When applying for a PhD, in the UK, it's almost certain you will need to write a research proposal. This proposal should be something you discuss with a potential PhD supervisor and should be something clearly aligned with their existing interests.
The proposal should:
- Situate itself within the current literature, citing several papers (at least 5) that are most relevant, as well as other papers in the general field. This should ideally include at least one paper from your proposed supervisor as it will then show why working with that supervisor makes sense for you and them;
- Demonstrate that you have read and understood (some of) these papers you are citing, and discuss their implications for your proposed work;
- Propose a number of research questions and potential directions for tackling them in your PhD. The more specific and concrete the better, but sometimes at this stage it might only be possible to point to a general direction of inquiry. Some high-level grand goals can be fine as long as there