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The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields and dust properties, while posing a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process must be studied thoroughly. The magnetically enhanced radiative torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Here, we aim to test the MRAT mechanism in starless cores using the multi-wavelength polarisation from optical to submillimetre. Our numerical modelling of dust polarisation using the MRAT theory demonstrates that the alignment efficiency of starlight polarisation (p_ext/A_V) and the degree of thermal dust polarisation (p_em) first decrease slowly with increasing visual extinction (A_V) and then fall steeply as ∝A_v^-1at largeA_Vdue to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope (A_V^loss) increases with maximum grain size. By applying physical profiles suitable for a starless core, 109 in the Pipe nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation in the R band (0.65 μm) and the H band (1.65 μm), as well as emission polarisation in the submillimetre (870 μm). Successful modelling of observational data requires perfect alignment of large grains, which serves as evidence for the MRAT mechanism, and an increased maximum grain size with higher elongation at higherA_V. The latter reveals the first evidence for a new model of anisotropic grain growth induced by magnetic grain alignment. This paper introduces the framework for probing the fundamental physics of grain alignment and dust evolution using multi-wavelength dust polarisation (GRADE-POL), and it is the first of our GRADE-POL series. 

ABSTRACT The interstellar medium is threaded by a hierarchy of filaments from large scales (∼100 pc) to small scales (∼0.1 pc). The masses and lengths of these nested structures may reveal important constraints for cloud formation and evolution, but it is difficult to investigate from an evolutionary perspective using single observations. In this work, we extract simulated molecular clouds from the ‘Cloud Factory’ galactic-scale ISM suite in combination with 3D Monte Carlo radiative transfer code polaris to investigate how filamentary structure evolves over time. We produce synthetic dust continuum observations in three regions with a series of snapshots and use the filfinder algorithm to identify filaments in the dust derived column density maps. When the synthetic filaments mass and length are plotted on an mass–length (M–L) plot, we see a scaling relation of L ∝ M0.45 similar to that seen in observations, and find that the filaments are thermally supercritical. Projection effects systematically affect the masses and lengths measured for the filaments, and are particularly severe in crowded regions. In the filament M–L diagram we identify three main evolutionary mechanisms: accretion, segmentation, and dispersal. In particular we find that the filaments typically evolve from smaller to larger masses in the observational M–L plane, indicating the dominant role of accretion in filament evolution. Moreover, we find a potential correlation between line mass and filament growth rate. Once filaments are actively star forming they then segment into smaller sections, or are dispersed by internal or external forces.