Adam Reiss on the Hubble Tension and Puzzles in Cosmology

Adam Reiss won the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe. Subsequent precision measurements of the universe’s present rate of expansion have revealed a discrepancy, called the “Hubble Tension”, between measurements based upon the cosmic microwave background radiation and from supernovæ in distant galaxies. This discrepancy now stands at greater than five standard deviations (compared to the estimated error bars of the two methods) and indicates either some unexplained systematic error in one or both forms of measurement or the need for revision to models of the evolution of the universe.

In this one hour conversation, Prof. Riess describes ongoing observations with the James Webb Space Telescope which have, so far, only confirmed the magnitude of the discrepancy.


Interesting topic. It is just a pity that Brian Keating’s unfortunate interview style can make it more difficult to get a full picture of the issue. The Hubble Tension is significant, but not large – 73-76 kilometers per second per million parsecs versus 67-69 kilometers per second per million parsecs.

I can’t help noticing cosmology’s heavy dependence on the red shift phenomenon. Clearly, the Doppler effect is the principal contributor – but what if it is not 100% of the explanation? What if there were a tiny “frictional” loss of energy by photons traversing parsecs worth of cosmic distance through the aether-like sea of virtual quantum particles in “empty” space? It would be an effect too small to be measured in any laboratory – but it might change our views on distances in the cosmos, and hence on the rate of acceleration.


This is called the “tired light” hypothesis, which was originally proposed by Fritz Zwicky in 1929 as an alternative explanation for the redshifts of distant galaxies to the expansion of the universe. The concept was invoked in the 1950s by advocates of the steady state cosmological model to explain redshifts.

The tired light model has been considered falsified by a variety of observations. First of all, scattering of photons that reduced their energies should blur the images of sources, increasingly with distance. This is not observed. The surface brightness of objects should not vary with distance due to tired light, but this is observed. Tired light predicts no time dilation of events in distant galaxies, but this is observed. Further, tired light should cause a broadening of spectral lines for distant objects, but none is observed.

Tired light has been considered as completely falsified as an alternative to cosmic expansion since the 1970s. It is always possible to invoke a tired light mechanism much smaller than would be needed to explain redshifts and argue that it is too small to detect with present-day instruments, but it is doubtful an effect so small as to evade detection could cause the discrepancies in the two methods of measuring the Hubble parameter or those which indicate the expansion of the universe is accelerating.


Exactly! “Tired light” in the original sense of a complete alternative explanation for observed red shifts is an obvious non-starter. That is not what is being discussed here.

The separate possibility that there is some “frictional” loss of energy from photons as they speed through nominally-empty space deserves some consideration. Presumably, such minor loss of energy would result in the wavelength of those electromagnetic vibrations becoming slightly longer. The most obvious fact about our understanding of the Universe is that it is overwhelmingly empty space. Distances are incomprehensibly large, such that even a trivially tiny “frictional” energy loss per meter would mount up.

The Hubble Tension is statistically significant, but modest in real terms – a 10% difference in acceleration over a distance of 3,260,000 light years. Unfortunately, moving beyond conceptual speculation to quantitative analysis would require a very deep dive into the mathematics to determine what level of energy loss could be consistent with that data. That is above most of our pay grades. But it might make a good project for one of those “Top STEM Doctoral Students” who prefers a 40-hour work week.


But regardless of the proposed form of “friction”, if the photon propagating over cosmological distance is losing energy (equivalent to its wavelength becoming longer), then unless you’re willing to give up conservation of energy (for which there is no experimental evidence whatsoever, despite exquisitely precise null experiments being used to look for it), then whatever causes the photon to lose energy must involve a scattering event in which the particle with which the photon scattered would take away some of the momentum of the photon and cause it to blur. As it happens, very similar processes are proposed by theories that predict Lorentz invariance violation on the cosmological scale. A variety of tests from the 1960s to the present day have found no evidence whatsoever for any of the phenomena which would indicate such a violation, and would also result from photons losing energy over long distances.

The only known mechanisms which can cause redshift without disturbing the trajectory of the photon are motion or stretching of spacetime due to cosmological expansion.


Emphasis on the word “known”. But that leaves us with the puzzle of the Hubble Tension – which is observed but for which there is currently no known reason. That means something is happening which our current theories have not predicted, which implies our current theories are incomplete. In science, there is always a need for open minds to address new unexpected observations. Otherwise, we would still be talking about phlogiston.

I remembered reading something in the Urantia Book about our erroneous red shift measurements. Searching for the reference to refresh my memory, I found a paper from a UB reader about this very problem: Explaining Large Doppler Redshifts Without a Big Bang. There are a lot of crackpots reading the UB, including myself, but who knows, maybe Phil is on to something?