Formation and Evolution of Antimatter Objects

TL;DR

This work investigates whether antimatter domains in a baryon-asymmetric early universe can undergo gravitational collapse and form antimatter stars. By adopting CP-invariant thermodynamics and symmetry with matter, it shows that antimatter gas clouds with initial conditions around $Z\approx20$ and masses near $\sim 5\times10^3\,M_\odot$ can become gravitationally unstable (consistent with $M_J$ and $M_{BE}$) and proceed through a quasi-isothermal collapse driven by $\bar{H}_2$ cooling to an adiabatic protostellar core. The sequence mirrors Population III star formation, but the key unresolved step is whether anti-nuclear fusion can ignite core burning; if anti-fusion is viable, the collapse is predicted to yield antistars with masses $\gtrsim 22\,M_\odot$. Observationally, the existence of such objects would be constrained by high-energy $\gamma$-ray or X-ray signals from matter–antimatter annihilation at domain boundaries or during accretion, providing an avenue to test early-universe phase transitions and fundamental symmetry in nuclear processes.

Abstract

The fundamental question of baryogenesis and the problem of matter-antimatter asymmetry motivate this study into the formation and evolution of antimatter objects in the early Universe. Hypothesize is the existence of isolated antimatter domains in a baryon-asymmetric Universe that survive until the era of first star formation ($Z \approx 20$). By assuming CPT-symmetry, the thermodynamics, mechanics, and energy dynamics of an antimatter gas cloud (composed of antihydrogen and antihelium) are treated symmetrically to their primordial matter counterparts. Analysis demonstrates the physical feasibility of the gravitational collapse process for a conservatively estimated antimatter domain ($\approx 5 \times 10^3 M_{\odot}$). The initial conditions easily satisfy the Jeans and Bonnor-Ebert mass criteria, indicating a high propensity for instability and runaway collapse. The subsequent dynamical evolution, driven by $\bar{H}_2$ cooling, is predicted to proceed identically to that of Population III star formation, leading to the formation of a dense, adiabatic anti-protostellar core. The theoretical viability of a true antistar hinges upon a critical assumption: the physical possibility of antinuclear fusion (e.g., the antiproton cycle) under extreme core conditions. Assuming this symmetry holds, the collapse is predicted to yield massive antistars ($\gtrsim 22 M_{\odot}$). This suggests that if antimatter domains formed in the early Universe, they likely underwent stellar formation. Observational constraints on the existence of these objects must rely on the detection of characteristic high-energy $γ$-ray or X-ray signals resulting from matter-antimatter annihilation at the domain boundaries or during mass accretion.

Figures (4)

• Figure 1: The figure show the variation of density of a cloud with ideal gas (for ideal gas, $n \rightarrow \infty$). The gas in the primordial clouds can be approximately considered as an ideal gas, since a low density gas occupy a large volume. The figure is obtained through plotting the Lane-Emden equation. Here diffenently coloured lines corresponds to different value of the mean molecular mass $\mu$. Blue line with $\mu = 0.5$ models the early universe, orange line with $\mu = 2.3$ models an average cloud in current universe and green line with $\mu = 4$ shows clouds with high metallicity. For a stable gas cloud with same boundary ccondition, it is see that the clouds were significatly bigger in the early universe.
• Figure 2: Variation of the temperature and molecular fraction with the increase in number density as simulated for primordial matter stars (Yoshida et al., 2006).
• Figure 3: The following figure shows the major nuclear reactions that start to occur in the core during the nuclear burning. The set of reactions on the left is the ones that start first. The set of reactions on the right is the one that becomes dominant after an increase in metallicity due to fusion reactions (Kippenhahn & Weigert, 2013).
• Figure 4: Graph showing the region determining whether the core becomes degenerate or not. The graph plots the Equation \ref{['eqn:central temp p']}. There is existence of two regions. One in the bottom right is due to the creation of electron degeneracy and thus represents a region where if the curve of stellar evolution enters, the star forms a brown dwarf (Kippenhahn & Weigert, 2013).