Strongly gravitational lensed SNe Ia as multi-messengers: Direct test of
  the Friedmann-Lema\^{\i}tre-Robertson-Walker metric
  We present a new idea of testing the validity of the
Friedman-Lema\^{\i}tre-Robertson-Walker metric, through the multiple
measurements of galactic-scale strong gravitational lensing systems with type
Ia supernovae in the role of sources. Each individual lensing system will
provide a model-independent measurement of the spatial curvature parameter
referring only to geometrical optics independently of the matter content of the
universe. This will create a valuable opportunity to test the FLRW metric
directly. Our results show that with hundreds of strongly lensed SNe Ia
observed by LSST, one would produce robust constraints on the spatial curvature
with accuracy $\Delta \Omega_k=0.04$ comparable to the Planck 2015 results.
