1. In an isotrophic and homogeneous universe, why would the expansion of space always be vectored outward. Would it not also push galaxies inward?

2.

3. Originally Posted by THoR
In an isotrophic and homogeneous universe, why would the expansion of space always be vectored outward. Would it not also push galaxies inward?
It's not vectored in any direction. At every point in space, there is more space being created, so everything is expanding away from everything else. However, the force of this expansion is so weak that gravity easily overcomes it, holding things together, out to about 200 million lys.

4. If you look at the mathematics of General Relativity, it is clear that space must be either expanding or contracting; it requires an implausible balancing act of forces to keep it unchanging. We don't really know what started the initial expansion and it used to be thought that it would eventually slow and start contracting (but the evidence seems to be against that now).

5. Like these people say - relativity
Your inward is someone elses outward -anywhere in the universe can be considered the centre

6. Originally Posted by AlexG
It's not vectored in any direction.
Correct me if I am wrong, but I was brought up to believe that a vector is a mathematical object that has both a certain magnitude AND a certain direction

7. Originally Posted by Strange
If you look at the mathematics of General Relativity, it is clear that space must be either expanding or contracting;.
Then you are more clever than me. This NOT clear to me from the field equations. Please explain, using mathematics for preference

8. Originally Posted by Guitarist
Originally Posted by Strange
If you look at the mathematics of General Relativity, it is clear that space must be either expanding or contracting;.
Then you are more clever than me.
Definitely not

This NOT clear to me from the field equations. Please explain, using mathematics for preference
I'm afraid the maths is beyond me. But this was my understanding of the implications of the Friedmann–Lemaître–Robertson–Walker metric from reading this paper: [0707.0380] Expanding Space: the Root of all Evil?
It is worth starting from first principles
and asking what the general relativistic picture
of cosmology actually contains. The adoption of the
cosmological principle, in that the Universe is homogeneous
and isotropic, restricts the form of the underlying
geometry of the Universe, expressed in terms of the
FRW metric. With this metric, the continuity equation
demonstrates that in other than finely-tuned or
contrived examples, the density and pressures of cosmological
fluids must change over cosmic time, and it
is this change that represents the basic property of an
expanding (or contracting) universe.

9. It was in 1927 that Georges Lemaitre showed that static solutions to Einsteins field equations were unstable, and that the universe must either expand or contract. The expansion was confirmed two years later.

10. The universe can not be expanding from a single point if galaxies collide. There must have been more than one big bang, there are problems with the theory, or the observations of astronomers are in error. Have a look at this photo and the conclusions/assumptions of the astronomers.

A Grazing Encounter Between two Spiral Galaxies | ESA/Hubble

11. It is your lack of understanding of the issues involved that causes your error, jetstove. Of course galaxies can collide in an expanding universe, as anyone who actually understands cosmology would know. So, instead of immediately coming out saying "it must be wrong" it would pay you to first get a good understanding of how universal expansion works.

In a cluster of galaxies, which are bound to each other by gravity, the expansion of the universe has no effect. Galaxies within these clusters can collide, just like Andromeda is going to collide with our own galaxy.

Where two clusters of galaxies are so separated that they are not bound by gravity, the distance between those clusters increases due to the expansion of the universe.

I won't apologise for my tone - I am sick of having to correct people who naysay the current cosmological model without the proper appreciation of all the subtleties involved.

Instead of coming in with "something must be wrong", why not instead ask "why do we think this is correct?"

12. Originally Posted by SpeedFreek
It is your lack of understanding of the issues involved that causes your error, jetstove. Of course galaxies can collide in an expanding universe, as anyone who actually understands cosmology would know. So, instead of immediately coming out saying "it must be wrong" it would pay you to first get a good understanding of how universal expansion works.

In a cluster of galaxies, which are bound to each other by gravity, the expansion of the universe has no effect. Galaxies within these clusters can collide, just like Andromeda is going to collide with our own galaxy.

Where two clusters of galaxies are so separated that they are not bound by gravity, the distance between those clusters increases due to the expansion of the universe.

I won't apologise for my tone - I am sick of having to correct people who naysay the current cosmological model without the proper appreciation of all the subtleties involved.

Instead of coming in with "something must be wrong", why not instead ask "why do we think this is correct?"

The current theory says that the universe is expanding and taking the galaxy clusters with them. No where does it mention that galaxys are moving together (colliding). So I guess wiki must be wrong or the astronomers are wrong or the single big bang theory is wrong. When was the last time you heard of a head-on collision between two vehicles going in the same direction? If you have more information on galactic collisions, please share it. I think it would make good reading. Thanks.

From Wiki:

"Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.[7]If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures,[8][9][10] and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model."

According to the Big Bang model, the Universe expanded from an extremely dense and hot state and continues to expand today. A common analogy explains that space itself is expanding, carrying galaxies with it, like spots on an inflating balloon. The graphic scheme above is an artist's concept illustrating the expansion of a portion of a flat universe.

source link: Big Bang - Wikipedia, the free encyclopedia

13. Originally Posted by jetstove
So I guess wiki must be wrong or the astronomers are wrong or the single big bang theory is wrong.
Nope. You seem to have missed out one possibility there.

One problem with Wikipedia is that, even when it is accurate, it is highly simplified. But another problem is that it is so easy to miss some important details.

From Wiki:

"Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point
Just thought I would highlight the bit you seem to have missed.

14. Originally Posted by jetstove
From Wiki:

"Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.[7]If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures,[8][9][10] and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model."
This is what I said. If a galaxy or cluster of galaxies is very distant, it will recede due to the expansion of the universe. You should be asking yourself what causes galaxies to cluster, rather than them being randomly distributed across the universe. The answer to that is gravity. So in a cluster, galaxies can move towards each other due to gravity, but over the large distance between clusters, the universe expands.

The expansion of the universe is cumulative - a simplified explanation is that the rate increases with distance, so locally gravity wins out over expansion.

Here is a quote from a famous article published in Scientific American, by two well known cosmologists, based on their work on the subject:

The expansion of our universe is much like the inﬂation of a balloon. The distances to remote galaxies are increasing. Astronomers casually say that distant galaxies are “receding” or “moving away” from us, but the galaxies are not traveling through space away from us. They are not fragments of a big bang bomb. Instead the space between the galaxies and us is expanding. Individual galaxies move around at random within clusters
The article: http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

The scientific paper: [astro-ph/0310808] Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe

Now then, when a galaxy is moving away from us and we measure the spectrum of the light coming from it, we find that spectrum to be redshifted. On the other hand if a galaxy is moving towards us, we measure a blueshift.

Curious About Astronomy: Why are there blue shifted galaxies?
Almost all galaxies are redshifted because of the Hubble expansion of the universe. Only a handful of the most nearby galaxies are blue-shifted. You see, in addition to the apparent motion imparted to galaxies due to universal expnasion, individual galaxies also have their own intrinsic, what we call "peculiar" motions. This is not because they are peculiar, as in strange, but rather because each galaxy is in motion irrespective of the universe's expansion, and each galaxy has its own unique velocity.

Generally, that velocity is some hundreds of kilometers per second. In regions close enough to our own galaxy where the Hubble expansion results in less outward expansion than this, the galaxies' peculiar velocities (if they are large enough and sufficiently towards us) can overcome that expansion, resulting in a blue-shift.
The peculiar motions of galaxies are due to the gravitational conditions local to them - ie. the close by galaxies. That is why galaxies can collide.

Oh, and before you ask, early in the universes history gravity didn't really come into effect until everything was already pretty spread out and all it could do was slow down the rate of expansion somewhat. Whilst all this was going on, gravity was able to do its work, so anywhere that was any denser than a place closeby would become more dense, leaving gaps! The seeds of large scale structure formed early on, so it is thought that by the time galaxies formed they were already in clusters, rather than evenly distributed.

So, the distance between clusters increases, whilst galaxies swirl around within their local group.

Originally Posted by jetstove
So I guess wiki must be wrong or the astronomers are wrong or the single big bang theory is wrong

Nope. It's you. Your guess is wrong.

I should say here that it is good that you are thinking critically, and if you are interested in science that is a good quality to have. But don't worry, astronomers haven't missed something so obvious.

15. Can I ask a question here or is it best to start a new thread, Apologies i'm new here but i have a question about Hubble and the theory that the universe and inflation is accelerating.

Is it true or not that visually objects further away seem to be moving at a higher rate of speed?

Assuming the answer coming is going to be yes, that objects 3 times as far away are moving 3 times as fast (which is what i have picked up) would it not be logical to assume that the universe is in fact slowing down and not accelerating?

My reason for thinking this is that objects we see are actually light particles created thousands and millions of years ago, those older photons of light we actually see as moving faster (the further ones away) were actually created in a younger universe. This would mean the further away we look, the quicker things seem to be moving because the light we are seeing shows us what the galaxies were doing all those years ago.

Put simply imagine a ruler,

distance=speed=time ago
10cm=100kmh=1,000 years ago
100cm=1,000kmh=10,000 years ago
1000cm=10,000kmh=100,000 years ago

The scale is incorrect, i'm just using it as some kind of easier to understand example..

So 10cm away is moving at 100kmh 100cm away is moving at 1,000kmh 1000cm away things seem to be moving at 10,000kmh, those light photons we see at 1,000cm are actually where they were and doing what they were doing 100,000 years ago, while at 100cm we see things moving as they were 10,000 years ago and 10cm were only 1,000 years ago. I hope this makes sense to everyone else, i'm finding it hard to explain. Simply its a time paradox, what we are seeing is the past the further away we look so things would appear to be moving quicker in a universe which is slowing down instead of accelerating, no?

To touch on other points of your discussion i would like to know,

Could it be the true red/blue shift is actually starting to show the universe is starting to crunch already, where the closest objects are already showing signs of coming back together while the further objects we see were once moving away from everything else? I do sort of understand the inflation theory, just maybe shedding a different perspective on the whole matter as even in out local galaxy things are always moving towards something and generally the only way to move away is to be influenced by some outside force, Dark energy is only dark because no one can explain it or how its working. I don't mean to piss on anybodys boots here, just trying to get a couple of things out of my head which have bugged me for years.

Thanks for any replies (if i get any)

16. The acceleration is not an artifact of distance or where that galaxy was when it emitted the light. Expansion is a certain value change in length per time period everywhere and it is this rate which is accelerating.

17. So in reality it is time which is accelerating? space which is expanding, together spacetime is both accelerating and expanding? Trying to over simplify here sorry im not a scientist, just a keen observer.

My understanding of time is the measurement of movement of somehthing interacting with something else, Its fluid running in many different speeds and directions. If nothing moved then time would not be apparent.

Weed usually solves these problems for me by shutting me up inside and out lol. as you can probably tell its been a while and my mind is starting to work again :P

18. Not a scientist either. Nor a pot smoker.

That is as good a definition of time as any.

It is the rate of expansion that is accelerating, not time though.

19. Time to make a toy model to illustrate the expansion of the universe! (Some of you will have seen me post this before)

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 havebeen 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 lengths 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.

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.

This is all highly simplified, of course, but I hope it goes some way to helping you understand how the expansion of the universe works - basically the whole thing scales up, which means the further away something is "now", the faster it would have had to move, if it were moving through space to get there (which it wasn't!). The acceleration is in the rate the whole thing scales up (which is the same thing that Kalster said!). In any kind of expanding universe, recession speed always increases with distance - this is not the same thing as the acceleration of the expansion.

20. Originally Posted by Titan
Is it true or not that visually objects further away seem to be moving at a higher rate of speed?

Assuming the answer coming is going to be yes, that objects 3 times as far away are moving 3 times as fast (which is what i have picked up) would it not be logical to assume that the universe is in fact slowing down and not accelerating?
I think you might be confusing the linear increase of speed with distance (which is a result of expansion at a constant rate) and an accelerating rate of expansion.

Lets assume the rate of expansion is constant (which was the initial conclusion from Hubble's data). Imagine a series of points that are, say, 1 light year (ly) apart (as with your example, the numbers are just made up for simplicity):

A..B..C..D..E.. .. .. Z ..

At the start the distance from A to B is 1 ly; the distance from A to C is 2 ly, etc.

Now, if after a year the distance between each point has increased by 10% then the distance from A to B will be 1.1 ly. The speed of recession of B is 0.1 ly/year. Now, point C has moved to 2.2 ly from A so its speed of recession is 0.2 ly/year. And so on. Point Z will appear to be receding at 2.5 ly/year (i.e. faster than the speed of light). However, all that has happened is that the distance between each point has increased by the same amount.

What was discovered (relatively) recently is that the relationship is not completely linear and is best explained by the rate of expansion increasing for the last few billion years.

Does that help?

21. It does help, opens up a couple more questions but i guess ill just stick to being an observer and leave the techie stuff to those in the know. Its not a matter of how quickly or slowly an object travel through space, rather how space increases the distance and how it would multiply over that distance. cheers for your input

22. The further away something is from us, the more space is between us. It is all space which expands. So the more space that is between us, the more new space is generated. Hence the further away something is, the higher we measure the rate of seperation.

When the recession rates of very far away objects were measured, they were higher than they should have been if the expansion was slowing. Calculation indicates that up to 7 billion years ago, the expansion was beginning to slow, and then it began to accelerate.

23. Originally Posted by SpeedFreek
Time to make a toy model to illustrate the expansion of the universe! (Some of you will have seen me post this before)

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 havebeen 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 lengths 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.

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.

This is all highly simplified, of course, but I hope it goes some way to helping you understand how the expansion of the universe works - basically the whole thing scales up, which means the further away something is "now", the faster it would have had to move, if it were moving through space to get there (which it wasn't!). The acceleration is in the rate the whole thing scales up (which is the same thing that Kalster said!). In any kind of expanding universe, recession speed always increases with distance - this is not the same thing as the acceleration of the expansion.

This is a particularly good explanation as you dumbed it down enough for me to understand.

I've now just got to get my head around the possibility that it may or may not have expanded to infinity.

24. Originally Posted by Chrisgorlitz
I've now just got to get my head around the possibility that it may or may not have expanded to infinity.
I don't think it can expand to infinity. Either the "grid" was infinite to begin with (and all the points got further apart) or it was finite and still is.

If you don't see how an infinite space can expand, look up Hilbert's hotel.

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