It seems that the nerves connected to the tiny muscles responsible for goosebumps also connect to hair follicle stem cells, which are the cells responsible for hair growth. So, in response to cold, the nerve tells the tiny muscles in the skin to contract (causing goosebumps) and the same nerve activates hair follicle stem cells for new hair growth.

Wondering about goosebumps? Of course you are – Harvard Health

https://www.health.harvard.edu/blog/wondering-about-goosebumps-of-course-you-are-2020080320688

What are goosebumps? Why do we get them? Do they serve a purpose? Some of these questions can be answered, others can’t. But a recent study in mice links goosebumps to stem cells responsible …