Early Cosmology Constrained
  We investigate our knowledge of early universe cosmology by exploring how
much additional energy density can be placed in different components beyond
those in the $\Lambda$CDM model. To do this we use a method to separate early-
and late-universe information enclosed in observational data, thus markedly
reducing the model-dependency of the conclusions. We find that the 95\%
credibility regions for extra energy components of the early universe at
recombination are: non-accelerating additional fluid density parameter
$\Omega_{\rm MR} < 0.006$ and extra radiation parameterised as extra effective
neutrino species $2.3 < N_{\rm eff} < 3.2$ when imposing flatness. Our
constraints thus show that even when analyzing the data in this largely
model-independent way, the possibility of hiding extra energy components beyond
$\Lambda$CDM in the early universe is seriously constrained by current
observations. We also find that the standard ruler, the sound horizon at
radiation drag, can be well determined in a way that does not depend on
late-time Universe assumptions, but depends strongly on early-time physics and
in particular on additional components that behave like radiation. We find that
the standard ruler length determined in this way is $r_{\rm s} = 147.4 \pm 0.7$
Mpc if the radiation and neutrino components are standard, but the uncertainty
increases by an order of magnitude when non-standard dark radiation components
are allowed, to $r_{\rm s} = 150 \pm 5$ Mpc.
