Are Galaxies Independent ?

[modeller]

Do any galaxies touch or overlap each other ?

[jim360]

Galaxies can interact with each other just like anything else, only probably more violently. We're due our own collision with Andromeda, apparently, in a few billion years or so.

I think others have collided but the time scale of such collisions is very long so they are probably not too dramatic in a single picture. I'll try to dig one or two up for you:

http://en.wikipedia.org/wiki/Mice_Galaxies

http://en.wikipedia.org/wiki/NGC_520

[jomifl]

They do indeed inter-react, there are some good computer models of galaxies 'colliding' although they mostly just pass through one another, being mostly empty space.

[Zacs-Master]

That's pretty amazing isn't it ( that they pass through one another). Just shows how vast and empty interstellar space is. I like things which give a little 'scale' to the universe.

[jim360]

The gravity effects ten to mean that they join up rather than just pass through -- but certainly very few of any objects in the galaxies will collide. It is fairly incredible, though. How empty space is indeed.

[jomifl]

If this link works it might be interesting
http://isc.astro.cornell.edu/~spoon/crashcourse/collisions.html

[jim360]

I am going to have so much fun with that later. Thanks!

[Clanad]

Probable collided galaxies here:

http://csep10.phys.utk.edu/astr162/lect/galaxies/colliding.html

[Spoonboy]

The crab nebula is the collision of two galaxies. I'd love to come back in about four and a half billion years and see Andromeda after colliding with our Milky Way galaxy.

[Teddio]

Isn't the Crab Nebula a supernova remnant?

[Mojo-Jo-Jo]

Yes it is Teddio.

[ToraToraTora]

The Crab nebula is not a collsion between 2 galaxies, it's the remains of a supernova that was actually seen in the 11th century on our calendar.

Do a search for "galaxy collisions" there are many photgraphed through hubble. When galaxies collide there are very few actual collisions but there is a gravitational dance that will rip entire solar systems out of one galaxy and into another. When it all calms down you are left with a large blob of stars merged rather than the traditional spiral. It is thought that over time these do become spirals though much bigger than before. It's hard to see from inside but our own galaxy actually has a couple of smaller "sub" galaxies that may well be the remnants of previous collisions. The LMC for example.

[Old_Geezer]

Space is expanding so usually galaxy move away from each other, but some move fast enough in the opposite direction to buck the trend. Stick around and se Andromeda hit the Milky Way eventually. However since most of a galaxy is the space between things, one assumes few spectacular collisions.

[modeller]

It would appear that gravity is a mirror image of dark energy.
Is that a correct assumption , with dark energy being the stronger.

[jim360]

I don't think anyone has the foggiest yet. Probably the other way round, in some sense -- i.e. dark energy is just inverted gravity, but there's no consensus on this.

Have you ever done integral calculus? If so, here's a simple-ish way to explain how dark energy might arise.

Whenever anyone talks about equations for just about anything in physics they nearly always mean equations of a form:

rate of change of quantity X = something else.

The rate of change bit implies differentiation, so that if you were ever going to solve these equations you'd have to integrate at some point. The general form is then:

X = integral of something

If you have done integration then you will remember that if we don't have any specific boundary conditions then you have to write something like:

integral of x = (1/2)x^2 + constant

and in A-level exams people lose marks just for forgetting to put "+ c" on the end of their results! It's annoying, anyway, to remember, but has to appear, as whenever you differentiate just a number you get zero. And as integration is the inverse of differentiation you must remember to add the number on.

Now of course in General Relativity we are dealing with four-dimensional space time but the basic form of the equations is essentially the same, i.e rate of change of space and time is given by the distribution of energy and mass in the Universe ("matter bends space, and this is how" is effectively all the equations say).

We can differentiate the equations of general relativity once more. Unsurprisingly the amount of energy and matter in the Universe is assumed to be constant -- where else can it go? -- so one side vanishes completely and so the other side must also vanish. But then we have a law that says basically:

Overall "rate of change" of the curvature = zero

This is a law that we can then integrate back up, but in doing so we have to allow for that "+ constant" again. A more complicated "+constant", but the same idea. You have to allow for it for the equations of General Relativity to be in their most general form. In fact this "+constant" is just the cosmological constant that is believed to be the driving force for dark energy.

[mibn2cweus]

modeller
Question Author
It would appear that gravity is a mirror image of dark energy.
Is that a correct assumption , with dark energy being the stronger.
21:32 Fri 24th May 2013

Gravity is associated with mass and obeys the inverse square law.
'Dark energy' appears to be evenly distributed through space.
The prevalent force, by far, where mass is relatively near by is gravity.