**Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence** This paper investigates the decoherence of quantum fields in both flat and de Sitter space, focusing on how unobserved fields interact with observed fields during inflation. The study computes the rate at which unobserved fields cause decoherence in observed fields, particularly in de Sitter space, where the process is highly efficient once modes pass the Hubble scale. This leads to secular growth, indicating the breakdown of perturbative methods on a time scale much longer than the Hubble time. The paper shows how to match perturbative evolution at early times with a late-time Lindblad evolution, allowing reliable resummation of perturbative results beyond the perturbative regime. It demonstrates that super-Hubble modes are predominantly decohered by other super-Hubble modes, closing a potential loophole in recent calculations of the late-time purity of primordial fluctuations. The study considers two types of interactions: mixing and cubic. For mixing, the decoherence rate is calculated using the Bunch-Davies vacuum, showing that the purity of the system decreases over time. For cubic interactions, the results are similar but involve more complex calculations with environmental correlators. In de Sitter space, the analysis reveals that the decoherence rate depends on the mode's co-moving momentum and the background geometry. The results show that the purity of the system evolves in a way that is consistent with the breakdown of perturbative methods at late times, but the resummed approach allows for reliable predictions. The paper also discusses the implications of these results for the quantum nature of primordial fluctuations, emphasizing the importance of understanding decoherence in the context of inflationary cosmology. The findings highlight the need for resummed methods to accurately describe the late-time behavior of quantum fields in de Sitter space.**Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence** This paper investigates the decoherence of quantum fields in both flat and de Sitter space, focusing on how unobserved fields interact with observed fields during inflation. The study computes the rate at which unobserved fields cause decoherence in observed fields, particularly in de Sitter space, where the process is highly efficient once modes pass the Hubble scale. This leads to secular growth, indicating the breakdown of perturbative methods on a time scale much longer than the Hubble time. The paper shows how to match perturbative evolution at early times with a late-time Lindblad evolution, allowing reliable resummation of perturbative results beyond the perturbative regime. It demonstrates that super-Hubble modes are predominantly decohered by other super-Hubble modes, closing a potential loophole in recent calculations of the late-time purity of primordial fluctuations. The study considers two types of interactions: mixing and cubic. For mixing, the decoherence rate is calculated using the Bunch-Davies vacuum, showing that the purity of the system decreases over time. For cubic interactions, the results are similar but involve more complex calculations with environmental correlators. In de Sitter space, the analysis reveals that the decoherence rate depends on the mode's co-moving momentum and the background geometry. The results show that the purity of the system evolves in a way that is consistent with the breakdown of perturbative methods at late times, but the resummed approach allows for reliable predictions. The paper also discusses the implications of these results for the quantum nature of primordial fluctuations, emphasizing the importance of understanding decoherence in the context of inflationary cosmology. The findings highlight the need for resummed methods to accurately describe the late-time behavior of quantum fields in de Sitter space.