What is the observational evidence for the Hubble relationship?

DeiRenDopa

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"How do you know that the redshift is correctly measuring time and distance?"

That is a quote from a recent post by JEROME DA GNOME. In a subsequent post, he clarified his question by making an explicit reference to "Hubble's law", and I offered to start a new thread on the topic, introducing the physics and astronomy relevant to it, and the observational evidence for it.

Although the title of the thread says "observational evidence", it's appropriate to start with the theory of General Relativity (GR), because the astronomical observations are consistent with a universe describable using GR.

So, as background JEROME, here is the latest version of Clifford Will's The Confrontation between General Relativity and Experiment.

Next, the historical context - Hubble's original paper, and the observations he made that lead him to write it.
 
I think they key is understanding the role of Cepheid variables in pre-Hubble/expansion distance calculations. We understand how far away Cepheids are, and we see that their red shifting and the red shifting of their nebulae (galaxies) is proportional to their distance, nicely plotted.

Like everything else in science, this doesn't prove expansion, but the explanation that velocity away from us is proportional to distance is so good that any other model has a lot of work to do.

Simon Singh's book "Big Bang" is thick, but it's a good read for a layperson who wants to know about the independent lines of reasoning and evidence that converged on the current hyperinflation model.
 
Hubble, Edwin, "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae" (1929) Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168–173 (link to PDF version) is the paper that started it all.

He takes the observed redshift of some ~20 galaxies which have what he considers to be reliable estimates of distance, and plots them (redshift on the vertical axis, distance on the horizontal one). Note that the terminology may seem a little strange to us today - "radial velocity" vs "redshift", "extra-galactic nebula" vs "galaxy".

From the plot (data actually), he derives what we today call H0, the Hubble constant - the slope of the line of best fit. His derived value (500 or 530 km/s/Mpc) is far too high, and much of the next several decades of astronomy is concerned with getting a better estimate of this.

The main 'errors' in Hubble's original paper are in the estimates of distance to the ~20 galaxies; he was working at the end of the astronomical distance ladder, and it turns out several lower rungs were wrong.

Today we can get estimates of the distance to some galaxies (and clusters) largely independently of the (historical) distance ladder, via a range of ingenious applications of textbook physics - lensed quasars, the Sunyaev-Zel'dovich effect, a quasi-parallax method or two, and the CMB, to give just shorthands. While the uncertainties on each of these direct methods are larger than those on reported in the final Hubble Key Project paper (and at least one contains a degeneracy that must be broken though other observations), they all give consistent estimates of H0.

Note that Hubble was well aware of the work done just a few years earlier on application of GR to cosmology; here is how the final paragraph begins:
The outstanding feature, however, is the possibility that the velocity-distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space.
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Next, a look at that heavily cited final HKP paper ("Freedman et al. (2001)").
 
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It should also be noted that we now have much evidence corraborating distances as measured by Hubble's law. There are Cepheid variables as blutoski mentioned, but there are also the light curves from supernovae as well that match up quite nicely.

Overall, there are a variety of techniques astronomers use to determine distances. Some are more useful in certain situations than others, but they do seem to match up with predictions made by Hubble's Law.

So when many different methods converge and provide relatively consistent results, it's generally a good sign that you're on the right track.
 
Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant, by Freedman et al. (2001) (link to ADS page on it; click on "Full Refereed Journal Article (PDF/Postscript)", on the left near the top, to get the full paper).

I can thoroughly recommend this paper.

Not only is it a relatively easy read, not only has it gathered over 1200 citations (and counting), but it also gives a nice summary of the various methods used to estimate distances to extragalactic objects, and the systematic and random uncertainties associated with each.

Here's the abstract:
We present here the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant. We summarize our method, the results, and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. Our results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60-400 Mpc. The analysis presented here benefits from a number of recent improvements and refinements, including (1) a larger LMC Cepheid sample to define the fiducial period-luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. We adopt a distance modulus to the LMC (relative to which the more distant galaxies are measured) of μ0(LMC)=18.50+/-0.10 mag, or 50 kpc. New, revised distances are given for the 18 spiral galaxies for which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we find values (in km s-1 Mpc-1) of H0=71+/-2 (random)+/-6 (systematic) (Type Ia supernovae), H0=71+/-3+/-7 (Tully-Fisher relation), H0=70+/-5+/-6 (surface brightness fluctuations), H0=72+/-9+/-7 (Type II supernovae), and H0=82+/-6+/-9 (fundamental plane). We combine these results for the different methods with three different weighting schemes, and find good agreement and consistency with H0=72+/-8 km s-1 Mpc-1. Finally, we compare these results with other, global methods for measuring H0. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.
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As both blutoski and MattusMaximus have already noted, and as you'd have discovered if you'd read the classic Hubble paper, Cepheids play a leading role in estimating distances of extra-galactic objects.

The HKP (Freedman et al. (2001)) paper reports the results of a lot of time on the Hubble Space Telescope observing Cepheids in a number of relatively nearby galaxies (but beyond the Local Group).

Other methods discussed, albeit only briefly, in Freedman et al. (2001) are:

* Type Ia Supernovae
* Tully-Fisher relation
* Fundamental Plane for Elliptical Galaxies
* Surface Brightness Fluctuations
* Type II Supernovae
* Sunyaev-Zel'dovich effect
* Time Delays for Gravitational Lenses.

There's an intriguing section (8.1.1) entitled "NGC 4258: Comparison of a Maser and Cepheid Distance". The section begins as follows:
Given the current uncertainties and systematics affecting the local distance scale, it would be highly desirable to have geometric methods for measuring distances, independent of the classical distance indicators. A very promising new geometric technique has recently been developed and applied to the galaxy NGC 4258, a galaxy with an inner disk containing H2O masers (Herrnstein et al. 1999).
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This 'geometric (maser) technique' is very interesting, and I'll discuss it in a later post; I'll also cover the Sunyaev-Zel'dovich effect and gravitational lenses, as these are largely independent methods of estimating distance.

This webpage, by John Huchra (one of the "et al." of the HKP paper above), gives a nice, concise summary of what H0 is, some early history, and how the estimates of its value have changed over time.
 
But you can't prove that pixies aren't making the redshift. The expansion of the universe in un-needed if we assume pixies are doing it. Thereby from Occam's razor the universe is not expanding.

;)

The proof of alternate redshifts is that there are associations (optical not statistical) between different redshift objects. The proof of alternate theories of intrinsic redshift is that these associations exist. There is no evidence to support either, other than the optical alignment.
 
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Dancing David said:
But you can't prove that pixies aren't making the redshift. The expansion of the universe in un-needed if we assume pixies are doing it. Thereby from Occam's razor the universe is not expanding.

;)

The proof of alternate redshifts is that there are associations (optical not statistical) between different redshift objects. The proof of alternate theories of intrinsic redshift is that these associations exist. There is no evidence to support either, other than the optical alignment.
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Well, what can I say?

I intended this thread to be about the observational evidence for the Hubble relationship; if anyone else wants to start a thread on the observational basis for alternatives, go for it!

Note that explaining the Hubble relationship - by applying GR to the universe, deriving an expanding solution, etc, for example - is a separate (though closely related) topic.
 
robinson said:
Not true.
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What evidence is there of intrinsic redshifts other than the optical alingment of Arp galaxies and QSOs. If that is wwhat you reffered to.

I am curious if there is any, BAC never had any.

If DRD wants we can take this to another thraed.
 
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DeiRenDopa said:
He takes the observed redshift of some ~20 galaxies which have what he considers to be reliable estimates of distance, and plots them (redshift on the vertical axis, distance on the horizontal one).
Click to expand...

How was the distance determined to be reliable?

What are we using for measurement outside of redshift?
 
Estimating the Hubble constant from gravitational lens time delays.

I had hoped to be able to find a webpage, with visuals, describing the principle in simple terms; so far, I have been unsuccessful. If anyone comes across such a webpage, please post it!

The principle, in simple terms: if a distant object is 'lensed' by a foreground object, to form two or more distinct images when seen by us here on Earth, the light (x-rays, radio, etc) we receive in each lensed image has travelled a slightly different path to get to us. If we know the difference in path lengths, and something about the basic geometry of the lens, we can estimate the distance to the lensed object (not the lens!) ... without the need for any intermediate rung on the astronomical distance ladder.

How to determine the difference in the path lengths? If the source (the distant object) is variable, then the path length difference will show up as time offsets ('time delays') in the lightcurves of each image (a plot of brightness against time): path difference = time delay x speed of light.

In practice it's not that simple, for a variety of theoretical and practical reasons.

This paper by Kochanek and Schechter gives an excellent summary, albeit rather technical; this later paper by Schechter gives more details of the many difficulties with actually using this method, and presents what Schechter considers to be the (then) only determination of H0 from a 'golden' lens (it's consistent with that estimated by Freedman et al. (2001)). Interestingly, one of the major difficulties is that most quasars are insufficiently variable, so most lensed quasars are unsuitable for this approach!

(And yes, I think it's the same Paul L. Schechter that the Schechter function is named after ...)
 
DeiRenDopa said:
Estimating the Hubble constant from gravitational lens time delays.

I had hoped to be able to find a webpage, with visuals, describing the principle in simple terms; so far, I have been unsuccessful. If anyone comes across such a webpage, please post it!

The principle, in simple terms: if a distant object is 'lensed' by a foreground object, to form two or more distinct images when seen by us here on Earth, the light (x-rays, radio, etc) we receive in each lensed image has travelled a slightly different path to get to us. If we know the difference in path lengths, and something about the basic geometry of the lens, we can estimate the distance to the lensed object (not the lens!) ... without the need for any intermediate rung on the astronomical distance ladder.

How to determine the difference in the path lengths? If the source (the distant object) is variable, then the path length difference will show up as time offsets ('time delays') in the lightcurves of each image (a plot of brightness against time): path difference = time delay x speed of light.

In practice it's not that simple, for a variety of theoretical and practical reasons.

This paper by Kochanek and Schechter gives an excellent summary, albeit rather technical; this later paper by Schechter gives more details of the many difficulties with actually using this method, and presents what Schechter considers to be the (then) only determination of H0 from a 'golden' lens (it's consistent with that estimated by Freedman et al. (2001)). Interestingly, one of the major difficulties is that most quasars are insufficiently variable, so most lensed quasars are unsuitable for this approach!

(And yes, I think it's the same Paul L. Schechter that the Schechter function is named after ...)
Click to expand...


The principal is not in question. It's accuracy and usefulness is.


How was the distance determined to be reliable?

What are we using for measurement outside of redshift?
 
JEROME DA GNOME said:
DeiRenDopa said:
He takes the observed redshift of some ~20 galaxies which have what he considers to be reliable estimates of distance, and plots them (redshift on the vertical axis, distance on the horizontal one).
Click to expand...
How was the distance determined to be reliable?

What are we using for measurement outside of redshift?
Click to expand...
That's a great question JdG! :)

It can, of course, only be answered historically, in the sense of 'how did Hubble, at the time, come to conclude that the estimates of the distances to those ~20 galaxies were reliable?'

The answer will take some time to write, and will necessarily involve rather a lot of the basic physics that goes into astronomy, as well as quite a bit of background (to adequately capture the state of astronomy in the 1920s).

I'm happy to spend several hours researching this, and several more to write up a post on it ... but I'd like your feedback first please - is the historical answer what you are looking for? Or is it, perhaps, more a question of how the distance to nearby galaxies (not just the ~20 Hubble used) can be reliably estimated, today?

The answer to the second question overlaps with the first, but differs in many important respects, not least in that there are, today, several independent methods of estimating such distances (making them considerably more robust than Hubble's were, or ever could be).
 
DeiRenDopa said:
I'm happy to spend several hours researching this, and several more to write up a post on it ... but I'd like your feedback first please - is the historical answer what you are looking for? Or is it, perhaps, more a question of how the distance to nearby galaxies (not just the ~20 Hubble used) can be reliably estimated, today?

The answer to the second question overlaps with the first, but differs in many important respects, not least in that there are, today, several independent methods of estimating such distances (making them considerably more robust than Hubble's were, or ever could be).
Click to expand...

The historical answer is most relevant as the principal from which the answer was built defines much of our current understanding of cosmology.
 
JEROME DA GNOME said:
So you do not know either. Belief in the great Book is all that is needed! :rolleyes:
Click to expand...

I do know. Or at least, I have a reasonable idea if your question was essentially asking "How do we measure distances other than using redshift"? I just couldn't be bothered to write a long and detailed explanation for someone who was just gonna tell me "you have failed" like you love to do.
 
JEROME DA GNOME said:
DeiRenDopa said:
Estimating the Hubble constant from gravitational lens time delays.

I had hoped to be able to find a webpage, with visuals, describing the principle in simple terms; so far, I have been unsuccessful. If anyone comes across such a webpage, please post it!

The principle, in simple terms: if a distant object is 'lensed' by a foreground object, to form two or more distinct images when seen by us here on Earth, the light (x-rays, radio, etc) we receive in each lensed image has travelled a slightly different path to get to us. If we know the difference in path lengths, and something about the basic geometry of the lens, we can estimate the distance to the lensed object (not the lens!) ... without the need for any intermediate rung on the astronomical distance ladder.

How to determine the difference in the path lengths? If the source (the distant object) is variable, then the path length difference will show up as time offsets ('time delays') in the lightcurves of each image (a plot of brightness against time): path difference = time delay x speed of light.

In practice it's not that simple, for a variety of theoretical and practical reasons.

This paper by Kochanek and Schechter gives an excellent summary, albeit rather technical; this later paper by Schechter gives more details of the many difficulties with actually using this method, and presents what Schechter considers to be the (then) only determination of H0 from a 'golden' lens (it's consistent with that estimated by Freedman et al. (2001)). Interestingly, one of the major difficulties is that most quasars are insufficiently variable, so most lensed quasars are unsuitable for this approach!

(And yes, I think it's the same Paul L. Schechter that the Schechter function is named after ...)
Click to expand...
The principal is not in question. It's accuracy and usefulness is.


How was the distance determined to be reliable?

What are we using for measurement outside of redshift?
Click to expand...
Huh? :confused:

JEROME DA GNOME, did you actually open and read the two papers I provided links to? If you did, it seems you didn't understand very much of what you read ...

Why? Because large parts of each paper are devoted to addressing, six ways to Sunday, exactly your first question ("How was the distance determined to be reliable?")! :p

And your second question ("What are we using for measurement outside of redshift?") makes no sense at all, in the context of the post of mine you are quoting ... "we" are using time delays in the light curves of separate images of multiply-lensed quasars (plus some other things, which are described in great detail in the two papers) to determine distance ... the redshift of the quasar is irrelevant for determining distance (it's only relevant for estimating H0).

But perhaps you did read the papers, but did not understand them.

If so, would you mind please saying so, and I'll try to walk you through the key parts as best I can. To help me do this, would you mind letting me know what level of math you are comfortable with?
 
JEROME DA GNOME said:
DeiRenDopa said:
I'm happy to spend several hours researching this, and several more to write up a post on it ... but I'd like your feedback first please - is the historical answer what you are looking for? Or is it, perhaps, more a question of how the distance to nearby galaxies (not just the ~20 Hubble used) can be reliably estimated, today?

The answer to the second question overlaps with the first, but differs in many important respects, not least in that there are, today, several independent methods of estimating such distances (making them considerably more robust than Hubble's were, or ever could be).
Click to expand...
The historical answer is most relevant as the principal from which the answer was built defines much of our current understanding of cosmology.
Click to expand...
Thanks for responding so quickly.

Unfortunately, I'm going to have to ask you to explain your answer ... to me it makes little sense ...

First, did you mean "principle" (and not "principal")?

Second, "the [principle] from which the answer was built" only "defines much of our current understanding of cosmology" in a very general way - the methods used to estimate distance, for extra-galactic objects, are independent of the principle involved in coming up with "the Hubble relationship".

Empirically, anyone can plot any two quantities of a set of objects, on an ordinary piece of graph paper (or the modern equivalent, using Microsoft's XL, say) - you could, to take a ridiculous example, plot the number of times the word "London" is used on your favourite TV channel, per hour, against time of day.

Theoretically, the Hubble relationship is of great interest as a possible test of GR ... as I indicated already.
 
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