What is the reason for density perturbations in the CMB?

[timmdeeg]

I could not find an answer by checking the web. Could these perturbations originate from primordial black holes? What else?

 

[PeterDonis]

I could not find an answer by checking the web.

Try this series of Insights articles:

https://www.physicsforums.com/insights/poor-mans-cmb-primer-part-0-orientation/

Articles 3, 4, and 5 talk about the perturbations.