The Cosmic Microwave Background power spectrum exhibits a series of acoustic peaks corresponding to different scales of temperature fluctuation, and the positions of these peaks (specifically the angular scales at which they occur) depend on the geometry of the universe and the sound speed of acoustic oscillations in the primordial plasma. Lambda-CDM predicts specific peak positions based on the assumed flat geometry, matter density, and the standard model of how acoustic waves propagated before recombination, but observations show subtle discrepancies in where peaks actually appear compared to predictions, with the acoustic scale appearing systematically shifted relative to what the model expects based on other cosmological parameters (Addison 2018; Planck Collaboration 2018). Lambda-CDM struggles because the acoustic peak positions constrain fundamental cosmological parameters, and any discrepancy between observed and predicted peak locations suggests either that the assumed flat geometry is incorrect, or that the sound speed in the primordial plasma was different than the model assumes, or that the epoch of recombination or the baryon-to-photon ratio differs from predictions—all of which would indicate tensions in the standard model's internal consistency.