We introduced a series of new energy sources SU(6), SU(12),..... etc. beyond the standard model of physics.

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Well; it first depends on how you count: According to échelle longue, one trillion is 1018. According to échelle courte, one trillion is 1012. Next it depends on which parts of the electromagnetic spectrum contributes to suntan; that ought to be known to those in the know (which doesn't include me).

Anyway, let me try to estimate the contribution from just one extra-galactic source, the Andromeda galaxy. It is about a factor 1.64 ×1011 further away than the Sun, so light from it is diluted by a factor 2.70 × 1022 relative to light from the Sun. On the other hand, it contains about 1012 stars, so maybe its total energy output is also 1012 times that of the Sun. So, if we get our fair share of that output, which ought to have about the same spectral composition as sunlight (on slightly blueshifted), it should make up about a fraction 3.7 × 10-11 of the suntan contribution. So, assuming échelle courte counting, your estimate may be about right, but perhaps a little bit low (since there are additional extra-galactic suntan sources than Andromeda).

According to this paper (DOI: 10.3847/0004-637X/827/2/108), every second Earth is hit by about 10 billion photons that originated beyond the Milky Way. This represents just ten-trillionths of the total number in terms of square meter hit... or about 0.000000001 percent :)

@ Mikael. An informative paper (which takes some time and efforts to digest). I did not find exactly the number(s) you mention in the article(?). The CMB photons seems to be excluded from these counts. Just as well; they don't contribute to suntan anyway...

@Kåre :) It's certainly not information that jumps out from the pages. Back in 2016 I read a blog review/summary of this paper where the author derived the numbers... I was a bit fascinated that someone actually went through all that effort just to investigate extragalactic-suntan-contribution and tried to derive it from the paper myself. Not an easy task as the main objective of the paper was not explicitly suntan contribution. My conclusion was that the numbers were there (more or less), but that they were quite subjective and dependent on estimation.

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The deep problem behind this question is of course why one cannot suntan at night (with noticeable results). As was already asked by Olbers some 200 years ago. I would say this is a fortunate effect of the universe expansion (I don't care much for suntanning).

But in the article http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1991ApJ...367..399W&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

Paul Wesson points out that the extragalactic starlight is not that much influenced by the expansion; the finite age of the universe, and the stars in it, seem like a more relevant explanation. I think this is true for starlight, since most observed galaxies are not very much redshifted, and they must all have been born after the dark ages.

However, if the 2.7 K cosmic microwave background had not been redshifted by a factor 1100 since decoupling, Olbers would not have needed to ponder why the night sky is so dark...

Werner Heisenberg emphasized the fact that there are sentences that are syntactically correct and nevertheless bare of any sense. The present question is an perfect examplification of this.

If the question would be:

What’s the fraction of our suntan that extragalactic photons account for?

one could safely answer 'less then 1/1000 of a percent'.

If one asks for numbers of contributing extragalactic photons a possible answer could be 'with probability 99 percent no single molecule contributing to skin tanning was formed under the influence of an extragalactic photon during a human life time'. My guess would be that one could replace 99 by 99.999999....

Noble women in past centuries nearly completely avoided suntan by using umbrellas when walking in sunshine!