ARlogo Annu. Rev. Astron. Astrophys. 2017. 55:343-387
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5. SUMMARY AND OUTLOOK

Small-scale structure sits at the nexus of astrophysics, particle physics, and cosmology. Within the standard ΛCDM model, most properties of small-scale structure can be modeled with high precision in the limit that baryonic physics is unimportant. And yet, the level of agreement between theory and observations remains remarkably hard to assess, in large part because of hard-to-model effects of baryonic physics on first-principles predictions. Given the stakes – absent direct detection of dark matter on Earth, indirect evidence from astrophysics provides the strongest clues to dark matter's nature – it is essential to take potential discrepancies seriously and to explore all avenues for their resolution.

We have discussed three main classes of problems in this review: (1) counts and (2) densities of low-mass objects, and (3) tight scaling relations between the dark and luminous components of galaxies. All of these issues may have their origin in baryonic physics, but they may also point to the need for a phenomenological theory that goes beyond ΛCDM. Understanding which of these two options is correct is pressing for both astrophysics and particle physics.

In our opinion, the search for abundant dark matter halos with inferred virial masses substantially lower than the expected threshold of galaxy formation (Mvir ∼ 108 M) is the most urgent calling in this field today. The existence of these structures is an unambiguous prediction of all WIMP-based dark matter models (though it is not unique to WIMP models), and confirmation of the existence of dark matter halos with M ∼ 106 M or less would strongly constrain particle physics of dark matter and effectively rule out any role of dark matter free-streaming in galaxy formation. Here, too, accurate predictions for the number of expected dark subhalos will require an honest accounting of baryon physics – specifically the destructive effects of central galaxies themselves (e.g., Garrison-Kimmel et al. 2017b). Of nearly equal importance is characterizing the central dark matter density structure of very faint (M ≲ 106 M) galaxies, as a prediction of many recent high-resolution cosmological simulations within the ΛCDM paradigm is that stellar feedback from galaxies below this threshold mass should not modify their host dark matter halos’ cuspy density profile shape. The detection of ubiquitous cores in very low-mass galaxies therefore has the potential to falsify the ΛCDM paradigm.

While some of the tests of the paradigm are clear, their implementation is difficult. Dark matter substructure is extremely diffuse compared to baryonic matter, making its detection highly challenging. The smallest galaxies have very few stars to base accurate dynamical studies upon. Nevertheless, a variety of independent probes of the small-scale structure of dark matter are now feasible, and the LSST era will likely provide a watershed for our understanding of the nature of dark matter and the threshold of galaxy formation. It is not far-fetched to think that improved astrophysical data, theoretical understanding, and numerical simulations will provide a definitive test of ΛCDM within the next decade, even without the direct detection of particle dark matter on Earth.


Disclosure Statement

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.


Acknowledgments

It is a pleasure to thank our collaborators and colleagues for helpful discussions and for making important contributions to our perspectives on this topic. We specifically thank Peter Behroozi, Brandon Bozek, Peter Creasey, Sandy Faber, Alex Fitts, Shea Garrison-Kimmel, Andrea Macciò, Stacy McGaugh, Se-Heon Oh, Manolis Papastergis, Marcel Pawlowski, Victor Robles, Laura Sales, Eduardo Tollet, Mark Vogelsberger, and Hai-Bo Yu for feedback and help in preparing the figures. MBK acknowledges support from The University of Texas at Austin, from NSF grant AST-1517226, and from NASA grants HST-AR-12836, HST-AR-13888, HST-AR-13896, and HST-AR-14282 from the Space Telescope Science Institute (STScI), which is operated by AURA, Inc., under NASA contract NAS5-26555. JSB was supported by NSF grant AST-1518291 and by NASA through HST theory grants (programs AR-13921, AR-13888, and AR-14282) awarded by STScI. This work used computational resources granted by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575 and ACI-1053575. Resources supporting this work were also provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center.

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