Regarding being systematically wrong, the rebuttal paper is essentially arguing that, by ignoring other cosmological measurements, the original study refuting a

Regarding being systematically wrong, the rebuttal paper is essentially arguing that, by ignoring other cosmological measurements, the original study refuting acceleration is potentially falling prey to systematics. The other cosmological measurements that are typically used rely on completely different observing techniques and physical processes. Baryon acoustic oscillations[0] (BAO) rely on there being a predicted characteristic length scale based on when the baryons and dark matter de-couple from each other. By measuring the large-scale distribution of galaxies you can measure this characteristic length. The cosmic microwave background[1] (CMB) is looking at the redshifted thermal emission from the hot plasma which filled the Universe during its youth. The brightness fluctuations correspond to temperature fluctuations in the early Universe. Both the BAO and CMB are sensitive to the energy content of the universe and the relative balance between baryonic matter, dark matter, and any dark energy. Examining the BAO signal as a function of time may provide constraints on the acceleration (if the characteristic length scale grows faster than expected from a non-accelerating universe, I believe). Additionally, the power spectrum of the cosmic microwave background is sensitive to the existence of dark energy[2, e.g., pg 24]. Because these three measurements (BAO, CMB, Type Ia supernovae) are looking at completely different things (characteristic separation between galaxies, temperature distribution of hot plasma when the universe was 380,000 years old, and apparent vs intrinsic brightness for a special class of supernovae) which can be linked back to cosmological model parameters, it is unlikely that the systematics of all three would conspire to provide a false signal of dark energy. So, using all three as simultaneous constraints makes it far less likely to be mislead by systematics inherent in a single one of those three datasets. Another aspect of the rebuttal (which I don't really understand because I don't work on Type Ia supernovae) is that the N16 authors made an assumption for the luminosity-corrections of the Type Ia supernovae that could increase the derived luminosity; this would reduce any signal of acceleration. The authors of the rebuttal also argue that this is part of what resulted in the decreased significance found by N16. So, from my understanding, the rebuttal is essentially arguing that N16 incorrectly computed the dark energy signal by using an incorrect correction to the Type Ia supernovae and by ignoring additional cosmological datasets. [0] https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations [1] https://en.wikipedia.org/wiki/Cosmic_microwave_background https://en.wikipedia.org/wiki/Cosmic_microwave_background [2] http://www.quantumfieldtheory.info/cmb.pdf http://www.quantumfieldtheory.info/cmb.pdf