Red-Shifts as Loss of Energy in Transit
Well, perhaps the nebulae are all receding in this peculiar manner. But the notion is rather startling. The cautious observer naturally examines other possibilities before accepting the proposition even as a working hypothesis. He recalls the alternative formulation of the law of red-shifts - light loses energy 'in proportion to the distance it travels through space. The law, in this form, sounds quite plausible. Internebular space, we believe, cannot. be entirely empty. There must be a gravitational field through which the light-quanta travel for many millions of years before they reach the observer, and there may be some interaction between the quanta and the surrounding medium. The problem invites speculation, and, indeed, has been carefully examined. But no satisfactory, detailed solution has been found. The known reactions have been examined, one after the other - and they have failed to account for the observations. Light may lose energy during its journey through space, but if so, we do not yet know how the loss can be explained.
The observer seems to face a dilemma. The familiar interpretation of red-shifts leads to rather startling conclusions. These conclusions can be avoided by an assumption which sounds plausible but which finds no place in our present body of knowledge. The situation can be described as follows. Red-shifts are produced either in the nebulae, where the light originates, or in the intervening space through which the light travels. If the source is in the nebulae, then red-shifts are probably velocity-shifts and the nebulae are receding. If the source lies in the intervening space, the explanation of red-shifts is unknown but the nebulae are sensibly stationary.
https://ned.ipac.caltech.edu/level5/Sept04/Hubble/Hubble2_5.html
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The Dilemma
Thus the familiar interpretation of red-shifts as velocity-shifts leads to strange and dubious conclusions; while the unknown, alternative interpretation leads to conclusions that seem plausible and even familiar. Theories may be revised, new information may alter the complexion of things, but meanwhile we face a rather serious dilemma. Some there are who stoutly maintain that the earth may well be older than the expansion of the universe. Others suggest that in those crowded, jostling yesterdays, the rhythm of events was faster than the rhythm of the spacious universe today; evolution then proceeded apace, and, into the faint surviving traces, we now misread the evidence of a great antiquity. Our knowledge is too meagre to estimate the value of such speculations, but they sound like special pleading, liked forced solutions of the difficulty. The fundamental question is the interpretation of red-shifts.
https://ned.ipac.caltech.edu/level5/Sept04/Hubble/Hubble2_12.html
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The Alternative Laws of Red-Shifts
(a) Red-Shifts not interpreted as Velocity-Shifts
If red-shifts are not velocity-shifts, light loses energy strictly in proportion to the distance it travels through space. As light streams in from the remote nebulae in all directions, each million years of the light-paths subtracts the same fraction of energy from the quanta. We may not know how the reduction is accomplished, but we do know that the action is everywhere uniform. Within the small uncertainties of the measures the sample of the universe that can be explored with spectrographs is thoroughly homogeneous.
(b) Red-Shifts interpreted as Velocity-Shifts
Moreover, the receding nebulae could be traced backward in time to a very remarkable epoch, about 1,860 million years ago. At that epoch the nebulae would all have been found in our immediate neighbourhood. The present distribution would be reproduced if all the nebulae, being then in a compact cluster, had suddenly started to recede at different velocities. The fastest would now be the most distant, while the slowest would still be in our vicinity. Evidently, the present distances would be proportional to the velocities. The initial instant, the famous t0, is back in time about 1,860 million years. In the favoured, expanding worlds of relativistic cosmology, the period was generally called `the age of the universe', the time-interval since the expansion began.
Efforts were made to crowd a vast multitude of events into the brief span, but serious difficulties were encountered. These difficulties often led to the expectation that, when departures from the approximate linear 'velocity-distance relation' were finally detected, they would prove to be negative. In other words, it was expected, or hoped, that further investigations would show that the expansion began very slowly but continually accelerated until it reached the enormous speed we observe today. Consequently, the true age of the universe would be much greater than that estimated from the provisional linear relation.
Well, we have the first intimation of the nature of the departures - and they are positive. Long ago when the light we now record left the very distant nebulae, the expansion was more rapid than it is today. The time-scale is not lengthened; on the contrary, it is materially shortened.
FROM:
THE OBSERVATIONAL APPROACH TO COSMOLOGY
By Edwin Hubble
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Red-shifts
The entire pattern of absorption lines, all details in a spectrum, appear to have been shifted towards the red
Although the spectra of the sun and of the nebulae exhibit the same pattern of absorption lines, there is one remarkable difference. The lines in the nebular spectra, in general, are not in their normal positions; they are displaced towards the red end of the spectrum to positions representing wave-lengths somewhat longer than normal. The entire pattern of absorption lines, all details in a spectrum, appear to have been shifted towards the red. These displacements are commonly known as red-shifts. They are characteristic features in the spectra of all nebulae except a few that are in the immediate neighbourhood of our own stellar system.
Each line in a given spectrum is shifted by a certain constant fraction of its normal wave-length lambda. The linear shifts vary with, the wave-lengths, but the fractional shift, dlambda / lambda, remains constant. Therefore, the red-shift in a particular nebula is specified by dlambda / lambda, and it is dlambda / lambda which varies from nebula to nebula.
The study of many nebulae has shown that, on the average, the red-shifts increase with the apparent faintness of the nebulae in which they are measured. Therefore, we conclude that, on the average, red-shifts increase with distance. Extensive investigations have demonstrated that the relation is approximately linear,
Equation
This relation is called the law of red-shifts.
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