two heavier neutrinos are closer in mass) ordering, is actually realized in nature. The smallness of neutrino masses is a puzzling fact by itself, and might point to the fact that neutrinos do not acquire mass exclusively through their coupling to the Higgs boson, but that instead the mass generation mechanism is related to some high-energy scale, far higher than the electroweak scale probed in accelerators on Earth.
Cosmological observations are a powerful probe of neutrino masses, thanks to the peculiar effect that such light particles have on structure formation, slightly hindering the clustering of matter at small scales due to their large thermal velocities. In fact, cosmology currently provides the strongest constraints on neutrino masses: the 2018 Planck data, together with measurements of baryon acoustic oscillations (BAO), constrain the sum of neutrino masses m to be below 0.12 eV (95% CL). This value is very close to 0.1 eV, the value that could allow to discriminate between the two possibilities for neutrino mass ordering. An accurate measurement of the small-scale CMB lensing, that probes the distribution of matter between us and the last scattering surface, together with baryon acoustic oscillation data or galaxy lensing/clustering data would allow to reach a higher sensitivity on neutrino masses, with an uncertainty ( m) = 0.02 eV or better. Given that the results of oscillation experiments imply m> 0.06 eV, future-generation CMB experiments might be able to finally provide a statistically significant (>3) measurement of neutrino masses, as well as evidence for the normal mass ordering if the sum of the masses is close to 0.06 eV.
LiteBIRD will not measure the small-scale pattern of CMB anisotropies necessary to probe the CMB lensing effect, that will instead be probed by ground-based experiments like the future CMB-S4 experiment. However, a precise (cosmic-variance limited) measurement of the optical depth to reionization like that provided by LiteBIRD is of paramount importance to reach the sensitivity to m quoted above. In fact, the suppression of anisotropy power caused by reionization can somehow “confuse” the signature of massive neutrinos. Observing the large-scale polarization anisotropies, unaccessible from the ground, as LiteBIRD is specifically designed to do, would provide a measurement of the reionization peak and an independent, cosmic-variance limited (() = 0.002) estimate of , minimizing the “confusion” effect described above.
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