Fast galaxy bars continue to challenge standard cosmology
  Many observed disc galaxies harbour a central bar. In the standard
cosmological paradigm, galactic bars should be slowed down by dynamical
friction from the dark matter halo. This friction depends on the galaxy's
physical properties in a complex way, making it impossible to formulate
analytically. Fortunately, cosmological hydrodynamical simulations provide an
excellent statistical population of galaxies, letting us quantify how simulated
galactic bars evolve within dark matter haloes. We measure bar strengths,
lengths, and pattern speeds in barred galaxies in state-of-the-art cosmological
hydrodynamical simulations of the IllustrisTNG and EAGLE projects, using
techniques similar to those used observationally. We then compare our results
with the largest available observational sample at redshift $z=0$. We show that
the tension between these simulations and observations in the ratio of
corotation radius to bar length is $12.62\sigma$ (TNG50), $13.56\sigma$
(TNG100), $2.94\sigma$ (EAGLE50), and $9.69\sigma$ (EAGLE100), revealing for
the first time that the significant tension reported previously persists in the
recently released TNG50. The lower statistical tension in EAGLE50 is actually
caused by it only having 5 galaxies suitable for our analysis, but all four
simulations give similar statistics for the bar pattern speed distribution. In
addition, the fraction of disc galaxies with bars is similar between TNG50 and
TNG100, though somewhat above EAGLE100. The simulated bar fraction and its
trend with stellar mass both differ greatly from observations. These dramatic
disagreements cast serious doubt on whether galaxies actually have massive cold
dark matter haloes, with their associated dynamical friction acting on galactic
bars.
