Previously I've discussed on here an understanding by my Rosh Yeshiva, Rabbi Yochanan Zweig, Shlit"a, on the concept of עצם היום in conjunction with the Supernal Light of the first day of creation, the general theory of which I've reproduced below from my previous discussion.

In a different Shiur, he used this understanding as the basis for a reconciliation of the Torah's dating of the universe at c. 5780 years with science's dating at c. 13.8 billion years. After presenting his skeleton of an answer, I had set to work in calculating if this reconciliation actually checks out with science's understanding of the early universe, if all the dates discussed throughout Creation line up, or if only the initial date of Creation checks out. With his blessing, I've copied my calculations below showing just how perfectly this lines up. Additionally I've added some of my own notes within Rav Zweig's discourse to explain it more formally in scientific terminology.

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Properties of the Supernal Light

Chagigah 12a.9 describes the light created on Day One as follows:

א"ר אלעזר אור שברא הקב"ה ביום ראשון אדם צופה בו מסוף העולם ועד סופו

R' Elazar said, "The light which Hashem created on the first day — Adam saw with it from the end of the world to its end."

As the Gemara goes on to describe, this special light was hidden away for future generations.

What does it mean that Adam could see from one end of the world to the other with this light? After all, the earth is spheroidal, not flat. The earth doesn't have ends! Rather, the expression must be from one end of time to the other, that Adam was able to use the Supernal Light to see the future!¹

Now, based on Einstein's Relativity Theory, we know that the faster an object moves with respect to a given observer, the less time appears to pass from the perspective of the moving object. Specifically, as the speed approaches c, the speed of light in a vacuum, the change in time from the perspective of the moving object approaches 0. Now, we see events by light bouncing off of objects into our retinas and stimulating the optic nerve; in order to see something in the future, as Adam was able to, Special Relativity requires that the Supernal Light enabling this would have to be able to travel faster than c! Theoretical physicists would call the Supernal Light a theoretical particle called a tachyon.²

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What is יום?

In Bereishis 1:5, the Torah writes:

וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם וְלַחֹשֶׁךְ קָרָא לָיְלָה‏

G-d called the light "Day," and the darkness He called "Night."

So the Torah expressly defines יום as based on the light created on the first day.

We, used to ordinary photonic light, define a "day" as the amount of time required for one photon of light to travel a fixed distance, which given SI definitions of units of time and distance, is approximately 26 billion kilometers. In the same amount of time, a tachyon could travel significantly farther (though backwards in time, as explained above). How far, exactly? Based solely on the Gemara in Chagigah, we don't know. But we do know the following:

ויהי ערב ויהי בקר יום אחד

And it was evening, and it was morning; one day.

If a "day" in the Torah is defined based on the Supernal tachyonic light, it follows that each day must be significantly longer. Consider: our definition of a "day" is the amount of time that a photon needs to travel 26 billion km. How far can a tachyon travel in the same amount of time? Significantly further. Now how long does it take a photon to travel the distance that the Supernal Light could cover in what we call 24 hours? That amount of time is what the Chumash calls a "day."

This is where Rav Zweig's Shiur ended. But we can go further: given the remaining data we are given in the text, let's figure out how long a day is, and how it lines up with the scientific understanding of prehistory.

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How old is Earth?

Rashi writes on Day Four (1:14):

מִיֹּום רִאשׁוֹן נִבְרְאוּ וּבָרְבִיעִי צִוָּה עֲלֵיהֶם לְהִתָּלוֹת בָּרָקִיעַ, וְכֵן כָּל תּוֹלְדוֹת שָׁמַיִם וָאָרֶץ נִבְרְאוּ מִיּוֹם רִאשׁוֹן, וְכָל אֶחָד וְאֶחָד נִקְבַּע בַּיוֹם שֶׁנִּגְזַר עָלָיו, הוּא שֶׁכָּתוּב אֵת הַשָּׁמַיִם, לְרַבּוֹת תּוֹלְדוֹתֵיהֶם, וְאֵת הָאָרֶץ, לְרַבּוֹת תּוֹלְדוֹתֶיהָ:‏

From day one were [the luminaries] created, and on the fourth [Hashem] commanded regarding them to hang in the sky. Likewise all of the offspring of Heavens and Earth were created from day one, and each one was fixed on the day which was decreed upon it. This is that which was written, "[es] the heavens," to include their offspring, "and [es] the land," to include her offspring.

Rashi here refers to the exegetical concept that, at least according to some opinions in the Gemara, whenever the grammatical marker "es" (את) appears in the text, it can be expounded to include something not mentioned in the verses. This particular theme, that everything was created on day one and put in its place on each one's day, is something Rashi references as well on v. 24).

If everything was already prepared by Day One, it follows that the Creation story does not open with the Big Bang. Rather, it begins with the formation of Earth, c. 4.5 billion years ago.

Rashi alludes to this understanding in one of his opening comments, back on v. 1:

וְאִ"תֹּ לְהוֹרוֹת בָּא שֶׁאֵלּוּ תְּחִלָּה נִבְרְאוּ, וּפֵרוּשׁוֹ בְּרֵאשִׁית הַכֹּל בָּרָא אֵלוּ [...] אִ"כֵּ, תְּמַהּ עַל עַצְמְךָ, שֶׁהֲרֵי הַמַּיִם קָדְמוּ, שֶׁהֲרֵי כְתִיב וְרוּחַ אֱלֹהִים מְרַחֶפֶת עַל פְּנֵי הַמָּיִם, וְעַדַיִין לֹא גִלָּה הַמִּקְרָא, בְּרִיאַת הַמַּיִם מָתַי הָיְתָה, הָא לָמַדְתָּ, שֶׁקָּדְמוּ הַמַּיִם לָאָרֶץ, וְעוֹד, שֶׁהַשָּׁמַיִם מֵאֵשׁ וּמַיִם נִבְרְאוּ‏

If you will say that [v. 1] comes to teach that [heavens and earth] were created first, and [the verse's] explanation is "In the beginning (of everything) He created these," [...] if so, ask yourself: Water came first, for it is written, "And the Spirit of Hashem hovered over the water," yet the Passuk has not yet revealed when the creation of the water was. From here you see that water preceded land! Further, the heavens were created from fire and water [and therefore fire and water preceded the heavens].

Here Rashi explicitly addresses the elephant in the room: there were creations before Day One. It follows, then, that the Torah does not open with the creation of the universe!

Rashi makes another important point on 1:14:

להבדיל בין היום ובין הלילה. מִשֶּׁנִּגְנַז הָאוֹר הָרִאשׁוֹן, אֲבָל בְּשִׁבְעַת יְמֵי בְרֵאשִׁית שִׁמְּשׁוּ הָאוֹר וְהַחֹשֶׁךְ הָרִאשׁוֹנִים יַחַד בֵּין בַּיּוֹם וּבֵין בַּלַּיְלָה

[The luminaries were set] to separate between day and night — from when the Original Light was hidden. But for the seven days of creation, the Original Light and Dark served together, both by day and by night.

There's several key points in this Rashi. Rashi emphasizes that for the entirety of the seven days of Creation, the Supernal Light was around (rather than being hidden sometime on Day Six). He does say seven days, not six, but nevertheless the Torah only uses the expression “one day,” “a second day,” “a third day,” etc. only up to “the sixth day.”

So what do we have so far?

• Days in Creation are counted not as 24-hour intervals, but as much longer intervals, calculated based on the Supernal Light.
• The Supernal Light served for all seven days of Creation.
• Each day, presumably, is the same length.
• The Torah opens not with the creation of the universe, but with the creation of Earth.
• Therefore, the six days of Creation cumulatively span approximately 4.5 billion years, and therefore each day was approximately 750 million years.

Technically this figure is slightly high. The planet was already in existence with plenty of water by the time the Chumash opens, so really we should be starting sometime later. In the earlier days of Creation this doesn’t make too much of a difference, but by Day Five or so the error begins to add up. I will be fudging the numbers there a bit, where events that should fall in Day Six’s domain I ascribe to Day Five.

With this figure in hand, let's analyze each day, from the Beginning.

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Day One: 4.50-3.75 billion years ago

Earth already exists by this point. There's already a sun in the sky, as well as the rest of the solar system. Approximately 25 million years after the formation of Earth, a large asteroid dubbed Theia is thought to have struck the planet, and this collision is what is thought to have created our Moon.

During this time, archaeological evidence suggests that the Earth spent its time being constantly bombarded by extraterrestrial objects, hence a subset of this Hadean period — itself named for the Greek god of the Underworld — being called the Heavy Bombardment. Meanwhile, Earth was swamped by extreme volcanic activity, yet evidence exists for water being on Earth's surface even earlier than the Theia impact. Indeed, there was water covering the surface, but more importantly there was chaotic molten liquid covering the entire surface. And then Hashem said, "Let there be light," which once again refers not to literal light, but to the Supernal Light.³

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Day Two: 3.75-3.00 billion years ago

During this time the atmosphere is still deoxygenated. Here we begin to see the origins of life — archaic life, to be precise, thriving on the extreme environment that was literal Hell on Earth. In the meantime, there's still plenty of volcanic activity going on; continental crusts begin to form, but are quickly destroyed because of a combination of temperature, speed of motion, and chemical composition, not to mention the Late Heavy Bombardment. It's fair to say that at this stage there is no solid land.

The רקיע, lit. "something which floats," is typically translated as a firmament. Whatever the רקיע is, Hashem called it שמים, "heavens." Heaven is inherently a spiritual construct; it's well beyond my grasp to explain how a spiritual creation such as the heavens could be created with fire and water, or how it can divide between the lower waters and upper waters, whatever that means. Instead I'll appeal to Ramban on these verses: these are such esoteric concepts that those who know don't tell and those who tell don't know.

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Day Three: 3.00-2.25 billion years ago

The spawning of photosynthetic microbes by the end of Day Two led to lots of things happening at around the same time on Day Three. This led to the formation of an oxygenated atmosphere, filled with greenhouse gases like methane, carbon dioxide, and ammonia. This, in turn, affected the temperature to allow for the tectonic plates to start solidifying into solid ground, forming the beginnings of continents.

In other words, plant life did not exist until much later, but photosynthetic life and solid ground did exist. The potential for plant life, however, did not come about until after there was solid ground, even though photosynthetic life came even before solid ground.

Rashi later writes, on 2:5:

וְכָל עֵשֶׂב הַשָּׂדֶה טֶרֶם יִצְמַח, עֲדַיִן לֹא צָמַח, וּבְג' שֶׁכָּתוּב תּוֹצֵא הָאָרֶץ, לֹא יָצְאוּ, אֶלָּא עַל פֶּתַח קַרְקַע עָמְדוּ עַד יוֹם שִׁשִּׁי:‏

"And all the herbage of the field, before it sprouted," and still it has not sprouted! On the third, that it is written "and the land sprouted," — they did not yet sprout, but rather was ready at the opening of the ground until day six.

Taking some liberties with this Rashi, I interpret this to mean that the chain of cause and effect which led to the creation of plant life began on day three with the creation of photosynthetic life and an atmosphere, while day six marked the creation of plant matter. As we will see later, the second half of this assertion checks out as well, but in the meantime we're leaving the planet for a bit.

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Day Four: 2.25-1.50 billion years ago

This is the day of prehistoric climate change. The increase of photosynthetic life on Day Three led to the majority of life on earth being wiped out; having not evolved photosynthesis, most species viewed oxygen as being toxic. To this day, only about 15 species of archaea are still extant.

The abundance of oxygen in the atmosphere led to the formation of the ozone layer. As is the process to this day, oxygen molecules, when stimulated by ultraviolet radiation from the sun, is split into single atoms of oxygen, which will bind to form ozone. Eventually it’ll break down back into oxygen molecules, at which point the cycle repeats.

Meanwhile, methane in the atmosphere was reacting with the oxygen being produced in abundance, producing carbon dioxide and water vapor — both greenhouse gases, but nowhere near as potent as methane. The Earth quickly cooled, and this first so-called Snowball Earth formed the earliest glaciers. (It’s possible it was less a Snowball and more a so-called Slushball, where the poles were icy, but plenty of liquid water could be found near the equator.)

As a result, eukaryotic cells began forming during this time. Some fungi already existed as early as late Day Three, but only in mid-Day Four times were mitochondria and nuclei formed, paving the way for proper complex life.

Year lengths were essentially fixed from the beginning of the planet’s history, but day lengths were still evolving; around this time Earth’s day would’ve been around 20 hours long (from 4-6 hours following the Theia impact on Day One). On Day Four, one might say that the relationship between life and celestial bodies evolved to the point of allowing complex life to form.

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Day Five: 1.50 billion-750(ish) million years ago

Plant, animal, fungal, and protist cells all existed, though still as unicellular organisms. The first multicellular organism was probably some sort of algae, and it’s thought that by a billion years ago animals had evolved to multicellularity as well. Tectonic plates during this time formed a supercontinent termed Rodinia, perhaps with previous supercontinents as well, but Rodinia broke apart toward the end of Day Five.

Following another Snowball Earth or two, new diversity of life began to form. Proper invertebrate animal life began to develop toward the end of the Proterozoic period, around 600 million years ago. Fungi had been colonizing the land since about a billion years ago, and plants since about 700 million years ago, but tetrapods would still have a ways to come. Unfortunately this is the one and only point in this chronology that I cannot defend: no form of flying life that could satisfy ועוף יעופף would come along until too far into Day Six.

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Day Six: The rest of prehistory

Arthropods may have appeared as early as 530 million years ago, but tetrapods certainly appeared by 375 million years ago. First they were amphibians, then reptiles. Around this time plants evolved seeds, helping their spread on land. There was plenty of ירק עשב for the animals to eat!

Around 300 million years ago is when mammals began to evolve, but diversification didn’t really occur until after the dinosaurs. Flying creatures evolved around the same time, but birds so to say only evolved about 150 million years ago.

More Snowball Earths, meteors, and depletion of greenhouse gases from evolution of trees around 130 million years ago led to five mass extinctions during this time, the most well-known being the most recent — the K-T extinction that killed the dinosaurs, and 75% of all life in general. But following these extinctions led to the evolution of diverse groups of mammals. Grass began to evolve around this time as well, and humans began to evolve around 6 million years ago. Hominids evolved around two million years ago, and modern humans are thought to have evolved just 200,000 years ago.

1. NB: That is how Rav Zweig posed this step of the proof. Coming from a perspective where Chazal can be wrong about science because their goal is to teach moral lessons, not scientific ones, I respectfully questioned the validity of this question. Instead I brought a proof to this step of the discussion from Nazir 1:3: "סְתָם נְזִירוּת שְׁלשִׁים יוֹם. אָמַר הֲרֵינִי נָזִיר אַחַת גְּדוֹלָה, הֲרֵינִי נָזִיר אַחַת קְטַנָּה, אֲפִלּוּ מִכָּאן וְעַד סוֹף הָעוֹלָם, נָזִיר שְׁלשִׁים יוֹם; An unspecified Nazir-vow lasts thirty days. [If] he said, 'I am hereby a Nazir for a large period,' 'I am hereby a Nazir for a small period,' even 'From here until the end of the world' — he is a Nazir for thirty days." We see from this Mishnah that when Chazal use the expression סוף העולם, it means in time, not in space. Perhaps in a different D'var Torah I may discuss my interpretation in which the word עולם always means in time and never in space, at least in its usage in the Chumash. ↩

2. If tachyons actually existed in our universe, it'd create massive problems, the ability to see the future being one of the major ones. Consider all the classic issues with backwards time travel — being able to cause an event which prevents yourself from ever traveling back in the first place (Grandfather Paradox), or having particles and concepts which appear to come from nowhere (Bootstrap Paradox), being among of the more famous ones. Tachyons would allow these sorts of causality violations. Can you imagine what would happen if this power fell into the wrong hands? While Rav Zweig didn't make this point, this understanding of the Hidden Light makes it obvious why, exactly, it was hidden in the first place. In the words of the Gemara ad. loc., "[Hashem] looked upon the generation of the Flood and the generation of the Dispersion and saw that their ways were corrupt; He arose and concealed it from them" (Chagigah ad. loc.). Can you imagine what such a greedy, selfish, corrupt society would do with such a power? ↩

3. Considering the nature of tachyons perhaps it went the other way — Hashem created them in the future, and this was as far back as they could reach before being destroyed. One theory, which of course by the non-existent nature of tachyons can't be experimented, could be that the same way any "normal" particle becomes annihilated upon striking its own antiparticle, such as the interaction between an electron and a positron, perhaps there is similarly an "Ohr HaGanuz" particle as well as an "anti-Ohr HaGanuz" particle which annihilated each other. Such a theory would also explain how the Supernal Light could travel back in time to Adam, yet none of the generations in between Adam and the events he's witnessing could benefit of this Light. The same way that the imbalance between matter and antimatter allows, well, us to exist, an imbalance between the Ohr HaGanuz and anti-Ohr HaGanuz would allow Adam to use the light, then the antiparticles to annihilate the Ohr HaGanuz on the future-end as well. Yes, that should lead to a massive causality violation. Did you expect time travel to not make your head hurt? Did you not read footnote 2? ↩