Define the baryon asymmetry, which sometimes is called the baryon density, as the baryons to photons ratio $\eta = n_b/n_\gamma$.

I found in Ref. 1, published in 2018, that the value of $\eta$ is of order $ 6.1 \pm 0.3 \times 10^{-10} $ by observing the cosmic microwave background experimentally by Planck satellite. However in Ref. 2, published this year, $\eta \approx 8.6 \pm 0.1 \times 10^{-11} $ as observed experimentally as well according to the particle data group (PDG).

So the value of $\eta$ is which one of these? Is it the recent one? Or the procedure of observation is different in each study?


  1. N. E. Mavromatos, Matter-antimatter asymmetry in the universe via string-inspired CPT violation at early eras, J. Phys.: Conf. Ser. 952 012006, arXiv:1708.08350.
  2. S. A. Shapira, Current bounds on baryogenesis from complex Yukawa couplings of light fermions, arXiv:2106.05338.

First one is the ratio of baryon asymmetry to photon density, the second is baryon asymmetry to entropy density. According to wiki present-day entropy density $s$ is related to photon density $n_\gamma$ as $s=7.04n_\gamma$. This reconciles the two values more or less.

  • $\begingroup$ Hay @Kosm, thanks for the answer. May you let me know how did you know the difference between both? Have you checked the papers themselves or you have returned for the papers cited references for each $\eta$ value. Cause I'm spending good time now try to understand how to get the predicted or observed value. Please if you have help about this thread physics.stackexchange.com/questions/663837/…, or this: physicsforums.com/threads/… $\endgroup$
    – Dr. phy
    Sep 3 at 17:32
  • 1
    $\begingroup$ @Dr.phy I just looked at the equations in those papers, they clearly show which asymmetry parameter they are using (ratio by radiation or entropy density) $\endgroup$
    – Kosm
    Sep 3 at 18:09

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