Early seeds of axion miniclusters

By A. Vaquero, J. Redondo, Julia Stadler. Journal of Cosmology and Astroparticle Physics, 2018

191 citations recorded by the source corpus.

We study the small scale structure of axion dark matter in the post-inflationary scenario, which predicts the formation of low-mass, high density clumps of gravitationally bound axions called axion miniclusters. To this end we follow numerically the cosmological evolution of the axion field and the network of strings and domain walls until the density contrast is frozen. Our simulations, comprising up to 81923 points, are the largest studies of the axion field evolution in the non-linear regime presented so far. Axitons, pseudo-breathers of the Klein-Gordon equation, are observed to form in our simulation at late times. Studying their properties analytically and numerically, we observe that in particular the earliest axitons contribute to density perturbations at the typical length scale of miniclusters. We analyse the small scale structure of the density field, giving the correlation length, power spectrum and the distribution of high density regions that will collapse into axion miniclusters. The final density field of our simulations can be used to calculate the minicluster mass fraction in simulations including gravity. In particular, we find that typical minicluster progenitors are smaller than previously thought and only of moderate, 𝒪(1) overdensity. We expect these miniclusters to have a rich sub-structure, emerging from small-scale fluctuations produced in the collapse of the string-wall network and from axitons.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/8397af092f8f938f03e8e401aaf11433838359f9

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Citations in the corpus

The landscape of QCD axion models (2020, methodology): “Starting from the axion power spectrum, the collapse of non-linear density fluctuations during radiation domination that lead to the formation of axion miniclusters has been recently assessed through a semi-analytical Press-Schechter approach [335–337] or by numerical simulations [201, 202].”
More axions from strings (2020, background): “e. The deformation of the spectrum above could have important implications for the properties of the small scale structures produced by the axion inhomogeneities known as mini-clusters [46] (see also [24,25,47,48] for recent studies). Figure 4 also shows the role of oscillons. These only aect the sp”
Relaxion stars and their detection via atomic physics (2019, background): “These miniclusters have been recently shown to form self-gravitating boson stars on short timescales (compared to galaxy lifetimes), through a process described by gravitational relaxation of effective quasiparticles [41, 42]; this has been investigated assuming both idealized initial conditions [43”