Forward Rate f(t,T)
  ^
  |       t1 < t2 < t3 (Time progression)
  |      / 
  |     /  f(t1, T)  --->  Initial Forward Curve observed at t1
  |    /   /
  |   /   /   f(t2, T)  --->  Evolved Forward Curve at t2
  |  /   /   /
  | /   /   /   f(t3, T)  ->  Evolved Forward Curve at t3
  |/___/___/________________________> Maturity T
  t1  t2  t3
  
  HJM SDE: df(t,T) = [ σ_f(t,T) ∫_t^T σ_f(t,s) ds ] dt + σ_f(t,T) dW_t
  
  *The drift is entirely deterministic given the volatility structure σ_f(t,T).
  *This ensures the absence of arbitrage across all maturities simultaneously.