Starship Asterisk*

by **Osage** » Wed May 12, 2010 8:41 pm

Thanks, I have a habit of thinking in linear three dimensional terms. That explains it nicely.

by **Chris Peterson** » Wed May 12, 2010 5:14 am

Rabelad wrote:Please explain how galactic collisions could occur when the Big Bang theory says that all matter in the universe was ejected from a giant explosion. If that is the case then all matter would be flying outward from the central point of the primordial explosion and each would be ejected at it's own unique angle. If that be the case then and none should interfere with another, as the Big Bang would have directed each at an angle that could not cross the path of any other. Ok, so gravitational forces somehow get to affect the galaxies, forces that are perhaps much stronger than the momentum of the ejected matter from the Big Bang. So if that be the case then such collisions should have happened much earlier in the evolution of the universe because they were all so much closer to each other, say, a couple of billion years back and not in these later times.

Most galaxies are part of clusters- groupings that are gravitationally bound. That means that the cluster components orbit one another in a complex dance (and are not getting farther apart as the Universe expands). This can go on for billions of years with no collision taking place. Therefore, we can see recent collisions, and there are collisions that will occur in the future (such as between the Milky Way and the Andromeda galaxy). There is also solid evidence in looking at the structure of galaxies that many of them have been involved in collisions in the distant past.

by **Rabelad** » Wed May 12, 2010 5:09 am

Please explain how galactic collisions could occur when the Big Bang theory says that all matter in the universe was ejected from a giant explosion. If that is the case then all matter would be flying outward from the central point of the primordial explosion and each would be ejected at it's own unique angle. If that be the case then and none should interfere with another, as the Big Bang would have directed each at an angle that could not cross the path of any other. Ok, so gravitational forces somehow get to affect the galaxies, forces that are perhaps much stronger than the momentum of the ejected matter from the Big Bang. So if that be the case then such collisions should have happened much earlier in the evolution of the universe because they were all so much closer to each other, say, a couple of billion years back and not in these later times.

by **Chris Peterson** » Tue May 11, 2010 2:50 am

neufer wrote:However, the general terminology states that our telescopes are virtual time machines that allow us to see into the past.
Hence, our telescopes do NOT allow us to see the much larger "visible?" universe that is "now".

That's fine. The point remains, we can only see each three-dimensional point at one time. That is very different than the way we see space, where every (x,y,z) is accessible to us.

by **neufer** » Tue May 11, 2010 2:27 am

Chris Peterson wrote:
neufer wrote: We can't see along the time axis or along any space axis...but that is not really the issue.
We can certainly see along the spatial axes. We can see any point in the observable universe, as some (x,y,z) at t=now (in the most common sense, that we always see now; you can adjust for distance if you like, it doesn't change the argument). The point is, any given point (x,y,z) we can only see at one time, whatever you want to call that time.

I am quite happy to call the past light cone as "now" and claim that the true "visible" universe is 13.7 billion light years in radius.

However, the general terminology states that our telescopes are virtual time machines that allow us to see into the past.
Hence, our telescopes do NOT allow us to see the much larger "visible?" universe that is "now".

Chris Peterson wrote:
neufer wrote:The reason we can't see the center because the CMBR plasma gets in the way.
We still wouldn't see a center in the sense the OP was asking about. Because there is no center that can be defined in three dimensions for our universe.

On that we agree.

by **Chris Peterson** » Tue May 11, 2010 2:04 am

neufer wrote: We can't see along the time axis or along any space axis...but that is not really the issue.

We can certainly see along the spatial axes. We can see any point in the observable universe, as some (x,y,z) at t=now (in the most common sense, that we always see now; you can adjust for distance if you like, it doesn't change the argument). The point is, any given point (x,y,z) we can only see at one time, whatever you want to call that time. We can't explore that point in the t dimension. We can explore the entire Universe in the x, y, and z directions.

The reason we can't see the center because the CMBR plasma gets in the way.

We still wouldn't see a center in the sense the OP was asking about. Because there is no center that can be defined in three dimensions for our universe.

by **neufer** » Tue May 11, 2010 1:16 am

Chris Peterson wrote:
neufer wrote: Everywhere one looks is in the direction of the big bang center at t=0; isn't it?
If you like. I don't really see any problem with what I said originally, though.
We can't see the center because it is on the time axis, and we can't see along that axis.

We can't see along the time axis or along any space axis...but that is not really the issue.

The reason we can't see the center because the CMBR plasma gets in the way.

by **Chris Peterson** » Tue May 11, 2010 1:04 am

neufer wrote:But that is not what you said!

Everywhere one looks is in the direction of the big bang center at t=0; isn't it?

If you like. I don't really see any problem with what I said originally, though. We can't see the center because it is on the time axis, and we can't see along that axis.

by **neufer** » Mon May 10, 2010 11:55 pm

Chris Peterson wrote:
neufer wrote:I thought that the center at t=0 is in the ONLY direction that we can see.
Not at all. As we look at distant objects, we see their three-dimensional state at some time t<now, but that isn't the same as seeing along the time axis of spacetime.

But that is not what you said!

Everywhere one looks is in the direction of the big bang center at t=0; isn't it?

by **Chris Peterson** » Mon May 10, 2010 10:12 pm

neufer wrote:I thought that the center at t=0 is in the ONLY direction that we can see.

Not at all. As we look at distant objects, we see their three-dimensional state at some time t<now, but that isn't the same as seeing along the time axis of spacetime.

by **neufer** » Mon May 10, 2010 10:02 pm

Chris Peterson wrote:The Big Bang describes an "explosion" in four dimensions, not three. So there is no sphere of expansion, and never was. The expansion we see is taking place on the three-dimensional surface of a four-dimensional hypersphere. The center is at t=0 - a direction we can't see.

I thought that the center at t=0 is in the ONLY direction that we can see.

by **Chris Peterson** » Mon May 10, 2010 9:28 pm

Osage wrote:From the Big Bang onward, it would seem to me that all significant mass (ie galaxies) would be travelling outward spherically from the point of the explosion with that sphere having an ever-expanding radius. I can see how individual nova could send mass in different directions, but as far as galaxy size aggregates are concerned it seems they should be travelling along non-intersecting vectors. Plus, without friction, shouldnt they all be travelling at the same velocity, therefore no rear-end collisions? What caused this deviation from the initially expanding sphere?

The Big Bang describes an "explosion" in four dimensions, not three. So there is no sphere of expansion, and never was. The expansion we see is taking place on the three-dimensional surface of a four-dimensional hypersphere. The center is at t=0 - a direction we can't see. There is no three-dimensional center of expansion, which is why it appears everything is moving away from everything else, and why objects with greater separation are moving apart faster.

by **Osage** » Mon May 10, 2010 5:59 pm

From the Big Bang onward, it would seem to me that all significant mass (ie galaxies) would be travelling outward spherically from the point of the explosion with that sphere having an ever-expanding radius. I can see how individual nova could send mass in different directions, but as far as galaxy size aggregates are concerned it seems they should be travelling along non-intersecting vectors. Plus, without friction, shouldnt they all be travelling at the same velocity, therefore no rear-end collisions? What caused this deviation from the initially expanding sphere?

Thanks,
Osage

by **ems57fcva** » Fri May 07, 2010 11:57 pm

I will stand corrected. I was looking at the Hubblesite picture of these galaxies (http://hubblesite.org/newscenter/archiv ... rmat/zoom/), and found that they do have short, stubby secondary tidal tales. Still, they are nothing like the secondary tidal tales in the simulations. Furthermore, they are distinct from the bridge of gas and starts joining the galaxies, making that bridge even more inconsistent with the simulations.

EMS

by **ems57fcva** » Fri May 07, 2010 9:20 pm

JohnD wrote:Thank you, ems! I liked that very much, esp. the CGI rotates between actual photos, demonstrating I hope to verks how the tails do look.

I still think the second video is very, very similar to the original APOD, stop it at 6 seconds and see. [sp. corrected]

I'm not going to deny the similarity, especially at low resolution. Certainly the simulations of the early parts of the interactions are more-or-less correct, based on their correctly generating long tidal tails coming off of the trailing sides of the interacting galaxies. However, at 6 seconds the colliding galaxies are tearing each other up, and turning into an elliptical mass. There is no bridge, just chaos. On the other hand, NGC 4038 (on the right) seems to me to be fairly intact, and remains distinct from NGC 4039 (on the left and behind the left end of the bridge). Also, at 6 seconds into the simulation the secondary tidal tails (which are collinear with the primary tidal tails) are still being reabsorbed, but in the Antennae there is no sign of them. Instead there is this bridge which joins the interacting galaxies and does so in each case at a substantial angle from the orientation of the tidal tails as they join their parent galaxies.

This may impress you as just a bunch of details, but do recall that "the devil is in the details".

BTW - With the video I posted, I really suspect that the most developed collision is actually the third one shown, since none of the others had developed a connecting bridge yet.

EMS

by **JohnD** » Fri May 07, 2010 7:46 pm

Thnak you, ems! I liked that very much, esp. the CGI rotates between actual photos, demonstrating I hope to verks how the tails do look.

I still think the scond video is very, very similar to the origianl APOD, stop it at 6 seconsd and see.

John

by **ems57fcva** » Fri May 07, 2010 7:03 pm

JohnD wrote: Loads of simulations on internet video, like this: http://www.youtube.com/watch?v=RBFc1ldP_Ps
I presume they are based on basic physics and they look very like the APOD.
OR, this one is IDENTICAL! http://www.youtube.com/watch?v=P8S-RW9Veks

Neither one of those simulations shows a bridge developing between the galaxies as is seen in this image! Furthermore, the second simulation (which you claim to be "IDENTICAL") has two secondary tidal tails appearing on the opposite sides of the galactic cores from the primary tails. No such tails are present in this image! Instead, the "X" shape is created by the crossing of the tidal tails after the galaxies have passed.

To top it off, there is another You Tube video show comparing galaxy collisions and actual photos as http://www.youtube.com/watch?v=lXy3B2K4 ... re=related. In the third comparison, the simulation shows two disconnected tidal tails, while the comparison photo shows a unified bridge between the galaxies, and some evidence for large chunks of matter starting to move into that bridge.

Let's just say that I see these simulations as doing a very poor job of reflecting reality. There is something missing from them. Whether it is dark matter, colliding gas being left in the middle, or what I cannot say.

by **Hofi** » Fri May 07, 2010 6:21 pm

Last year, there was an eppisode of "Hubble's Universe Unfiltered" which discussed the topic of merging galaxies. There was also an animation which, in my opinion, was quite good.
You can find it here. Hope it helps.

by **JohnD** » Fri May 07, 2010 6:13 pm

verk wrote:Can I ask some basic questions?
If this really is the photo of 2 galaxies that have collided, why do the curvatures in the tails become sharper as they collide? It seems the opposite would happen. Since we suppose that the galaxies are attracted to each other due to gravitational forces, the curvature lines should become more straight as they get closer to each other, but in fact it looks like the opposite.

verk,
Loads of simulations on internet video, like this: http://www.youtube.com/watch?v=RBFc1ldP_Ps
I presume they are based on basic physics and they look very like the APOD.
OR, this one is IDENTICAL! http://www.youtube.com/watch?v=P8S-RW9Veks

No one has asked: What would it be like to be on a planet in thos galaxies?
No stellar collisions, but lots of star forming, energy all over the place.
COULD there be life on such a planet. or would that whole region of space be sterilised?

John

by **Chris Peterson** » Fri May 07, 2010 5:30 pm

Byran wrote:I do like the image. I am new to learning astronomy. Are the individual stars (foreground stars) in the Milky Way? :?

Yes, all of the stars are in the foreground, and in the Milky Way. Except... if you look closely you'll see that some of those "stars" are actually distant galaxies.

by **Byran** » Fri May 07, 2010 4:35 pm

I do like the image. I am new to learning astronomy. Are the individual stars (foreground stars) in the Milky Way?

Byran

by **Chris Peterson** » Fri May 07, 2010 2:56 pm

biddie67 wrote:A very non-technical WOW!! What a great picture. So many questions - I was wondering also how the two galaxies' central core/black holes could possibly not interact with each other and even pull together.

The gravitational effect of the central black holes is small compared with the galaxies as a whole. And the galaxies have been interacting for billions of years- orbiting each other in a complex way that has led up to a collision (possibly not for the first time). Black holes don't pull together any more than any other objects operating under the influence of gravity. They were in orbit around each other when the galaxies were distant, and the only difference when the galaxies are colliding is that they are nearer each other in that orbit.

by **Chris Peterson** » Fri May 07, 2010 2:52 pm

JMM wrote:What happens when two spiral galaxies with presumptive central black holes colide? I realize that the galaxies are primarily empty space, but do we know if the black holes merge or the gravitational forces are counterbalanced by net forces that repulse?

In terms of their impact on galaxies, even supermassive black holes are rather minor and boring. They typically only have the mass of some millions of stars- a tiny fraction of the entire galaxy. When two galaxies collide, the dynamics are pretty much the same whether or not there are central black holes, because the gravity fields of the galaxies are pretty much the same either way. Only in a tiny region very close to the black hole is the gravity field sufficiently distorted to get interesting effects.

You talk about new star formation by merger of dust clouds, but the central black hole interaction must create huge and measurable changes also.

Only in the very rare case where two galaxies collide in such a way that their central black holes pass very close to each other. In that case, the two black holes may enter a tight orbit around each other, and may even merge after some time.

What about the large amount of "Dark Matter" and "Dark Energy" that permeates and surrounds the galaxies? Do they become measurable in some way during this galactic "accident"?

There is no dark energy around galaxies. But the structure of galactic collisions does reveal much about their dark matter halos, and is part of the evidence supporting the existence of dark matter.

by **ems57fcva** » Fri May 07, 2010 2:48 pm

The thing that impresses me is the bridge of gas and stars between the cores of the galaxies. To get a better look at it, click on the photo or go to http://apod.nasa.gov/apod/image/1005/NGC4038_ssro.jpg and zoom in on the cores of the galaxies. The impression I have is that the material is being funneled into a concentration of mass on the left side of the bridge. I assume that this concentration is newly forming galactic center which is in front of the galactic core on the left. (The other possibility is that the material is being funneled into the center of the galaxy on the left, but I don't expect that to be the case unless the left galaxy is much more massive than the one on the right.)

by **biddie67** » Fri May 07, 2010 2:21 pm

A very non-technical WOW!! What a great picture. So many questions - I was wondering also how the two galaxies' central core/black holes could possibly not interact with each other and even pull together.

Imagine if somehow two black holes were aligned in opposite directions/spins/forces and would just null each other out.

But then what if they were aligned in the same orientation - would they become so powerful that they could pull in most, if not all, of the matter in each of the original galaxies.

I know, I know - more imagination than technical expertise ........

Starship Asterisk*

APOD: The Antennae (2010 May 07)

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