@b_cavello "P vs. NP" is a hard problem. Factoring large numbers / finding large primes is a hard problem. Travelling Salesman is a hard problem. Sustainable energy-positive nuclear fusion is a hard problem. Only one of those might be a big problem as I see it.
Addressing inequality or global warming are big problems. Addressing resource or environmental sink exhaustion are big problems. A former CEO (perhaps of GM or GE) had said of climate change: "We know the answer to the question; we don't like it." The solution to climate change is simple: reduce emissions. The problem in its impacts and coordination challenges is however big.
A big problem is one whose resolution would have a huge impact on future conditions.
I've asked the question a few times at HN and elsewhere:
https://hn.algolia.com/?q=what%20are%20the%20big%20problems
#BigProblems should turn up Fedi instances, probably from some former accounts of mine.
