Hacker News Reader

The basic principle is that space itself is expanding—the empty stuff between all those stars, planets, and galaxies. Not only that, but the expansion of space itself? It’s accelerating. And the further away something is, the faster it’s accelerating. Allow this play out over time and with acceleration unbounded, space itself starts to expand faster than the speed of light. And since the speed of light is our universal speed limit… that means the far reaches of universe will forever be out of reach. Eventually our own night sky will go dark (minus the stuff closest to us like the sun, Milky Way, and Andromeda) as more and more objects continue their acceleration away from us.

More at https://en.wikipedia.org/wiki/Expansion_of_the_universe

One consequence of which is that if intelligent life arises in the Milky Way again after that point, they will likely have no way of knowing that there ever were other galaxies beyond the local group.

We're somewhat lucky to be around to see the universe when it's comparatively young.

Can't that be true for us, though? If we had JWT 4 billion years ago, who knows what we would have seen.

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

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

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

https://www.forbes.com/sites/startswithabang/2018/05/12/ask-...

Imagine you’re swimming in a magical pool with some friends and the entire pool doubles in size over the course of a second.

Your friend right next to you who was one foot away is now two feet. The absolute distance just grew by 1 foot, a rate of 1 foot/second.

Your friend across the pool who was 10 feet is now 20 feet away. Absolute distance just grew by 10 feet, or a rate of 10 ft/sec.

You probably mean the pool quadruples in size, meaning it doubles in length.

No, I mean doubles across all axes. 1 unit cubed becomes 2 units cubed.

It's like a balloon. Take two distances when balloon is empty. Let's say furthest points of each other (10cm) and halfway points (5cm) When you inflate the balloon the distance of furthest points increased to 20cm. But the 5cm is 8cm and not 10cm. The same in space. the further you go the faster it expands. The event horizon is the point where the expansion is faster than the speed of light. Any photon emitted will never reach us. The crazy thing is that according to some theories even the space between atoms will be beyond the event horizon.

Here's a link about this idea by Ethan Siegel, and its HN discussion thread (thanks 'petilon for helping me remember the author):

https://medium.com/starts-with-a-bang/the-disappearing-unive... ("The Disappearing Universe")

https://news.ycombinator.com/item?id=7896776 (64 comments)