Detecting the Gravitational Red Shift Dispersion Effect:
A test to distinguish between two red-shift models
Spectral lines from distant, extra-galactic objects are shifted towards the red. While the currently dominating model ascribes these redshifts to expansion of the Universe (expansional redshift), a competing model from the time of discovery of redshifts ascribes them to energy losses in passing through gravitational fields (gravitational redshift). A method is described for distinguishing between these models.
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Improvements in telescope design in the 1920s enabled observations to be made of objects outside our local galaxy.
In 1929, Edwin Hubble established1 that lines in the spectra of extra-galactic objects were shifted towards the red, and that the extent of this shift appeared to increase with increasing distance of the object from Earth.
The Doppler Effect causes a redshift in radiation from an object moving away from the observer, and it was widely assumed at the time that the redshifts were due to the extra-galactic objects moving away from the Earth. If the more distant objects were receding more quickly, this implied that the Universe as a whole was expanding, and this formed the first basis for the Expanding Universe model.
However, even in the 1930s, expansion was not the only explanation put forward to explain the redshifts, and other researchers such as Fritz Zwicky put forward the view2 that the light from distant galaxies had been red-shifted by its movement through the gravitational fields between the source and observer. A study3 by Ten Bruggencate of redshifts in light from two groups of objects at similar intergalactic distances, but chosen so the light from one group had to pass through denser gravitational fields than the other, did support the gravitational red shift model.
The fact that light passing through a gravitational field loses energy is not really in dispute at this time. For example, Hawking describes observations4 from apparatus in Earth satellites which demonstrates this effect.
Nevertheless, since the 1930s the common view that these redshifts are due to expansion has gained the high ground and is very seldom questioned in the scientific community. It is of interest that, in spite of common assertions in popular literature that "Edwin Hubble discovered that the Universe is expanding", Hubble himself never accepted5 the expansion model.
There is a practical test which could distinguish between the expansional and the gravitational redshift models. If the redshifts were really due to recession, then their values would have to be exactly the same across the entire spectrum of the object, because they would reflect a physical movement of the object.
On the other hand, if redshifts were due to the action of intervening gravitational fields, then this action could well vary with wavelength, and the redshifts would have slightly different values at different ends of the same object's spectrum.
Three cases can be distinguished in searching for a 'gravitational redshift dispersion effect'.
1. The effect is real and appreciable. In this case, it should be verifiable from existing spectral records.
2. The effect is real, but small. In this case, special tests might need to be devised. Obviously a small effect might still be found for an available very wide spectrum, perhaps extending from the ultraviolet through to microwave, and for the most distant objects, where more gravitational fields would have to be passed through.
3. The effect is not real. In this case, measurements should at least demonstrate that any effect must be below a certain limit.
Clearly a positive result in searching for gravitational redshift dispersion has major implications for cosmology. Some of these implications are examined elsewhere6.
1. E. Hubble. A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae. Proceedings of The National Academy of Sciences, 1929. p.168. Online at: www.pnas.org/cgi/content/full/101/1/8.
2. F. Zwicky. On the Red Shift of Spectral Lines through Interstellar Space. Proceedings of the National Academy of Sciences of the United States of America, Vol. 15, No. 10 (Oct. 15, 1929), pp. 773-779. Online at: www.pnas.org/cgi/reprint/15/10/773.pdf.
3. P. Ten Bruggencate. The Radial Velocities of Globular Clusters. Proceedings of the National Academy of Sciences of the United States of America, Vol. 16, No. 2 (Feb. 15, 1930), pp. 111-118. Online at http://www.pnas.org/cgi/reprint/16/2/111 as111.pdf.
4. Stephen W Hawking. A Brief History of Time. Bantam Books, London, 1989, pp. 35, 90, 97.
5. Edwin Hubble. http://en.wikipedia.org/wiki/Edwin_Hubble.
6. David Noel. The Placid Universe Model. http://www.aoi.com.au/bcw/Placid.
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