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We perform the first self-consistent, time-dependent, multi-groupcalculations in two dimensions (2D) to address the consequences of using theray-by-ray+ transport simplification in core-collapse supernova simulations.Such a dimensional reduction is employed by many researchers to facilitatetheir resource-intensive calculations. Our new code (F{\sc{ornax}}) implementsmulti-D transport, and can, by zeroing out transverse flux terms, emulate theray-by-ray+ scheme. Using the same microphysics, initial models, resolution,and code, we compare the results of simulating 12-, 15-, 20-, and25-M$_{\odot}$ progenitor models using these two transport methods. Ourfindings call into question the wisdom of the pervasive use of the ray-by-ray+approach. Employing it leads to maximum post-bounce/pre-explosion shock radiithat are almost universally larger by tens of kilometers than those derivedusing the more accurate scheme, typically leaving the post-bounce matter lessbound and artificially more "explodable." In fact, for our 25-M$_{\odot}$progenitor, the ray-by-ray+ model explodes, while the corresponding multi-Dtransport model does not. Therefore, in two dimensions the combination ofray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2Dsupernova dynamics can result in quantitatively, and perhaps qualitatively,incorrect results.

Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?