Abstract
The CMB is a powerful probe of early-universe physics but is only observed
after passing through large-scale structure, which changes the observed spectra
in important model-dependent ways. This is of particular concern given recent
claims of significant discrepancies with low redshift data sets when a standard
$Łambda$CDM model is assumed. By using empirical measurements of the CMB
lensing reconstruction, combined with weak priors on the smoothness of the
lensing spectrum, foregrounds, and shape of any additional integrated
Sachs-Wolfe effect, we show how the early-universe parameters can be
constrained from CMB observations almost independently of the late-time
evolution. This provides a way to test new models for early-universe physics,
and measure early-universe parameters, independently of late-time cosmology.
Using the empirical measurement of lensing keeps the size of the effect of
late-time modelling uncertainty under control, leading to only modest increases
in error bars of most early-universe parameters compared to assuming a full
evolution model. We provide robust constraints on early-$Łambda$CDM model
parameters using the latest Planck PR4 data and show that with future data
marginalizing over a single lensing amplitude parameter is sufficient to remove
sensitivity to late-time cosmological model only if the spectral shape matches
predictions.
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