Wasn't it Paul Jones who wrote:
On 15 Sep, 14:05, Mike Williams kindly
replied to say:

Relativity.

Electromagnetic radiation always travels at the speed of light relative
to any observer. So radiation that set off from the big bang 13.7
billion years ago arrives at the speed of light.

This reflects my lack of understanding. My question is not really
about how fast it arrives but how does it arrive here? If I went off
into space and shone a beam of coherent light from my laser in the
diametrically opposite direction to where an observer was positioned,
then he/she wouldn't be able to detect it (would he?). Shouldn't all
the photons from the big bang are travelling away from us like the
galaxies?

The flash from the big bang wasn't coherent.

At one time the universe was full of incandescent opaque ionised plasma.
Photons were being created randomly by the hot ions, but they didn't get
far because they got absorbed by other ions in the plasma.

As the universe expanded and cooled, it reached a point where the ions
were able to form into neutral atoms (hydrogen and a bit if helium)
producing a transparent gas. Many of the photons that were in flight at
that time, travelling from every point in the universe in every possible
direction, were suddenly free to travel immense distances.

Some of the photons from regions that are now 13.7 billion light years
away from here happened to be pointing in our direction.

--
Mike Williams
Gentleman of Leisure