The standard model
The standard model of physics, namely the framework of laws at the foundation of modern physics, has reigned supreme since the 1970s, confirmed to great precision in a vast array of experimental tests. Among other things, the standard model predicted the existence of the Higgs boson, which was experimentally discovered in 2012, nearly 50 years after it was first predicted.
Yet physicists have recognized for many years that the standard model cannot be the final answer. For example, quantum theory and general relativity are known to be mathematically incompatible. String theory and Loop quantum gravity are being explored as potential frameworks to resolve this incompatibility, but neither is remotely well-developed enough to qualify as a new “theory of everything.” Other difficulties may exist as well.
But there is only so far that mathematical analysis can go in the absence of solid experimental results. As Sabine Hossenfelder has emphasized, beautiful mathematics published in a vacuum of experimental data can lead physics astray.
The Hubble constant anomaly
One significant experimental anomaly that does not appear to be going away, and which may point to a fundamental weakness in the standard model, is the discrepancy in values of the age of the universe, or, equivalently, in the Hubble constant, based on different experimental approaches.
The Hubble constant $H_0$ is a measure of the rate of expansion of the universe, and is directly connected to estimates of the age $A$ of the universe via the relation $A = 1 / H_0$. Units must be converted here, since the age of the universe is normally cited in billions of years, whereas the Hubble constant is usually given in kilometers per second per megaparsec (a megaparsec is $3.08567758128 \times 10^{19}$ km). Also, an adjustment factor is normally applied to this formula to be fully in conformance with the big bang model.
The trouble is, the best current experimental results give conflicting values for the Hubble constant, and thus, equivalently, for the age of the universe. See this previous Math Scholar article for an overview of the problem, as of August 2019.
Conflicting results
One method to determine $H_0$ is based on the flat Lambda cold dark matter (Lambda-CDM) model of the universe, combined with careful measurements of the cosmic microwave background (CMB) data from the Planck satellite. The latest (2018) result from the Planck team yielded $H_0 = 67.4 \pm 0.5$, which corresponds to $13.77$ billion years for the age of the universe.
Another approach is to employ a more traditional astronomical technique, based on observations of Cepheid variable stars, combined with parallax measurements as a calibration. In 2016, a team of astronomers using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope obtained the value $H_0 = 73.24 \pm 1.74$, corresponding to $12.67$ billion years for the age of the universe.
Clearly, these two sets of values differ by significantly more than the combined error bars of the two measurements. What is going on?
New results for the Hubble constant anomaly
In an attempt to resolve the “Hubble tension,” as this controversy is now called, several research teams, using different approaches, have launched studies hoping to resolve the issue. But rather than resolve the issue, their latest results only deepen the controversy.
In March 2019, a research team working with the Hubble Space Telescope reported that based on observations of 70 long-period Cepheid variable stars in the Large Magellanic Cloud, they were able to refine their estimate to $H_0 = 74.03 \pm 1.42$. Needless to say, this new result does not help to resolve the discrepancy of the Cepheid group’s result with the Planck team’s result — it moves in the other direction.
In July 2019, a group headed by Wendy Freedman at the University of Chicago reported results from another experimental approach, known as the “Tip of the Red Giant Branch” (TRGB). Their approach, which is analogous to but independent from the approach taken with Cepheid variable stars, is to analyze a surge in helium burning near the end of a red giant star’s lifetime. Using this scheme, they reported $H_0 = 69.8 \pm 1.7$. This is slightly more than the Planck team value ($67.8$), but not nearly enough to close the gap with the Cepheid approach.
Another group, called $H_0$ Lenses in COSMOGRAIL’s Wellspring (HoLiCOW) [yes, that is the acronym], also announced results in July 2019. Their study is based on gravitational lensing of distant quasars by an intervening galaxy. When this happens, multiple time-delayed images of the galaxy appear at the edges of the intervening galaxy, when viewed by earth-bound astronomers. The HoLiCOW project’s latest result is $H_0 = 73.3 \pm 1.76$, which is reasonably close to the Cepheid result, but not to the Planck result.
Latest results (August 2020)
In February 2020, the group headed by Wendy Freedman at the University of Chicago updated their TRGB study with additional consistency checks. Their updated result was $H_0 = 69.6 \pm 1.7$, a value slightly larger than their earlier figure, but still hopelessly inconsistent with the Cepheid value.
In July 2020, a team based at Princeton University announced a new result, based on the same Lambda-CDM model as the Planck team, but using the Atacama Cosmology Telescope (ACT) in Chile. Their result is $H_0 = 67.6 \pm 1.1$. This is within $0.3\%$ of the Planck team’s result.
A group headed by researchers at the University of Oregon also reported results in July 2020. They employed the “baryonic Tully-Fisher relation” (bTFR) as a distance estimator. Using 50 galaxies with accurate distances (from either Cepheid or TRGB measurements), they calibrated the bFTR on a large scale. After applying this calibrated bTFR model to 95 independent galaxies, they found $H_0 = 75.1 \pm 2.3$.
Needless to say, researchers are perplexed by the latest reports: the Planck team (based on the Lambda-CDM model) reports $H_0 = 67.4 \pm 0.5$; the Princeton group reports $H_0 = 67.6 \pm 1.1$; the Chicago team reports (updated) $H_0 = 69.6 \pm 1.7$; the HoLiCOW team reports $H_0 = 73.3 \pm 1.76$; the Cepheid team reports $H_0 = 74.03 \pm 1.42$; and the Oregon team reports $H_0 = 75.1 \pm 2.3$. Obviously these results cannot all simultaneously be correct. For example, the Oregon team’s figure ($75.1$) represents a five-sigma discrepancy from the Planck figure ($67.4$).
See this Quanta Magazine article for an overview of the experimental results as of February 2020 (prior to the two July 2020 studies mentioned above).
Are the physical models wrong?
While each of these teams is hard at work scrutinizing their methods and refining their results, researchers are increasingly considering the unsettling possibility that one or more of the underlying physical theories are just plain wrong, at least on the length and time scales involved.
Key among these theories is the Lambda-CDM model of big bang cosmology. Yet physicists and cosmologists are loath to discard this model, because it explains so much so well:
- The cosmic microwave background radiation and its properties.
- The large-scale structure and distribution of galaxies.
- The present-day observed abundances of the light elements (hydrogen, deuterium, helium and lithium).
- The accelerating expansion of the universe, as observed in measurements of distant galaxies and supernovas.
As Lloyd Knox, a cosmologist at the University of California, Davis, explains,
The Lambda-CDM model has been amazingly successful. … If there’s a major overhaul of the model, it’s hard to see how it wouldn’t look like a conspiracy. Somehow this ‘wrong’ model got it all right.
Various modifications to the Lambda-CDM model have been proposed, but while some of these changes partially alleviate the Hubble constant discrepancy, others make it worse. None are taken very seriously in the community at the present time.
Adam Riess, an astronomer at Johns Hopkins University in Baltimore, Maryland, is reassured that the Princeton ACT team’s result was so close to the Planck team’s result, and he hopes that additional experimental results will close the gap between the competing values. Nonetheless, he ventures, “My gut feeling is that there’s something interesting going on.”
For additional details and discussion, see this Scientific American article, this Quanta article and this Nature article.
Caution
In spite of the temptation to jump to conclusions, throwing out the standard model or big bang cosmology, considerable caution is in order. After all, in most cases anomalies are eventually resolved, usually as some defect of the experimental process or as a faulty application of the theory.
A good example of an experimental defect is the 2011 announcement by Italian scientists that neutrinos emitted at CERN (near Geneva, Switzerland) had arrived at the Gran Sasso Lab (in the Italian Alps) 60 nanoseconds sooner than if they had traveled at the speed of light. If upheld, this finding would have constituted a violation of Einstein’s theory of relativity. As it turns out, the experimental team subsequently discovered that the discrepancy was due to a loose fiber optic cable that had introduced an error in the timing system.
A good example of misapplication of underlying theory is the solar neutrino anomaly, namely a discrepancy in the number of observed neutrinos emanating from the interior of the sun from what had been predicted (incorrectly, as it turned out) based on the standard model. In 1998, researchers discovered that the anomaly could be resolved if neutrinos have a very small but nonzero mass; then, by straightforward application of standard model, the flavor of neutrinos could change enroute from the sun to the earth, thus resolving the discrepancy. Takaaki Kajita and Arthur McDonald received the 2015 Nobel Prize in physics for this discovery.
In any event, sooner or later some experimental result may be found that fundamentally upsets currently accepted theoretical theories, either for a specific framework such as Lambda-CDM big bang cosmology, or even for the foundational standard model. Will the “Hubble tension” anomaly ultimately overturn these basic theories? Only time will tell.