APOD: CMB Dipole: Speeding Through the... (2022 Apr 03)

[Iksarfighter]

Few people only really understand tidal phenomens.

[johnnydeep]

That's closer. It's actually pretty simple. The gravitational force on the Earth is stronger on the side closer to the Moon than on the other. Add the two force vectors, and you have a net force that pulls the ocean (and the solid Earth, as well) into an oblong shape. Nothing to do with centrifugal force. It would happen even if the Earth and Moon were stationary (assuming a mechanism to keep them from falling into each other).

The same even if the moon and earth were not rotating or orbiting each other? That can't be right. If, for the sake of argument, there was a zero mass infinitely strong pole holding the earth and moon in position relative to each other, wouldn't all the oceans of the earth eventually migrate to the side closest to the moon?

You're ignoring the effect of Earth's own gravity on the oceans. Simplify the system. Replace the Earth with a big blob of Jello, held at a fixed distance from the Moon with a giant stick. What shape will that blob assume?

Yes, that makes it make a lot more sense. Or even better, just replace the Earth with a big globe of water!. The only shape that it could possibly form is an oblate spheroid. All the water certainly wouldn't run to the other side!

[Chris Peterson]

The same even if the moon and earth were not rotating or orbiting each other? That can't be right. If, for the sake of argument, there was a zero mass infinitely strong pole holding the earth and moon in position relative to each other, wouldn't all the oceans of the earth eventually migrate to the side closest to the moon?

You're ignoring the effect of Earth's own gravity on the oceans. Simplify the system. Replace the Earth with a big blob of Jello, held at a fixed distance from the Moon with a giant stick. What shape will that blob assume?

Yes, that makes it make a lot more sense. Or even better, just replace the Earth with a big globe of water!. The only shape that it could possibly form is an oblate spheroid. All the water certainly wouldn't run to the other side! :-)

The reason I chose Jello over water is because there's no obvious way to hold a sphere of water at a fixed distance from a tidal mass. So a gel works better for my example. But yes, a blob of water does the same thing if you can stop it from falling into the Moon.

[johnnydeep]

Well, those two observers are seeing different CMBs, so that complicates things. And you need to be clear about what the two observers know about each other already.

What do you mean by different CMBs? Let's use my hypothetical "pole" idea from my post above about the earth/moon/tides. Suppose an infinitely strong zero mass pole was holding the Milky Way and Andromeda (say) in fixed position relative to each other. Wouldn't an observer in each galaxy (discounting galaxy rotation, etc) see the CMB Doppler shifted the same amount in all directions? And if so, if one measured a zero shift, wouldn't the other also?

Again, an observer in our galaxy sees a different CMB than an observer in the Andromeda Galaxy, because we each have our own unique observable universes. Of course, as close as we are, they only differ very slightly, so they will appear very similar. But each of us still can see parts of the Universe forever hidden from the other.

And again, I still don't understand your different CMBs. Hmm, the CMB isn't just a thin shell, is it. It suffuses all space, right? In the sense that those first photons are flying around everywhere in all parts of the universe (and they're losing energy over time due to the expansion of space). And you'd see different photons, Doppler shifted differently, depending on where you happen to be and how you are moving relative to those photons. Is that what you mean?

[Iksarfighter]

I have calculated that for 2 spheres with same apparent diameter (Sun, Moon...), the ratio of their tidal effect is equal of the ratio of their density. That works well with Sun and Moon
It implies also that if you hold at the end of your arm a small sphere with the density of the moon and if this sphere occults exactly and totally the Moon, then this small sphere produce the same tidal effect on your eye than the Moon does.

[johnnydeep]

You're ignoring the effect of Earth's own gravity on the oceans. Simplify the system. Replace the Earth with a big blob of Jello, held at a fixed distance from the Moon with a giant stick. What shape will that blob assume?

Yes, that makes it make a lot more sense. Or even better, just replace the Earth with a big globe of water!. The only shape that it could possibly form is an oblate spheroid. All the water certainly wouldn't run to the other side!

The reason I chose Jello over water is because there's no obvious way to hold a sphere of water at a fixed distance from a tidal mass. So a gel works better for my example. But yes, a blob of water does the same thing if you can stop it from falling into the Moon.

I silently dispensed with the pole and just assumed it was orbiting normally just like the Earth does.

[Chris Peterson]

Yes, that makes it make a lot more sense. Or even better, just replace the Earth with a big globe of water!. The only shape that it could possibly form is an oblate spheroid. All the water certainly wouldn't run to the other side! :-)

The reason I chose Jello over water is because there's no obvious way to hold a sphere of water at a fixed distance from a tidal mass. So a gel works better for my example. But yes, a blob of water does the same thing if you can stop it from falling into the Moon.

I silently dispensed with the pole and just assumed it was orbiting normally just like the Earth does. :-)

Which is fine. I just wanted to remove the orbiting part to take any issue of centrifugal force out of things.

[Chris Peterson]

What do you mean by different CMBs? Let's use my hypothetical "pole" idea from my post above about the earth/moon/tides. Suppose an infinitely strong zero mass pole was holding the Milky Way and Andromeda (say) in fixed position relative to each other. Wouldn't an observer in each galaxy (discounting galaxy rotation, etc) see the CMB Doppler shifted the same amount in all directions? And if so, if one measured a zero shift, wouldn't the other also?

Again, an observer in our galaxy sees a different CMB than an observer in the Andromeda Galaxy, because we each have our own unique observable universes. Of course, as close as we are, they only differ very slightly, so they will appear very similar. But each of us still can see parts of the Universe forever hidden from the other.

And again, I still don't understand your different CMBs. Hmm, the CMB isn't just a thin shell, is it. It suffuses all space, right? In the sense that those first photons are flying around everywhere in all parts of the universe (and they're losing energy over time due to the expansion of space). And you'd see different photons, Doppler shifted differently, depending on where you happen to be and how you are moving relative to those photons. Is that what you mean?

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

[Ann]

Again, an observer in our galaxy sees a different CMB than an observer in the Andromeda Galaxy, because we each have our own unique observable universes. Of course, as close as we are, they only differ very slightly, so they will appear very similar. But each of us still can see parts of the Universe forever hidden from the other.

And again, I still don't understand your different CMBs. Hmm, the CMB isn't just a thin shell, is it. It suffuses all space, right? In the sense that those first photons are flying around everywhere in all parts of the universe (and they're losing energy over time due to the expansion of space). And you'd see different photons, Doppler shifted differently, depending on where you happen to be and how you are moving relative to those photons. Is that what you mean?

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

But our observable universe is moving with respect to the CMB?

Right?

Ann

[Chris Peterson]

And again, I still don't understand your different CMBs. Hmm, the CMB isn't just a thin shell, is it. It suffuses all space, right? In the sense that those first photons are flying around everywhere in all parts of the universe (and they're losing energy over time due to the expansion of space). And you'd see different photons, Doppler shifted differently, depending on where you happen to be and how you are moving relative to those photons. Is that what you mean?

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

But our observable universe is moving with respect to the CMB?

Right?

Ann

No. The CMB represents the most distant edge of the observable universe we are able to see in electromagnetic radiation.

[Iksarfighter]

Maybe some day advanced neutrino detector be able to see past these 13.6 billion years ?

[Chris Peterson]

Maybe some day advanced neutrino detector be able to see past these 13.6 billion years ?

That would be one possibility. Gravitational radiation is another.

[Iksarfighter]

Maybe some day advanced neutrino detector be able to see past these 13.6 billion years ?

That would be one possibility. Gravitational radiation is another.

Oh yes ! Thx for info.

[Chris Peterson]

Maybe some day advanced neutrino detector be able to see past these 13.6 billion years ?

That would be one possibility. Gravitational radiation is another.

Oh yes ! Thx for info.

Of course, the actual edge of the observable universe is just 380,000 light years beyond the currently visible edge. Not very far, but lots of interesting stuff happened in that short period.

[MarkBour]

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

How would those photons be different if they had been in flight for 20 billion years?

[Chris Peterson]

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

How would those photons be different if they had been in flight for 20 billion years?

How could they have been in flight for greater than the age of the Universe? If the redshift led to that conclusion, we'd revise the age of the Universe.

[MarkBour]

The CMB is defined by photons that have been in flight for 13.6 billion years. We see their origin in a shell around us.

How would those photons be different if they had been in flight for 20 billion years?

How could they have been in flight for greater than the age of the Universe? If the redshift led to that conclusion, we'd revise the age of the Universe.

I think I'm questioning how rational the current interpretation of high-redshift data is.
If I look at this article: https://en.wikipedia.org/wiki/List_of_t ... al_objects
I see the following table:

Code: Select all

                                        Light travel 
       Name              Redshift (z)  distance (Gly)        Type                   Notes
----------------------  -------------  --------------   ---------------  --------------------------------
HD1                       z = 13.27        13.5         Galaxy           Formulative understanding
GN-z11                    z = 11.09        13.39        Galaxy           Confirmed galaxy
MACS1149-JD1              z = 9.11         13.26        Galaxy           Confirmed galaxy
EGSY8p7                   z = 8.68         13.23        Galaxy           Confirmed galaxy
A2744 YD4                 z = 8.38         13.20        Galaxy           Confirmed galaxy
MACS0416 Y1               z = 8.31         13.20        Galaxy           Confirmed galaxy
GRB 090423                z = 8.2          13.18        Gamma-ray burst
EGS-zs8-1                 z = 7.73         13.13        Galaxy           Confirmed galaxy
z7 GSD 3811               z = 7.66         13.11        Galaxy           Galaxy
J0313-1806                z = 7.64                      Quasar            
z8 GND 5296               z = 7.51         13.10        Galaxy           Confirmed galaxy
A1689-zD1                 z = 7.5          13.10        Galaxy           Galaxy
GS2_1406                  z = 7.452        13.095       Galaxy           Galaxy
SXDF-NB1006-2             z = 7.215        13.07        Galaxy           Galaxy
GN-108036                 z = 7.213        13.07        Galaxy           Galaxy
BDF-3299                  z = 7.109        13.05        Galaxy            
ULAS J1120+0641           z = 7.085        13.05        Quasar            
A1703 zD6                 z = 7.045        13.04        Galaxy            
BDF-521                   z = 7.008        13.04        Galaxy            
G2-1408                   z = 6.972        13.03        Galaxy            
IOK-1                     z = 6.964        13.03        Galaxy           Lyman-alpha emitter
LAE J095950.99+021219.1   z = 6.944        13.03        Galaxy           Lyman-alpha emitter — Faint galaxy

Starting at the bottom, a z of around 7 is interpreted as light from 13 billion years ago.
Moving to the top, it is quite non-linear, and we're up to the latest discoveries with z=11 and z=13 that are interpreted as light traveling just a "little" longer, 13.2 and 13.5 billion years.

~~~~~~~~~~

Okay, perhaps you would want to stop reading at this point. The following are some ruminations of a non-professional, but a person who is interested as they watch the progress of this science, philosophically. I wonder how various ideas can actually run up against the data and withstand the shock with just minor adjustment, or instead get refuted by the data. But I really don't have enough understanding to be worth listening to on this topic. I certainly don't have any credentials to cause anyone to think they ought to listen to me.

The question I would like to ask, is:

Okay, we just saw a photon from a galaxy that astronomers say travelled for 13 billion years and landed in our telescope. A little ways away from it, another photon from the same source happened to miss our telescope. Let that photon continue to fly through space for another 7 billion years. What will be its redshift then? Can anyone answer this?

I assume most cosmology astronomers can indeed give me an answer, that yes, they think it will have z= (some value, call it r).
If they can't then I don't think they have even a basically useful theory about the age of the objects they're seeing.

On the other hand, if they can, then I'm waiting for the JWST to see an object with z = r, and to realize that it's a full galaxy.
But when they add said object to their chart, they're likely to just say:

Oh, z=<r>? That's just a little older than z=13. We'll calculate it at about 13.6 billion years.
Ummm . . . But we're surprised to find an apparently complex galaxy at such an early date after the big bang. We'll have to revise some of our numbers about the big bang a little. Or maybe we'll have to change our description of the cosmic inflation epoch to handle this.

If all of this should come to pass, I would then say that they're kind of stuck in their paradigm, unable to see beyond it (I meant that metaphorically, but it works literally, too).

But I think a rival theory will perhaps arise to challenge it. This all circles back (for me) to

• How old, and from what distance, is the radiation of the CMB, really?
• What opportunity does the JWST present for astronomy and cosmology? Will it continue to fill in a picture of cosmology that is already hard to change, or will it cause a revolution in our understanding?

I like placing bets on these things (just a "gentleman's bet" today). I think the idea that the CMB is proof of a big bang 13.8 billion years ago is wrong. I think it will eventually be understood as light from stars and such that is hundreds of billions of years or trillions of years old. Of course at this point I'm way out on a limb and the likelihood that I'm right in these assertions is very low. And if by some luck those hunches turn out to be correct, they will only become believed by anyone from the research of some who will have much more understanding and do lots more work than I have. Still, it's fun to muse.

[Chris Peterson]

I think I'm questioning how rational the current interpretation of high-redshift data is.

Very rational. It hangs on all of the underlying theory of the lambda-CDM model, which is itself very well supported by multiple independent lines of evidence. There are a few parameters which remain uncertain, such as the actual geometry of the Universe (assumed to be flat, but possibly not) and the exact current value of the Hubble constant (which is not constant with time). The redshift calculation is based on how the model has the Hubble constant changing. But the parameters are sufficiently well known now that the relationship between redshift and light travel time is reasonably accurate. Changing the parameters within their error space isn't going to radically change the relationship.

The question I would like to ask, is:

Okay, we just saw a photon from a galaxy that astronomers say travelled for 13 billion years and landed in our telescope. A little ways away from it, another photon from the same source happened to miss our telescope. Let that photon continue to fly through space for another 7 billion years. What will be its redshift then? Can anyone answer this?

Sure. Redshift is created by the expansion of the Universe, and the current expansion rate (and certainly that for many billions of years into the future) is known with a low error. So it's a simple calculation to determine how much redshift a photon will experience over the next 7 billion years of travel time.

Of course at this point I'm way out on a limb and the likelihood that I'm right in these assertions is very low.

I agree. ;-)

[alter-ego]

How would those photons be different if they had been in flight for 20 billion years?

How could they have been in flight for greater than the age of the Universe? If the redshift led to that conclusion, we'd revise the age of the Universe.

I think I'm questioning how rational the current interpretation of high-redshift data is.
If I look at this article: https://en.wikipedia.org/wiki/List_of_t ... al_objects
I see the following table:

Code: Select all

                                        Light travel 
       Name              Redshift (z)  distance (Gly)        Type                   Notes
----------------------  -------------  --------------   ---------------  --------------------------------
HD1                       z = 13.27        13.5         Galaxy           Formulative understanding
GN-z11                    z = 11.09        13.39        Galaxy           Confirmed galaxy
MACS1149-JD1              z = 9.11         13.26        Galaxy           Confirmed galaxy
EGSY8p7                   z = 8.68         13.23        Galaxy           Confirmed galaxy
A2744 YD4                 z = 8.38         13.20        Galaxy           Confirmed galaxy
MACS0416 Y1               z = 8.31         13.20        Galaxy           Confirmed galaxy
GRB 090423                z = 8.2          13.18        Gamma-ray burst
EGS-zs8-1                 z = 7.73         13.13        Galaxy           Confirmed galaxy
z7 GSD 3811               z = 7.66         13.11        Galaxy           Galaxy
J0313-1806                z = 7.64                      Quasar            
z8 GND 5296               z = 7.51         13.10        Galaxy           Confirmed galaxy
A1689-zD1                 z = 7.5          13.10        Galaxy           Galaxy
GS2_1406                  z = 7.452        13.095       Galaxy           Galaxy
SXDF-NB1006-2             z = 7.215        13.07        Galaxy           Galaxy
GN-108036                 z = 7.213        13.07        Galaxy           Galaxy
BDF-3299                  z = 7.109        13.05        Galaxy            
ULAS J1120+0641           z = 7.085        13.05        Quasar            
A1703 zD6                 z = 7.045        13.04        Galaxy            
BDF-521                   z = 7.008        13.04        Galaxy            
G2-1408                   z = 6.972        13.03        Galaxy            
IOK-1                     z = 6.964        13.03        Galaxy           Lyman-alpha emitter
LAE J095950.99+021219.1   z = 6.944        13.03        Galaxy           Lyman-alpha emitter — Faint galaxy

Starting at the bottom, a z of around 7 is interpreted as light from 13 billion years ago.
Moving to the top, it is quite non-linear, and we're up to the latest discoveries with z=11 and z=13 that are interpreted as light traveling just a "little" longer, 13.2 and 13.5 billion years.

~~~~~~~~~~
...

The question I would like to ask, is:

Okay, we just saw a photon from a galaxy that astronomers say travelled for 13 billion years and landed in our telescope. A little ways away from it, another photon from the same source happened to miss our telescope. Let that photon continue to fly through space for another 7 billion years. What will be its redshift then? Can anyone answer this?

Yes, I can answer that. You've picked an interesting redshift example. Viewing z = 7 (now) won't change much over 7 Gyr. The cosmic acceleration causes redshift minima to occur at different times, i.e. minima times are dependent on z. Several years ago, I had a sudden interest in redshift evolution over time for a fixed observer (e.g. the milky way). It culminated in the plot below. I find it very interesting as it reveals characteristics I wasn't expecting. I won't elaborate any further now, and although I can calculate your specific example, I interpolated between the closest two curves for this post.

Assuming you're adding 7 Gyr to present time, the answer is still close to 7. Note, dz/dt depends on when the first observation is made.
The two vertical lines mark now (13.72 Gyr) and +7Gyr (20.72 Gyr). The big arrows point to the two redshifts.
These calculations assume a flat spacetime. Also note, the evolution of the CMB redshift is plotted. It is the first electromagnetic radiation to fill the Universe. It was emitted roughly 380,000 years after the Big Bang, and therefore it's redshift (z ≈1100) cannot be exceeded by any visible object.
I'm expecting Webb to make discoveries pertinent to the onset and duration of the Dark Ages. I think the current thought is the first stars formed around 100 Myr after the BB (z~30)
 

[Ann]

How could they have been in flight for greater than the age of the Universe? If the redshift led to that conclusion, we'd revise the age of the Universe.

I think I'm questioning how rational the current interpretation of high-redshift data is.
If I look at this article: https://en.wikipedia.org/wiki/List_of_t ... al_objects
I see the following table:

Code: Select all

                                        Light travel 
       Name              Redshift (z)  distance (Gly)        Type                   Notes
----------------------  -------------  --------------   ---------------  --------------------------------
HD1                       z = 13.27        13.5         Galaxy           Formulative understanding
GN-z11                    z = 11.09        13.39        Galaxy           Confirmed galaxy
MACS1149-JD1              z = 9.11         13.26        Galaxy           Confirmed galaxy
EGSY8p7                   z = 8.68         13.23        Galaxy           Confirmed galaxy
A2744 YD4                 z = 8.38         13.20        Galaxy           Confirmed galaxy
MACS0416 Y1               z = 8.31         13.20        Galaxy           Confirmed galaxy
GRB 090423                z = 8.2          13.18        Gamma-ray burst
EGS-zs8-1                 z = 7.73         13.13        Galaxy           Confirmed galaxy
z7 GSD 3811               z = 7.66         13.11        Galaxy           Galaxy
J0313-1806                z = 7.64                      Quasar            
z8 GND 5296               z = 7.51         13.10        Galaxy           Confirmed galaxy
A1689-zD1                 z = 7.5          13.10        Galaxy           Galaxy
GS2_1406                  z = 7.452        13.095       Galaxy           Galaxy
SXDF-NB1006-2             z = 7.215        13.07        Galaxy           Galaxy
GN-108036                 z = 7.213        13.07        Galaxy           Galaxy
BDF-3299                  z = 7.109        13.05        Galaxy            
ULAS J1120+0641           z = 7.085        13.05        Quasar            
A1703 zD6                 z = 7.045        13.04        Galaxy            
BDF-521                   z = 7.008        13.04        Galaxy            
G2-1408                   z = 6.972        13.03        Galaxy            
IOK-1                     z = 6.964        13.03        Galaxy           Lyman-alpha emitter
LAE J095950.99+021219.1   z = 6.944        13.03        Galaxy           Lyman-alpha emitter — Faint galaxy

Starting at the bottom, a z of around 7 is interpreted as light from 13 billion years ago.
Moving to the top, it is quite non-linear, and we're up to the latest discoveries with z=11 and z=13 that are interpreted as light traveling just a "little" longer, 13.2 and 13.5 billion years.

~~~~~~~~~~
...

The question I would like to ask, is:

Okay, we just saw a photon from a galaxy that astronomers say travelled for 13 billion years and landed in our telescope. A little ways away from it, another photon from the same source happened to miss our telescope. Let that photon continue to fly through space for another 7 billion years. What will be its redshift then? Can anyone answer this?

Yes, I can answer that. You've picked an interesting redshift example. Viewing z = 7 (now) won't change much over 7 Gyr. The cosmic acceleration causes redshift minima to occur at different times, i.e. minima times are dependent on z. Several years ago, I had a sudden interest in redshift evolution over time for a fixed observer (e.g. the milky way). It culminated in the plot below. I find it very interesting as it reveals characteristics I wasn't expecting. I won't elaborate any further now, and although I can calculate your specific example, I interpolated between the closest two curves for this post.

Assuming you're adding 7 Gyr to present time, the answer is still close to 7. Note, dz/dt depends on when the first observation is made.
The two vertical lines mark now (13.72 Gyr) and +7Gyr (20.72 Gyr). The big arrows point to the two redshifts.
These calculations assume a flat spacetime. Also note, the evolution of the CMB redshift is plotted. It is the first electromagnetic radiation to fill the Universe. It was emitted roughly 380,000 years after the Big Bang, and therefore it's redshift (z ≈1100) cannot be exceeded by any visible object.
I'm expecting Webb to make discoveries pertinent to the onset and duration of the Dark Ages. I think the current thought is the first stars formed around 100 Myr after the BB (z~30)
 

Click to view full size image

I'm so impressed!

Alter-ego, I wish you could sit down with me and give me the sort of information about your chart that a math idiot's brain can process.

For example, what does the light green curved line mean, the one that starts at redshift 1, curves gently downward, and starts rising slowly at the present time, at z= 0.5 when the age of the Universe is 13.72 Gyr?

There is another line above the light green one, an aqua-colored line, which intercepts the present time at z= 1.75 (if I'm reading it correctly). And there is a dark green curved line above the aqua-colored one. And then there are many other curved lines that start to the right of the light green, the aqua-colored and the dark green lines.

Is the curved red line extra important? What does it mean?

I don't understand it at all, but I am, indeed, so impressed.

I think I "understand" - well, "expect" is a better word - the fact that the redshift will be much the same 7 billion years from now as it is today, because the redshift really increases "almost exponentially" the closer we get to the Big Bang, but from then it just keeps slowing down.

Anyway. Fantastic.

Ann

[johnnydeep]

Ann wrote above:

I think I "understand" - well, "expect" is a better word - the fact that the redshift will be much the same 7 billion years from now as it is today, because the redshift really increases "almost exponentially" the closer we get to the Big Bang, but from then it just keeps slowing down.

Isn't it the exact opposite? That is, since the expansion rate of the universe is increasing everywhere, so will the values of redshift over time. Or, stated another way: we have to wait less now for the wavelength of traveling light to increase by the same factor it did before.

But maybe I'm totally missing what the redshift value is measuring (despite reading and not entirely understanding) alter-ego's post above).

[alter-ego]

Ann wrote above:

I think I "understand" - well, "expect" is a better word - the fact that the redshift will be much the same 7 billion years from now as it is today, because the redshift really increases "almost exponentially" the closer we get to the Big Bang, but from then it just keeps slowing down.

Isn't it the exact opposite? That is, since the expansion rate of the universe is increasing everywhere, so will the values of redshift over time. Or, stated another way: we have to wait less now for the wavelength of traveling light to increase by the same factor it did before.

But maybe I'm totally missing what the redshift value is measuring (despite reading and not entirely understanding) alter-ego's post above).

First, to clarify, you should consider the plotted redshifts are for single observer at different observation times. Anywhere in our galaxy can be a fine observation location. The horizontal axis is the observation time as measured from the BB. The first observation is now, and the second observation is 7 Gyr later. As you know, current ΛCDM model uses lambda (Λ) Einstein's cosmological constant as the parameter to model cosmic expansion.

• Looking at any redshift curve, when sloping downward to the right (negative slope for increasing observation time), cosmic expansion is slowing down. The higher the redshift, the faster the deceleration, the faster wavelength shortens.
• At the valley of these curves, redshift changes very slowly with time. I.e. there is very little cosmic expansion between two observations made within this valley. Consequently the wavelength changes very slowly.
→ Right now, a star with z = 2.26 has no significant wavelength change over a 5 Gyr range (11.4 Gyr to 16.4 Gyr)
• Moving forward in time through this minimum redshift period, cosmic expansion turns around and begins to increase. The redshift curves slope upward to the right (positive slope). The higher-z redshifts change faster with time, and the wavelength reddens between observations.

Ann - If I interpret your comment right, the answer to the slowing down is not due to a steady exponential slow-down. It's due ti cosmic acceleration driving the transition from slowing down to speeding up. The z = 7 case happens to fall very close to, if not within, a redshift valley.

Johny - Yes, the universe is, and always has been, expanding, but the transition from slowing down to speeding up (the redshift valley) is what creates the "zero" redshift / wavelength change for long times. For z ≤ 2.26, space expansion is past the valleys and is speeding up. For z > 2,26, space expansion is still slowing down. Pick a high-z example: Today, a measured z = 25 drops to z = 16 in another 10 Gyr. The wavelength negligibly changes over 5 Gyr range.

[Ann]

Maybe I get a little bit of what you are saying, alter-ego.

If I get you correctly, the reason why the redshift increases so enormously the closer we get to the Big Bang is that the Universe underwent this incredible exponential growth spurt called inflation, and when that epoch was over the Universe was "braking hard". Instead of increasing exponentially in size "indefinitely" (wonder what that would have led to?) the Universe settled down, expanding very moderately.

During the epoch of inflation (and, in fact, during the epoch of "braking hard" after the inflation), the redshift changed dramatically, simply because of how much the actual growth of the Universe changed in a short time.

We are now in an epoch of acceleration. However, the acceleration has not "picked up speed" yet, and it has not yet affected the size of the Universe very much. Therefore, we are located in a "redshift valley", where the redshift stays relatively constant over billions of light-years.

Is that correct?

Ann

[johnnydeep]

Maybe I get a little bit of what you are saying, alter-ego.


Evolution of the Universe Shutterstock.png

If I get you correctly, the reason why the redshift increases so enormously the closer we get to the Big Bang is that the Universe underwent this incredible exponential growth spurt called inflation, and when that epoch was over the Universe was "braking hard". Instead of increasing exponentially in size "indefinitely" (wonder what that would have led to?) the Universe settled down, expanding very moderately.

During the epoch of inflation (and, in fact, during the epoch of "braking hard" after the inflation), the redshift changed dramatically, simply because of how much the actual growth of the Universe changed in a short time.

We are now in an epoch of acceleration. However, the acceleration has not "picked up speed" yet, and it has not yet affected the size of the Universe very much. Therefore, we are located in a "redshift valley", where the redshift stays relatively constant over billions of light-years.

Is that correct?

Ann

I'm still trying to digest alter-ego's posts - which are giving me heartburn - but inflation happened long before the universe became transparent to radiation (at about 380000 years). From Wikipedia:

Inflation stopped at around the 10⁻³³ to 10⁻³² seconds mark, with the universe's volume having increased by a factor of at least 10⁷⁸.

And a little later:

As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.[36]

I would think that T=0 for all redshift calculations starts at the "recombination" mark at 379000 years. Or I supposed it would actually be at the "decoupling" mark "shortly afterward". Again from Wikipedia:

Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since, though growing less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation). This is the source of the alternative term relic radiation. The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling.

[MarkBour]

First, let me say that I appreciate these answers, from Chris and alter-ego. Also, the further comments that Ann and johnnydeep have added.

An easy response to my post would have been something like: "No, you just need to take one or more courses that cover this, then you'd get it. Until then, just keep your uneducated views to yourself." I'm grateful that this is not what I got, but that you took the time to read it, consider, and respond.

I think I'm questioning how rational the current interpretation of high-redshift data is.

Very rational. It hangs on all of the underlying theory of the lambda-CDM model, which is itself very well supported by multiple independent lines of evidence.
. . .

Great. I'd love nothing better than to learn more about the ΛCDM model and how, from the sequence of observations and thought, this model came to be the dominant accepted theory. Watching YouTube videos by physics popularizers hasn't gotten me anywhere in this regard. I've also read a few books, aimed at the layman, that got into these topics, and have just found myself repeating over and over again: "Why do you think that?"

More helpfully, I just read the Wikipedia article on ΛCDM. A lot of it is very concise and large chunks of it are just beyond my ability to understand without further study. It does give an opening summary of the main points as to why the ΛCDM is the currently most-accepted model, so those are topics I probably should try to follow further. The article does seem to be very balanced, though. There are quite a number of objections and difficulties mentioned throughout. And, as if to support my skepticism, there is this one small section nearly at the end, that summarizes my gut-level concern about it:

Excerpt from: https://en.wikipedia.org/wiki/Lambda-CDM_model
Unfalsifiability
It has been argued that the ΛCDM model is built upon a foundation of conventionalist stratagems, rendering it unfalsifiable in the sense defined by Karl Popper.[77]

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The question I would like to ask, is:

Okay, we just saw a photon from a galaxy that astronomers say travelled for 13 billion years and landed in our telescope. A little ways away from it, another photon from the same source happened to miss our telescope. Let that photon continue to fly through space for another 7 billion years. What will be its redshift then? Can anyone answer this?

Yes, I can answer that. You've picked an interesting redshift example. Viewing z = 7 (now) won't change much over 7 Gyr. The cosmic acceleration causes redshift minima to occur at different times, i.e. minima times are dependent on z. Several years ago, I had a sudden interest in redshift evolution over time for a fixed observer (e.g. the milky way). It culminated in the plot below. I find it very interesting as it reveals characteristics I wasn't expecting. I won't elaborate any further now, and although I can calculate your specific example, I interpolated between the closest two curves for this post.

Assuming you're adding 7 Gyr to present time, the answer is still close to 7. Note, dz/dt depends on when the first observation is made.
The two vertical lines mark now (13.72 Gyr) and +7Gyr (20.72 Gyr). The big arrows point to the two redshifts.
These calculations assume a flat spacetime. Also note, the evolution of the CMB redshift is plotted. It is the first electromagnetic radiation to fill the Universe. It was emitted roughly 380,000 years after the Big Bang, and therefore it's redshift (z ≈1100) cannot be exceeded by any visible object.
I'm expecting Webb to make discoveries pertinent to the onset and duration of the Dark Ages. I think the current thought is the first stars formed around 100 Myr after the BB (z~30)
 
(image not replotted)

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Alter-ego, I wish you could sit down with me and give me the sort of information about your chart that a math idiot's brain can process.
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Ann

Right on, Ann! Please let me attend that session, too ...

As I look at that diagram, if I understand it at all, I find two things that are amazing.

1. One is your statement, alter-ego, indicated by the arrows:
   An object we can observe today, with z=7, we would still be able to see 7 billion years from now, if it didn't stop emitting, and it would still have z=7. (!)
2. If we found an object right now showing at z=600, that object's z-shift is predicted to be diminishing rapidly at this time, and will bottom out way, way down at about z=8, before rising again. (!)