Macro Dark Matter

Abstract: Dark matter is a vital component of the current best model of our universe, $\Lambda$CDM. There are leading candidates for what the dark matter could be (e.g. weakly-interacting massive particles, or axions), but no compelling observational or experimental evidence exists to support these particular candidates, nor any beyond-the-Standard-Model physics that might produce such candidates. This suggests that other dark matter candidates, including ones that might arise in the Standard Model, should receive increased attention. Here we consider a general class of dark matter candidates with characteristic masses and interaction cross-sections characterized in units of grams and cm$^2$, respectively -- we therefore dub these macroscopic objects as Macros. Such dark matter candidates could potentially be assembled out of Standard Model particles (quarks and leptons) in the early universe. A combination of Earth-based, astrophysical, and cosmological observations constrain a portion of the Macro parameter space. A large region of parameter space remains, most notably for nuclear-dense objects with masses in the range $55 - 10^{17}$ g and $2\times10^{20} - 4\times10^{24}$ g, although the lower mass window is closed for Macros that destabilize ordinary matter.

Overview of "Macro Dark Matter"

The paper titled "Macro Dark Matter," authored by Jacobs, Starkman, and Lynn, offers a comprehensive exploration of a class of dark matter candidates termed "Macros," characterized by macroscopic properties such as mass and interaction cross-sections. This work challenges the conventional focus on candidates like WIMPs and axions, suggesting that macroscopic dark matter, potentially composed of Standard Model particles, should receive more attention due to the lack of definitive observational or experimental support for other candidates.

Characterization of Macros

Macros are defined by their mass scale, approximately ranging from $55$ grams to $10^{34}$ grams, and interaction cross-sections measured in $cm^2$. The paper delineates two primary possibilities for dark matter interaction:
1. Intrinsically weakly interacting due to small interaction cross-sections.
2. Effectively weakly interacting due to being massive, hence having lower number density and reduced cross-section ($$).

Macros, which may exhibit nuclear densities ($3.6 \times 10^{14} g/cm^3$), are argued to be potentially consistent with the absence of beyond-the-Standard-Model physics, thereby possibly comprising Standard Model quarks and leptons.

Constraints on Macro Parameter Space

The research systematically evaluates constraints on the Macro parameter space arising from diverse sources:

1. Astrophysical and Earth-Based Observations: Experiments and observations, including ancient mica studies and Skylab space station data, establish limits on Macro masses and cross-sections, ruling out certain ranges below $10^{17}$ GeV ($\approx 2 \times 10^{-7}$ g).

2. Cosmological and Large-Scale Structure: Constraints derived from the micro- and femtolensing, CMB data, and large-scale structure simulations specify the allowed interaction cross-sections, narrowing permissible regions for Macro existence, particularly for elastic interactions.

3. Self-interaction and Interaction with Baryons: Limits arising from gravitational amplitude impact rates and thermal equilibrium arguments suggest that Macros with significant interaction cross-sections may alter the baryon number density or cosmic structures, further enhancing constraints.

Model-Specific Considerations

The paper introduces several models (I, II, and III) to estimate the electromagnetic properties of Macros and their interaction with baryons. Model II, where Macros absorb nucleons distinctly, allows theoretical predictions concerning primordial nucleosynthesis discrepancies, potentially improving lower mass constraints by several orders of magnitude.

Theoretical and Practical Implications

The findings suggest substantive regions of dark matter parameter space remain unexplored or insufficiently constrained, highlighting potential future directions in experimental and theoretical astrophysics. They also speculate on how non-fluid behavior due to macroscopic masses might impact observed cosmic and local phenomena, opening up areas for investigation in astrophysics and cosmology.

Conclusion

The paper effectively broadens the spectrum of dark matter research, urging the scientific community to consider macroscopic candidates as viable constituents of the cosmic dark matter milieu. While constraints from available data exclude specific windows, notably those below the solar mass scale, the investigation invites novel observational strategies to probe the unconstrained domains, potentially leveraging local solar neighborhood phenomena. Future work should seek to further validate these findings through targeted experiments and advanced simulations.