The crisis in cosmology – revisited.

Graham writes ...

Those of you who are regular visitors to this blog page may recall a post (1) in August 2023 when the so-called ‘Crisis in Cosmology’, or more formally what the cosmologists call ‘the Hubble tension’, was introduced and discussed.  If you are not, then may I suggest that you have a read of the previous post to get a feel for the nature of the issue raised? Also please note that some sections of the previous post have been repeated here to make a coherent story.
 
It concerns the value of an important parameter which describes the current rate of expansion of the Universe called Hubble’s constant, which is usually denoted by Ho (H subscript zero). This is named after Edwin Hubble, the astronomer who first experimentally confirmed that the Universe is expanding. The currently accepted value of H0 is approximately
 
70 km/sec per Megaparsec.

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Edwin Hubble, and his famous pipe! Credit: Public Domain and Space Telescope Science Institute (STScI) (background image).

As discussed in the book (2) (pp. 57-59), Hubble discovered that distant galaxies were all moving away from us, and the further away they were the faster they were receding. This is convincing evidence that the Universe is, as a whole,  expanding (2) (Figure 3.4). The value of H0 above says that speed of recession of a distant galaxy increases by 70 km/sec for every Megaparsec it is distant. As explained in (1), a Megaparsec is roughly 3,260, 000 light years.
 
Currently there are two ways to establish the value of Ho. The first of these, that is sometimes referred to as the ‘local distance ladder’ (LDL) method, is the most direct and obvious. This is essentially the process of measuring the distances and rates of recession of many galaxies, spread across a large range of distances, to produce a plot of points as shown below. The ‘slope of the plotted line’ gives the required value of Ho.

The slope of the best fit line gives the value of Hubble's constant. Credit: Rebecca Smethurst.

Max Planck. Credit: Public Domain

The second method employs a more indirect technique using the measurements of the cosmic microwave background (CMB).  As discussed in the book (2) (pp. 60-62) and in the May 2023 blog post, the CMB is a source of radio noise spread uniformly across the sky that was discovered in the 1960s. At that time, it was soon realised that this was the ‘afterglow’ the Big Bang. Initially this was very high energy, short wavelength radiation in the intense heat of the early Universe, but with the subsequent cosmic expansion, its wavelength has been stretched so that it currently resides in the microwave (radio) part of the electromagnetic spectrum. The most accurate measurements we have of the CMB was acquired by the ESA Planck spacecraft , named in honour of the physicist Max Planck who was a pioneer in the development of quantum mechanics (as an aside, I couldn’t find a single portrait of Max smiling!). The ‘map’ of the radiation produced by the Planck spacecraft is partially shown below, projected onto a sphere representing the sky. The temperature of the radiation is now very low, about 2.7 K (3), and the variations shown are very small – at the millidegree level (4). The red areas are the slightly warmer, denser regions and the blue slightly cooler. This map is a most treasured collection of cosmological data, as it represents a detailed snap-shot of the state of the Universe approximately 380,000 years after the Big Bang, when the cosmos became transparent to the propagation of electromagnetic radiation.

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The Planck spacecraft. Credit: ESA

Tiny temperature variations in the Cosmic Microwave Background captured by the ESA Planck spacecraft, projected onto a sphere representing the sky. Credit: ESA

To estimate the value of H0 based on using the CMB data, cosmologists use what they refer to as the Λ-CDM (Lambda-CDM) model of the Universe (5) – this is what I have called ‘the standard model of cosmology’ in the book (2) (pp. 63 – 67, 71 – 76). The idea is that of using the CMB data as the initial conditions, noting that the ‘hot’ spots in the CMB data provide the seeds upon which future galaxies will form. The Λ-CDM model is evolved forward using computer simulation to the present epoch. This is done many times while varying various parameters, until the best fit to the Universe we observe today is achieved. This allows us to determine a ‘best fit value’ for H0 which is what we refer to as the CMB value. For those interested in the detail, please go to (1).
 
The ‘crisis’ referred to above arose because the values of Ho, determined by each method, do not agree with each other,
 
Ho = 73.o km/sec per Mpc (LDL),     Ho = 67.5 km/sec per Mpc (CMB).

The 'evolution' of values of Ho since 2000. Credit: Jian-Ping Hu & Fa-Yin Wang, Hubble Tension: The evidence of new physics, 2023.

Not only that, but the discrepancy is statically very significant, with no overlap of the estimated error bounds of the two estimates. So how can this mismatch between the two methodologies be resolved? It was soon realised that the implications of this disparity was either (a), the LDL method for estimating cosmic distances is flawed, or (b), our best model of the Universe (the Λ-CDM model) is wrong. Option (b) on the face of it sounds like a bit of a disaster, but since the birth of science centuries ago this has been the way that it makes progress. The performance of current theories is compared to what is going on in the real world, and if the theory is found wanting, it is overthrown and a new theory is developed. And in the process of course there is the opportunity, in this case, to learn new physics.

The James Webb Space Telescope (JWST). Credit: NASA, ESA, CSA

Looking at the options, it would seem that the easier route is to check whether we are estimating cosmic distances accurately enough. Fortunately, we have a shiny new spacecraft available, that is, the James Webb Space Telescope (JWST), to help in the task. When I described the LDL method of estimating Ho above, it looks pretty straight forward, but it is not as easy as it sounds – measuring huge distances to remote objects in the Universe is problematic. The metaphor of a ladder is very apt as the method of determining cosmological distances involves a number of techniques or ‘rungs’. The lower rungs represent methods to determine distances to relatively close objects, and as you climb the ladder the methods are applicable to determining larger and larger distances. The accuracy of each rung is reliant upon the accuracy of the rungs below, so we have to be sure of the accuracy of each rung as we climb the ladder. For example, the first rung may be parallax (accurate out to distances of 100s of light years), the second rung may be using cepheid variable stars (2) (p. 58) (good for distances of 10s of millions of light years), and so on. Please see (1) for details. The majority of these techniques involve something called ‘standard candles’. These are astronomical bodies or events that have a known absolute brightness, such as cepheid variable stars, and Type Ia supernovae (the latter can be used out to distances of a billions of light years). The idea is that if you know their actual brightness, and you measure their apparent brightness as seen from Earth, you can estimate their distance. It is also interesting to note that a difference of 0.1 magnitude in the absolute magnitude of a ‘standard candle’, due to a discrepancy in estimating its distance, can lead to a 5% difference in the value of Ho. In other words, a value of Ho = 73 versus Ho = 69! It would seem the route of investigating the accuracy of estimating cosmic distances is fertile ground for a variety of reasons.

Wendy Freedman of the University of Chicago. Credit: Wendy Freedman and American Physical Society.

And this is exactly what Wendy Freedman, and her team of researchers, at the University of Chicago did. However, I should say that the results that now follow are not peer-reviewed, and therefore may change. The story henceforth is based on a 30-minute conference paper presentation at the American Physical Society meeting in April 2024. Interestingly, the title of her paper was “New JWST Results: is the current tension in Ho signalling new physics?”, which suggests that the original intention, at the time of the submission of the paper’s title and abstract, was to focus on objective (b) as mentioned above – in other words, looking at the implications of the standard model of the Universe being wrong. But in fact the focus is on (a) – an investigation of the accuracy of measuring distances. I can identify with this – when the conference deadline is so early that you’re not sure yet where your research is going!

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So, what did Freedman’s team do and achieve? They used two different ‘standard candles’ to recalibrate the distance ladder with encouraging results. The first of these are TRGB (Tip of the Red Giant Branch) stars. Without going into all the details, this technique assumes that the brightest red giant stars have the same luminosity and can therefore be used as a ‘standard candle’ to estimate galactic distances. The second class is referred to as JAGB (J-region Asymptotic Giant Branch) stars that are a class of carbon-rich stars that have near-constant luminosities in the near-infrared part of the electromagnetic spectrum. Clearly, these are useful as standard candles, and are also good targets for the JWST which is optimised to operate in the infrared. The team observed Cepheid variable, TRGB and JAGB stars in galaxies near enough for the JWST to be able to distinguish individual stars to determine the distances to these galaxies. Encouragingly, the results from each class of object gave consistent results for the test galaxies. Once a reliable distance to a particular galaxy was found, the team was able to recalibrate the supernova ‘standard candle’ data, which could then be used to re-determine the distances to very distant galaxies.
 
After all that, they were able to recalculate the current expansion rate of the Universe as
 
Ho = 69.1 ± 1.3 km/sec per Mpc
 
The results of the study are encapsulated in the diagram below, which shows that the new result agrees with the CMB data calculation (labelled ‘best model of the Universe result’ in the diagram) within statistical bounds.
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The value of Ho in units of km/sec per Mpc. Left is the existing CMB value, centre is the LDL value derived by the new (Freedman) study, and far right is the old LDL value based on Hubble space telescope data. Credit: Wendy Freedman and the American Physical Society.

So, is that the end of the story? Well, as regards this study, it is yet to be peer-reviewed so things could change. Another aspect is that the apparent success here may encourage other groups to look back at their (predominately Hubble Telescope) data to recalibrate their previous estimates of galactic distances. So, I think this has a long way to run yet, but for now the Freedman Team should be congratulated in their efforts to ease the so-called ‘crisis in cosmology’!
 
Graham Swinerd
 
Southampton, UK
June 2024
 
(1) Blog post August 2023, www.bigbangtobiology.net.
(2) Graham Swinerd & John Bryant, From the Big Bang to Biology: where is God?, Kindle Direct Publishing, November 2020.
(3) The Kelvin temperature scale is identical to the Celsius scale but with zero Kelvin at absolute zero (-273 degrees Celsius). Hence, for example, water freezes at +273  K and boils at +373 K.
(4) A millidegree is 1 thousandths of a degree.
(5) Here CDM stands for cold dark matter, and the Greek upper-case Lamba (Λ) refers to Einstein’s cosmological constant, which governs the behaviour of dark energy.