Where things are.

[Michael Kieth Adams]

The more distance away from us something is, the faster it moves away from us.    Things are moving slower near to us.  Am I wrong to believe that the huge speeds we see happened long ago and the fact that things near us are slower has to mean that things are slowing down.  Closer is more recent, far away is long ago.  Do I misunderstand ?

[saac]

I think what you are looking at is the expansion of the universe which gives rise to distant objects having a higher recessional velocity than nearby objects.  This is measured in the red shift of distant galaxies which is one of the supporting pieces of evidence to support the expansion of the universe.  My understanding is  that it doesn't point to things slowing down rather than the stretching of the space between the objects. 

Jim

Edited December 16, 2020 by saac

[Jiggy 67]

9 minutes ago, saac said:

I think what you are looking at is the expansion of the universe which gives rise to distant objects having a higher recessional velocity than nearby objects.  This is measured in the red shift of distant galaxies which is one of the supporting pieces of evidence to support the expansion of the universe.  My understanding is  that it doesn't point to things slowing down rather than the stretching of the space between the objects. 

Jim

I love that Jim, great explanation.....now explain the influence of dark energy on the expansion of the Universe 😀😀

[wulfrun]

As an anology which you can probably visualise in your head. Partly blow up a balloon. Mark a red dot on it somewhere and a load of black ones at reasonable distances. Blow the balloon up a lot more. Notice that the dots have all got further from oneanother. Notice that the dots farthest from your red, reference dot have moved further than the ones nearby. The fabric of spacetime is expanding in the same fashion but with more dimensions.

Edited December 16, 2020 by wulfrun

[saac]

Lol , I'll  try but don't go cashing any cheques on this   Dark energy is a candidate for causing what appears to be an acceleration in the rate of expansion.  The detail  has still to be written !  

Jim

[wulfrun]

7 minutes ago, Jiggy 67 said:

I love that Jim, great explanation.....now explain the influence of dark energy on the expansion of the Universe 😀😀

Refrerring to my analogy above, someone's turning up the air pressure inflating the balloon. We don't yet know how that comes to be. 🙂

[Michael Kieth Adams]

I know what you are talking about but the fact is  what we see far away happened long ago, what happens right here in our region of space time is slower, was fast is slow, slowing down.  I know the balloon illustration but far away things move faster than close ones.  Long ago things fast recent things slower, slowing down  ( not stopping, yet).  I’m sure there is a flaw in my logic but where?

Edited December 17, 2020 by Michael Kieth Adams
Mispelling

[wulfrun]

4 hours ago, Michael Kieth Adams said:

I know what you are talking about but the fact is  what we see far away happened long ago, what happens right here in our region of space time is slower, was fast is slow, slowing down.  I know the balloon illustration but far away things move faster than close ones.  Long ago things fast recent things slower, slowing down  ( not stopping, yet).  I’m sure there is a flaw in my logic but where?

For starters, imagine you were in a distant galaxy, looking at the milky way - your reasoning would reach the same (wrong) conclusion about Earth/Milky Way. It's correct that looking further corresponds to longer ago, simply because of the finite speed light travels at. What you can't deduce is who's moving: you, the other end or both. Chances are very high the answer is both. There's no absolute position in spacetime and so you can only conclude that local objects are closer to co-moving than distant ones. That's true everywhere. I think the flaw in your logic is that you are taking the view that you are in some special place that is stationary/slow moving. You have no special viewpoint, you are being carried along in (expanding) spacetime like everything else.

Edited December 17, 2020 by wulfrun
clarity

[Davey-T]

As mentioned in a recent SGL Stargazine episode there is no speed limit on the expansion of the Universe so it can expand faster than the speed of light but those on galaxies moving apart don't realise they are moving at that speed as their whole environment is "moving".

Dave

[andrew s]

13 hours ago, Michael Kieth Adams said:

The more distance away from us something is, the faster it moves away from us.    Things are moving slower near to us.  Am I wrong to believe that the huge speeds we see happened long ago and the fact that things near us are slower has to mean that things are slowing down.  Closer is more recent, far away is long ago.  Do I misunderstand ?

It a brain mangler thinking on the scale of the Universe.

Firstly, we believe the Universe is the same everywhere ( homogeneous and isotropic) on large scales and we have evidence to support this. However, it is true only for observation made at the same time (i.e. age of the Universe by observers co-moving with the expansion of the Universe). Which is a fancy way of saying they would not measure and red or blue shift in the CMB. On earth we do but it is subtracted from the images you normally see. So at the same epoch the expansion will look the same from everywhere and they will measure the same value for the Hubble "constant". Note the Hubble constant is not constant!

Secondly, in the past the expansion (as measured by the Hubble constant) was significantly higher than it is now (this has nothing to do with the proposed early cosmic inflation phase). So yes the expansion is slower now than before.

Thirdly, the Hubble constant is asymptotically tending to a constant value even with the "acceleration of the universe" . Sound odd but its because its a ratio. Imagine you are driving your car and accelerating you velocity increase but so does the distance traveled. The Hubble is like taking your velocity and dividing by the distance traveled.

Fourthly, as you say, the further we look out into the observable universe the further back in time we look. In practice its very hard to account for all this and you need a model of the Universe to calculate distances. This is currently done based on the LCDM concordance model.

Regards Andrew

Edited December 17, 2020 by andrew s

[Michael Kieth Adams]

The universe is not the same everywhere.  The early universe was quite a bit different than it is now.  The early expansion was faster than light, it is slower than that now.    Would we be able to see something moving faster than light?  Maybe parts of the universe expand at different speeds in the same space.  Maybe dark energy and dark matter are parts of local space moving faster than light.  
Thanks for reading and thinking about my ideas.  I really appreciate it.  Mike

[Moonshed]

The most recent and accurate measurements we have show that the rate of expansion of the universe is increasing, which came as something as a surprise to the scientists,  “dark energy” was invented as a means of explaining this. 

When studying the rotational speeds of galaxies is was realised that there was not enough visible matter to be able to create enough gravity to hold them together, “Dark Matter” was invented as a means of explaining this.

All galaxies are moving away from us, apart from a few very local ones, as can be seen by their red shift, and the further they are from us the faster they are moving away. This is due to the expansion of the universe. To help visualise this imagine having a long piece of black elastic and paint white dots on it all equally spaced apart, they are galaxies, the elastic the fabric of the universe. Hold it up to your eye and stretch the far end. You will notice as you look along it’s length that the further away each dot is the faster it is moving.

 

[andrew s]

18 minutes ago, Michael Kieth Adams said:

The universe is not the same everywhere.  

What I said was it is assumed to be the same ( on large scales ) at the same epoch. If this were not the case the the CMB would not be as uniform as it is.

21 minutes ago, Michael Kieth Adams said:

The early universe was quite a bit different than it is now.

Yes it started in a hot dense state and has been cooling ever since.  

 

22 minutes ago, Michael Kieth Adams said:

 The early expansion was faster than light, it is slower than that now.    Would we be able to see something moving faster than light? 

The expansion is of the "scale factor" not a normal expansion as say when you heat a bar of metal.  As in the example of the balloon being blown up, the spot on the surface get further apart but locally the spots don't move.  If the expansion of the scale factor is positive then as you look further away they appear to be receding faster and faster (Hubble's Law). Eventually you reach what is call the Hubble sphere where the speed of recession is equal to the speed of light. We can currently see objects receding out to about 3c. 

As I said it was very fast initially, as measured by the Hubble constant,  and is slowing down. However, there is no single speed as it depends on how far away you look. 

See this for some background Scale_factor_(cosmology)

40 minutes ago, Michael Kieth Adams said:

 Maybe parts of the universe expand at different speeds in the same space.  Maybe dark energy and dark matter are parts of local space moving faster than light.  
 

I don't understand what these bits mean. 

Regards Andrew

[Moonshed]

59 minutes ago, Michael Kieth Adams said:

The universe is not the same everywhere.  The early universe was quite a bit different than it is now.  The early expansion was faster than light, it is slower than that now.    Would we be able to see something moving faster than light?  Maybe parts of the universe expand at different speeds in the same space.  Maybe dark energy and dark matter are parts of local space moving faster than light.  
Thanks for reading and thinking about my ideas.  I really appreciate it.  Mike

On the large scale the universe IS the same everywhere.

Yes, the early universe was different to how it is now, it began as being infinitely dense and infinitely hot and since that time has been expanding and cooling, forming matter, stars, planets, galaxies and so on.

I don’t know what you mean saying the early expansion was faster than light, unless you are referring to the inflationary epoch?

We can see galaxies that are moving faster than light.

The universe cannot expand at different speeds in the same space. Not sure what you mean by that.

I don’t understand what you mean suggesting maybe dark energy and dark matter are parts of local space moving faster than light. 
 

It’s a complicated subject that gets written and rewritten as newer data is collected and new discoveries made. It’s a subject I love and I can see it’s got you thinking about it too. Keep researching!

 

[Michael Kieth Adams]

Things do not have to be moving at the same velocities in the same space.   I know that the idea of an explosion is not even close to the reality of the Big Bang, but if you looked at the tracks of an explosion, some would end close to the center point and others farther away.   Perhaps the far parts that we see are parts that have slowed down enough for us to see them.   The world around us seems homogenous, but we know there are parts of it that we cannot see.   Is the dark universe homogenous?   We should be able to tell by gravitational effects.   Parts of the universe around us move at different speeds.  We believe that moving very fast changes the effects of time.   If dark matter were  moving faster than light it might not behave as we expect.   Viewpoint might determine what we see.  I ask again, would we be able to see something moving faster than light?  I suspect not.  We know that the universe is very different from what we see, maybe it depends on how fast things are moving.

[andrew s]

1 hour ago, Moonshed said:

The most recent and accurate measurements we have show that the rate of expansion of the universe is increasing, which came as something as a surprise to the scientists,  “dark energy” was invented as a means of explaining this. 

This is true as is the fact the Hubble constant is falling asymptotically.

How can they both be true? While the "rate of expansion" in the quote refers to the scale factor 'a',  the expansion rate as measured by the Hubble constant is the ratio of the rate of change of a to a i.e. ( da/dt)/a.

The acceleration in a is positive i.e. ( d2a/dt^2 > 0)

The recessional velocity of a galaxy at a given distance (v = HD) is getting slower in units of Km/s per MPc

Strange but true.

Regards Andrew

Edited December 17, 2020 by andrew s

[andrew s]

3 minutes ago, Michael Kieth Adams said:

Things do not have to be moving at the same velocities in the same space.   I know that the idea of an explosion is not even close to the reality of the Big Bang, but if you looked at the tracks of an explosion, some would end close to the center point and others farther away.   Perhaps the far parts that we see are parts that have slowed down enough for us to see them.   The world around us seems homogenous, but we know there are parts of it that we cannot see.   Is the dark universe homogenous?   We should be able to tell by gravitational effects.   Parts of the universe around us move at different speeds.  We believe that moving very fast changes the effects of time.   If dark matter were  moving faster than light it might not behave as we expect.   Viewpoint might determine what we see.  I ask again, would we be able to see something moving faster than light?  I suspect not.  We know that the universe is very different from what we see, maybe it depends on how fast things are moving.

I not sure I can unpick this but I will try.

When we say the universe is homogeneous and isotropic we mean anyone anywhere will see essentially the same thing on a large enough scale. This seemsis by the uniformity of the CMB to 1 part in 10^5. There is no special place. No center of an explosion. 

Nothing material can travel at or faster than the speed of light and from its gravitational effect we know dark matter is material in this sense.

Hot relativistic (moving close to the speed of light) dark matter has and is being researched but cold (slow moving) dark matter fits better to current observations.

In our cosmological models based on GR time dilation due to velocity and gravity are accounted for.

What we see does depend on how fast we are moving but movement has to be relative to something. I am doing all possible speeds now relative to one thing or another in the Universe (but less than c). We know how to account for this.

Regards Andrew

 

[saac]

23 hours ago, Michael Kieth Adams said:

The more distance away from us something is, the faster it moves away from us.    Things are moving slower near to us.  Am I wrong to believe that the huge speeds we see happened long ago and the fact that things near us are slower has to mean that things are slowing down.  Closer is more recent, far away is long ago.  Do I misunderstand ?

I don't believe that this is a correct interpretation . The speed to which you are referring is, as described above, a consequence of the expansion of the space that the objects inhabit.  The expansion of our local space, the space we inhabit and share with our galactic neighbours,  does not dominate rather gravity is the more dominant influence.  So nearer things are not slowing down compared to more distant (earlier) objects they simply inhabit a different space which in turn imparts the "local" expansion signature onto those objects which we observe in red-shift.  The more distant objects carry the signature of their "more distant" expansion of space, again in the form of red shift,  and they inhabit space at a steeper part of the expansion curve.  Remember also that it is space and not the objects therein that is expanding.  I think you are right to assume though that the oldest objects in our universe are furthest away from us .  Even that interpretation may even be fraught with difficulty as, as far as I understand it, it is not really possible to use a  notion of simultaneity across the expanse of the universe - the moment of "now" here on Earth does not translate to the same point in time at the other end of the universe !   My understanding here may be incorrect , happy to be educated, but I believe there is no fixed or absolute point of universal time. 

Jim

Edited December 17, 2020 by saac