A daft question…..

[Stu]

I ask this with some trepidation, not least because I suspect the answers may well be beyond the capacity of my pea-sized brain to process!

I may even have asked this before and forgotten, so my apologies if I have.

My brain says:

When we look at distant objects we see that they are moving away from us due to the expansion of the universe.

The further away the objects are, the faster they are accelerating away from us.

The further away from us the objects are, the further back in time they are.

The furthest objects we see are accelerating the most rapidly, and are also the closest to the Big Bang that we can observe.

If you reverse this process, the younger the objects are, the slower they are accelerating.

Now, the bit I can’t get my head around… why does this not tell us that the acceleration is slowing down over time rather than speeding up?

What do we know or assume about how fast they are accelerating away from us now, even though we can’t see them?

Simple answers on a postcard please 🙏 👍

[Ags]

"If you can understand the answer, you haven't asked the right question."

[vlaiv]

Don't mix change in acceleration and change in speed.

Acceleration is change in speed - or more precisely increase in speed.

5 minutes ago, Stu said:

The further away the objects are, the faster they are accelerating away from us.

Not true - at least, not plainly obvious.

Correct sentence that is plainly obvious should be - further away objects are - faster they are moving away from us. Speed not acceleration.

6 minutes ago, Stu said:

The further away from us the objects are, the further back in time they are.

Correct

6 minutes ago, Stu said:

The furthest objects we see are accelerating the most rapidly, and are also the closest to the Big Bang that we can observe.

Again not true, or rather not obvious - what is obvious is that they have biggest speed away from us.

7 minutes ago, Stu said:

If you reverse this process, the younger the objects are, the slower they are accelerating.

Again - speed not acceleration

8 minutes ago, Stu said:

Now, the bit I can’t get my head around… why does this not tell us that the acceleration is slowing down over time rather than speeding up?

What do we know or assume about how fast they are accelerating away from us now, even though we can’t see them?

Two things here.

First speed - fact that more distant objects are going faster and closer object slower does not have to do anything with speed of acceleration (if it is increasing, is constant or is decreasing). It has to do with the fact that universe is expanding at some rate.

You can sort of equate this with everyone moving away from their neighbor at some speed. Why do we then see distant things moving faster? Well - because they are not our neighbors.

Imagine this:

A - B - C

B sees A moving away and C moving away from them at some speed.

C will see B moving away from them at the same speed - but then, they must see A moving away at twice that speed.

This can extend to a chain of galaxies and further down the chain you go - faster galaxy moves away from us - because each move at the same speed one from another.

Now to acceleration.

Acceleration would be small change in above neighbor to neighbor speed.

We can't see that, or rather - it takes complex math, or complex diagram to be drawn in order to see if things are accelerating or decelerating or staying the same (constant speed).

This can be read of the diagram. If you plot distance vs time - you can always see if object is accelerating or decelerating at particular instant in time.

This is what constant speed looks like - distance between two things increases linearly with time. No acceleration.

This is what acceleration looks like on such graph:

It is change in the slope of the graph that represents acceleration - if it is curved upwards - it is gaining speed - or accelerating. If it is curved downwards - it is slowing down / decelerating.

You might have seen image like this that explains how universe (according to our models) behaved in past:

Curve on this graph has meaning - and meaning is the one I described above.

When graph is curving "downwards" (not necessarily pointing down - just bending in that direction) - we have deceleration, when it is curving upwards - we have acceleration.

From above image - you can read how acceleration changed over the age of the universe.

First there was big bang and then there was massive acceleration in expansion speed - this is called inflation - you see strong curving upward, but then we see curving downward in "dark ages" period. Universe was still expanding at this stage but this expansion was slowing down. Then after period of reionization, structure of universe changed (mass/density thing, or how much radiation vs regular matter there is) and universe started accelerated expansion once again - but at a much slower rate than in inflation period (mechanism that drives accelerated expansion is different).

So universe was always expanding - it always had some speed between neighboring galaxies - but this speed of recession changed during the lifetime of universe - it was rapidly accelerating, then it was decelerating and now we are in very mild acceleration epoch.

Sorry if this was too complex - but I don't really know how to answer your question simpler than this.

[Ags]

Is an alternate explanation possible that there is no expansion at all but "light gets tired" and over the course of billions of years simply shifts to a lower frequency, just like neutrinos change between different types as they travel?

Edited March 31, 2022 by Ags

[Stu]

24 minutes ago, vlaiv said:

Don't mix change in acceleration and change in speed.

Acceleration is change in speed - or more precisely increase in speed.

Not true - at least, not plainly obvious.

Correct sentence that is plainly obvious should be - further away objects are - faster they are moving away from us. Speed not acceleration.

Correct

Again not true, or rather not obvious - what is obvious is that they have biggest speed away from us.

Again - speed not acceleration

Two things here.

First speed - fact that more distant objects are going faster and closer object slower does not have to do anything with speed of acceleration (if it is increasing, is constant or is decreasing). It has to do with the fact that universe is expanding at some rate.

You can sort of equate this with everyone moving away from their neighbor at some speed. Why do we then see distant things moving faster? Well - because they are not our neighbors.

Imagine this:

A - B - C

B sees A moving away and C moving away from them at some speed.

C will see B moving away from them at the same speed - but then, they must see A moving away at twice that speed.

This can extend to a chain of galaxies and further down the chain you go - faster galaxy moves away from us - because each move at the same speed one from another.

Now to acceleration.

Acceleration would be small change in above neighbor to neighbor speed.

We can't see that, or rather - it takes complex math, or complex diagram to be drawn in order to see if things are accelerating or decelerating or staying the same (constant speed).

This can be read of the diagram. If you plot distance vs time - you can always see if object is accelerating or decelerating at particular instant in time.

This is what constant speed looks like - distance between two things increases linearly with time. No acceleration.

This is what acceleration looks like on such graph:

It is change in the slope of the graph that represents acceleration - if it is curved upwards - it is gaining speed - or accelerating. If it is curved downwards - it is slowing down / decelerating.

You might have seen image like this that explains how universe (according to our models) behaved in past:

Curve on this graph has meaning - and meaning is the one I described above.

When graph is curving "downwards" (not necessarily pointing down - just bending in that direction) - we have deceleration, when it is curving upwards - we have acceleration.

From above image - you can read how acceleration changed over the age of the universe.

First there was big bang and then there was massive acceleration in expansion speed - this is called inflation - you see strong curving upward, but then we see curving downward in "dark ages" period. Universe was still expanding at this stage but this expansion was slowing down. Then after period of reionization, structure of universe changed (mass/density thing, or how much radiation vs regular matter there is) and universe started accelerated expansion once again - but at a much slower rate than in inflation period (mechanism that drives accelerated expansion is different).

So universe was always expanding - it always had some speed between neighboring galaxies - but this speed of recession changed during the lifetime of universe - it was rapidly accelerating, then it was decelerating and now we are in very mild acceleration epoch.

Sorry if this was too complex - but I don't really know how to answer your question simpler than this.

Thank you Vlad! I can’t pretend to have read your answer yet, but will do when I get a change and see if I can take it in. My biggest mistake was clearly in the speed/acceleration part so hopefully it will make sense once it out that right in my head! 👍👍

[vlaiv]

14 minutes ago, Ags said:

Is an alternate explanation possible that there is no expansion at all but "light gets tired" and over the course of billions of years simply shifts to a lower frequency, just like neutrinos change between different types as they travel?

I don't think we can just say that light gets tired. What we can say is this, and it again boils down to the same thing - there is no expansion but instead space between galaxies is stretching.

With this continuous stretching of space, light gets stretched as it moves thru it and wavelengths become longer - red shifted.

However, effects of the two are the same - we see different densities of galaxies depending on how far into the past we look - we see red shifts when we look far into the past.

In fact - I think preferred point of view is that space is being stretched rather than "things are flying apart".

This "space is being stretched" - also makes sense in context of new research directions. There is now interesting hypothesis being explored, and I like the way it sounds and what it means. Hypothesis is that space and time are not fundamental but arise from quantum entanglement.

I think it is neat idea as it would explain "elasticity" of space and time (as being of a certain level of entanglement / phase correlation and so on) - that is why we see gravity affecting things (presence of things that are entangled) and the reason why would space be able to stretch between galaxies.

By the way - stretch is probably not the best term - many people use term "to be created" - more space is being created between galaxies. Again - there is no sensible way to justify that point of view if space is fundamental, but if it is consequence of entanglement - then why would not it be created depending how fields entangle ...

 

[powerlord]

today's PBS SpaceTime relevant here:

[jetstream]

@vlaiv What would be the meaning of entanglement in this context?

20 minutes ago, vlaiv said:

By the way - stretch is probably not the best term - many people use term "to be created" - more space is being created between galaxies.

I presume that when more "room" is created between galaxies that the galaxies themselves remain as is? ie no more space in them or "stretching"

[Stu]

@vlaiv, I’ve now digested your post and so understand it, fortunately, thank you.

I shall ponder on it further and read other response before asking anything further   

[Stu]

45 minutes ago, vlaiv said:

By the way - stretch is probably not the best term - many people use term "to be created" - more space is being created between galaxies

The analogy I often hear for this is of a balloon surface stretching, but your comment above implies this is not correct? Also, with a balloon, the more it stretches, the thinner it gets but space is just space, does it actually have a density as such which is reducing as space expands, or….?????

[vlaiv]

18 minutes ago, jetstream said:

I presume that when more "room" is created between galaxies that the galaxies themselves remain as is? ie no more space in them or "stretching"

Yes that is correct. Usual explanation given is that gravity is strong enough to hold galaxy together, while weak enough to be able to stop things "flying apart".

It is a bit like pulling on a rubber band and pulling on a piece of chewing gum.

Rubber band is strong enough for regular pool to be stopped - it will expand a bit, but after that it will stay the same, but if you pull on chewing gum - it does not have enough force to resist and will start to get ever so stretched.

20 minutes ago, jetstream said:

@vlaiv What would be the meaning of entanglement in this context?

Ok, that is somewhat hard to explain, but let's start like this.

Entanglement between two entities is introduction of correlation between their properties.

Say you have something that is can be of any color. Two such things entangled would have some level of similarity in color - say if one has warm color - other would have warm color as well and same for cold color - if first has cold color (like blue or green) - other will have cold color as well.

That is weak correlation.

Entanglement just means some level (strong or weak) of correlation or anti correlation between entities (anti correlation is when property in one entity correlates to what we consider opposite property in another entity - like if one is cold color then you know other will be warm - or warm - cold combination).

Now, I've postulated long time ago when I thought about things - that you need entities and relationships to form existence.

Entanglement represents different type of relationships that can form, and there exist a relationship between elements - that acts like 3d space (you can be left of something, right of something, up/down, forward/ backward and there is order and distance).

I have to go now, but will expand more later.

[Adam J]

1 hour ago, Stu said:

I ask this with some trepidation, not least because I suspect the answers may well be beyond the capacity of my pea-sized brain to process!

I may even have asked this before and forgotten, so my apologies if I have.

My brain says:

When we look at distant objects we see that they are moving away from us due to the expansion of the universe.

The further away the objects are, the faster they are accelerating away from us.

The further away from us the objects are, the further back in time they are.

The furthest objects we see are accelerating the most rapidly, and are also the closest to the Big Bang that we can observe.

If you reverse this process, the younger the objects are, the slower they are accelerating.

Now, the bit I can’t get my head around… why does this not tell us that the acceleration is slowing down over time rather than speeding up?

What do we know or assume about how fast they are accelerating away from us now, even though we can’t see them?

Simple answers on a postcard please 🙏 👍

The key thing that you need to grasp is that space is expanding between objects as opposed to objects moving through space. So the more space there is between objects the faster that space is expanding and so the more space there is. Its effectively a run away process. 

Adam 

 

[Zermelo]

changes in speed vs changes in acceleration:

a financial analogy: inflation is a measure of price change - it's the rate of change of increase of prices. A fixed, positive inflation value implies that prices are rising at a constant rate - say, 4% p.a.   An increasing inflation rate means that prices are increasing ever faster. A decreasing (but still positive) rate means that prices are still rising, but at a slower rate going forward. I recall a letter to a newspaper, complaining "I could understand prices going up when inflation was going up, but then why aren't they coming down now that inflation is falling?" - confusion between the rate of change of prices, and the rate of change of price increases.

Until recently, it was believed that the universe's expansion was decelerating (i.e. a negative acceleration value). This was based on the perceived average density of matter across the universe (and its mutual gravitational effect). In that scenario, the long term prospects could be one of:
(a) universe continues to expand without limit, if the deceleration is not enough to overcome the existing expansion
(b) universe continues to expand, but approaches a fixed limit size, in the case of a 'critical' behaviour of the acceleration (corresponding to a critical average density)
(c) universe stops expanding and then contracts (i.e. negative expansion), resulting in a "big crunch"

However, observations since 1998 have shown that the acceleration is actually increasing, meaning that the expansion will continue unchecked. The causes of this are still being debated, but that's where "dark energy" comes in.

btw, if the rate of change of position is called velocity, and the rate of change of velocity is called acceleration, then the rate of change of acceleration (which will be non-zero, if the acceleration isn't constant) is called jerk, which is then the third derivative of position with respect to time. (and it doesn't stop there - the rate of change of jerk is called jounce, or snap; the fifth derivate is sometimes called crackle, and the sixth pop. I'm not making it up).

 

[andrew s]

3 hours ago, jetstream said:

I presume that when more "room" is created between galaxies that the galaxies themselves remain as is? ie no more space in them or "stretching"

The effective "stretching" force due to the expansion is orders of magnitude smaller than the electromatic or nuclear forces so has no effect on normal objects.

For gravitationally bound object you have to go to scales larger than galaxies or close cluster for it to have a noticible effect.

For example galaxies in our local cluster have a mix of red and blue shifts but as you go further away they all go red.

 

3 hours ago, Ags said:

Is an alternate explanation possible that there is no expansion at all but "light gets tired" and over the course of billions of years simply shifts to a lower frequency, just like neutrinos change between different types as they travel?

This has been seriously studied but predictions based on it don't match the observations.

Regards Andrew 

This gives a good readable overview on the problems with tired light theories.

Edited March 31, 2022 by andrew s

[powerlord]

talking of which, you see they found a star 12.9B light years away?

https://www.theguardian.com/science/2022/mar/30/hubble-space-telescope-captures-distant-star-earendel

[Zermelo]

17 minutes ago, powerlord said:

talking of which, you see they found a star 12.9B light years away?

https://www.theguardian.com/science/2022/mar/30/hubble-space-telescope-captures-distant-star-earendel

 

[jetstream]

2 hours ago, vlaiv said:

I have to go now, but will expand more later.

Perfect, thank you.

[Marvin Jenkins]

STU! What have you done? Unleashing a question like that on people like me. I am tired and have a head ache already.

I am off to join the Flat Earthers🤣

Marvin

[vlaiv]

7 hours ago, Stu said:

The analogy I often hear for this is of a balloon surface stretching, but your comment above implies this is not correct? Also, with a balloon, the more it stretches, the thinner it gets but space is just space, does it actually have a density as such which is reducing as space expands, or….?????

Balloon is both good and bad analogy.

It is good as it shows how certain speed of neighbors moving apart can lead to increase in speed at more separated galaxies. It also shows that there is case where everything moves away from everything else - that is usually hard for people to imagine.

It is bad analogy in number of ways

- it is 3d object but uses 2d analogy. Your question shows what sort of confusion this leads to as you instinctively asked about thickness of rubber and what is the meaning of that. You should only look at a surface of balloon and forget that it has thickness or even that it is a 3d object for analogy to work properly.

- it implies stretching of sorts and that does not correspond to reality. If we were to draw a ruler with marks between two galaxies on balloon itself - it would also stretch as balloon is being blown - but the number of marks would remain the same. That sort of stretching and bending of things - could be happening but we would not be able to tell in any way as it would preserve distances - there would still be 10 marks between two galaxies - regardless of how much air is there in balloon.

Much better explanation is that space is being created between galaxies and it can be related to balloon case - not by focusing on balloon fabric that stretches - but merely the surface of balloon and actual distance between galaxies along the surface of the balloon.

If you blow balloon to larger size and take tape measure and measure distance between two galaxies along the surface of the balloon (and not "inside" surface - like drawn onto it) - then you will see that distance actually grew as there is now more millimeters between galaxies

As for density of space - space itself is empty so it can't have density. However there is something that has density in actual equations used in cosmological model that explains this - and it has name cosmological constant or popularly called - dark energy.

Whatever that is - it is modeled and behaves as if it it has constant density - there is always constant amount of that stuff per some cubic unit of volume.

This is one of reasons people prefer not to think of space as stretching but rather being created between galaxies.

If you take two galaxies and they are in one moment - 10 units of length away from each other and you take 10 units of volume along those ten units of length (like 10 meters of distance and you take 10 cubic meters of space 1m in height, 1m in width and 10m in length) - it will contain 10 units of this cosmological constant. As two galaxies move apart and now there is 11 units of length between them - there is also going to be now 11 units of dark energy or cosmological constant between these two galaxies - as if someone "inserted" one volume of space with same properties as all other volumes of space.

That is how dark energy behaves in equations and this is why we say that space is both expanding - but not stretching, because this mysterious stuff has constant density - it is somehow tied to amount of space that exists between things.

[saac]

9 hours ago, Stu said:

If you reverse this process, the younger the objects are, the slower they are accelerating.

Now, the bit I can’t get my head around… why does this not tell us that the acceleration is slowing down over time rather than speeding up?

What do we know or assume about how fast they are accelerating away from us now, even though we can’t see them?

Simple answers on a postcard please 🙏 👍

 

Stu I think what you are effectively asking is what evidence do we have that the rate of expansion is increasing (accelerating) .   The Hubble -Lemaitre law documents the expansion of the universe - evident from observations as you say that the most distant objects are moving away at a higher velocity (recessional velocity).   The rate of this expansion (the gradient of the Hubble-Lemaitre graph) has since been shown to be increasing; another way of saying this is that the rate of expansion is accelerating.  Graphically this is often depicted in diagrams as shown below where the curve of the expansion line steepens.  So we have the Hubble-Lemaitre law telling us that the universe is expanding (more distant objects have higher recessional velocity) but where is the evidence that the rate of expansion is accelerating.  Evidence of an accelerated expansion originated in 1998 from the Supernovae Cosmology Project (https://en.wikipedia.org/wiki/Supernova_Cosmology_Project) which, as far as I can tell, employed a similar but more refined method than Hubble based on observations of Type 1s supernovae. In effect they they measured distance and recessional velocity like Hubble but more accurately.  Their findings contradicted the expectation of the time that the rate of expansion was reducing due to gravitation attraction.  Their measurements showed instead an acceleration in the rate of expansion in turn leading to a revival of the cosmological constant and ultimately paving the way for theories of dark energy to emerge. 

Jim 

 

 

Edited March 31, 2022 by saac

[Stu]

15 minutes ago, vlaiv said:

Balloon is both good and bad analogy.

It is good as it shows how certain speed of neighbors moving apart can lead to increase in speed at more separated galaxies. It also shows that there is case where everything moves away from everything else - that is usually hard for people to imagine.

It is bad analogy in number of ways

- it is 3d object but uses 2d analogy. Your question shows what sort of confusion this leads to as you instinctively asked about thickness of rubber and what is the meaning of that. You should only look at a surface of balloon and forget that it has thickness or even that it is a 3d object for analogy to work properly.

- it implies stretching of sorts and that does not correspond to reality. If we were to draw a ruler with marks between two galaxies on balloon itself - it would also stretch as balloon is being blown - but the number of marks would remain the same. That sort of stretching and bending of things - could be happening but we would not be able to tell in any way as it would preserve distances - there would still be 10 marks between two galaxies - regardless of how much air is there in balloon.

Much better explanation is that space is being created between galaxies and it can be related to balloon case - not by focusing on balloon fabric that stretches - but merely the surface of balloon and actual distance between galaxies along the surface of the balloon.

If you blow balloon to larger size and take tape measure and measure distance between two galaxies along the surface of the balloon (and not "inside" surface - like drawn onto it) - then you will see that distance actually grew as there is now more millimeters between galaxies

As for density of space - space itself is empty so it can't have density. However there is something that has density in actual equations used in cosmological model that explains this - and it has name cosmological constant or popularly called - dark energy.

Whatever that is - it is modeled and behaves as if it it has constant density - there is always constant amount of that stuff per some cubic unit of volume.

This is one of reasons people prefer not to think of space as stretching but rather being created between galaxies.

If you take two galaxies and they are in one moment - 10 units of length away from each other and you take 10 units of volume along those ten units of length (like 10 meters of distance and you take 10 cubic meters of space 1m in height, 1m in width and 10m in length) - it will contain 10 units of this cosmological constant. As two galaxies move apart and now there is 11 units of length between them - there is also going to be now 11 units of dark energy or cosmological constant between these two galaxies - as if someone "inserted" one volume of space with same properties as all other volumes of space.

That is how dark energy behaves in equations and this is why we say that space is both expanding - but not stretching, because this mysterious stuff has constant density - it is somehow tied to amount of space that exists between things.

Thanks Vlad. That makes total sense, even if it is hard to comprehend the concept of space being created, I get what you are saying 👍👍

[vlaiv]

5 hours ago, jetstream said:

Perfect, thank you.

Ok, not sure if I can sensibly continue where I left off

I was trying to maybe offer hint of explanation to what would mean that entanglement gives rise to space and time.

I might be waaay off with this - as it is area of active research, but I do believe it is related to relationships or especially concept of mathematical relation.

Certain types of relations give rise to certain structures. I'll give you two simple using binary relations. Binary relation is just simply mapping of elements between two sets. It can be thought of as on/off association - either there is link (correlation in entanglement lingo) between elements of the set or there is not.

Such relations can have specific properties - like symmetry.

Symmetry is property that if A is in relation with B then B is in relation to A and this holds for any A - element of first set and B - element of second set.

To give you an example in everyday life - such relation is "being brother to" - if we observe all male set.

If Adam is Benjamin's brother - then Benjamin is also Adam's brother.

There is Transitivity or transitive relation.

If A relates to B and B relates to C then A relates to C.

Examples of such relationships is "being longer than" in set of swords, or being descendant in human population.

If we take another property like Reflexivity - that simply means that any element is in relation with itself, and we combine those three properties of relations - we get raise to a structure of sorts

Relation that is reflexive, transitive and symmetric - defines what it means to be equal.

We can change one of above to something else - and we will have different structure. Say we introduce Anti symmetry - being opposite from symmetry - if A is in relation to B then B can't be in relation to A, then we have new structure that has Transitivity, Reflexivity and Anti Symmetry - and such relation gives raise to order.

Example would be less or equal to in set of numbers. If we have such relation over set of something - then we can order them according to this relation.

Space is nothing more than set of relations - how is anything position with respect to other things - in front, behind, left, right, far, close, ...

Time is same thing but for events - in fact, space time is "space" for events - which has such properties, and if entanglement creates relations between elements - it can give rise to structures and space-time can be resulting complex structure that arises because things entangle.

Makes sense?

[Stu]

15 minutes ago, saac said:

 

Stu I think what you are effectively asking is what evidence do we have that the rate of expansion is increasing (accelerating) .   The Hubble -Lemaitre law documents the expansion of the universe - evident from observations as you say that the most distant objects are moving away at a higher velocity (recessional velocity).   The rate of this expansion (the gradient of the Hubble-Lemaitre graph) has since been shown to be increasing; another way of saying this is that the rate of expansion is accelerating.  Graphically this is often depicted in diagrams as shown below where the curve of the expansion line steepens.  So we have the Hubble-Lemaitre law telling us that the universe is expanding (more distant objects have higher recessional velocity) but where is the evidence that the rate of expansion is accelerating.  Evidence of an accelerated expansion originated in 1998 from the Supernovae Cosmology Project (https://en.wikipedia.org/wiki/Supernova_Cosmology_Project) which, as far as I can tell, employed a similar but more refined method than Hubble based on observations of Type 1s supernovae. In effect they they measured distance and recessional velocity like Hubble but more accurately.  Their findings contradicted the expectation of the time that the rate of expansion was reducing due to gravitation attraction.  Their measurements showed instead an acceleration in the rate of expansion in turn leading to a revival of the cosmological constant and ultimately paving the way for theories of dark energy to emerge. 

Jim 

 

 

Thanks Jim.

This shows accelerated expansion in the present, which implies we see higher expansion locally that in the most distant regions, or am I misinterpreting the diagram/what you are saying?

Do we know anything about what is happening to those galaxies now ie 12 or 13 billion odd years from when we are seeing them now….?

[vlaiv]

3 minutes ago, Stu said:

Thanks Vlad. That makes total sense, even if it is hard to comprehend the concept of space being created, I get what you are saying 👍👍

Yes, indeed - if we think of space and time as this background that is given and that things happen in - much like theater stage, then it is sort of very unintuitive to think of space being created.

That is one of the reasons I find notion of space and time being product of something else very plausible.

[mikeDnight]

9 hours ago, Ags said:

Is an alternate explanation possible that there is no expansion at all but "light gets tired" and over the course of billions of years simply shifts to a lower frequency, just like neutrinos change between different types as they travel?

Spot on!  Photons that leave a distant galaxy or star are not the same photons that reach us. Each interaction between high energy photons with other particles or subatomic particles, causes a loss in the energy level of the photons, causing a move towards the red end of the spectrum, so the more distant the object the stronger the move towards the red. Therefore red is an indicator of distance and not of recession. All these fancy hypotheses remind me of the Copernican model of the solar system, clever but overly complicated and ultimately wrong. 🥄