Except the galaxies do not travel. Galaxies are more or less stationary, it's the space inbetween that expands. Think in terms of raisins in a dough or points on expanding baloon. Neither of them could be said to travel, yet they can be seen to become more and more far away.

Technically, galaxies do travel:

> In physical cosmology, peculiar velocity refers to the components of a galaxy's velocity that deviate from the Hubble flow. ....

> Galaxies are not distributed evenly throughout observable space, but are typically found in groups or clusters, where they have a significant gravitational effect on each other. Velocity dispersions of galaxies arising from this gravitational attraction are usually in the hundreds of kilometers per second, but they can rise to over 1000 km/s in rich clusters.

-- https://en.wikipedia.org/wiki/Peculiar_velocity

Granted, 1000 km/s isn't even 1% of light speed, so clearly this motion couldn't contribute much to a distant object's recession.

That's why I wrote "more or less stationary", which is describing 1000km/s compared to double the speed of light that would come from naive calculation.

Ah, fair enough. I glossed right over that qualification, which is not a smart thing to do on HN--a credit to the quality of discourse here :)

and yet you stated nothing about this factor in your original comment.

No, space inbetween is stationary. It's galaxies that shrinks. Think of ice in water.

What’s the difference, observationally?

Shrinking releases energy. Expanding consumes energy.

Energy from where?

You aren't stationary and space itself is expanding.

The metaphor I normally see is a balloon being blown up.

Importantly, the expansion of the universe is necessary. Under special relativity two things could be moving at .9c toward one another and their velocities won’t somehow add to 1.8c in relative velocity. Light will still travel at c from one to the other.

It's called "mathemagick".

No, the universe is expanding. No magic required.

Infinite amount of dark mathemagic is required for faster than light expansion of nothing.

Not really.

FTL speed requires infinite amount of energy in this iteration of physics, except for case when vacuum is not empty, but even in this case medium cannot move at supersonic speed of medium, while still in one piece. If far away vacuum moves at FTL speed, then there must be "sonic" barrier somewhere.

It's the fabric of spacetime that's expanding. Nothing physical is moving and thus no energy is necessary.