[tambourine_man]

He’s referring to the cosmic event horizon

“ In an expanding universe, the speed of expansion reaches — and even exceeds — the speed of light, preventing signals from traveling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves, which travel at the speed of light.”

https://en.m.wikipedia.org/wiki/Event_horizon

[davidvarela_us]

I was just going to ask about gravity. TIL that gravity travels at the speed of light. Thanks!

[weego]

Or light travels at the speed of gravity but for a different order of discovery and convention.

Massless particles all travel at the speed at which causality can propagate through the universe (though they themselves do not experience speed because they do not experience time). The fact we reference it as the speed of light is just scientific idiom