# Thread: Is there REALLY no center to the Universe?

1. I get it, everything is expanding. The point where expansion started WAS everything, therefore there can't be a center, for the center is everything.

But... everything away from us is moving faster than us, and everything behind us is moving slower, hence the illusion of everything appearing to be moving away from us, just at different rate depending on where you're looking.

Example. (Letters represent objects such as galaxies, and the periods are a reference of measurement)

Past= ..A...B....C -> Present= ...A.....B.......C -> Future (we assume) .....A.........B..............C

This is calculated by observing redshifts and distances. I also understand that.

But at any point do the redshifts suddenly increase drastically, or double? Or can we not observe that far away yet. As my example shows above, all letters are moving away from an unknown point. But at the moment of the big bang, energy was everywhere in all directions, wouldn't there be an increase in the Hubble's constant when calculating distances away and redshifting data? I explain below:

Same example as above: Past= (what is out here? Shouldn't things be expanding in all directions?) ..A...B....C or x..A...B...C What's to the left of x? Shouldn't at some point we detect increase in redshifts because if everything truly went in all directions, we'd get to a point where things would start expanding in the other direction. OR is this impossible because we can't observe faster than the speed of light, but what if we meet/observe that light from that object traveling in that other direction eventually. Let me explain:

F............E......D...(x)...A......B............ C

So, hypothetically, if we were object B and we looked to C, we'd see it moving/accelerating faster than us, therefore going away from us. If we look at A, it is moving slower than us, therefore appears to be moving sightly farther away from B's perspective. But then again, hypothetically, if we observe far enough, wouldn't we eventually get to an object such as D, which moving in the opposite direction as B, have a higher redshift (than what we would expect given it's distance away from us) than A because it truly is moving in the opposite direction, not just MUCH slower compared to A. If that is the case, and all things did expand in all directions, wouldn't we get to an exact point where that redshift starts to increase not consistently with prior observations or Hubble's law, implying that exact point could be a center to this Universe to where everything is moving away from? If not, from how it seems, everything we observe is expanding away in relatively a similar direction (not in the opposite direction), minus gravitational influences, which implies maybe it's all just moving away horizontally from a wall?

Help! Mind is blown.

2.

3. Ahead and behind us ? There is is no ahead and behind.

4. I guess you're right, but I still don't think that takes away from the validity of the question.

5. Time for me to spam you with one of my old essays!

Let's make a model.

Now to model an expanding space we need to assign coordinates within that space. For the moment, forget about any edges to that space, we don't need edges, we just need coordinates in order to measure the expansion of space. Galaxies come later, so for now just imagine a 3 dimensional grid. At each grid intersection we will assign a coordinate, a point, a dot. Let's say each intersection point is 1 meter apart.

Put yourself on a point somewhere in this space. Whatever axis you look along you see neighbouring points 1, 2, 3, 4, 5 etc meters away, receding off into the distance. Then we introduce some expansion. Let's say the space grows to 10 times its original size in 1 second! That seems fast perhaps, but this is just a model with easy numbers. The key thing to remember is that the grid expands with the space.

So, here we are, still sitting on our point (but it could have been any point) 1 second later. Now lets look along an axis. We see those neighbouring points are now 10, 20, 30, 40, 50 etc meters away. The space increased to 10 times its original size, and so did the distance between each intersection point on that grid.

Our nearest neighbouring point has receded from 1 to 10 meters in 1 second, so it has receded at 9 meters per second. The next point away has receded from 2 to 20 meters in 1 second, so that point receded at 18 meters per second. The fifth point has moved from 5 to 50 meters away in 1 second, so that one has receded at 45 meters per second. The further away you look, the faster a point will seem to have receded!

And the view would be the same, whatever viewpoint you choose in the grid! There is no "centre" of expansion, no origin point within that grid - the whole thing, the whole space has expanded from something where the spaces between things were really small to something where the spaces between things are much larger. The expansion of that space has carried matter and energy along for the ride.

Remember I said the grid of points receded off into the distance.. well a point that was initially 33,000,000 meters away will have moved away to 330,000,000 meters in 1 one second, meaning that it has receded at 300,000,000 meters per second - the speed of light! Any point initially more distant than 33,000,000 meters away from another point will have receded from that point faster than the speed of light. That is the distance were an object recedes at light speed in this "little" model of expansion. If you look at a point that has receded at the speed of light, then from that point, the point you are on has receded at the speed of light. But no object would be moving through space faster than light, no photon would ever overtake another photon, it all just gets carried along by the cosmic flow.

Now I know this is a very simple model, dealing with a simple 10 times expansion in 1 second. This might seem very different from a universe where the rate of expansion was slowing from immense speed and then starting to accelerate, but if you start your grid very small and apply different rates of expansion to that grid, incrementally, over different rates of time, to simulate slowing it down and then speeding it up, when you look at the end result it is essentially the same. (Whenever there is a change in the rate of expansion, it is the rate of expansion for the whole grid that changes).

You might be asking how useful this model actually is. Well you can substitute different distance measures and time-scales if you like but the principle remains. If you sprinkle galaxies throughout the grid and then expand that grid such that the galaxies move with the expansion, you would find that galaxies interact gravitationally with their near neighbours. The further apart galaxies are when they form, the less the gravitational attraction between them. If they are less than a certain distance apart, the galaxies will move towards each other and cluster together, but if there is enough distance they will be moved apart by the expansion of the universe.

Galaxies at the edge of clusters might have some attraction to their neigbouring clusters, but that is countered by the gravity of the closer galaxies in their own cluster. Thus, the edges of the clusters seem to stretch out, "filament like", towards others in a manner reminiscent of the spiders web structures of the SDSS survey.

We end up with clusters of gravitationally-bound galaxies and increasing distance between the centres of those clusters, in a universe where there is no "origin point" or centre of expansion. The whole thing was the origin point and we have no way of knowing how much larger than our observable part of it the whole thing is. We don't even know if it has an edge, and it doesn't actually need one, mathematically. It is not quite as simple as saying "if it has an overall shape, it must have a centre", unfortunately.

6. And if you want to know how to relate the above to what we see when we look out into the universe, then....

This is a space-time diagram.

Image credit - Prof. Mark Whittle, Mark Whittle's Home Page - from the extragalactic astronomy section

So, what are we looking at here?

Well, let's start at the top left, where it says Here & Now. The vertical axis that "Here & Now" sits on marks light travel time, or lookback time. As you trace that axis downwards you are looking backwards in time, and you reach the origin at the Big Bang something over 13 billion years ago.

This vertical axis itself represents our worldline - it represents "here" across the history of the universe, at rest in relation to the expansion of the universe.

The horizontal axes, both higher and lower, represent proper distance - that is distance as measured by a ruler anchored "here", and the ruler does not expand with the universe.

What expands is the proper distance to other galaxies, which are shown as blue worldlines. Notice that all worldlines converge at the Big Bang.

So, how does this all relate to what we see?

Well, the red line shows what we see, here & now, which is known as our past-light cone. The light of all the galaxies we can see right now traced a path along that line towards us, from the place where the worldines of those galaxies intersected that red line. The whole of the red line represents the theoretical path of a photon released at recombination (when the Cosmic Microwave Background was originally released), relatively close to "here".

Let's start with the middle galaxy, one we measure here & now as having a redshift of z=1. See where it crosses our lightcone, marked - this represents the distance at the time the light was emitted. If you trace a vertical line downwards from there, it intersects the horizontal axis at 5.2 billion light-years. That was the proper distance to that galaxy (), at the time the light was emitted. If you instead trace from the intersection point horizontally towards the vertical axis, it shows that the light took 7.3 billion years to reach us (shown as ).

Now look along the upper horizontal axis and see where that galaxy's blue worldline intersects it at 10 billion light years - that is the distance to the galaxy "now", which is known as the co-moving radial distance, or the distance at the time the light is observed ().

So, when we are looking at a galaxy with a redshift of z=1, we are seeing a galaxy as it was 7.3 billion years ago, when it was 5.2 billion light-years away. Today, it would be something around 10 billion light-years away, due to the continued expansion of the universe since the light we see was originally emitted.

Now look at a more distant galaxy, the right hand blue worldline with a redshift of z=6.8. It crosses our light cone 12.6 billion years ago, when it was only 3.6 billion light-years away, but today it is 28 billion light-years away.

That galaxy has always been more distant than the galaxy we see at z=1, but we see them at different times in the history of the universe. Our light cone cuts a slice through the universe all the way back to the time of recombination, when everything, including all the stuff that made up those galaxies, was very close to "here" indeed.

Here is another space-time diagram, this time showing the apparent recession speeds:

If you look at the v=c line (which represents the Hubble distance, the distance where an object apparently recedes at c), you will see it intersects our past light cone at a redshift between z=1 and z=2, which has a proper distance (along the bottom axis) of between 5 and 6 billion light-years. If you look along the vertical axis, you will see that all galaxies with v>c have lookback times of more than 9 billion years.

I should add here that the redshift factor z actually represents the amount that the universe has scaled up between the time the light from that galaxy was emitted, and today as we detect that light. It represents the increase in scale factor in the form 1+z. So the universe today is twice as large as it was at the time the light we see was originally emitted from a galaxy with a redshift of z=1, and the universe today is 3 times as large as it was when the light we see was originally emitted from a galaxy with a redshift of z=2, and so on.

I know this is a lot to take in, so I would be happy to explain any part of it in more detail, or in a more user friendly way if you want. Feel free to ask.

7. Thanks for the response! You put a lot of effort in to this and I appreciate it. So, to business

Now look at a more distant galaxy, the right hand blue worldline with a redshift of z=6.8. It crosses our light cone 12.6 billion years ago, when it was only 3.6 billion light-years away, but today it is 28 billion light-years away.
Under that distinction it would be proof that it is indeed moving away from us in an opposite direction, and/or our prediction of the Universes age is flawed. My math could be wrong, but say you have one thing traveling the speed of light in one direction, and another things traveling the speed of light in the other direction, those things would be moving away from each other faster (2x) than the speed of light. Given that the Universe is a projected 13.7 billion years old, how is it possible a galaxy is now 28billion ly away from "here and now"? Maybe I'm totally not comprehending the example

8. In the second diagram, what are the lines marked "horizon" and "todays horizon"?

9. Originally Posted by Cudamerica
Thanks for the response! You put a lot of effort in to this and I appreciate it. So, to business

Now look at a more distant galaxy, the right hand blue worldline with a redshift of z=6.8. It crosses our light cone 12.6 billion years ago, when it was only 3.6 billion light-years away, but today it is 28 billion light-years away.
Under that distinction it would be proof that it is indeed moving away from us in an opposite direction, and/or our prediction of the Universes age is flawed. My math could be wrong, but say you have one thing traveling the speed of light in one direction, and another things traveling the speed of light in the other direction, those things would be moving away from each other faster (2x) than the speed of light. Given that the Universe is a projected 13.7 billion years old, how is it possible a galaxy is now 28billion ly away from "here and now"? Maybe I'm totally not comprehending the example
It is possible for galaxies to be more than 13.7 billion light-years away from each other in a universe that is only 13.7 billion years old, because unlike normal motion through the universe, the expansion of the universe is not limited by the speed of light. Not that anything ever overtakes a photon, anyway! (As I explained in my first post)

http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf

(Note: the first page is blank)

10. Originally Posted by KJW
In the second diagram, what are the lines marked "horizon" and "todays horizon"?
Here is another diagram that shows the distinction in a clearer form.

The line marked "todays horizon" in that 2nd diagram that you asked about is the traditional way to show the worldline of an object comoving with the expansion of the universe and currently at the emission coordinate for the CMB photons we detect today (also known as the "surface of last scattering" and currently estimated to be 46 billion light-years away). In simple terms, it shows the history of expansion for an object currently in the region of space where the CMB we currently detect originally came from. The upper line simply marked "horizon" on that diagram and represented by the green dotted line in the diagram above is an alternative way to plot the particle horizon as a function of time. This removes the need to plot a new worldline for a comoving object in order to calculate where the particle horizon was at an earlier time in the history of the universe.

So basically, the straight line marked horizon shows the history of that conceptual horizon itself (it is an observer dependent horizon which propagates through space at c) whereas the curved line shows how any matter that is now at that horizon got there.

11. Center is still relative to an outer edge. You are just assuming an outer edge and so there must be a center.

Take this poor example. Imagine you are a fish in the middle of the ocean.

You are wondering ,which way is land.

There might be an island in one direction.
And there might be a continent in another.

All in all there is land in all directions, a surrounding edge.

The center will also be irregular depending on directions and distances of the surroundings.

Net result is that the center point is not a single point.

12. Originally Posted by Magic Pixel
Center is still relative to an outer edge. You are just assuming an outer edge and so there must be a center.
There is no boundary to the universe; no one here is assuming such a thing.

13. Would the center of the universe be original point of the start of the big bang? Does the universe expand semetrically? I couldn't even present a good guess.

14. So is the (visible) universe the only thing in the (visible) universe that does not have a center of mass?

15. Originally Posted by Magic Pixel
Would the center of the universe be original point of the start of the big bang?
The original "point" of the big bang (if there was such a thing) has now expanded and makes up the universe. So, in that sense, that point is now everywhere.

Does the universe expand semetrically?
Apparently. The variations in the cosmic microwave background are really tiny.

16. Originally Posted by Laurieag
So is the (visible) universe the only thing in the (visible) universe that does not have a center of mass?
The centre of mass of the visible universe is "here" (wherever you are). Because the visible universe is, by definition, centred on the observer.

17. Originally Posted by Strange
The centre of mass of the visible universe is "here" (wherever you are). Because the visible universe is, by definition, centred on the observer.
I don't particularly like your assertion Strange.

While the entire universe may be homogeneous and isotropic with no center of mass being able to be calculated (on assumptions based on BB cosmology), the center of mass of the observable universe is something that could be calculated. A series of calculations could be made based on the masses observed in astronomical epochs and the various centers of mass for each epoch could be compared to see if there was some kind of relationship or any consistency between them.

Obviously, if someone has already done this survey/calculation and proved your assertion, then you should be able to provide a reference to the published results?

18. Originally Posted by Laurieag
Originally Posted by Strange
The centre of mass of the visible universe is "here" (wherever you are). Because the visible universe is, by definition, centred on the observer.
I don't particularly like your assertion Strange.

While the entire universe may be homogeneous and isotropic with no center of mass being able to be calculated (on assumptions based on BB cosmology), the center of mass of the observable universe is something that could be calculated. A series of calculations could be made based on the masses observed in astronomical epochs and the various centers of mass for each epoch could be compared to see if there was some kind of relationship or any consistency between them.

Obviously, if someone has already done this survey/calculation and proved your assertion, then you should be able to provide a reference to the published results?

"The observable matter is spread homogeneously (uniformly) throughout the Universe, when averaged over distances longer than 300 million light-years."
"The observable matter of the Universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content."
Universe - Wikipedia, the free encyclopedia

"The observable universe is very nearly homogeneous and isotropic."
Research

"The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million lightyears) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized in accordance with the Cosmological Principle. At this scale, no pseudo-random fractalness is apparent. The superclusters and filaments seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. "
Observable universe - Wikipedia, the free encyclopedia

19. Originally Posted by Laurieag
Originally Posted by Strange
The centre of mass of the visible universe is "here" (wherever you are). Because the visible universe is, by definition, centred on the observer.
I don't particularly like your assertion Strange.

While the entire universe may be homogeneous and isotropic with no center of mass being able to be calculated (on assumptions based on BB cosmology), the center of mass of the observable universe is something that could be calculated. A series of calculations could be made based on the masses observed in astronomical epochs and the various centers of mass for each epoch could be compared to see if there was some kind of relationship or any consistency between them.
If the universe is homogeneous and isotropic, then the "centre of mass" of the observable universe would be where the observer is. Or are you assuming the observable universe has a physical edge?

20. Originally Posted by KJW
If the universe is homogeneous and isotropic, then the "centre of mass" of the observable universe would be where the observer is. Or are you assuming the observable universe has a physical edge?
Obviously there is some limitation to our observational limits so if you wish to call that an edge I have no problem with your interpretation.

Philosophical considerations aside, do you know of any scientific measurements that have determined what you and Strange claim or is it just based on faith?

Observable universe - Wikipedia, the free encyclopedia
The Virgo Supercluster – home of Milky Way – is marked at the center, but is too small to be seen in the image.

21. Originally Posted by Laurieag
Philosophical considerations aside, do you know of any scientific measurements that have determined what you and Strange claim or is it just based on faith?

"The observable matter is spread homogeneously (uniformly) throughout the Universe, when averaged over distances longer than 300 million light-years."
"The observable matter of the Universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content."
Universe - Wikipedia, the free encyclopedia

"The observable universe is very nearly homogeneous and isotropic."
Research

"The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million lightyears) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized in accordance with the Cosmological Principle. At this scale, no pseudo-random fractalness is apparent. The superclusters and filaments seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. "
Observable universe - Wikipedia, the free encyclopedia

22. Research

Soon-to-be-released data from Planck and other CMB experiments, the results of on-going large-scale structure surveys, and more futuristic measurements of the 21 cm Hydrogen line promise to drastically improve our understanding of the early universe. Since energies far greater than those possible in particle accelerators (such as the LHC) were achieved in the early universe, these datasets represent some of the only known opportunities to confront the most fundamental theories of nature with observation. This is a crucial time to both be determining the consequences of theory, and designing search strategies for observation.