David Wands and collaborators gave a conservation-based account of long-wavelength fluctuations in linear cosmological perturbation theory. Their 2000 work treated slowly varying regions as locally homogeneous expanding universes and identified the conditions for conserved uniform-density curvature perturbations, without selecting particular gravitational field equations.[ref]
A new approach to the evolution of cosmological perturbations on large scales by David Wands and others. Abstract and separate-universe interpretation
The important qualifications are negligible spatial gradients and negligible non-adiabatic pressure perturbations; large wavelength alone is insufficient.
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David Wands is a theoretical physicist known for his work in cosmology, particularly in relation to the early universe and the nature of dark energy. He has contributed to the understanding of the dynamics of the universe and has published research on various aspects of cosmology, including inflationary models and the multiverse hypothesis.