The sun, moon, planets, and stars, I thought, were created on a later day, and in the firmament itself. Not above the firmament, not underneath the firmament, but in the firmament. Ge 1:19 -- and the evening and the morning were the fourth day.
Yes, which is why your 5cm thickness hypothesis may be off by about 15 billion light years. I am presently working on calculating the quantity of water which Genesis says lies above the firmament. So far, I've devoted the last 30 years of my life to this problem. Soon I shall be able to publish my findings. Hint -- the mass of such extra firmamento fluid is sufficient to cause the redshift of distant stars.