Hubble's law

[Alan Grayson]

Is it correct to interpret Hubble's law as a linear relationship between recessional velocity and distance, both in the present, NOW, but NOT to imply that in the very early universe galaxies were receding at near light speed? AG

[Alan Grayson]

On Thursday, June 5, 2025 at 8:35:56 PM UTC-6 Alan Grayson wrote:

Is it correct to interpret Hubble's law as a linear relationship between recessional velocity and distance, both in the present, NOW, but NOT to imply that in the very early universe galaxies were receding at near light speed? AG

My problem is this; the light from distant galaxies is highly red-shifted, indicating a huge recessional velocity, but at a much earlier time. But at that time, galaxies were fairly close to each other, so the model of recessional velocity being primarily an artifact of geometry and expansion seems in doubt. What's the resolution? AG 

[John Clark]

On Sat, Jun 7, 2025 at 1:59 AM Alan Grayson <agrays...@gmail.com> wrote:

> My problem is this; the light from distant galaxies is highly red-shifted, indicating a huge recessional velocity, but at a much earlier time.

No. The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

John K Clark    See what's on my new list at  Extropolis

q19

[Alan Grayson]

You say we're observing how that galaxy looked 13.5 billion years ago, but that the redshift being observed today, gives us the recessional velocity today? Seems contradictory. AG 

q19

[John Clark]

On Sat, Jun 7, 2025 at 8:05 AM Alan Grayson <agrays...@gmail.com> wrote:

>> The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

> You say we're observing how that galaxy looked 13.5 billion years ago,

Yes.

 

> but that the redshift being observed today, gives us the recessional velocity today?

Not exactly. Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding. The movement through space can never be faster than the speed of light nor can we communicate faster than the speed of light, but space itself is free to expand at any speed.  

> Seems contradictory. AG 

It's not. 13.5 billion years ago when that light was emitted it was in the ultraviolet, but during its journey space has been expanding, so the wavelength of the light has been expanding, so now the light is in the infrared not the ultraviolet. 

[Alan Grayson]

On Saturday, June 7, 2025 at 6:28:41 AM UTC-6 John Clark wrote:

On Sat, Jun 7, 2025 at 8:05 AM Alan Grayson <agrays...@gmail.com> wrote:

>> The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

> You say we're observing how that galaxy looked 13.5 billion years ago,

Yes.

 

> but that the redshift being observed today, gives us the recessional velocity today?

Not exactly. Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

The redshift gives us a combination of expansion of space and the recessional velocity through space. But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal? I am having difficulty resolving the rapid recessional velocity implied by Hubble's law, whether it's now or in the past. AG

 

The movement through space can never be faster than the speed of light nor can we communicate faster than the speed of light, but space itself is free to expand at any speed.  

> Seems contradictory. AG 

It's not. 13.5 billion years ago when that light was emitted it was in the ultraviolet, but during its journey space has been expanding, so the wavelength of the light has been expanding, so now the light is in the infrared not the ultraviolet. 

If photons have no measured extent, which I think Brent concedes, the model of their "waves" being stretched as the universe expands, does not explain their loss of energy. AG 

[Brent Meeker]

On 6/10/2025 7:58 PM, Alan Grayson wrote:

On Saturday, June 7, 2025 at 6:28:41 AM UTC-6 John Clark wrote:

On Sat, Jun 7, 2025 at 8:05 AM Alan Grayson <agrays...@gmail.com> wrote:

>> The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

> You say we're observing how that galaxy looked 13.5 billion years ago,

Yes.

 

> but that the redshift being observed today, gives us the recessional velocity today?

Not exactly. Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

The redshift gives us a combination of expansion of space and the recessional velocity through space.

That's assumed to be zero.

But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal?

No one denies that.  The galaxy is further away now and receding faster now and hence still obeying Hubble's law.

I am having difficulty resolving the rapid recessional velocity implied by Hubble's law, whether it's now or in the past. AG

 

The movement through space can never be faster than the speed of light nor can we communicate faster than the speed of light, but space itself is free to expand at any speed.  

> Seems contradictory. AG 

It's not. 13.5 billion years ago when that light was emitted it was in the ultraviolet, but during its journey space has been expanding, so the wavelength of the light has been expanding, so now the light is in the infrared not the ultraviolet. 

If photons have no measured extent, which I think Brent concedes, the model of their "waves" being stretched as the universe expands, does not explain their loss of energy. AG

They have extent in the direction of travel.

Brent

[Alan Grayson]

On Tuesday, June 10, 2025 at 9:27:11 PM UTC-6 Brent Meeker wrote:

On 6/10/2025 7:58 PM, Alan Grayson wrote:

On Saturday, June 7, 2025 at 6:28:41 AM UTC-6 John Clark wrote:

On Sat, Jun 7, 2025 at 8:05 AM Alan Grayson <agrays...@gmail.com> wrote:

>> The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

> You say we're observing how that galaxy looked 13.5 billion years ago,

Yes.

 

> but that the redshift being observed today, gives us the recessional velocity today?

Not exactly. Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

The redshift gives us a combination of expansion of space and the recessional velocity through space.

That's assumed to be zero.

But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal?

No one denies that.  The galaxy is further away now and receding faster now and hence still obeying Hubble's law.

How can it be receding near light speed NOW, if we're measuring the red shift in the PAST? AG 

I am having difficulty resolving the rapid recessional velocity implied by Hubble's law, whether it's now or in the past. AG 

The movement through space can never be faster than the speed of light nor can we communicate faster than the speed of light, but space itself is free to expand at any speed.  

> Seems contradictory. AG 

It's not. 13.5 billion years ago when that light was emitted it was in the ultraviolet, but during its journey space has been expanding, so the wavelength of the light has been expanding, so now the light is in the infrared not the ultraviolet. 

If photons have no measured extent, which I think Brent concedes, the model of their "waves" being stretched as the universe expands, does not explain their loss of energy. AG

They have extent in the direction of travel.

Has that been measured? AG 

Brent

[Brent Meeker]

On 6/10/2025 9:54 PM, Alan Grayson wrote:

On Tuesday, June 10, 2025 at 9:27:11 PM UTC-6 Brent Meeker wrote:

On 6/10/2025 7:58 PM, Alan Grayson wrote:

On Saturday, June 7, 2025 at 6:28:41 AM UTC-6 John Clark wrote:

On Sat, Jun 7, 2025 at 8:05 AM Alan Grayson <agrays...@gmail.com> wrote:

>> The James Webb telescope recently found a galaxy that had a red shift of 14.44, from that number astronomers calculate that it took light 13.5 billion years to reach us, so we're observing how that galaxy looked 13.5 billion years ago. However during that 13.5 billion years the universe has not only been expanding it's been accelerating, so back then the universe was expanding slower not faster than it is now. Today that galaxy is not 13.5 billion light years from us, it is 34.7 billion light years from us. Even if we could travel at the speed of light we could never reach that galaxy in a finite number of years, and any galaxy that has a red shift greater than 1.8 is forever out of our reach.

> You say we're observing how that galaxy looked 13.5 billion years ago,

Yes.

 

> but that the redshift being observed today, gives us the recessional velocity today?

Not exactly. Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

The redshift gives us a combination of expansion of space and the recessional velocity through space.

That's assumed to be zero.

But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal?

No one denies that.  The galaxy is further away now and receding faster now and hence still obeying Hubble's law.

How can it be receding near light speed NOW, if we're measuring the red shift in the PAST? AG

It's receding faster than light speed now.  It's gone from the observable universe.

Brent

[Alan Grayson]

I don't doubt that, but it doesn't address my question. We see the light from the distant PAST, and it's hugely red shifted, yet we conclude that that is a measure of its recessional velocity NOW. AG 

[John Clark]

On Tue, Jun 10, 2025 at 10:58 PM Alan Grayson <agrays...@gmail.com> wrote:

>> Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

> The redshift gives us a combination of expansion of space and the recessional velocity through space.

Yes.

 

> But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal?

Deny? What are we denying? I don't understand the question.  

 

I am having difficulty resolving the rapid recessional velocity implied by Hubble's law, whether it's now or in the past. AG

The "Hubble constant" is a poor name because it's not constant, a better name would be the Hubble parameter. When people speak about the Hubble constant what they usually mean is the rate at which the universe is expanding right now, the symbol for that is H0.

> If photons have no measured extent, which I think Brent concedes, the model of their "waves" being stretched as the universe expands, does not explain their loss of energy. AG 

Yes it does explain how photons lose their energy as the universe expands because in addition to saying that photons have no measurable extent Quantum Mechanics ALSO says that the formula for finding the energy in a photon is:

E = hc/λ

λ is the wavelength, c is the speed of light, and h is Planck's constant. 

And countless experiments have proven that Quantum Mechanics made the correct prediction about how much energy a photon will have. If all this seems very strange that's only because it is, the universe is under no obligation to be understandable by human beings. 

John K Clark    See what's on my new list at  Extropolis

7ow

[Alan Grayson]

On Wednesday, June 11, 2025 at 6:08:13 AM UTC-6 John Clark wrote:

On Tue, Jun 10, 2025 at 10:58 PM Alan Grayson <agrays...@gmail.com> wrote:

>> Velocity is about objects moving through space, but the redshift tells us how much space itself has been expanding.

> The redshift gives us a combination of expansion of space and the recessional velocity through space.

Yes.

 

> But since we're observing the galaxy as it was, about 10 billion years ago, how can we deny that it's now receding at near light speed if that's what our measurements plainly reveal?

Deny? What are we denying? I don't understand the question.  

For distant galaxies we're observing the past, which shows a large redshift, which means a large recessional velocity in the PAST, but then you conclude that same recessional velocity is occurring NOW, in the present. AG 

[John Clark]

On Wed, Jun 11, 2025 at 8:22 AM Alan Grayson <agrays...@gmail.com> wrote:

> For distant galaxies we're observing the past, which shows a large redshift, which means a large recessional velocity in the PAST, 

No! We're observing the light from that galaxy NOW, and today space has expanded a great deal more than it had 10 billion years ago when that light was emitted; back then it was ultraviolet, now it's infrared. We are observing how the galaxy looked 10 billion years ago, except that the color is different, however Einstein and Quantum Mechanics can tell us how to color correct for that and get a more accurate picture.   

John K Clark    See what's on my new list at  Extropolis

7mh

[Alan Grayson]

When viewing celestial objects, it's routinely claimed that what we observe, is how something looked in the past. The farther away it is, the further in the past is what the observation reveals. But now you've turned this on its proverbial head; namely, the redshift observed is a measure of its recessional velocity NOW. How are these contradictory interpretations resolved? TY, AG 

7mh

[John Clark]

On Thu, Jun 12, 2025 at 4:59 AM Alan Grayson <agrays...@gmail.com> wrote:

> When viewing celestial objects, it's routinely claimed that what we observe, is how something looked in the past.

Yes.

The farther away it is, the further in the past is what the observation reveals.

Yes.

> But now you've turned this on its proverbial head; namely, the redshift observed is a measure of its recessional velocity NOW.

Unless you're talking about very nearby galaxies such as Andromeda, which is actually blueshifted, cosmic redshift is NOT caused by the recessional velocity of something moving through space, it is caused by the amount of expansion of space itself that has occurred during the billions of years that it took the light to reach us. 

 

>How are these contradictory interpretations resolved? TY, AG 

What contradictory interpretations?  The light from distant galaxies is always damaged in one way or another, sometimes they run into dust clouds and only their infrared light makes it through, and sometimes the images of galaxies are distorted by gravitational lensing. But if we know there was a dust cloud between us and the galaxy then nobody believes the galaxy produced no optical or ultraviolet light, and if the light passed near a massive object then nobody believes the galaxy really had such a weird shape, we know the distortion was caused by a gravitational lens. 

 John K Clark    See what's on my new list at  Extropolis

zx5

[Alan Grayson]

On Thursday, June 12, 2025 at 5:05:27 AM UTC-6 John Clark wrote:

On Thu, Jun 12, 2025 at 4:59 AM Alan Grayson <agrays...@gmail.com> wrote:

> When viewing celestial objects, it's routinely claimed that what we observe, is how something looked in the past.

Yes.

The farther away it is, the further in the past is what the observation reveals.

Yes.

> But now you've turned this on its proverbial head; namely, the redshift observed is a measure of its recessional velocity NOW.

Unless you're talking about very nearby galaxies such as Andromeda, which is actually blueshifted, cosmic redshift is NOT caused by the recessional velocity of something moving through space, it is caused by the amount of expansion of space itself that has occurred during the billions of years that it took the light to reach us. 

That's one interpretation. Another is that the redshift from distant galaxies indicates a rapid expansion at the time of the BB. AG 

[Liz R]

I asumed it isn't a linear relationship due to the universe's expansion rate varying with time, however, that isn't apparently quite correct...

According to:

https://bigthink.com/starts-with-a-bang/hubble-constant-changes-time/

...the constant will always be measured as constant, but that result will vary over (cosmological) time scales.