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In the primordial Universe, neutrino decoupling occurs only slightly before electron-positron annihilations, leading to an increased neutrino energy density with order $10^{-2}$ spectral distortions compared to the standard instantaneous…
After a brief review of the cosmological lithium problem, we report a few recent attempts to find theoretical solutions by our group at Texas A&M University (Commerce & College Station). We will discuss our studies on the theoretical…
We construct high-precision models of the Universe that contain radiation, a cosmological constant, and periodically distributed inhomogeneous matter. The density contrasts in these models are allowed to be highly non-linear, and the…
Progress in observational cosmology over the past five years has established that the Universe is dominated dynamically by dark matter and dark energy. Both these new and apparently independent forms of matter-energy have properties that…
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) have a long history together in the standard cosmology. The general concordance between the predicted and observed light element abundances provides a direct probe of…
The origin of the ultra high energy cosmic rays via annihilation of heavy stable, fermions "f", of the cosmological dark matter (DM) is studied. The particles in question are supposed to be created by the scalaron decays in $R^2$ modified…
Our ignorance of the period between the end of inflation and the beginning of Big Bang Nucleosynthesis limits our understanding of the origins and evolution of dark matter. One possibility is that the Universe's energy density was dominated…
We show that Big Bang Nucleosynthesis (BBN) significantly constrains axion-like dark matter. The axion acts like an oscillating QCD $\theta$ angle that redshifts in the early universe, increasing the neutron-proton mass difference at…
We show that density spikes begin to form from dark matter particles around primordial black holes immediately after their formation at the radiation-dominated cosmological stage. This follows from the fact that in the thermal velocity…
We study the big-bang nucleosynthesis (BBN) scenario with late-decaying exotic particles with lifetime longer than $\sim 1$ sec. With a late-decaying particle in the early universe, predictions of the standard BBN scenario can be…
Primordial or Big Bang nucleosynthesis (BBN) is one of the three historical strong evidences for the Big-Bang model together with the expansion of the Universe and the Cosmic Microwave Background radiation (CMB). The recent results by the…
Long-lived radioactive by-products of nucleosynthesis provide an opportunity to trace the flow of ejecta away from its sources for times beyond where ejecta can be seen otherwise. Gamma rays from such radioactive decay in interstellar space…
Multidimensional modification of gravity with a smaller mass scale of the gravitational interaction is considered. Stable by assumption dark matter particles could decay via interactions with virtual black holes. The decay rates of such…
We study cosmological scenarios in which high-energy neutrinos are emitted from the decay of long-lived massive particles at the cosmic time later than a redshift of 10^6. The high-energy neutrino events recently observed by the IceCube…
Big Bang Nucleosynthesis imposes stringent bounds on light sterile neutrinos mixing with the active flavors. Here we discuss how altered dispersion relations can weaken such bounds and allow compatibility of new sterile neutrino degrees of…
We review some recent determinations of the amount of dark matter on galactic, cluster, and large scales, noting some puzzles and their possible resolutions. We discuss the interpretation of big bang nucleosynthesis for dark matter, and…
We present what we know on nucleosynthesis in the Universe and hypotheses that have been made in this regard. A brief description of the Universe's evolution during its different stages is offered, indicating which are the periods and…
Low-scale supersymmetry breaking in string motivated theories implies the presence of O(100) TeV scale moduli, which generically lead to a significant modification of the history of the universe prior to Big Bang Nucleosynthesis. Such an…
We show that the predicted primordial helium production is significantly reduced when new measurements of the neutron lifetime and the implied enhancement in the weak reaction rates are included in big-bang nucleosynthesis. Therefore, even…
A complete model for the origin of high-energy >~10^{14} eV) cosmic rays from gamma-ray bursts (GRBs) and implications of this hypothesis are described. Detection of high-energy neutrinos from GRBs provide an unambiguous test of the model.…