by toejam » Tue Oct 16, 2007 8:20 pm
toejam wrote:Chris Peterson wrote:toejam wrote:What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?
The expansion of space is nowhere near strong enough to overcome the local effects of gravity. That's why the Earth isn't expanding with the Universe, and it's why groups of galaxies can be gravitationally bound, allowing for some collisions. Also, even galaxies that are not gravitationally bound can still have motion paths that intersect, even in an expanding Universe.
The expansion of space is presumably uniform, but the distribution of material is not, and neither is the distribution of velocities.
Thank you Chris.
It boggles my mind that the Milky Way and the Andromeda Galaxy, some 2 MILLION light-years apart, are approaching one another at 120 km/sec owing purely to gravitational attraction at that distance. I am told (not my calculation) that they will collide in a mere 5 BILLION years. There's something to worry about!!!
![Very Happy :D](./images/smilies/icon_biggrin.gif)
More questions.
Oct 12 APOD
http://antwrp.gsfc.nasa.gov/apod/ap071012.html
States that the galaxies there have had close encounters in the past.
Well, now they are far apart, so gravity did not cause their motion to halt when they were close. Why? If gravity is responsible for their motion towards one another it should not allow them to "overshoot" in this way. Should they not have comingled?Or will theyfall back together again, because of gravitational attraction? I have not ever seen any statement that this happens, although there have been many mentions of galaxy "capture" and many of galaxies "having passed through" one another. What keeps them going? Are they going to reverse?
[quote="toejam"][quote="Chris Peterson"][quote="toejam"]What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?[/quote]
The expansion of space is nowhere near strong enough to overcome the local effects of gravity. That's why the Earth isn't expanding with the Universe, and it's why groups of galaxies can be gravitationally bound, allowing for some collisions. Also, even galaxies that are not gravitationally bound can still have motion paths that intersect, even in an expanding Universe.
The expansion of space is presumably uniform, but the distribution of material is not, and neither is the distribution of velocities.[/quote]
Thank you Chris.
It boggles my mind that the Milky Way and the Andromeda Galaxy, some 2 MILLION light-years apart, are approaching one another at 120 km/sec owing purely to gravitational attraction at that distance. I am told (not my calculation) that they will collide in a mere 5 BILLION years. There's something to worry about!!! :D[/quote]
More questions.
Oct 12 APOD
http://antwrp.gsfc.nasa.gov/apod/ap071012.html
States that the galaxies there have had close encounters in the past.
Well, now they are far apart, so gravity did not cause their motion to halt when they were close. Why? If gravity is responsible for their motion towards one another it should not allow them to "overshoot" in this way. Should they not have comingled?Or will theyfall back together again, because of gravitational attraction? I have not ever seen any statement that this happens, although there have been many mentions of galaxy "capture" and many of galaxies "having passed through" one another. What keeps them going? Are they going to reverse?