Session S09: Early Universe, Dark Energy, and the CMB

Live

The cosmological $\Lambda CDM$ model provides a remarkable description of numerous astronomical observations related to the expansion of the Universe after the Big Bang. A deficiency of the $\Lambda CDM$ model is that Dark Energy or, equivalently, the cosmological constant $\Lambda $ is merely an adjustable parameter with no explanation for either its magnitude or its physical properties. In this contribution we describe the cosmological consequences of the Born self-energy, associated with the electric field, which surrounds a finite-sized electron. In the current formulation of QED mass renormalization implies that energy is not conserved for a classical electron. A necessary condition, which restores energy conservation, is that the Born self-energy be added to the rest mass energy of the electron. It is found that this Born self-energy, for a finite-sized electron, reproduces many of the features attributed to Dark Energy including its magnitude, the equation of state, as well as, the observed acceleration-deceleration transition at a red-shift of approximately 0.8. Further details about this model can be found in Astrophys. Space Sci. 365:64 (2020).   

Live

While the production of bosons during inflation has received substantial attention, the production of fermions has been comparatively neglected. Although they cannot be produced resonantly, fermions can be produced during axion (pseudoscalar) inflation through a derivative coupling. Despite the lack of resonant enhancement, fermions can have phenomenologically interesting consequences. In particular, the cosmic microwave background (CMB) power spectrum fluctuations can be dominated by the sourced contribution from this fermion production, with the familiar vacuum fluctuations subdominant. Unlike in bosonic production, this scenario satisfies non-Gaussianity constraints, in addition to being consistent with the measured spectral tilt. Therefore, this scenario is consistent with all observational data, and we cannot say with confidence whether the observed CMB fluctuations are sourced or vacuum fluctuations. Implications for primordial gravitational waves will also be discussed.   

Live

The non-canonical scalar model is one of the interesting models in the class of scalar-tensor theories. This model not only explains the nature of dark energy but also provides an answer to the origin of dark matter. In addition, it can be used to describe the formation of structures. We study the scalar perturbations in this model and show that in the framework of the spherical collapse model, the formation of structures can be explained. We show that the formation of structures in this model occurs earlier than predicted in the standard cosmological model. We obtain the parameters of spherical collapse, such as spherical overdensity in the framework of the non-canonical scalar model. We present the results of the analysis of the tensor perturbations, which are assumed to be traceless and transverse. We find the effect of these perturbations on the action up to the second-order and obtain the mass of gravitational waves. We demonstrate that by choosing the proper values for the potentials and the parameters of the model, the calculated value for the mass of gravitational waves is highly consistent with the observations of Laser Interferometer Space Antenna (LISA), Square Kilometre Array (SKA), and Parkes Pulsar Timing Array (PPRA).   

Live

We explore Beyond Standard Model (BSM) physics models of out-of-equilibrium particle decay in the early universe around the time of Big Bang Nucleosynthesis (BBN). In particular, we look into the decay of massive neutral fermions (e.g., ``sterile neutrinos'') into Standard Model particles that will heat the photon-electron-positron-baryon plasma during the BBN epoch and produce non-thermal high-energy neutrinos and antineutrinos of all flavors. By considering the production and scattering of these high-energy neutrinos, we investigate the implications of this process on cosmological observables such as the number of relativistic degrees of freedom, $N_{\rm eff}$, and the sum of the neutrino masses, and further discuss possible effects of changing the time-temperature relation and a non-thermal high-energy distribution of neutrinos and antineutrinos on BBN yields.   

Live

The theory of primordial inflation has been highly successful in resolving theoretical difficulties, as well as having its general predictions confirmed by observations. However, higher precision measurements of the CMB now disfavor many potentials that specify the behavior of the expansion. One scenario of interest is warm inflation, which contrasts itself from standard inflation by having friction (i.e. the inflaton converting its energy into radiation) during the expansion. This scenario provides a mechanism to gradually transition to a radiation dominated universe. I will show how warm inflation, with a linear dissipation, changes which potentials are allowed by CMB data. In the case of warm natural inflation, the symmetry breaking scale can now remain below the Planck energy.   

Live

The quantum kinetic equations (QKEs) self-consistently describe the coherent quantum mechanical evolution and kinetic evolution induced by scattering of particles. We solved the QKEs numerically for neutrinos in the hot and dense early universe where both neutrino flavor oscillation and scattering are important in the evolution of the neutrino states. The results show that the neutrino states reach an approximate “equilibrium” where the coherent and scattering effects balance. In this talk, we will present approximate analytical solutions to the QKEs in this environment to better quantify and understand these numerical results.   

Live

In a recent paper, we presented a new model of cosmology based on the idea of a universe with time-varying curvature dominated by vacuum energy acting as its own source. In this model, the universe began with an exponential Plank era inflation before transitioning to a spacetime described by Einstein’s equations. While no explicit model of the Plank era is as yet known, a number of its properties are established. It is shown that structures came into existence during that inflation that were later responsible for all cosmic structures. A new solution of Einstein’s equations incorporating time-varying curvature is presented which predicts that the scaling was initially power law with a parameter of gamma = 0.5 before transitioning to a present-day exponential expansion. A formula relating the curvature to the vacuum energy density appears as part of the solution. A non-conventional model of nucleosynthesis provides a solution of the matter/antimatter asymmetry problem and a non-standard origin of the CMB. The CMB power spectrum is shown to be a consequence of the same large structures and of uncertainties also embedded in the vacuum during the initial inflation. Using Einstein’s equations, it is also shown that so-called dark matter is, in fact, vacuum energy.   

Live

Precision measurements of the cosmic microwave background (CMB) polarization yield tremendous insights into the Standard Model, $\Lambda$CDM, and the history of our universe. While small-aperture polarimeters have traditionally focused on "B-mode" searches for signatures of primordial gravitational waves, the large angular scale E-mode power spectrum offers probes of interesting physics such as axion-like dark matter candidates, WIMP annihilations, and isocurvature perturbations. In this talk I will introduce the motivation for a ground-based large angular scale E-mode search. I will then outline how we leverage the fast mapping speed of the BICEP/Keck series of small aperture telescopes in a new wide-area scan strategy and present initial performance metrics from demonstration scans. Finally, I will discuss our projected sensitivity to E-mode science targets.   

Live

In recent decades, measurements of the CMB angular power spectra have yielded ever-narrowing constraints on cosmological parameters. As measurements of the CMB signal approach the cosmic variance limit, a new source of information will become necessary to further improve constraints. The Rayleigh scattering of CMB photons off of the neutral hydrogen produced during recombination is a promising option. Rayleigh scattering effectively creates an additional scattering surface after recombination that encodes new cosmological information, including the expansion and ionization history of the universe. Thus a first detection of Rayleigh scattering is a tantalizing target for next-generation CMB experiments. We have developed a Rayleigh scattering forecasting pipeline that includes instrumental effects (e.g., atmospheric noise) and astrophysical foregrounds (e.g., Galactic cirrus, CIB, thermal Sunyaev-Zel'dovich effect). We forecast the Rayleigh scattering detection significance for several upcoming ground-based experiments, including SPT4, Simons Observatory, and CMB-S4, and examine the limitations from different astrophysical foregrounds and potential mitigation techniques.  We further discuss how to optimize the detection significance in future experiments.