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We study the late-time evolution of the central regions of two Milky Way-likesimulations of galaxies formed in a cosmological context, one hosting a fastbar and the other a slow one. We find that bar length, R_b, measurementsfluctuate on a dynamical timescale by up to 100%, depending on the spiralstructure strength and measurement threshold. The bar amplitude oscillates byabout 15%, correlating with R_b. The Tremaine-Weinberg-method estimates of thebars' instantaneous pattern speeds show variations around the mean of up to~20%, typically anti-correlating with the bar length and strength. Throughpower spectrum analyses, we establish that these bar pulsations, with a periodin the range ~60-200 Myr, result from its interaction with multiple spiralmodes, which are coupled with the bar. Because of the presence of odd spiralmodes, the two bar halves typically do not connect at exactly the same time toa spiral arm, and their individual lengths can be significantly offset. Weestimated that in about 50% of bar measurements in Milky Way-mass externalgalaxies, the bar lengths of SBab type galaxies are overestimated by ~15% andthose of SBbc types by ~55%. Consequently, bars longer than their corotationradius reported in the literature, dubbed "ultra-fast bars", may simplycorrespond to the largest biases. Given that the Scutum-Centaurus arm is likelyconnected to the near half of the Milky Way bar, recent direct measurements maybe overestimating its length by 1-1.5 kpc, while its present pattern speed maybe 5-10 km/s/kpc smaller than its time-averaged value.

Fluctuations in galactic bar parameters due to bar-spiral interaction