On the validity of the Born approximation for weak cosmic flexions

Bjoern Malte Schaefer, Lavinia Heisenberg, Angelos F. Kalovidouris, David J. Bacon

Research output: Contribution to journalArticlepeer-review


Weak lensing calculations are often made under the assumption of the Born approximation, where the ray path is approximated as a straight radial line. In addition, lens–lens couplings where there are several deflections along the light ray are often neglected. We examine the effect of dropping the Born approximation and taking lens–lens couplings into account, for weak lensing effects up to second order (cosmic flexion), by making a perturbative expansion in the light path. We present a diagrammatic representation of the resulting corrections to the lensing effects. The flexion signal, which measures the derivative of the density field, acquires correction terms proportional to the squared gravitational shear; we also find that by dropping the Born approximation, two further degrees of freedom of the lensing distortion can be excited (the twist components), in addition to the four standard flexion components. We derive angular power spectra of the flexion and twist, with and without the Born approximation and lens–lens couplings and confirm that the Born approximation is an excellent approximation for weak cosmic flexions, but may fail in the strong lensing regime.
Original languageEnglish
Pages (from-to)455-467
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date11 Feb 2012
Publication statusPublished - 11 Feb 2012


  • gravitational lensing: weak
  • methods: analytical
  • large-scale structure of Universe.


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