An exponential improvement for Ramsey lower bounds

We prove a new lower bound on the Ramsey number $r(\ell, C\ell)$ for any constant $C > 1$ and sufficiently large $\ell$, showing that there exists $\varepsilon=\varepsilon(C)> 0$ such that $$r(\ell, C\ell) \geq \left(p_C^{-1/2} + \varepsilon\right)^\ell,$$ where $p_C \in (0, 1/2)$ is the unique solution to $C = \frac{\log p_C}{\log(1 - p_C)}$. This provides the first exponential improvement over the classical lower bound obtained by Erd\H{o}s in 1947.