Universe expansion and merging galaxies

[vincentm]

According to the Raisin pudding theory all raisins (galaxies) are to be receding from one another, so why is it that there are exceptions for example the Andromeda Galaxy that is heading towards the Milky way at 50 kilometers a second? and other such galaxies merging together if they are to be headed away from each other?

Are the raisins going to just be galactic clusters?

 

[Chronos]

Yes, there are many examples. Gravitational attraction is stronger than the Hubble flow in local clusters. Will cite papers if desired. By the way, welcome to PF. Well constructed questions, like yours, are welcome here. You will find many like minded people here.

 

[vincentm]

oh chronos I'm loving this site, i been interested in cosmology for awhile now, but i suck at maths. So i am going to be taking some Algebra classes, and i also picked up quite a few books, i think this will help me in my studies in this field of science

 

[hellfire]

Expansion of space is explained by general relativity: Assuming the cosmological principle in space (homogeneity and isotropy in the spatial distribution of matter) one arrives to a solution of Einstein equations in which space can expand or contract. Observations tell us space expands.

The validity of this description is given by the validity of the assumptions used for its derivation: In a scale where the distribution of matter is not homogeneous and isotropic, space must not expand (it may, however). Observations tell us that the present universe is homogeneous and isotropic as a whole at scales greater than 100 Mpc (326 million light years).

 

[Chronos]

Everybody sucks at math, at first. Math is hard, as a personal friend suggests. Hellfire is very good at math, take his advice. He is very competent.

 

[vincentm]

Expansion of space is explained by general relativity: Assuming the cosmological principle in space (homogeneity and isotropy in the spatial distribution of matter) one arrives to a solution of Einstein equations in which space can expand or contract. Observations tell us space expands.

The validity of this description is given by the validity of the assumptions used for its derivation: In a scale where the distribution of matter is not homogeneous and isotropic, space must not expand (it may, however). Observations tell us that the present universe is homogeneous and isotropic as a whole at scales greater than 100 Mpc (326 million light years).

I'm not understanding the terms homogenous and Isotropic.

 

[hellfire]

I'm not understanding the terms homogenous and Isotropic.

Homogeneous means with same properties in each point of space. Isotropic means with same properties in every direction of space. For the matter and energy distribution in space we may have an universe which is isotropic but not homogeneous if it looks the same in every direction but there are e.g. gradients of energy density in the line of sight with spherical symmetry around the Earth (such an universe would violate the Copernican principle). Moreover we may have an universe which is homogeneous but not isotropic in case e.g. it would exist an homogeneous magnetic field in space pointing in one specific direction. The cosmological principle implies both, homogeneity and isotropy in space.

 

[vincentm]

Homogeneous means with same properties in each point of space. Isotropic means with same properties in every direction of space. For the matter and energy distribution in space we may have an universe which is isotropic but not homogeneous if it looks the same in every direction but there are e.g. gradients of energy density in the line of sight with spherical symmetry around the Earth (such an universe would violate the Copernican principle). Moreover we may have an universe which is homogeneous but not isotropic in case e.g. it would exist an homogeneous magnetic field in space pointing in one specific direction. The cosmological principle implies both, homogeneity and isotropy in space.

Thanks, I think that helps explains things more clearly to me.

 

[Phobos]

Welcome to Physics Forums, vincentm!

Hellfire is getting into the finer points of cosmology (something we love doing here), but like Chronos said, sometimes in a localized region gravity wins out over the overall expansion of space. There are several galaxies in our "Local Group" that are bound together via gravity (mostly small galaxies...with the 2 big players being the Milky Way and the Andromeda galaxies).

 

[SpaceTiger]

For the matter and energy distribution in space we may have an universe which is isotropic but not homogeneous if it looks the same in every direction but there are e.g. gradients of energy density in the line of sight with spherical symmetry around the earth

Before I respond to vincentm, I want to stray from hellfire's terminology a bit. When I say the universe is isotropic, I'm going to mean that it looks the same in all directions at all places in the universe.

Thanks, I think that helps explains things more clearly to me.

If I'm correctly interpreting that as sarcasm, then consider the following analogy. Let's say that you're standing in a forest with trees lined up in a regular and periodic fasion, and also that the forest goes on infinitely in all directions. Then, consider what it would mean if the forest had the following properties:

1) Completely flat terrain.
2) The forest sits on an infinite inclined plane (like the side of a mountain).
3) The forest is on a hill that extends downward infinitely from some peak.

Now, if I say that

1) Homogeneity implies that the forest appears the same from every point.
2) Isotropy implies that, from any given point in the forest, the forest looks the same in all directions.

Can you take each of the examples I gave and say whether they're homogeneous, isotropic, both or neither? Sorry to give you homework, but I think this is easier to understand if you think it through yourself.

 

[matt.o]

it just means that if you look north and than south you should observe that same type of properties (isotropy) and then if you move a million parsecs north to some point you should see the same properties as if you moved a million parces south (homogenous). ie. the universe is the same at every point in space and in every direction you look.

 

[matt.o]

what do you know, 2 people got in before me whilst I was typing!

 

[vincentm]

If I'm correctly interpreting that as sarcasm,

no i wasn't trying to be sarcastic, my apologies, i was simply trying to find an :unsure: icon.

I'll just lay off the emoticons for now, anyway i joined this site because i wanted to have a valuable resource for such studies, and this is a very good one. I mostly just lurk here and read alot. I'm not going to discuss things i don't know nothing about, but i will ask if i don't know anything. I appreciate your help ST your very knowledgeable about cosmology.

 

[SpaceTiger]

no i wasn't trying to be sarcastic, my apologies, i was simply trying to find an :unsure: icon.

In that case, anyone is free to respond to my analogy. I'm curious as to whether everyone will agree on the answer.

 

[SpaceTiger]

In that case, anyone is free to respond to my analogy.

Come on, someone give it a shot. I won't bite...

 

[LURCH]

Let's say that you're standing in a forest with trees lined up in a regular and periodic fasion, and also that the forest goes on infinitely in all directions. Then, consider what it would mean if the forest had the following properties:

1) Completely flat terrain.
2) The forest sits on an infinite inclined plane (like the side of a mountain).
3) The forest is on a hill that extends downward infinitely from some peak.

Now, if I say that

1) Homogeneity implies that the forest appears the same from every point.
2) Isotropy implies that, from any given point in the forest, the forest looks the same in all directions.

Can you take each of the examples I gave and say whether they're homogeneous, isotropic, both or neither? Sorry to give you homework, but I think this is easier to understand if you think it through yourself.

1)On completely flat terrain, the forrest would be both homogenous and isotropic, because it would look the same in all directions and, if I relocate, it would still look the same.

2)The inclined plane, the problem is a little more difficult, because if I lean to match my incline with that of the plane, it could appear to be the infinitely flat foprrset from question "1)". However, since the idea of being "inclined" suggests that the plane of the ground's surface is not orthaganal to the direction of gravitational influence, I'll just make that assumtion and conclude that the forrest would appear homogenous, because anywhere I stand within the forrest, I look around and see the same basic properties, but not isotropic, because when I look in one direction, I see a forrest sloping uphill away from me, and when I look in the opposite direction, I see a forrest sloping downard away from me.

3)This last forrest would appear homogenous to an observer standing on the peak; everywhere I lokk I will see forrest sloping downward away from my location. It would not, however, be isotropic, because if I move down off the peak, I will see forrest slping up in one direction, and downward in the opposite direction. This would also mean that, from anyplace other than the peak, the universe is not homogenous. (BTW; Can anyone think of a situation where the forrest could be anisotropic while still being homogenous from all locations? Is that even possible?)

 

[Nereid]

In that case, anyone is free to respond to my analogy. I'm curious as to whether everyone will agree on the answer.

It depends

'lined up in a regular and periodic fashion'? 'every point'?

Clearly none of the forests will be homogeneous; even if 'points' can be on only the 'forest floor', you will see a very different forest if you are right next to a giant ash than a baby oak (and if all the trees are the same, then the forest will look different, depending on how close you are to the nearest).

Similarly wrt isotropy.

Looking solely at what 'regular periodic' does: what sort of 'regular periodic'? - some will allow sightlines to infinity, others not. In any case, I doubt any of the forests could be isotropic.

However, I suspect ST may have wanted us to ignore 'local' effects

 

[SpaceTiger]

However, I suspect ST may have wanted us to ignore 'local' effects

You suspect correctly. I invoked peroidicity only to simplify the picture in people's minds. In fact, if we consider local effects, the question is trivial for the universe as well.

As for LURCH's response, that's pretty much the answer I had in mind. It turns out that I could not give you an example that was isotropic but not homogeneous because the former automatically implies the latter.

 

[vincentm]

But doesn't the fact, of different types of galaxies, existing (barred, spiral, barred-spiral, elliptical etc..) not make the universe homogenic/isotropic. Meaning if i was in interstellar space (between galaxies) that i would not see the same "point"/"type of object" everywhere?

 

[Chronos]

You would see no statistically significant difference in their distribution no matter what direction you looked - assuming you don't cherry pick the sample data set.

 

[SpaceTiger]

But doesn't the fact, of different types of galaxies, existing (barred, spiral, barred-spiral, elliptical etc..) not make the universe homogenic/isotropic.

Yes, and on small scales, it isn't homogeneous and isotropic. When we describe the universe that way, we're usually talking about averaging over much larger scales (say, 100 times the distance to the nearest galaxy).