The latest observations of the Cosmic Microwave Background (CMB) anisotropies, along with Baryon Acoustic Oscillations (BAO) and weak lensing surveys, provide powerful insights into the properties of dark matter and its role in the early universe. These observations allow for the precise measurement of the universe’s expansion rate, structure formation, and the evolution of matter and radiation, placing significant constraints on the interactions, thermal relic density, and nature of dark matter. The potential existence of exotic dark matter candidates such as dark photons, ultra-light scalar fields, and primordial black holes introduces alternative models that could challenge or expand our understanding of dark matter. Here’s how these observations help refine our understanding of dark matter’s properties and its connection to cosmic inflation and the formation of the first structures:

1. CMB Anisotropies and Dark Matter

• The CMB provides a snapshot of the universe at approximately 380,000 years after the Big Bang, offering critical information about the distribution of matter, radiation, and the underlying physics governing cosmic expansion. The anisotropies (tiny temperature fluctuations) in the CMB arise from the interactions between photons and baryons before recombination.
• Dark matter influences the formation of these anisotropies through its gravitational effects. Its density and clustering properties impact the sound waves in the early universe’s plasma (known as baryon acoustic oscillations, or BAO), which leave an imprint on the CMB power spectrum.
• These imprints can be used to constrain the abundance and density fluctuations of dark matter, with CMB data providing strong limits on the cold dark matter (CDM) model. Anomalies in the CMB—such as deviations from the expected lensing of the CMB or small-scale power—could indicate the presence of exotic dark matter candidates.

2. Baryon Acoustic Oscillations (BAO) and Structure Formation

• BAO refer to periodic fluctuations in the density of visible matter (baryons) caused by sound waves traveling through the primordial plasma before recombination. These oscillations serve as a “standard ruler” that helps measure the expansion rate of the universe.
• The pattern of BAO, when combined with CMB data, provides a direct measurement of the matter density parameter (Ω_m) and the dark matter density (Ω_dm). Anomalies in the BAO measurement, especially at small scales, could suggest interactions or properties of dark matter that differ from those predicted by standard CDM.
• For exotic candidates like dark photons or ultra-light scalar fields, the sound waves in the early universe would behave differently due to the additional interactions or light mass of these particles. This could modify the sound speed in the early universe and alter the observed BAO patterns, constraining the viability of these candidates.

3. Weak Lensing Surveys and Structure Growth

• Weak gravitational lensing occurs when the gravitational field of large-scale structures (such as galaxy clusters) distorts the path of background light, allowing us to map the distribution of matter in the universe (including dark matter).
• The weak lensing surveys allow for precise measurements of galaxy shapes and the distribution of matter on cosmological scales. These surveys help determine how dark matter interacts with regular matter and how it clusters in large structures.
• Deviations in the lensing measurements can highlight differences in the clustering properties of dark matter or indicate the presence of additional forms of dark matter like dark photons, ultra-light scalar fields, or primordial black holes.
  • Dark photons could interact with standard matter via a new electromagnetic force, potentially altering the clustering of dark matter and its contribution to structure growth.
  • Ultra-light scalar fields could lead to fuzzy dark matter scenarios, where the dark matter behaves more like a fluid, suppressing small-scale structure formation and altering the growth of cosmic structures.
  • Primordial black holes (PBHs) could contribute to dark matter in a compact, non-interacting form and affect the growth of structure differently than CDM, leading to unique signatures in weak lensing maps.

4. Exotic Dark Matter Candidates

• Dark Photons:
  • Dark photons are hypothesized to be the gauge bosons of a new force that interacts with both dark matter and standard model particles. The kinetic mixing between dark photons and regular photons could potentially leave distinct signatures in CMB and BAO data, especially in the early universe. Such interactions could lead to deviations in the sound waves and matter distribution compared to CDM, offering clues about the presence of dark photons.
• Ultra-light Scalar Fields (Axions):
  • Ultra-light scalar fields, such as axions, are another potential dark matter candidate. These fields would have very small masses, which means they would not cluster as tightly as CDM. In the early universe, this could lead to fuzzy dark matter that behaves as a coherent wave rather than individual particles. This would suppress small-scale structure formation and alter the distribution of matter, as observed in both the CMB and BAO.
  • CMB anisotropies could be sensitive to the effects of these ultra-light scalar fields on the early universe’s thermal history. The lack of small-scale power seen in current surveys could be interpreted as a sign of such a component of dark matter.
• Primordial Black Holes (PBHs):
  • Primordial black holes could also be a component of dark matter. These black holes, formed in the early universe, would not interact via conventional forces and could act as dark matter candidates that do not participate in the normal formation of structures. If PBHs are abundant, they could leave distinctive signatures in weak lensing surveys, which map the matter distribution.
  • PBHs might also provide exotic features in the early universe dynamics, potentially influencing inflation and the formation of early structures in unique ways.

5. Dark Matter and Cosmic Inflation

• Cosmic inflation refers to the period of exponential expansion in the very early universe, driven by a hypothetical scalar field. The properties of dark matter could be connected to inflationary dynamics in the sense that certain types of dark matter candidates—especially light dark matter such as axions—could be produced during inflation.
• Inflationary models predict that the early universe was in a highly energetic state, and the interactions between dark matter particles and the inflaton (the field responsible for inflation) could leave imprints on the cosmic structure. For example, the energy density of dark matter at the end of inflation would set the stage for the formation of galaxies, clusters, and larger-scale structures.
• If dark matter is composed of exotic candidates like dark photons or ultra-light scalar fields, their properties could alter the inflationary dynamics, impacting both reheating and the formation of the cosmic structure.

The latest CMB anisotropies, BAO measurements, and weak lensing surveys provide critical constraints on the properties and interactions of dark matter. These observations help refine our understanding of how dark matter behaves in the early universe and its role in structure formation. Exotic dark matter candidates like dark photons, ultra-light scalar fields, and primordial black holes could offer alternative explanations for the small-scale anomalies observed in the cosmic structure. The interplay between dark matter and cosmic inflation provides an exciting avenue for future research, as the exact nature of dark matter continues to evolve beyond the standard CDM model.

The question of whether axions can account for dark matter is a complex issue that intersects with several fields of study, including cosmology, particle physics, and astrophysics. Constraints on dark matter, particularly axions, come from various sources, including the cosmic microwave background (CMB) power spectrum, large-scale galaxy surveys, and direct detection experiments like XENON1T, as well as astrophysical observations. Let’s break down the evidence and challenges related to axions as a potential dark matter candidate.

Axions as a Dark Matter Candidate

• Axions are hypothetical particles predicted by the Peccei-Quinn theory to solve the strong CP problem in quantum chromodynamics (QCD). These particles are ultra-light, and if they have the right properties, they could contribute to dark matter. Their extremely low mass and weak interactions with other particles make them an intriguing candidate for cold dark matter (CDM).

CMB Power Spectrum Constraints

• The CMB provides crucial insights into the early universe, particularly the fluctuations in the density of matter and radiation, which can be used to infer properties of dark matter. Key features of the CMB, like the angular power spectrum, depend on the density of different components of the universe, including dark matter.
• Axions (if they exist) can significantly affect the CMB power spectrum. Specifically:
  1. Axions as Cold Dark Matter (CDM): If axions make up dark matter, they would impact the early universe’s expansion rate and the growth of cosmic structures. Their presence would modify the sound horizon (the size of the largest sound waves in the early universe), which in turn would affect the CMB peaks.
  2. Axion Dark Matter Density: CMB data, particularly from Planck and WMAP missions, have been used to place upper limits on the density of axion-like particles (ALPs) in the universe. Constraints on dark matter from CMB observations suggest that axions could contribute to dark matter, but their mass must be extremely small (on the order of $10^{-22}eV10^\{-22\}\; \backslash text\{eV\}$10−22eV) for consistency with the observed CMB power spectrum.

Large-Scale Galaxy Surveys

• Surveys of large-scale cosmic structures, such as the Baryon Acoustic Oscillation (BAO) measurements and the Lyman-alpha forest in quasar spectra, provide further constraints on the properties of dark matter.
  • Axions’ Influence on Structure Formation: The presence of axions as dark matter would have different effects on structure formation compared to other dark matter models. Specifically, axions (due to their small mass) would suppress structure formation at smaller scales compared to cold dark matter. This would leave a distinct signature in the distribution of galaxies, halos, and the clustering of large-scale structures.
  • Large-scale surveys, including data from SDSS and DES, have found no significant deviation from the predictions made by the standard CDM model. The lack of evidence for extra suppression of small-scale structure supports the idea that axions must have a very small mass to avoid disrupting the observed cosmic structures.

Direct Detection Experiments (XENON1T)

• Direct detection experiments, such as XENON1T, search for interactions between dark matter particles and the standard model of particles. These experiments are sensitive to weakly interacting massive particles (WIMPs), but also test other candidates, including axions.
  • Axion Detection via Axion-Photon Coupling: Axions can interact with photons through an axion-photon coupling, a feature that allows axions to potentially be detected through photon conversion in strong magnetic fields.
  • XENON1T Results: In 2020, XENON1T set stringent limits on interactions between dark matter and nucleons, primarily aimed at WIMPs. However, its sensitivity to axions is less direct, though it has placed upper bounds on the possible axion-photon coupling, which limits the detectability of axions via direct detection experiments.
  • The mass of the axion affects how it could be detected. Ultra-light axions might not interact sufficiently in direct detection experiments like XENON1T, and the limits on axion-photon coupling are critical in determining whether axions are detectable in this manner.

Astrophysical Observations

• Axion-Photon Coupling: Astrophysical observations, such as the behavior of light passing through magnetic fields in galaxies or the supernova 1987A, can provide constraints on the axion-photon coupling constant. If axions are too efficient at converting into photons, they could have observable effects on stellar evolution or the cosmic microwave background.
  • Supernova 1987A: This supernova provided strong constraints on the axion’s interaction with photons. If axions were abundant and could efficiently convert into photons, they would carry away energy from the supernova, altering the light curve. The non-observation of such effects puts upper bounds on the axion-photon coupling.
  • Cosmic Magnetic Fields: Axion-photon interactions could also produce observable effects in galactic and intergalactic magnetic fields, but current astrophysical data have not shown any such evidence, further tightening the constraints on axion properties.

Challenges in Reconciling Findings

1. Mass Range and Detection: The mass of axions that would fit cosmological constraints from the CMB and large-scale surveys is extremely small (around $10^{-22}eV10^\{-22\}\; \backslash text\{eV\}$10−22eV). However, this small mass makes them very difficult to detect in direct detection experiments like XENON1T, which are designed for much heavier dark matter candidates like WIMPs.
2. Axion-Photon Coupling: The limits on the axion-photon coupling derived from astrophysical observations and direct detection experiments often conflict with the range needed for axions to be a significant dark matter component. If the axion-photon coupling is too strong, it would contradict astrophysical constraints, while if it’s too weak, axions may not be detectable by existing experiments.
3. Small-Scale Structure Suppression: While axions’ impact on large-scale structure formation is consistent with observations, their ability to suppress structure formation at smaller scales (such as in dwarf galaxies) has yet to be conclusively validated. This could be a challenge if axions are too light, as they might leave fewer structures or fail to form halos in ways that align with observations.

The constraints from the CMB, large-scale galaxy surveys, direct detection experiments, and astrophysical observations suggest that axions could contribute to dark matter, but their ultra-light mass poses challenges for direct detection and for reconciling all these findings. While their small mass allows them to fit with cosmological data and structure formation at large scales, their axion-photon coupling must be very weak to avoid conflicts with astrophysical limits. As a result, axions remain a viable but challenging candidate for dark matter, and more precise experiments and observations will be needed to further refine their properties and determine their role in the dark matter puzzle.