  In this work we present a "modest" holographic reconstruction of the bulk
geometry in asymptotically flat spacetime using the two-point correlators of
boundary quantum field theory (QFT) in asymptotically flat spacetime. The
boundary QFT lives on the null boundary of the spacetime, namely null infinity
and/or the Killing horizons. The bulk reconstruction relies on two unrelated
results: (i) there is a bulk-to-boundary type correspondence between free
quantum fields living in the bulk manifold and free quantum fields living on
its null boundary, and (ii) one can construct the metric by making use of the
Hadamard expansion of the field living in the bulk. This holographic
reconstruction is "modest" in that the fields used are non-interacting and not
strong-weak holographic duality in the sense of AdS/CFT, but it works for
generic asymptotically flat spacetime subject to some reasonably mild
conditions.
