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Yaron Sheffer
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A recent arXiv manuscript, arXiv:1801.03278, claims that a cosmic background radiation with a black body temperature of $T_{rm BB}$ ~ 500 K (440 F) was just barely visible to human eyes, thus fixing the onset of the Dark Ages at about 5 million years post recombination. This claim presents an insurmountable biophysical challenge, since even hotter bodies, such as 450 F pizzas, do not seem to be glowing in the dark. As volunteer referees we show that this claim is the result of employing an incorrect assumption. Via a corrected analysis we find that the Dark Ages must have had a significantly earlier start. A second, more descriptive claim, that a cosmic background radiation with $T_{rm BB}$ of 1545 K was as blinding to humans as is our own Sun, is based on the same assumption and may have to be revised.
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Einsteins genius and penetrating physical intuition led to the general theory of relativity, which incorporates gravity into the geometry of spacetime. However, the theory of general relativity leads to perspectives which go far beyond the vision of its creator. Many of these insights came to light only after Einsteins death in 1955. These developments were due to a new breed of relativists, like Penrose, Hawking and Geroch, who approached the subject with a higher degree of mathematical sophistication than earlier workers. Some of these insights were made possible because of work by Amal Kumar Raychaudhuri (AKR) who derived an equation which turned out to be a key ingredient in the singularity theorems of general relativity. This article explains AKRs work in elementary terms.
We study the chameleon field dark matter, dubbed textit{scalaron}, in $F(R)$ gravity in the Big Bang Nucleosynthesis (BBN) epoch. With an $R^{2}$-correction term required to solve the singularity problem for $F(R)$ gravity, we first find that the scalaron dynamics is governed by the $R^{2}$ term and the chameleon mechanism in the early universe, which makes the scalaron physics model-independent regarding the low-energy scale modification. In viable $F(R)$ dark energy models including the $R^{2}$ correction, our analysis suggests the scalaron universally evolves in a way with a bouncing oscillation irrespective of the low-energy modification for the late-time cosmic acceleration. Consequently, we find a universal bound on the scalaron mass in the BBN epoch, to be reflected on the constraint for the coupling strength of the $R^2$ term, which turns out to be more stringent than the one coming from the fifth force experiments. It is then shown that the scalaron naturally develops a small enough fluctuation in the BBN epoch, hence can avoid the current BBN constraint placed by the latest Planck 2018 data, and can also have a large enough sensitivity to be hunted by the BBN, with more accurate measurements for light element abundances as well as the baryon number density fraction.
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