Opinion: Cosmological experiments carried out with increasing precision are producing funny results that no longer fit the standard model of the Universe
By Dr. Eoin Ó Colgáin, Atlantic Technological University
The bulk of scientific research is "normal", to use a term from philosopher of science Thomas Kuhn. A collection of accepted questions, or paradigm, is in place and scientists make incremental improvements in knowledge to address the questions in a linear or continuous way.
But periods of normal science are bookended by "paradigm shifts", whereby there is a crisis, a resolution and radically new ideas emerge that seed another period of normal science. Of course, confusion is widespread between periods of normal science. The key point is that science progresses not only linearly, but also through dramatic changes in the accepted questions. Paradigm shifts invariably rewrite textbooks.
Right now, cosmology, the study of the origin and evolution of the Universe at large scales, appears to be in crisis and a paradigm shift is a distinct possibility. The crisis arises because experiments with increasing precision are giving funny results that no longer fit the standard model of the Universe.
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From RTÉ News in 2020, the European Space Agency has released the most detailed map yet of the Milky Way, shedding new light on the workings of our galaxy
The most notable unexpected result, or anomaly, is easily grasped. Looking out into deep space, the light from all galaxies is shifted to longer wavelengths, which one interprets as the galaxies moving away from the Milky Way with increasing speed. Since gravity attracts, galaxies moving away is only possible if the Universe is expanding. Once one accepts this idea, the shift to longer (red) wavelengths, called redshift, serves as a proxy for cosmic time; the more light is redshifted, the further one looks back in time.
Measuring this rate of expansion of the Universe has been a century old scientific endeavour in astronomy, but the problem is we now have two conflicting results. One of the results for the rate of expansion, or Hubble constant (denoted H0), is indirect. It makes use of tiny fluctuations in the temperature of light that we observe from the early Universe, the Cosmic Microwave Background. Here, the most constraining results come from the European Space Agency's Planck satellite, but one must assume the standard model is correct.
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From RTÉ News in 2020, marking 30 years of the Hubble telescope
On the other hand, astronomers can build a distance ladder in the local or nearby Universe to determine the same quantity directly without assuming a model. Building on pioneering work by Wendy Freedman and the Hubble Space Telescope, which was commissioned to determine H0 in the early 2000s, Adam Riess (Nobel 2011) has shrunk the errors enough that a 10% odd discrepancy with Planck is evident. This problem is commonly referred to as Hubble tension.
What could be wrong? The simplest explanation is that there is a problem with one of the experiments. Hints of this crisis emerged a decade ago. In the interim, the community has revisited experimental data and devised new experiments, most notably the James Webb Space Telescope, which apparently confirm the problems. Thus, experimental errors, even if at work, are not obvious.
This shifts the problem to potential shortcomings with the standard model. When one builds models of the Universe, mathematical consistency demands that H0 is observationally a constant and it cannot change with time or its proxy redshift.
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From RTÉ Radio 1's The Business, Dr Paddy Kavanagh from the Dublin Institute for Advanced Studies on his role in building the James Webb Space Telescope
An interesting point here is that mathematics and observation need not agree when one has the wrong model. One can in principle fit the standard model to astronomical data at different redshifts, corresponding to different cosmic times, and check if H0 is a constant. As trivial as this sounds, this idea has not been tested by the cosmology community. Nevertheless, Hubble tension points to a change in a quantity that must be constant. This marks a deep inconsistency for the standard model.
Since 2020, my collaborators and I have been testing whether H0, interpreted as a fitting parameter in the standard model, changes with redshift. Preliminary results suggest that H0 changes across several independent observables in a consistent manner. In contrast to the Hubble tension problem, where one is comparing different observables at different epochs, our research studies the same observables at different redshifts, so there is a sharper conflict between each observable and the standard model.
From ATU Podcast, is there a crisis in cosmology and does it really matter. With Dr. Eoin Ó Colgáin
This leaves one with the option of throwing away the data or throwing away the standard model. Obviously, throwing away one data set may be no great loss, but throwing away too much data is a price too high to pay in a data-driven field. This ultimately sinks the model. However, data can be fickle, and it will be important to revisit findings with bigger and better-quality data sets.
The Hubble tension debate ticks all the signatures of paradigm shift. Experiments are giving funny results. There is widespread confusion in the literature. What new paradigm will emerge is unclear, but it is conceivable that normal science in cosmology has come to an end, and revolution awaits.
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Dr. Eoin Ó Colgáin is a mathematical physicist at Atlantic Technological University.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
