Is "empty space" empty (only reasonable answers)

[Anomalocaris]

Seeing that galaxies are constantly merging the idea of an expanding universe seems illogical.

Huh?

The effect of gravity between any two groups of galaxies declines with square of distance between them. The effect of expansion of space between two groups of galaxies increases with distance between them.

So galaxies groups separated from each other by short distances, say millions to hundreds of millions of light years, tend to attract each other as strongly or more strongly than expansion of space would push them apart. So they tend to clump together under gravity in defiance of expansion of universe

Galaxies groups separated from each by great distances, say billions of light years, are unable to attract each other more strongly than expansion of space would push them apart. So they separate as expansion of universe overwhelms their mutural gravitational attraction.

[Wyrd of Gawd]

Sounds legit but you're forgetting that even the most distant galaxies clump together in clusters. So galaxies are merging all over the place. We are not the center of the universe.

[Logisch]

Sounds legit but you're forgetting that even the most distant galaxies clump together in clusters. So galaxies are merging all over the place. We are not the center of the universe.

I don't think he was asserting that we are. However, the expansion itself is the space between galaxies expanding. I think Janna Levin put it quite well with her analogy of cities and the distance between them expanding some time ago. But I recently read some other stuff of hers and liked this:

Space is not just an abstract notion but a mutable, evolving field. It can begin and end, be born and die. Space is curved, it is a geometry, and our experience of gravity, the pull of the earth and our orbit around the sun, is just a free fall along the curves in space. From this huge insight people realized the universe must be expanding. The space between the galaxies is actually stretching even if the galaxies themselves were otherwise to stay put.

Some are on a collision course, some are merging, others are moving away faster than others from us. If you put yourself in a galaxy that was in redshift from us, we would appear to redshift from the milky way. In most any direction you look from any galaxy, the universe appears to be expanding.

Here's a really good analogy:

[Anomalocaris]

Sounds legit but you're forgetting that even the most distant galaxies clump together in clusters. So galaxies are merging all over the place. We are not the center of the universe.

Nothing is implied about the center of universe. Distant galaxies only clump with galaxies near them. They don clump with galaxies near us. We only clump with galaxies near us, we don't clump with distant galaxies. Everywhere it is the same, Gravity dominate over short distances with govern objects in relative proximity of tens or hundreds of millions of light years. Everywhere expansion of universe trumps gravity over long distances of billions of light years.

If you look at galaxies within several million light years, almost all are coming closer towards our Milky Way under the influence of collective mass of all these galaxies, one of which is Milky Way. On a scale of tens to hundred of millions of years, majority of galaxies are still coming closer to us, because most of these galaxies within hundreds of millions of years are still mainly operating under eachother's gravitational influences. But expand the horizon to 5 billion light years, and you find the number of galaxies coming closer to us shrink to insignificance next to the number moving away. This story doesn't depend on the observer being on the Milky Way. No matter which galaxy you are in, the same story will repeat.

[Wyrd of Gawd]

She's insane. We know for a fact that galaxies don't move away from each other because we know that they merge together. We are merging with the Milky Way right now and the Milky Way will merge with Andromeda. We have pictures of galaxies merging.

[Anomalocaris]

She's insane. We know for a fact that galaxies don't move away from each other because we know that they merge together. We are merging with the Milky Way right now and the Milky Way will merge with Andromeda. We have pictures of galaxies merging.

No. 99.9999999% of all galaxies are moving away from our Milky Way, at a speed far in excess of the gravitational escape velocity of Milky Way, meaning they will never reverse course and come back towards us. Only 0.00000001% of all galaxies are moving with any hint of coming closer or merging with our Milky Way any time in the future.

It would be the same with any other galaxy.

On the scale of the visible universe, Expansion of universe vastly overmatched Mutual attraction between galaxies for all but the closest galaxies.

[Wyrd of Gawd]

The problem with gravity is that there seems to be a lot of different kinds of it. One kind can attract galaxies to merge. Another kind keeps our solar system in balance. Yet another kind keeps the stars strung out in arms as they swirl around the center of the galaxy. Then there's another kind that keeps the stars at the center of the galaxy from colliding with each other and actually causes them to create a giant ball. Gravity is some weird stuff.

[Anomalocaris]

No, it's all exactly the same kind of gravity, describable by a single common equation. No one has found another type of attractive force that can operate and appear to actually operate over cosmic distances yet.

[Cato]

The problem with gravity is that there seems to be a lot of different kinds of it.

I am sincerely interested in knowing where you learned this.

[max-greece]

Gotta go with one kind of gravity like others have.

My problem with gravity is.....it really doesn't seem to be a force at all.

Consider the equation:

F = G (M1+M2)/r^2

OK - G is a constant and has no units.
M1 + M2 are mass - Kg.
R is distance - m

So the units of gravity are Kg/m^2

The thing is:

F = ma.

M is mass - Kg.
A is acceleration - m/s^2

So the units of force are Kgm/s^2

In other words there is a time element to force but gravity lacks it.

Never made any fucking sense to me.