Day 1 "And God said, 'Let there be light'; and there was light. And God saw that the light was good; and God separated the light from the darkness. God called the light Day, and the darkness he called Night. And there was evening and there was morning, one day." - Genesis 1:3-5
Day 4 "And God said, 'Let there be lights in the firmament of the heavens to separate the day from the night; and let them be for signs and for seasons and for days and years, and let them be lights in the firmament of the heavens to give light upon the earth.' And it was so. And God made the two great lights, the greater light to rule the day, and the lesser light to rule the night; he made the stars also. And God set them in the firmament of the heavens to give light upon the earth, to rule over the day and over the night, and to separate the light from the darkness. And God saw that it was good." - Genesis 1:14-18
The Origin of Light
Analysis of the Nature of Energy, Matter, Light and Time
Extensive research in particle composition  has yielded insights into the interchangeability of energy and matter, that the smallest particles are made of energy packets called quanta. Essentially energy is the substrate of all forces and the matter they act upon. Matter has reliable structure  which has been observed to be highly stable and repeatable in its behavior. Physicists claim today that the Big Bang was the source of space, matter and time all three, and that none of them existed "as we know them now" prior to the Big Bang .
The Bible says in Genesis 1 that "The spirit of God moved" and this word "move" is flutter-like or back-and-forth wave motion. All forms of energy in the universe appear in waveforms. We may then postulate that the waveforms of raw energy, coursing from God's spirit, are the initial raw material God used to manufacture everything in the universe. God's spoken word "informed" this energy into particles (including photons), elements, molecules, compounds etc, all with rules and laws to govern their behavior. The consistency and cohesion of the Biblical doctrines likewise show that the Creation is not a spontaneous act but a grand architectural blueprint that pre-existed the Creation. God has created Time itself Titus 1:2 and is sovereign over it. As such, God is not subject to Time nor is he a "tinkering" God attempting to orchestrate things "along the way".
As structure and composition are functions of information, even at the atomic level, "Let there be light" is an example of energy itself being informed as to what structure to take and where it fits in the architecture. Energy supplied by God is the ultimate raw material necessary to create all things.
In Hebrews 1:3 we are told that the entire Creation is upheld, in a continuous fashion, by the "word of his power". That God's words literally and completely have informed the matter on what shape it should take, where it should take it, and keeps in in place firmly, throughout the cosmos at the deepest atomic level.
So we have God's word "informing" the energy on what shape to take, and God's word "holding" the shape firmly in place. Jesus says that the Father John 5:17 works, which dovetails with the claim that all of Creation is continually upheld by God's words.
As a note, an unspoken but fundamental premise of Old-Earth Creationism, Progressive Creationism etc. all make the general presumption that God is not sovereign over Time. That God is somehow "using" the processes of evolution to "bring about" his overall plan. When we read Revelation 13:8 and John 1:3 and Colossians 1:13-17 , a clear pattern emerges, that the Son of God, Jesus Christ, is the centerpiece of the architecture from beginning to end and all things in Scripture describe, defend or praise Him as a broad and deeply embedded pattern. The Creation is not a product of ongoing tinkering, but a carefully crafted and well-executed plan to bring glory to Jesus Christ. Reducing God to the role of "conductor" or even "absentee landlord" misses the point entirely.
Invention vs. Application
Some interpret the activities on day one to be those of an inventor who developed the reactions to produce light. Then on day 4 that invention was applied to the creation of the solar bodies that provide light today.
Universe Was Filled With Light
Cosmologists working with 'Big Bang' calculations indicate that a massive presence of photons would have crossed the entirety of the universe during the initial seconds of universal expansion. This would in fact have 'lit up' the entire universe, all at once. "...and there was light."
Sonoluminescence is the process by which acoustic energy is turned into light. This is done by introducing a frequency to water while a air bubble is present. The light produced actually comes from this bubble. Light could have been accomplished this way during the creation week when the earth was just a sphere of water before land appeared.
Light Came From God
God is often referred to as light. And because light that comes from God would be eternal just like God, it would know no bounds of time and could travel to the ends of space instantly. Revelation 21:23 reports how the New Jerusalem will be lit by the glory of God, and that in that time there will be no need for a sun.
"And the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof. And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honour into it. And the gates of it shall not be shut at all by day: for there shall be no night there." - Revelation 21:23-25 (KJV)
- Steven Weinberg's book The First Three Minutes.