Huge Hydrogen Cloud Will Hit Milky Way

diewlasing points us to a story about a hydrogen cloud, eleven thousand light-years long, which will collide with the Milky Way in a devastating crossfire of shock waves and star formation...in 20-40 million years. Mark your calendars. At least it will give us something to watch while we're waiting for Andromeda to hit us in a few billion years. Hopefully, it will look at least this cool. "The detailed GBT study dramatically changed the astronomers' understanding of the cloud. Its velocity shows that it is falling into the Milky Way, not leaving it, and the new data show that it is plowing up Milky Way gas before it as it falls. 'Its shape, somewhat similar to that of a comet, indicates that it's already hitting gas in our Galaxy's outskirts,' Lockman said. 'It is also feeling a tidal force from the gravity of the Milky Way and may be in the process of being torn apart. Our Galaxy will get a rain of gas from this cloud, then in about 20 to 40 million years, the cloud's core will smash into the Milky Way's plane,' Lockman explained."

Re:I don't think it means what you think it means.

[maxwell demon]

Does anyone else have a problem with the word "smashing" to describe the contact of two bits of not-quite-vacuum passing through each other?

You mean, like a stone smashing into a window? You don't actually think the electrons or atomic nuclei of the stone actually come into contact with the electrons or atomic nuclei of the window, do you?

Re:I don't think it means what you think it means.

[Anonymous Coward]

It's not a matter of coming into contact (at the smallest level, every elementary particle may well be mathematical points), but of getting close enough for an interaction force to be produced.

furlongs and donkey forthnights

[viking80]

Here is just some useful unit conversations:
suns = 2E30 kg
light year = 1E16 meters
So this cloud has a density of 28 H2 molecules per liter.
That is pretty good vacuum. Actually about a million times better vacuum than "deep vacuum" in outer space here in our solar system, which again is much better vacuum than what is achievable here on earth.

So this "collision" will be quite soft in terms of energy density: One feather landing on an area the size of the earth.

Re:Shot in the Dark

[Amorymeltzer]

Don't forget to multiply! It's actually 0.01% so it's more akin to throwing a baby at an 18-wheeler. Not a huge effect, but when it's traveling fast it might dent a small portion of it, which is all we would need, really.

However, I was more referencing the ability of the cloud to interact with everything else going on. A baby/bug will either bounce or splat against a truck/car but the gas can and will interact with nearby stars, solar systems, etc. THAT's what would be cool to measure - aside from the fact that it'd be very complex and unpredictable (although, by then, who knows) any minute deviation could give an indication of the amount of dark matter (not) floating around, for example.

Re:Gravitation is an interaction force

[torako]

Dark Energy is really just a term to describe the fact that we can't come up with an expanding universe even if we add up all the known effects that could cause an expansion. There isn't a working theory of Dark Energy yet, so while we know that "something" has to provide for the expansion of the universe, we still don't know what that might be.

The four forces are an entirely different matter. Electromagnetism, the strong and weak forces are summed up in the Standard Model of Particle Physics (in the form of the Electroweak Theory + Quantum Chromodynamics), which is very well tested and in fantastic agreement with experiments. Gravity doesn't fit into the mathematical framework of quantum mechanics, but the theory of General Relativity has been tested experimentally and is almost universally accepted.

So that's basically the reason... You have four interactions for which we have very well tested theories and mathematical tools, while we know almost knothing about Dark Energy except for the fact that we need it to make our cosmological models work