We investigate the effects of an early cosmological period, dominated by

primordial 2-2-holes, on axion dark matter. 2-2-holes emerge in quadratic

gravity, a candidate theory of quantum gravity, as a new family of classical

solutions for ultracompact matter distributions. These objects have the black

hole exterior without an event horizon and hence, as a probable endpoint of

gravitational collapse, they do not suffer from the information loss problem.

Thermal 2-2-holes exhibit Hawking-like classical radiation and satisfy the

entropy-area law. Moreover, these objects, unlike BHs, have a minimum allowed

mass and hence naturally give rise to stable remnants. In this paper, we

consider the remnant contribution to dark matter (DM) small and adopt the axion

DM scenario by the misalignment mechanism. We show that a 2-2-hole domination

phase in the evolution of the universe changes the axion mass window from the

dark matter abundance constraints. The biggest effect occurs when the remnants

have the Planck mass, which is the case for a strongly coupled quantum gravity.

The change in abundance constraints for the Planck mass 2-2-hole remnants

amounts to that of the Primordial Black Hole (PBH) counterpart. Therefore;

since we use the revised constraints on the initial fraction of 2-2-holes from

GWs, the results here can also be considered as the updated version of the PBH

case. As a result, the lower limit on the axion mass is found as $m_a sim

10^{-9}$ eV. Furthermore, the domination scenario itself constrains the remnant

mass $M_{mathrm{min}}$ considerably. Given that we focus on the pre-BBN

domination scenario in order not to interfere with BBN (Big Bang

Nucleosynthesis) constraints, the remnant mass window becomes $m_{mathrm{Pl}}

lesssim M_{mathrm{min}} lesssim 0.1;mathrm{g}$.

The article examines the effects of an early cosmological period, dominated by primordial 2-2-holes, on axion dark matter. These 2-2-holes are classical solutions in quadratic gravity and have a black hole exterior without an event horizon. They exhibit classical radiation and satisfy the entropy-area law. Unlike black holes, they have a minimum allowed mass, leading to stable remnants.

The paper focuses on the remnant contribution to dark matter and adopts the axion dark matter scenario by the misalignment mechanism. It shows that a domination phase of 2-2-holes in the evolution of the universe changes the axion mass window. The biggest effect occurs when the remnants have the Planck mass, which is the case for strongly coupled quantum gravity.

The article states that the revised constraints on the initial fraction of 2-2-holes from gravitational waves can be considered as the updated version of the primordial black hole case. As a result, it finds a lower limit on the axion mass of approximately ^{-9}$ eV.

The domination scenario itself constrains the remnant mass considerably, with a window of approximately $m_{mathrm{Pl}} lesssim M_{mathrm{min}} lesssim 0.1;mathrm{g}$. The focus is on the pre-BBN (Big Bang Nucleosynthesis) domination scenario in order to avoid interference with BBN constraints.

## Future Roadmap

### Potential Challenges

- Validation of the existence and properties of 2-2-holes in quadratic gravity through further research and experimentation.
- Confirmation of the dominance of 2-2-holes during a specific cosmological period through observational evidence.
- Refinement of gravitational wave constraints on the initial fraction of 2-2-holes.
- Verification of the revised constraints on the axion mass and remnant mass.
- Investigation of the implications of the domination scenario on other cosmological phenomena.

### Potential Opportunities

- Further exploration of the role of 2-2-holes in the early universe and their impact on dark matter.
- Development of new experimental methods and technologies to probe the properties of 2-2-holes.
- Improvement of observational techniques to detect signatures or effects of 2-2-hole domination in the cosmic microwave background radiation or other cosmological observations.
- Potential discovery or confirmation of the existence of axion dark matter and its mass range.
- Opportunity to refine our understanding of quantum gravity and its implications for cosmology.

## Conclusion

The article highlights the effects of primordial 2-2-holes on axion dark matter and presents a new perspective on the axion mass window. It suggests that 2-2-hole remnants can have a significant contribution to dark matter and sets a lower limit on the axion mass based on the domination scenario. The findings encourage further research and open up new opportunities for studying the properties of 2-2-holes and their implications for cosmology and quantum gravity.