Phase mixing and the Vlasov equation in cosmology
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arXiv
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| Natura: | Preprint |
| Pubblicazione: |
2025
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| _version_ | 1866917124019060736 |
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| author | Taylor, Martin Ruiz, Renato Velozo |
| author_facet | Taylor, Martin Ruiz, Renato Velozo |
| contents | We consider the Vlasov equation on slowly expanding isotropic homogeneous tori, described by the Friedmann--Lemaître--Robertson--Walker cosmological spacetimes. For expansion rate $t^q$, with $0< q<\frac{1}{2}$ (excluding certain exceptional values), we show that the spatial density decays at the rate $t^{-6q}$ and that, when the spatial average is removed, the density decays at an enhanced rate due to a phase mixing effect. This enhancement is polynomial for Sobolev initial data and super-polynomial, but sub-exponential, for real analytic initial data. We further show that, when the expansion rate is the borderline $t^{\frac{1}{2}}$ -- the rate which describes a radiation filled universe -- a degenerate phase mixing effect results in a logarithmic enhancement for Sobolev initial data and a super-logarithmic enhancement (in fact, a gain of $\exp(-μ(\log t)^ε)$ for some $μ,ε>0$) for analytic initial data. The proof is based on a collection of commuting vector fields, and certain combinatorial properties of an associated collection of differential operators. The vector fields are not explicit, but are shown to have good properties when $t$ is large with respect to the momentum support of the solution. A physical space dyadic localisation is employed to treat non-compactly supported (in particular, non-trivial real analytic) but suitably decaying solutions. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2512_04214 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Phase mixing and the Vlasov equation in cosmology Taylor, Martin Ruiz, Renato Velozo General Relativity and Quantum Cosmology Mathematical Physics Analysis of PDEs We consider the Vlasov equation on slowly expanding isotropic homogeneous tori, described by the Friedmann--Lemaître--Robertson--Walker cosmological spacetimes. For expansion rate $t^q$, with $0< q<\frac{1}{2}$ (excluding certain exceptional values), we show that the spatial density decays at the rate $t^{-6q}$ and that, when the spatial average is removed, the density decays at an enhanced rate due to a phase mixing effect. This enhancement is polynomial for Sobolev initial data and super-polynomial, but sub-exponential, for real analytic initial data. We further show that, when the expansion rate is the borderline $t^{\frac{1}{2}}$ -- the rate which describes a radiation filled universe -- a degenerate phase mixing effect results in a logarithmic enhancement for Sobolev initial data and a super-logarithmic enhancement (in fact, a gain of $\exp(-μ(\log t)^ε)$ for some $μ,ε>0$) for analytic initial data. The proof is based on a collection of commuting vector fields, and certain combinatorial properties of an associated collection of differential operators. The vector fields are not explicit, but are shown to have good properties when $t$ is large with respect to the momentum support of the solution. A physical space dyadic localisation is employed to treat non-compactly supported (in particular, non-trivial real analytic) but suitably decaying solutions. |
| title | Phase mixing and the Vlasov equation in cosmology |
| topic | General Relativity and Quantum Cosmology Mathematical Physics Analysis of PDEs |
| url | https://arxiv.org/abs/2512.04214 |