First post for me... \/

Anyways...I don't feel like questing through 5 million pages so I hope this isn't too much of a repeated question...

Where is the center of the universe? (besides me)
And what if anything might be located in the center? (besides chocolate filling)

Hi, Welcome!

Astronomer Ned Wright has some nifty graphics, which give an initial explanation of this:

http://www.astro.ucla.edu/~wright/nocenter.html

Some other descriptions you'll encounter are:

The universe has no center in the conventional sense (as in a conventional explosion.) Every given location in the observed universe appears to be expanding away from the rest of the observed universe on the broadest scale.

The universe is finite yet unbounded.

The universe is not an explosion (with a center, though it has an origin in time,) and is not an explosion filling a pre-existing space. It "is" space.

You can think of yourself at the center of the expansion - but realize that others will see themselves at the center of the expansion wherever they are. (If they can see the sky.) :wink:

You can visualize the expansion of the universe with an elastic ribbon with stars stuck along it every 5 inches. Pick any star and pretend it represents your home world. Now stretch the ribbon away from this star. You'll notice that the other stars appear to be moving away from you personally. But jump to any other "star" and stretch the ribbon again, and you get the same effect. No center, just expansion from everywhere. (*Some people use an inflating spotted balloon to symbolize no center and expansion.) The ribbon is better in my opinion.

Others have different ideas, and still others disagree completely with this notion. Most scientists accept the "Big Bang" theory however, (a bad name but we're stuck with it,) because it lines up with observation on many levels.

It’s a fascinating question, and people shouldn’t loose their temper over it, but some do.

Dragonlor

It is customary that you will receive the balloon analogy or 'every point is the valid center' but this is an especially hollow answer.

What you can do instead is notice that almost everything is rotating/revolving around something else,the moon around the Earth,the Earth around the Sun,the solar system around the galaxy and perhaps inquire if there is a greater rotation.

As Newton used swinging buckets and ships at sea as analogies to carry his points on planetary motion we too can use a modern form of these analogies,for instance,why hurricane development and structure bears a striking resemblence to galactic structure.

http://rst.gsfc.nasa.gov/Sect20/20.jpg

http://www.atmos.albany.edu/facstaff...10269812ir.gif

The point of the hurricane analogy is that the Earth's rotation gives them the familiar spiral arms while during an epoch when the galaxies were forming there must have been a greater rotation present,while not suggesting a universal center of any kind it is easier to work with galactic structure and formation with a greater rotation than appealing to 'dark matter' to do the job.

The difficulty is interpreting how we observe a greater rotation than galactic rotation,you can do it by translating the 'acceleration' in 'accelerating expansion' into rotation but this is a contentious issue at the moment in this forum in terms of the way clocks measure distance away from an axis.

Just to be careful,it is easier to speak of a greater rotation than galactic rotation rather than a universal 'center' and a greater rotation only in respect to galactic formation and structure.

While Universe rotation makes sense on some levels, the structure of the visible Universe doesn't have a structure that suggests rotation. Isn't it more of a bubbly structure?

I understand the no center concept but I'm not 100% sure I buy it. Far more informed cosmologists than I agree on the concept so I have to accept that they have good evidence for what they say.

But when the Universe was only a fraction of a second old, if one were in the Universe at that time, and, if one had the ability to observe whatever state of matter existed then, could you have seen from one side of the Universe to the other side? And wouldn't it have had a center?

Technically, it was supposed to still be the same in all directions but I do wonder about it when I am contemplating the Universe. :-?

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Beskeptical

Well the attempt to free up the information which would indicate a greater rotation did'nt go down too well, even in brief outlines, and as you can see it would only go so far as its influence galactic formation and structure.

Even in principle, determination of the position of galaxies is difficult to the nth degree.If you observe two supernovae occuring in two seperate parent galaxies you know that a local Milky Way reference star will have shifted its position against the supernova from when it is observed to when it actually occured.

If a star went supernova a million years ago,the position of its parent galaxy will have changed to a local star due to Milky Way rotation,call it foreground galactic rotation if you wish.The problem is when you observe another supernova simultaneously at a distance of 10 million light years distance against the same local star.The star will have rotated 10 million years against that parent galaxy so if you wish to keep to be consistent with the local star's rotation around the galactic axis/center something has to give.

I understand that it introduces modelling based on what the cosmos looks like now but again,I would only go so far as to consider how a greater rotation affects the formation and structure of galaxies without having to appeal to 'dark' solutions.

It may not be just a matter that people feel more comfortable with a greater rotation than galactic rotation but from the point of view of a geometer rather than a gravity physicist,observational data in recent years would seem to support it.

oriel36,

Your analogy of hurricanes to galaxies is invalid. While the resemblance between some spiral galaxies and hurricanes is striking, it does not mean that they share a similar cause.

One could as easily posit that the resemblance between the appearance of stars seen through a reflecting telescope with a three-vane secondary support and snowflakes implies similar origins.

Hurricanes are the result of Coriolis forces due to rotation of the Earth.

Galaxies rotate (technically, the matter in them revolves around the center) for the same reason that planets revolve around the Sun (and the Sun rotates). Any collapsing nebula must result in a rotating system unless the non-radial component of motion is absolutely zero. Any non zero residual will result in a rotating object or system. The spiral appearance in visible light is actually illusory, as the stellar density is actually quite uniform. The spiral pattern is due to density waves that result in waves of star formation. Massive stars are bright, but short lived. They delineate the location of the density waves. Less massive stars are longer lived and are evenly distributed throughout the disk, since they continue to shine long after the initiating wave has gone.

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Since the expansion is creating space how would you define the side? One theory that I've seen is that the universe folds in on itself. In other words, if one were to go far enough in one direction, one would eventually arrive back at their starting point, never reaching and end.

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oriel36 - interesting question you raised. Just how much are galaxies not where they appear to be? The rotation of the galaxy is not relevant to the question of the galaxies displacement from where it appears to be because you the observer see the light from the supernova when you see the light from the supernova. The question is if you could instantaneously track back along that light where would the galaxy that emitted it be now. The answer turns out to be pretty much the same distance. I did the math:

A galaxy 10 million light years away will have a distance of ~ 3.1 Mpc (1 Mpc= 3.26 million light years).

1 year = 3.16 x 10^7 seconds
1 kiloparsec = 3.09 x 10^16 km
1 kiloparsec = 3260 light years

So lets say the galaxy at 3.1 Mpc (10 million LY) has a tangential velocity of 300 km/sec (quite a bit larger than typical peculiar motions within 30 million light years)

The number of seconds that light will have traveled from the galaxy is:

seconds = (3.16 x 10^7 s/year) x (10x10^6 years) =

3.16 x 10^14 seconds

kilometers traveled = 300 km/sec x (3.16 x 10^14 sec) =

9.48 x 10^16 km

kpc traveled = (9.48 x 10^16 km) x 1kpc/3.09 x 10^16 km = ~3.1 kpc

At a distance of 3.1 Mpc, 3.1 kpc amounts to an angular measure of about 3.4 arc minutes. (Purely accidental that I picked a velocity in which the motion amounts to the same number as the distance - "3.1").

So the galaxy would only move about 3.4 arc min across the sky which is only about 10% of the size of the galaxy's disk. The Milky Way is a typical mid sized spiral and has a diameter of about 27000 kpc (~88,000 LY).

Now if the galaxy was even more distant its angular diameter will appear smaller, but so won't the angular distance it covers for a given speed of motion. So the answer really doesn't change. Galaxies pretty much are where they appear to be.

Dgruss

I still have to get the hang of the 'quote' feature so bear with me.


"oriel36 - interesting question you raised. Just how much are galaxies not where they appear to be?

Could you rephrase this.

In case I was not precise,when referenced off a local Milky Way star (in terms of when a star went supernova in another galaxy) there is a difference in the position of the galaxy to the local star from when it is observed to when it occured or more specifically where it was then and where it is now.

It only becomes interesting when two supernovae are observed simultaneously occuring in two different galaxies,for instance one at a million years and another at 10 million years.We observe them now but they occured 9 million years apart,if they are now observed close enough to a local Milky Way star,it stands to reason that the parent supernova galaxies will have changed their position relative to the rotation of the local star by different amounts.

To keep the rotation of the local star around the Milky Way axis consistent with its own rotation and for both galaxies and the observed supernova you have to alter the position of the galaxies to each other.



"The rotation of the galaxy is not relevant to the question of the galaxies displacement from where it appears to be because you the observer see the light from the supernova when you see the light from the supernova."

The rotation of the local stars around the galactic axis is quite relevant and it easier to say the supernova happened a million years ago but we only observe it now .



"The question is if you could instantaneously track back along that light where would the galaxy that emitted it be now. The answer turns out to be pretty much the same distance. I did the math: "

It is not a question of distance,it is a question of where the galaxy is now when referenced off the rotation of a local Milky Way star in terms of an observed supernova and the when the supernova occured.

If your math is telling you that the galaxies are in the same position while the local stars have rotated against them, you are in danger of creating a fixed galactic background,basically a bigger cosmological version of the 'fixed stars'.If you feel the galaxies are in the same position now this is your priviledge.

It seems that what you are referring to is the fact that if light travels 10 million light years, then during the 10 million years in which the light travels the object that emitted it has moved to a different location. So my calculation involved determining just how much the galaxy might have moved relative to where it appears to be when we receive the light - and the answer for the scenario I gave was "not very much". Ten million light years is not that far away when you start talking about galaxies.


Right, but it is observed NOW. And I calculated an example difference in angular position for you - about 3.4 arc minutes. And that was for a pretty high tangential velocity for a local galaxy.

A lot of factors come into play. The first of which is what is the proper motion of the galaxy across the sky. The 3.4 arc minutes I calculated for a 10 million year time fram amounts to about 2 hundred thousands of an arc second per year or 1.4 ten thousandths of an arc second in a 70 year on average human lifespan - not very much and below the positioning accuracy when galaxy positions are measured.


I guess I'm not clear what it is you're trying to show.


Galactic rotation is relevant if you're trying to track back to the position of the galaxy to where the stars were 1 million or 10 million years ago when the supernova actually happened. And yes, we observe it now even though it happened a million years ago.


Our galaxy is rotating at about 240 km s-1. If you do additional math you find that in 1 million years a star at the Sun's distance from the galactic center has completed 0.45% of an orbit.


How do you get that from my calculation. I said:

Now I actually meant "position" not "distance" although distance would be about the same too.

And I said this:

I never said the galaxies are not moving. Most of that post was an effort to estimate how much a galaxy might have moved! What I pointed out is that the changes in relative position are small on the time scales you're talking about.

And in fact in a human lifespan the galactic background is pretty "fixed". Even more so than the stars because the galaxies are so much farther away. Despite a galactic rotation of 240 km s-1 we have the same constellations that were created thousands of years ago and they look pretty much the same.

But I think our signals are partially crossed here. I'm not sure what your point is. Perhaps you could clarify?

Dgruss; "It seems that what you are referring to is the fact that if light travels 10 million light years, then during the 10 million years in which the light travels the object that emitted it has moved to a different location. So my calculation involved determining just how much the galaxy might have moved relative to where it appears to be when we receive the light - and the answer for the scenario I gave was "not very much". Ten million light years is not that far away when you start talking about galaxies. "

Even within the tiny confines of the solar system in comparison to the vast cosmological arena the effects of finite light distance are recognisible.Ole Roemer in his determination of how finite light distance affects the motion of the Io as we view it from Earth,used the variation in the distance between Earth and Jupiter in their respective orbits around the Sun to explain why Io did not appear when and where it should if its orbit around Jupiter is steady.This affect was known as the Equation of Light and is fully explained in this excellent book by Bernard Cohen.

http://dibinst.mit.edu/BURNDY/Online...mer/index.html

As a variation on this theme in a somewhat more developed way,it is possible to turn elements of the method of Roemer to the wider cosmos but perhaps I am not the best person to explain how to use it,it can happen that some people are good at explaining things while others have marbles in their mouth,the point is that the method has historical precedence even if it is unfamiliar to the contemporary mind,for instance many hear of the speed of light but not so many hear of the Equation of light.

"Right, but it is observed NOW. And I calculated an example difference in angular position for you - about 3.4 arc minutes. And that was for a pretty high tangential velocity for a local galaxy."

There is no need to 'shout',we see a star go supernova now but because of finite light distance we also know it occured a long time ago depending on the distance of the parent galaxy to ours.We also know that a local star would have rotated against the parent galaxy and therefore the position of the galaxy would have changed to the local star,the difference is generated from the difference of the supernova actually occuring to when it is observed here on Earth.The present view simply bypasses the changing reference of stellar rotation around the Milky Way axis and assumes that the galaxies have the same positional relationship to each other and to the local Milky Way star.

"A lot of factors come into play. The first of which is what is the proper motion of the galaxy across the sky. The 3.4 arc minutes I calculated for a 10 million year time fram amounts to about 2 hundred thousands of an arc second per year or 1.4 ten thousandths of an arc second in a 70 year on average human lifespan - not very much and below the positioning accuracy when galaxy positions are measured."

All of this can only be done in principle so "proper motion of the galaxy across the sky" for cosmological modelling on a galactic scale has no relevance no more than the proper motion of the Sun across the sky has no relevance to heliocentric modelling.

"Galactic rotation is relevant if you're trying to track back to the position of the galaxy to where the stars were 1 million or 10 million years ago when the supernova actually happened. And yes, we observe it now even though it happened a million years ago."

As I said ,it is difficult enough and nobody is being put in a position of defending or attacking,as far as I am concerned it should be just a topic worth discussing.If it were possible to consider a greater rotation than galactic rotation I am forced to give my reasons where and why observation suggests a greater rotation hence the reference to the method of Roemer as a point of departure for the effects of finite light distance on observation and the rotation of the local Milky Way stars as a reference for supernova data and the position of the parent galaxies to the local star in terms of observed supernova occurence and when it actually happened.


"I never said the galaxies are not moving. Most of that post was an effort to estimate how much a galaxy might have moved! What I pointed out is that the changes in relative position are small on the time scales you're talking about. "

The timescales I am using are enormous,if a star went supernova a million years ago and is only observed now,the local star would have rotated against the parent galaxy by that amount,similarily if a star went supernova a billion years ago and we only observe it now the local star will have rotated against that parent galaxy by that amount.

The problem is if you observe the two supernova simultaneously/now it is impossible to make them fit with the consistent rotation of the local star around the Milky Way axis.If you leave the position of the galaxies where they are, even if the supernova events occured 999 million years apart,you are in danger of either creating a ' background' galaxies or grinding Milky Way rotation to a halt.The acceptable solution would be to alter the position of galaxies to each other significantly and hence cosmological modelling.

"But I think our signals are partially crossed here. I'm not sure what your point is. Perhaps you could clarify?"

Maybe it is better to leave this be ,again,I am not the best person to explain things and after all , the topic is up for discussion as the rotation of the local stars around the galactic axis is already known but is bypassed in contemporary models in terms of what the cosmos actually looks like and what it's true motion is.

The all caps was for emphasis - not shouting.

So if I understand you right, you are concerned about these motions because you believe that if they were fully corrected for you could test for universal rotation?

Not entirely. One of the corrections made to the measured redshifts of external galaxies is a correction for the rotation of the Milky Way. The stars within the galaxy provide a reference point for where things appear to be as we see the light from those objects. I'm not entirely clear on how the position of the star when the light left the galaxy is relevant. I guess it goes back to your concern about observing two supernova at the same time that are at different distances?

Dgruss

"Not entirely. One of the corrections made to the measured redshifts of external galaxies is a correction for the rotation of the Milky Way. The stars within the galaxy provide a reference point for where things appear to be as we see the light from those objects. I'm not entirely clear on how the position of the star when the light left the galaxy is relevant. I guess it goes back to your concern about observing two supernova at the same time that are at different distances?"

Yes,I'm certain that you will recognise that things are more complicated even without any interpretation I may impose.Probably it needs someone who can outline the problem in more detail but the way I see it,it raises more possibilities than difficulties.

These things can give you a headache and somehow the way I explain my points does'nt help nevertheless it may be possible to return to them at some stage with a clearer way of presenting the material involved.

Thanks for all the replies to my topic. A good read so far.

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For another excellent read, purchase a copy of (WARNING!! WARNING!! WARNING!! Blatant plug approaching at warp speed!) Our Esteemed Host's book Bad Astronomy. There's an excellent chapter in there speciifically on this subject.

I tend to go back and reread good books just to see what I missed last time.

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No, it is quite easy. Measure the plates, correct for any proper motion of nearby stars in the field (if any) and you're good to go.
Wrong. It matters not one whit where the galaxy with the supernova is now, it matters not one whit where the reference star was one or ten or a hundred million years ago, the only things that matter are the directions we observe the light from the star and the supernova to be coming from now. They are the only things that matter in this context because they are the only things we can observe. All else is speculation. (Well, OK, we can also measure spectra, brightnesses, etc. )
I have no problem with our galaxy revolving about the center of mass of the Local Cluster, which in turn orbits the Virgo Cluster, which in turn orbits the Great Attractor. But apart from causing anisotropy in the cosmic background radiation, this motion has no effect upon positional astronomy.

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