When the matter-density in the universe dropped so low that photons could finally travel a significant distance without immediately running into another particle. We already had atoms at that point.
The CMB is an image what the universe looked like at that point... Hundreds of thousands of years after the Big Bang.
The Cosmic Neutrino Background was created a few seconds after the Big Bang, when the energy-density dropped so low that neutrinos could finally travel a significant distance without immediately running into another particle. We already had Quarks, Gluons and Leptons at that point.
If we could measure that more precisely, we could get an image of what the universe looked like at that point... Seconds after the Big Bang.
And even seconds after the Big Bang is already way, way, way, way too late to measure anything about the particle-physics right during the Big Bang.
The guys in the paper want to go to a time where even Quarks, Gluons and Leptons didn't exist yet.
Here's their conclusion:
We showed that holographic models based on three-dimensional perturbative QFT are capable of explaining the CMB data and are competitive to the ΛCDM model. However, at very low multipoles (roughly l<30), the perturbative expansion breaks down, and in this regime the prediction of the theory cannot be trusted.
Translation:
The theory can predict the fine details of the CMB but not the larger general shape.
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