# I Any direct measure of Universe expansion via galaxy subtended angles?

#### hkyriazi

Summary
Is there anyone examining a large sample of galaxies' subtended angles over time, as an independent test of the universe's supposed expansion?
As indicated in the title and summary, I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time. The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.

#### phinds

Gold Member
... the universe's supposed expansion?
Are you seriously questioning the expansion of the universe?

#### hkyriazi

Are you seriously questioning the expansion of the universe?
I suppose I am. A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible. I'd be happy to see any direct confirmation of the expansion, not just one based on individual galaxy subtended angle measurements over time.

#### phinds

Gold Member
I suppose I am.
Amazing. You are seriously suggesting that all of the thousands of scientists who have confirmed it in various way have no idea what they are doing but YOU know the truth. You REALLY might want to rethink that.

Staff Emeritus
Are you seriously questioning the expansion of the universe?
That's the thing you are questioning? My question for for @hkyriazi would be "Are you seriously asking if we can look at the sky today and look at it again tomorrow and tell that the galaxies are slightly smaller?"

#### phinds

Gold Member
That's the thing you are questioning? My question for for @hkyriazi would be "Are you seriously asking if we can look at the sky today and look at it again tomorrow and tell that the galaxies are slightly smaller?"
Well he did say up to 20 years

#### PeterDonis

Mentor
I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time.
Of course there is. What you describe is called "angular size distance" or "angular diameter distance" in cosmology and is one of the primary observational parameters used to constrain possible models of the universe.

#### PeterDonis

Mentor
The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.
Actually, such time periods are far too short compared to the age of the universe to show measurable change in angular sizes of individual galaxies. But we can simply look at different galaxies with widely varying redshifts and widely varying distances from us, and therefore look back through a time span of billions of years.

#### PeterDonis

Mentor
A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible.
Others are logically possible, but they have been considered over decades and ruled out based on observational evidence.

#### Bandersnatch

The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.
IIRC, expansion dominates observed motion of galaxies at scales >500 MPc. Let's say we want to use one such galaxy, picking one of the closest viable ones (let's say it's exactly at 500 MPc distance). The recessional velocity predicted by Hubble law should be ~35 000 km/s. Over 20-year time span, that's total receded distance of approx. $22*10^{12} km$, which is a grand total of 2 light years and change. This in turn means 1/750 000 000 change in distance and a corresponding change in the angular diameter of a diffuse object whose size is measured in arcseconds or fractions thereof.
This is not the kind of precision that's currently achievable.

I'd be happy to see any direct confirmation of the expansion

#### Buzz Bloom

Gold Member
Summary: Is there anyone examining a large sample of galaxies' subtended angles over time, as an independent test of the universe's supposed expansion?
As indicated in the title and summary, I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time.
Hi hkyriazi:

I am very surprised that no one has brought up the point I make below.

Galaxies do not expand in the manner that astronmers know that the universe expands. That is, stars in a galaxy to not move apart (due according to the phenomenon of the Hubble constant) with a speed proportional to distance. Objects in the universe that are gravitationally bound to each other do not speed away from each other proportionately to distance.

Regards,
Buzz

#### PeterDonis

Mentor
Galaxies do not expand in the manner that astronmers know that the universe expands.
That's right, and that's why the angular size of galaxies is a useful datum--because their actual physical size can be expected to be stable.

#### hkyriazi

IIRC, expansion dominates observed motion of galaxies at scales >500 MPc. Let's say we want to use one such galaxy, picking one of the closest viable ones (let's say it's exactly at 500 MPc distance). The recessional velocity predicted by Hubble law should be ~35 000 km/s. Over 20-year time span, that's total receded distance of approx. $22*10^{12} km$, which is a grand total of 2 light years and change. This in turn means 1/750 000 000 change in distance and a corresponding change in the angular diameter of a diffuse object whose size is measured in arcseconds or fractions thereof.
This is not the kind of precision that's currently achievable.

View attachment 246661
Thanks, Bandersnatch. Refreshing to get a calm and to-the-point response. Do you think current technology could make accurate enough measurements, and look at enough galaxies (there are, after all, quite a few of them) over that time period, to detect such tiny changes?

#### hkyriazi

Hi hkyriazi:

I am very surprised that no one has brought up the point I make below.

Galaxies do not expand in the manner that astronmers know that the universe expands. That is, stars in a galaxy to not move apart (due according to the phenomenon of the Hubble constant) with a speed proportional to distance. Objects in the universe that are gravitationally bound to each other do not speed away from each other proportionately to distance.

Regards,
Buzz
I'd assumed that galaxies are fairly stable in size, otherwise it'd be a confounding variable. The size change I was asking about is that due to them (galaxies not in our local cluster) being continually further away from us with time.

#### Bandersnatch

Do you think current technology could make accurate enough measurements, and look at enough galaxies (there are, after all, quite a few of them) over that time period, to detect such tiny changes?
I don't think so. If I'm juggling the numbers right, it's like trying to discern which object is larger - a 100 000 light-year diameter galaxy, or a 100 000.0001 light-year one, all from a distance of 150 000 000 light-years. That's a $10^{-9}$ of an arc second difference (or thereabouts). The diffraction limit on resolution of Earth-spanning radio interferometry is on the order of $10^{-6}$ arcsec.
How many galaxies you look at shouldn't matter.

Staff Emeritus
it's like trying to discern which object is larger - a 100 000 light-year diameter galaxy, or a 100 000.0001 light-year one
More to the point, galaxies don't have a boundary that sharp - a part per billion. Heck, the sun doesn't have a boundary that sharp (around a meter).

Staff Emeritus
Arguably, even the earth doesn't have a boundary that sharp! (A quarter-inch)

#### Bandersnatch

More to the point, galaxies don't have a boundary that sharp - a part per billion. Heck, the sun doesn't have a boundary that sharp (around a meter).
I was thinking, if you had that kind of resolving power, you could pick some arbitrary features in the galaxy to serve as the boundary. Individual stars even.

#### metastable

I suppose I am. A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible.
Others are logically possible, but they have been considered over decades and ruled out based on observational evidence.
By "others" are we referring to gravitational redshift? @PeterDonis are you saying all other logically possible interpretations have been ruled out?

@PeterDonis ^I will refer to the above thread in which you offered the following observation:

PeterDonis said:
If the separation distance is not so small, then the ships will cover a large enough region of spacetime during the flight time of any particular photon that they cannot be treated as all being at rest in a single inertial frame during that flight time. In that case, gravitational redshift will be observable.

Staff Emeritus
you could pick some arbitrary features in the galaxy to serve as the boundary. Individual stars even.
I think stars are moving 1-2 orders of magnitude too fast for that.

#### hkyriazi

Amazing. You are seriously suggesting that all of the thousands of scientists who have confirmed it in various way have no idea what they are doing but YOU know the truth. You REALLY might want to rethink that.
If I'm wrong, please do correct me, but from my study of the question, all the other "confirmations" are based on hypotheses that are simply *consistent* with it--for example, the baryon acoustic oscillations hypothesis to explain the CMB heterogeneity, and hot nucleosynthesis. I'm looking for direct, rather than circumstantial (interpretation-dependent), evidence of a universal expansion.

#### hkyriazi

How many galaxies you look at shouldn't matter.
How so? If one does a plot of, say, 1,000 galaxy subtended angles measured over time, one might expect to see a trend (significant correlation) even if individual measurements are too imprecise to say.

#### hkyriazi

By "others" are we referring to gravitational redshift? @PeterDonis are you saying all other logically possible interpretations have been ruled out?
FWIW, I assumed he meant Zwicky's "tired light" hypothesis, from 1929 IIRC.

#### George Jones

Staff Emeritus
Gold Member
FWIW, I assumed he meant Zwicky's "tired light" hypothesis, from 1929 IIRC.
And "tired light" theories are disfavoured by observations. Gravitational time dilation in expanding universe models predicts that the rate at which photons leave a distant source is larger than the rate at which photons are received here. This leads to observable effects that are not predicted by tired light theories.

Steven Weinberg, in his 2008 grad-level book "Cosmology", writes
"For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor (1+z)−1, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of (1+z)−1 in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of 1+z in the denominator.

Lubin and Sandage have used the Hubble Space Telescope to compare the surface brightness of galaxies in three distant clusters ... quite inconsistent with the behavior ... expected in a universe with 'tired light'. ...

In the standard big bang cosmology all rates observed from a distant source are slowed by a factor (1+z)−1, not just the rate at which photons are emitted.This slowing has been confirmed for the rate of decline of light from some of the Type Ia supernovae used by the Supernova Cosmology Project ... "

Staff Emeritus
How so? If one does a plot of, say, 1,000 galaxy subtended angles measured over time, one might expect to see a trend (significant correlation) even if individual measurements are too imprecise to say.
Is that like "we lose a little on every sale but we make it up on volume?"

You need to work out the numbers rather than just spitballing it and making us refute these claims. Using Bandersnatch's numbers, which I believe are too optimistic by a factor of at least 10 and more likely 100, you need ten million galaxies to do even a 30% measurement. And this ignores the problem that galaxies don't have the sharp edge you need.

The HUDF has 0.1% of that and took 4 months to image. So you need 4000 months - about 3 centuries - to collect enough data. Then you need to do it again.

"Any direct measure of Universe expansion via galaxy subtended angles?"

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