Editing Openai/6921ae73-3700-8000-8ab7-90012976bb8a (section)

=== Assistant: Short answer: way beyond anything we can really imagine – effectively near-infinite density at the very beginning, and … ===
Short answer: way beyond anything we can really imagine – effectively near-infinite density at the very beginning, and still insanely dense in the first tiny fractions of a second.

Let’s break it into pieces so it’s not just “infinite = shrug.”

==== According to general relativity, if you follow the equations all the way back to time = 0, you get a singularity: ====
* Volume → 0
* Density → ∞ (infinite)
* Temperature → ∞

But:
Most physicists think this “infinite density” is really a sign that our equations break down at that point. We need quantum gravity (a theory that combines quantum mechanics + gravity), and we don’t have the full one yet.

So:

: 

==== We can talk more sensibly from around the Planck time, roughly: ====
* t∼10−43t \sim 10^{-43}t∼10−43 seconds after the Big Bang.

At that time, a typical estimate for density is around the Planck density:

ρPlanck≈5×1096 kg/m3\rho_\text{Planck} \approx 5 \times 10^{96}\ \text{kg/m}^3ρPlanck≈5×1096 kg/m3
To compare:
* Water: ~1,000 kg/m³
* Neutron star: ~10¹⁷ kg/m³
* Early universe near Planck time: ~10⁹⁶ kg/m³

So the early universe was:
* Around 10⁷⁹ times denser than a neutron star.
* That’s a 1 followed by 79 zeros times denser. Completely off the charts.

==== As the universe expands, density drops. But in the early seconds: ====
* At 1 second after the Big Bang: - Density was still something like 10⁹ kg/m³ (ballpark). - That’s a billion times denser than water.
* At a few minutes, when nuclei are forming: - Still enormously denser and hotter than anything we see in normal stars.

So for a while, the entire universe was like a super-massive, ultra-hot fireball, far denser than anything we can build or see today.

==== Putting it simply: ====
* At t = 0: - Our current math says infinite density, which really means “we don’t know; physics breaks here.”
* At ~10⁻⁴³ seconds (Planck time): - Density ≈ 5 × 10⁹⁶ kg/m³ (Planck density), probably the densest physical state we can sensibly talk about with current theories.
* Today: - Average density of the universe is tiny (mostly empty space), around ~10⁻²⁶ kg/m³.

So the universe went from unimaginably dense and hot → to thin and cold on average, over 13.8 billion years.

If you want, next we can compare that density to some of your earlier questions, like:
* “What if we compressed the universe into the size of a penny?”
* Or “how dense is a black hole compared to the early universe?”