Cosmological signature of primordial black hole (PBH) dark matter and its form in early universe

Abstract

The nature of the dark sector remains one of the central open problems in modern cosmology. While the standard ΛCDM model provides an excellent description of a wide range of observations, recent high-precision data increasingly motivate extensions in which the properties of dark matter and dark energy deviate from their minimal assumptions. In this thesis, we investigate a class of scenarios in which scalar–fermion interactions modify both the early-time behavior of dark matter and the late-time dynamics of dark energy, and we confront these models with current cosmological data using Bayesian inference. We first examine whether the assumption that dark matter was cold and non-relativistic prior to recombination is directly required by existing observations. In a phenomenological mass-varying dark matter (MVDM) framework, we allow the dark matter mass to undergo a rapid transition from zero to a finite value at high redshift. The resulting free-streaming suppression of small-scale power produces characteristic phase shifts and power suppression in the CMB angular power spectra and reduces the late-time clustering amplitude. Using Planck 2018 CMB data supplemented by BAO, supernovae, and weak-lensing priors, we derive a lower bound on the transition redshift and find a mild reduction in S₈ relative to ΛCDM. Including high-multipole measurements from ACT significantly improves the constraints and yields a mild preference for the MVDM model over ΛCDM. We find a lower bound log₁₀(zₜ) > 4.55 and a mild preference for an eV-scale dark matter mass, with M ≈ 20–40 eV depending on the dataset combination. The inclusion of KIDS1000+BOSS+2dfLenS weak-lensing priors further reduces the inferred value of S₈, demonstrating that early-time departures from cold dark matter can partially alleviate the clustering tension. Building on this phenomenological framework, we then construct a concrete microscopic realization in which a Yukawa-type scalar–fermion interaction leads to the late formation of primordial black holes (PBHs). In this scenario, dark matter becomes non-relativistic at z_T ∼ 10⁶, triggering an exponential growth of perturbations in the radiation-dominated era due to a force stronger than gravity. This mechanism produces early dense halos that cool through scalar radiation and collapse into PBHs. We derive a non-monochromatic PBH mass function and show that it is narrow, with a peak in the sublunar window M_BH ∼ 10⁻¹⁶–10⁻¹⁴ M⊙ for z_T ≃ 10⁶. We find that these PBHs can constitute nearly the entire dark matter abundance, f(M) ≃ 1, while remaining consistent with current observational constraints. We further demonstrate the existence of a temporary static configuration in which an attractive scalar force balances Fermi pressure, and show that the resulting PBHs are expected to possess significant spin, providing a potential observational signature for future gravitational-wave experiments. In the second part of the thesis, we explore the implications of similar Yukawa-type interactions for late-time cosmic acceleration in light of recent DESI observations. We first study an interacting quintessence scenario in which the scalar field retains non-zero kinetic energy at the present epoch. The interaction modifies the relation between the microscopic equation of state of the field and the effective equation of state inferred by an observer, allowing the effective equation of state w_eff to cross the phantom divide while maintaining w_φ > −1 at all times. Solving the modified Klein–Gordon equation for exponential and polynomial self-interaction potentials, we show that w_eff crosses −1 around z ∼ 0.5, transitions from w_eff < −1 in the past to w_eff > −1 today, and can reach present-day values as large as w_eff ∼ −0.8. We find that the model remains consistent within the 2σ DESI confidence regions for a wide range of parameters, demonstrating that interacting quintessence with finite kinetic energy provides a viable explanation of the DESI-inferred dark energy evolution without introducing phantom instabilities. Finally, we perform a full Markov Chain Monte Carlo analysis of the chameleon dark energy model in which the scalar field oscillates around the evolving minimum of an effective potential and settles with vanishing velocity at the present epoch. Using combinations of Planck, SDSS BAO, DESI DR2 BAO, Pantheon+, and SH₀ES data, we obtain fully constrained posteriors for the dark-sector coupling without invoking any sign-switching mechanism or additional exotic assumptions. Dataset combinations including DESI DR2 BAO or SH₀ES show a mild preference for a non-zero interaction, with β = 0.256 (+0.092/−0.063) for Planck+DESI DR2 BAO+Pantheon+ and β = 0.335 (+0.082/−0.054) for Planck+SDSS BAO+Pantheon+SH₀ES. For these combinations, we obtain Δχ²_min = −4.75, −6.41 and ΔAIC = −0.75, −2.41, corresponding to weak-to-positive statistical evidence relative to ΛCDM. Although the present-day equation of state asymptotically approaches w_eff → −1, the scalar dynamics exhibit phantom crossing near z ∼ 0.5, in qualitative agreement with the DESI reconstruction. Taken together, the four investigations presented in this thesis establish a unified picture in which scalar–fermion interactions influence both the early thermal history of dark matter and the late-time dynamics of dark energy. The same physical mechanism is shown to generate early departures from cold dark matter, produce a viable primordial black hole population, and induce an effective phantom crossing in the dark energy sector. This unified perspective suggests that early- and late-time cosmological anomalies may share a common microscopic origin within the dark sector, and provides a concrete framework for testing this possibility with forthcoming cosmological surveys.