NGC281 is an HII (ionized atomic hydrogen) region in the constellation of Cassiopeia and part of the Perseus Spiral Arm
The time, or temperature, at which the radiation era ends is defined by the equality of the radiation energy density/c^2 and the matter density. However, this does not provide a unique definition because of the ambiguities regarding what is counted as radiation and what is counted as matter.
Equality of electromagnetic radiation and baryonic matter occurs at 615,000 years and 2130 K. However, if dark matter is included, equality occurs much earlier, at times of order 10,000 to 40,000 years.
Neither result is especially close to the time of recombination (185,000-363,000 years). Nevertheless, if the hypothesis is made that the time of matter-radiation equality is the same as the time of recombination, the photon:baryon ratio can be estimated and found to be within a factor of 2 of the observed value.
Putting this together with the results of Chapters 8, 9, 9B it appears that recombination, universal transparency and baryon-photon equality all occur at roughly the same time - nearly a triple coincidence. The significance of this is not clear.
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NGC281 is an HII (ionized atomic hydrogen) region in the constellation of Cassiopeia and part of the Perseus Spiral Arm