What Is the ‘Nothing-at-Stake’ Problem That PoA Aims to Address?

The 'nothing-at-stake' problem occurs in pure PoS systems where validators have no financial disincentive to vote on multiple competing chain histories. Since there is no cost to create an alternative block, a malicious validator could vote on every fork, potentially hindering consensus.

PoA mitigates this by incorporating PoW, which requires real-world energy expenditure for the initial block. This expenditure acts as a cost, making it non-trivial to fork the chain repeatedly.