Gravitational Redshift On a Cosmological Scale Verifies General Theory of Relativity

For the first time, gravitational redshift has been measured outside of the solar system, on a cosmological scale. This effect of gravity on light is predicted by the general theory of relativity, and these measurements from the Dark Cosmology Centre at the Niels Bohr Institute, published in Nature, match the predictions. If you’re familiar with redshift, feel free to skip ahead to the second quote block.

Redshift is the result of the Doppler effect on light. Wikipedia has a pretty solid explanation of the Doppler effect, which is when wave frequencies increase (and thus wavelengths decrease) as the source of the wave moves towards the observer, and vice versa for a source moving away:

When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore each wave takes slightly less time to reach the observer than the previous wave. Therefore the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced; so the waves “bunch together”. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is increased, so the waves “spread out”.

Light can be described as a wave, with different wavelengths for different colours. Blue has the smallest wavelength and highest frequency, red has the largest wavelength and smallest frequency. That means that, as described by the Doppler effect, when a source of light (or more generally, electromagnetic radiation) is moving away from an observer it will shift towards the red end of the spectrum. Since the universe is expanding, light from distant galaxies is redshifted, and the degree of redshift can tell you their distance: this is cosmological redshift, or Hubble’s Law.

Meanwhile there is gravitational redshift: light moving from a place of stronger to weaker gravity will be redshifted. To the best of my understanding, this is because time moves slower near stronger sources of gravity, which intuitively seems to make sense as an explanation for wavelength expanding as it moves away from the gravity source. Light also has to travel further when it goes near a large source of gravity, since gravity curves space as well as time.

Gravitational redshift is what’s explained and predicted by general relativity, and what these researchers observed. From Science Daily:

Radek Wojtak, together with colleagues Steen Hansen and Jens Hjorth, has analysed measurements of light from galaxies in approximately 8,000 galaxy clusters. Galaxy clusters are accumulations of thousands of galaxies, held together by their own gravity. This gravity affects the light being sent out into space from the galaxies.

The researchers have studied the galaxies lying in the middle of the galaxy clusters and those lying on the periphery and measured the wavelengths of the light.

“We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge,” explains Radek Wojtak.

They also calculated the mass of the galaxy cluster and from that its gravitational potential, and armed with this could reliably predict the redshift from different regions of the cluster. Einstein emerges victorious, yet again – so you can see why everyone is very skeptical about the now famous CERN neutrino experiment

In trying to wrap my head around this I discovered this site explaining relativity, time dilation and quantum theory in a pretty digestible form; you should check it out if you have any interest.


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