Updated cosmological constraints on Macroscopic Dark Matter
  We revise the cosmological phenomenology of Macroscopic Dark Matter (MDM)
candidates, also commonly dubbed as Macros. A possible signature of MDM is the
capture of baryons from the cosmological plasma in the pre-recombination epoch,
with the consequent injection of high-energy photons in the baryon-photon
plasma. By keeping a phenomenological approach, we consider two broad classes
of MDM in which Macros are composed either of ordinary matter or antimatter. In
both scenarios, we also analyze the impact of a non-vanishing electric charge
carried by Macros. We derive constraints on the Macro parameter space from
three cosmological processes: the change in the baryon density between the end
of the Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB)
decoupling, the production of spectral distortions in the CMB and the kinetic
coupling between charged MDM and baryons at the time of recombination. In the
case of neutral Macros we find that the tightest constraints are set by the
baryon density condition in most of the parameter space. For Macros composed of
ordinary matter and with binding energy $I$, this leads to the following bound
on the reduced cross-section: $\sigma_X/M_X \lesssim 6.8 \cdot 10^{-7}
\left(I/\mathrm{MeV}\right)^{-1.56} \, \text{cm}^2 \, \text{g}^{-1}$. Charged
Macros with surface potential $V_X$, instead, are mainly constrained by the
tight coupling with baryons, resulting in $\sigma_X/M_X \lesssim 2 \cdot
10^{-11} \left(|V_X|/\mathrm{MeV}\right)^{-2} \text{cm}^2 \, \text{g}^{-1}$.
Finally, we show that future CMB spectral distortions experiments, like PIXIE
and SuperPIXIE, would have the sensitivity to probe larger regions of the
parameter space: this would allow either for a possible evidence or for an
improvement of the current bounds on Macros as dark matter candidates.
