What about time in Hubble’s analysis of redshifts?
In 1929 Edwin Hubble observed galaxies beyond the Milky Way. He proposed a correlation between the redshift and the distance of these galaxies. The following picture is taken right out of the scientific paper in which Hubble presented his results.
A note on the headline “extra galactic nebulae”: back then galaxies were called nebulae because observations in high resolution were not yet possible and from what was available they appeared to be of a foggy structure. And “extra-galactic” just meaning “outside our own galaxy”, the Milky Way. Actually, this was one of Hubble’s major accomplishments. With his observations and measurements, he discovered the existence of other galaxies and that the universe goes on way beyond the Milky Way.
In this graph you can see the distance of the galaxies to earth on the horizontal axis. On the vertical axis the speed is given at which these galaxies are assumed to move away from us. Here it is crucial to notice that this already includes an interpretation of data! The actual underlying data points are the redshifts of the light of those galaxies.
“Redshift” means the light or more precise the electromagnetic energy we receive from there is shifted to the red end of the spectrum. From that velocities can be calculated assuming that this redshift is mostly caused by the so called Doppler-effect. This effect occurs then an object is moving away from an observer and because of this motion the wavelength of emitted radiation appears to be longer for the observer. But to this date nobody really knows if it is true or not that the Doppler-effect is the one and only cause for the observed redshift.
The final statement of this diagram and the whole paper: there is a linear relationship between distance and velocity. That means the further a galaxy is away the greater its redshift, as indicated by the lines in the graph.
This was the foundation for many theories that followed. For example: Because the velocities are increasing over distance an expansion of the whole universe must be the cause for these redshifts. And following that reasoning further one arrives at the big bang theory.
But as I read the article and looked at the diagram a question came to my mind. The speed of light is limited and the great distances of many million light years to other galaxies means that the information, the light, from these galaxies is very old. To make this clear I recalculated what the distance means for the age of the information reaching us – it’s really straight forward!
I added the information to the diagram:
The information reaching us from the galaxies in the left circle is about 0.9 million years old. The information from the galaxies in the second circle originated about 2.9 million years ago and from the galaxies in the last circle we know how they appeared 6.5 million ago.
So we know that some galaxies had a small redshift 0.9 million years ago and some other galaxies showed a much larger redshift 6.5 million years ago. We do not know how big the redshift of the galaxies to the right was at the time we got information about the galaxies to the left. And of course we do not know anything about the current state of these galaxies.
Drawing this information in one diagram suggests that this is the state of the universe as is it today and that the data can be compared without further thought. By doing so implicitly an assumption is made: in millions of years nothing changed about these galaxies, at least then it comes to redshift. I don’t know why this huge assumption is not mentioned in the article…
My point here is not to evaluate if this assumption is plausible or not. But such a huge guess needs to be stated clearly when a scientific result is presented. So what do you think about my observation in this article? Can the data be compared as shown in the original diagram or was it a rushed conclusion by Hubble?
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