  The cosmic microwave background (CMB) is sensitive to the recent phase of
accelerated cosmic expansion through the late-time integrated Sachs-Wolfe (ISW)
effect, which manifests as secondary temperature fluctuations on large angular
scales. However, the large cosmic variance from primary CMB fluctuations limits
the usefulness of this effect in constraining dark energy or modified gravity.
In this paper, we propose a novel method to separate the ISW signal from the
primary signal using gravitational lensing, based on the fact that the ISW
signal is, to a good approximation, not gravitationally lensed. We forecast how
well we can isolate the ISW signal for different experimental configurations,
and discuss various applications, including modified gravity, large-scale CMB
anomalies, and measurements of local-type primordial non-Gaussianity. Although
not within reach of current experiments, the proposed method is a unique way to
remove the cosmic variance of the primary signal, allowing for better CMB-based
constraints on late-time phenomena than previously thought possible.
