Dark Energy Survey Year 3 results: cosmological constraints from the
  analysis of cosmic shear in harmonic space
  We present cosmological constraints from the analysis of angular power
spectra of cosmic shear maps based on data from the first three years of
observations by the Dark Energy Survey (DES Y3). Our measurements are based on
the pseudo-$C_\ell$ method and offer a view complementary to that of the
two-point correlation functions in real space, as the two estimators are known
to compress and select Gaussian information in different ways, due to scale
cuts. They may also be differently affected by systematic effects and
theoretical uncertainties, such as baryons and intrinsic alignments (IA),
making this analysis an important cross-check. In the context of $\Lambda$CDM,
and using the same fiducial model as in the DES Y3 real space analysis, we find
${S_8 \equiv \sigma_8 \sqrt{\Omega_{\rm m}/0.3} = 0.793^{+0.038}_{-0.025}}$,
which further improves to ${S_8 = 0.784\pm 0.026 }$ when including shear
ratios. This constraint is within expected statistical fluctuations from the
real space analysis, and in agreement with DES~Y3 analyses of non-Gaussian
statistics, but favors a slightly higher value of $S_8$, which reduces the
tension with the Planck cosmic microwave background 2018 results from
$2.3\sigma$ in the real space analysis to $1.5\sigma$ in this work. We explore
less conservative IA models than the one adopted in our fiducial analysis,
finding no clear preference for a more complex model. We also include small
scales, using an increased Fourier mode cut-off up to $k_{\rm max}={5}{h{\rm
Mpc}^{-1}}$, which allows to constrain baryonic feedback while leaving
cosmological constraints essentially unchanged. Finally, we present an
approximate reconstruction of the linear matter power spectrum at present time,
which is found to be about 20\% lower than predicted by Planck 2018, as
reflected by the $1.5\sigma$ lower $S_8$ value.
