In baryon-isocurvature baryogenesis scenarios, macroscopic antimatter domains formed in the early Universe may, if they survive annihilation at their boundaries, undergo Population-III-like gravitational collapse at redshift z≈20z\approx20 into long-lived, fusion-powered antistars. The open problem is to establish this end to end: the parameter space of domain sizes, compositions, external irradiation, and matter contamination within which domains survive to the first-star-formation era; whether the surviving antimatter gas collapses and ignites sustained antinuclear fusion (the CPT mirror of ordinary stellar nuclear burning); and the resulting present-day antistar population with its annihilation gamma-ray, X-ray, and cosmic-ray antinuclei signatures, confronted with Fermi-LAT, microlensing, and AMS-02 constraints.

Background

Cosmological setting. Our Universe contains baryons with no comparably large antimatter regions; the origin of this matter-antimatter asymmetry is the baryogenesis problem, and the Standard Model produces too small an asymmetry. One family of beyond-the-Standard-Model scenarios predicts that, in addition to the observed global asymmetry, the very early Universe (t≲10−6t\lesssim10^{-6} s) contained separated regions of matter and antimatter: antimatter domains, pockets of antihydrogen and antihelium surrounded by ordinary matter, with matter-antimatter annihilation (the conversion of a particle-antiparticle pair into high-energy photons and pions) occurring only at their boundaries. Examples are the baryon-isocurvature scenario of Dolgov and Silk (1993), in which the early Universe contains fluctuations in baryon number at nearly fixed total energy density — generated by the Affleck-Dine mechanism, a hypothetical scalar-field condensate carrying baryon number — leaving locally antibaryon-dominated regions, and the inflationary CP-violation scenario of Khlopov, Rubin and Sakharov (2000). If such domains survive, they could seed antistars, stars made of antimatter whose energy source would be fusion of antinuclei, the CPT-mirror of ordinary stellar nuclear burning. Whether macroscopic domains can survive at all is tightly constrained: annihilation and diffusion in the pre-recombination plasma, big-bang-nucleosynthesis (BBN) light-element abundances (Kurki-Suonio and Sihvola 2000; Rehm and Jedamzik 2001), and the diffuse gamma-ray background all bound the allowed domain sizes and masses; Dolgov, Godunov, Rudenko and Tkachev (2015) constructed scenarios in which matter and antimatter domains separate without leaving unstable domain walls.

Star-formation template. The matter analogue of the problem is Population III (first-generation, metal-free) star formation. A primordial gas cloud collapses when its mass exceeds the Jeans mass (the mass above which self-gravity overwhelms pressure support for a gas of given temperature and density) and the Bonnor-Ebert mass (the maximum mass of a stable isothermal gas sphere in hydrostatic equilibrium); cooling by molecular hydrogen (Hˉ2\bar H_2 for its antimatter counterpart) then removes compressional heat and the cloud runs away to a dense protostellar core (Yoshida, Omukai, Hernquist and Abel 2006). Simulations including protostellar feedback find final stellar masses of order tens of solar masses (Stacy, Greif and Bromm 2012). By the CPT theorem, the statement that physical laws are invariant under simultaneous charge conjugation, parity flip, and time reversal, tested for example by ALPHA Collaboration antihydrogen 1S-2S spectroscopy at the parts-in-101210^{12} level (Ahmadi et al. 2018), an antimatter gas cloud should obey identical thermodynamics, mechanics, and reaction kinetics. The source study (Yadav 2026) verified at order-of-magnitude level that a conservatively estimated domain (∼5×103 M⊙\sim5\times10^{3}\,M_{\odot} of antimatter, comparable to the ≈5.6×103 M⊙\approx5.6\times10^{3}\,M_{\odot} Jeans and Bonnor-Ebert masses for primordial gas at z≈20z\approx20) satisfies the collapse criteria, and conjectured that the resulting antistars would mirror Population III stars (≳22 M⊙\gtrsim22\,M_{\odot}), while listing as open issues the fusion-ignition step, the unexplored domain-survival parameter space, and the observational strategy.

Observational status. Antistars have not been detected, but quantitative limits exist. A matter-antimatter interface produces a characteristic annihilation gamma-ray spectrum, and Dupourqué, Tibaldo and von Ballmoos (2021) used the 10-year Fermi-LAT (Large Area Telescope) gamma-ray source catalog to bound the local antistar fraction, finding for example f∗ˉ<2.5×10−6f_{\bar\ast}<2.5\times10^{-6} for a disk-like population. Bondar, Blinnikov, Bykov, Dolgov and Postnov (2022) showed that proton-antiproton atom formation in antistar environments produces narrow X-ray lines, most significantly the 1.73 keV L-line, accessible to XRISM/Athena-class spectroscopy. Bykov, Postnov, Bondar, Blinnikov and Dolgov (2023) connected hypothetical Galactic antistars to the tentatively reported AMS-02 (Alpha Magnetic Spectrometer) antihelium candidate events with explicit flux estimates. Dolgov, Novikov and Vysotsky (2013) proposed photon-polarization and neutrino signatures that would distinguish an antistar from an ordinary star. The unresolved question is whether any domain population that survives to the first-star era can be followed quantitatively from baryogenesis through collapse to a predicted, testable antistar population.

Problem Statement

Within baryon-isocurvature baryogenesis scenarios that produce macroscopic antimatter domains in the early Universe, determine, end to end, whether such domains can produce long-lived antistars and what their observable signatures would be. This comprises three linked deliverables serving one objective. (1) Survival: map the parameter space (domain size and mass, antimatter fraction and composition, external irradiation, boundary matter contamination) within which an antimatter domain embedded in the surrounding baryonic plasma survives boundary annihilation from the baryogenesis epoch down to the era of first-star formation at z≈20z\approx20, consistent with big-bang nucleosynthesis (BBN), cosmic microwave background (CMB), and diffuse gamma-ray limits. (2) Collapse and ignition: determine for which surviving initial conditions the antimatter cloud (antihydrogen and antihelium, cooled by Hˉ2\bar H_2) undergoes Jeans/Bonnor-Ebert gravitational collapse and protostellar contraction to ignite sustained antinuclear fusion, treating nuclear burning as the CPT mirror of Population III stellar evolution and including annihilation feedback from accretion of ambient matter. (3) Population and signatures: derive from (1) and (2) the predicted present-day antistar population and its annihilation gamma-ray, X-ray (for example the 1.73 keV proton-antiproton atom line), and cosmic-ray antinuclei signatures, with detection thresholds or exclusions for current instruments (Fermi-LAT, X-ray spectroscopy surveys, AMS-02, and microlensing, the gravitational amplification of background-star light by compact foreground objects). Deliverables (2) and (3) inherit the domain initial conditions fixed by (1), so a complete answer is a quantified feasibility-and-detectability map of primordial antistars rather than three independent calculations.

References

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