Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos (1110.2484v3)

Abstract: For a Gaussian spectrum of primordial density fluctuations, ultracompact minihalos (UCMHs) of dark matter are expected to be produced in much greater abundance than, e.g., primordial black holes. Forming shortly after matter-radiation equality, these objects would develop very dense and spiky dark matter profiles. In the standard scenario where dark matter consists of thermally-produced, weakly-interacting massive particles, UCMHs could thus appear as highly luminous gamma-ray sources, or leave an imprint in the cosmic microwave background by changing the reionisation history of the Universe. We derive corresponding limits on the cosmic abundance of UCMHs at different epochs, and translate them into constraints on the primordial power spectrum. We find the resulting constraints to be quite severe, especially at length scales much smaller than what can be directly probed by the cosmic microwave background or large-scale structure observations. We use our results to provide an updated compilation of the best available constraints on the power of density fluctuations on all scales, ranging from the present-day horizon to scales more than 20 orders of magnitude smaller.

• The paper refines constraints on the small-scale primordial power spectrum using gamma-ray observations of ultracompact minihalos.
• It employs models linking early-universe density fluctuations to UCMH formation post-recombination, supported by detailed numerical analyses.
• Findings indicate that gamma-ray bounds restrict UCMH abundance, which in turn tightens predictions for dark matter behavior and inflationary models.

Improved Constraints on the Primordial Power Spectrum at Small Scales from Ultracompact Minihalos

In the paper titled "Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos," Bringmann et al. provide a thorough investigation into the implications of density fluctuations in the early universe, particularly focusing on ultracompact minihalos (UCMHs) within the context of dark matter studies. UCMHs, hypothesized as dense, dark matter structures emerging shortly after matter-radiation equality, can significantly refine constraints on the primordial power spectrum—especially at scales too small to be detected by conventional cosmic microwave background (CMB) or large-scale structure observations.

Formation of UCMHs and Theoretical Implications

Bridging the gap between primordial power dynamics and present-day observations of small-scale structures, the authors explore scenarios where the Gaussian spectrum of primordial density fluctuations results in the formation of UCMHs rather than primordial black holes (PBHs). UCMHs are proposed to form due to density perturbations roughly three orders of magnitude smaller than those that could produce PBHs, collapsing post-recombination into miniscule, dense clusters exclusively composed of cold dark matter (CDM).

In the standard cosmological framework, dark matter is defined primarily by weakly-interacting massive particles (WIMPs). The gamma-ray luminosity of such UCMHs due to potential dark matter annihilation offers a novel probe into the cosmic abundance of these structures. The authors derive limits on the abundance of UCMHs and inferrably constrain the power spectrum of primordial density perturbations, emphasizing that such constraints are particularly tight at scales beyond the reach of typical CMB measurements.

Results and Numerical Insights

Bringmann et al. apply these theoretical constructs to observational data, utilizing the non-observation of gamma-ray sources by the Fermi-LAT instrument as a key metric to infer UCMH abundance. They show that for certain mass ranges, UCMHs must constitute less than a minute fraction of dark matter, refining existing small-scale cosmological constraints. The primordial power spectrum constraints deduced from this paper demonstrate that fluctuations at small scales can be significantly more constrained than previously believed—a result with profound implications for understanding structure formation in the early universe.

Numerical constraints highlight that the primordial perturbation spectrum cannot diverge rapidly from a nearly scale-invariant form without leading to excessive UCMH production, thus illuminating a previously underexplored aspect of cosmic evolution. Detailed calculations and fit functions for minimal density contrasts, required for UCMH collapse, further substantiate the underlying theoretical framework and indicate potential directions for future refinement of these observations.

Practical and Theoretical Implications

The findings advance our comprehension of small-scale cosmological parameters, positioning gamma-ray bounds from UCMH studies as a compelling complement to traditional methods such as PBH observations. By integrating UCMH formation scenarios into broader cosmological models, researchers can achieve a more nuanced understanding of density perturbations and their evolution from the early universe to large cosmic structures we observe today.

These constraints, especially impactful for inflationary models predicting additional power at small scales, underline important practical considerations for modeling dark matter behavior and the distribution of potential cosmic structures. This research opens pathways for future observational campaigns targeting the detection or further exclusion of UCMHs and reinforces the necessity for multidisciplinary approaches in cosmology, blending theoretical insights with astrophysical data.

Conclusion

In conclusion, Bringmann et al. effectively elevate the discourse surrounding primordial power spectra by incorporating UCMH dynamics into this analytical domain. Their paper exemplifies the synergy between theoretical inferences and observational constraints, offering a refined perspective on the universe's initial conditions and the resultant formation of complex structures—ultimately sculpting our cosmic landscape. Future exploration and nuanced probing at these scales hold potential for groundbreaking revelations in the field of cosmology and dark matter research.