Reality and Causality

But... it was limited to the Light Cone.

Yes. Even with such means of observation, all one could perceive was merely what lay within the Light Cone. If one could receive "light," then everything still remained within the speed of light.

As long as the Light Cone had not been broken, everything observable was merely the Observable Universe, not the universe itself.

The Observable Universe was the region from which light could not escape. For example, to the lifeforms within the Returning Ruins, the entire Returning Ruins was their "Observable Universe." Once one passed beyond the Returning Ruins' event horizon, light could no longer escape. Everything they could observe or interfere with would be forcefully repelled before that horizon. They could not see it, could not interfere with it, and could not even imagine it.

Although the universe itself was isotropic, they had no way to verify that. Even calculating spacetime curvature on grand scales made it difficult to truly perceive.

Though this distance of tens of billions of light-years was already unimaginably vast to those civilizations, it was still merely the tip of the universe's iceberg. Moreover, the universe continued expanding without cease, at a speed so unbelievable that light itself could never catch it, as though it remained in a state of inflation at every moment.

So-called inflation was the expansion from the universe's original volume-less singularity, expanding in an instant from far smaller than a proton to the scale of galaxies.

In less than 10^-33 seconds, the universe had expanded by 2^100 times. Nor had it stopped; it continued expanding, and now no one knew how many billions of years had passed.

The space brought about by eternal inflation formed the "existence" within the present universe. One could imagine just how boundless the universe was. If one