The Tip of the Red Giant Branch has pushed the Hubble tension into full blown crisis

The Hubble tension is the growing discrepancy between measurements of Hubble’s constant. This is the constant that tells us how fast the universe is expanding right now. Our best model of the universe as a whole, the cosmological constant (Λ) Cold Dark Matter or ΛCDM says that it is a constant everywhere in the universe.

We have two ways of measuring Hubble’s constant, and, if the ΛCDM were correct and our measurements were correct, then the two methods should agree with one another within the bounds of error.

They don’t, by a lot.

The tension between the two is so extreme that it is at the point of being six sigma or 1 in 285,715 likelihood that we would randomly see a discrepancy like that given the bounds of error on the two measurement methods.

This graph shows how much the two have deviated in recent years:

Hu, Jian-Ping, and Fa-Yin Wang. "Hubble tension: The evidence of new physics." Universe 9.2 (2023): 94.

The CMB measurements are based on observations of the Cosmic Microwave Background (CMB). This is the oldest light we can see, the microwaves that emerged when the universe first became transparent to light about 380,000 years after the Big Bang. By measuring temperature and polarization fluctuations in the CMB, we can determine what Hubble’s constant is compatible with the CMB. It turns out to be about 67.66.

The local distance ladder is more straightforward. You just measure the distances to objects further and further away and measure how fast they are moving away from us and you get a measurement for Hubble’s constant. That measurement is at around 73.04.

At a 2015 survey of conference attendees called Beyond ΛCDM, 69% of participants believed that new physics was required to solve the Hubble tension. Now, this clearly wasn’t a very scientific survey since people attending a conference called “Beyond ΛCDM” would likely be interested in theories that include new physics and not, say, that the tension is caused by something we don’t understand about how to measure things.

Not understanding how to measure things correctly is often called systematic error because such measurements tend to all agree with one another instead of random errors which tend to disagree. This is why it has become very important to try to measure both methods in many different ways that are independent of one another.

That is why the recent publication of measurement results using data from the James Webb and Hubble Space Telescopes called Tip of the Red Giant Branch (TRGB) is so important. It provides a completely independent way of measuring the distance ladder, and it agrees with other distance ladder measurements.

One of the biggest problems in astronomy is that space is really, really big. While the distances to closer stars can be measured using stellar parallax, by measuring how they move against the background of more distant stars as the Earth moves in its orbit, for anything further away than 100-1000 Parsecs we need to use other methods.

Over the last 100 years, astronomers have developed a variety of tools and methods for measuring distances. These are collectively called the Cosmic Distance Ladder.

When it comes to trying to measure distances to other galaxies, astronomers can rely on things that have a known, reliable brightness.

For stars and galaxies further away, up to about 40 Megaparsecs or about 100 million lightyears, they use Cepheid variables. These are stars that vary their brightness over time. The brighter they are the longer it takes for them to vary their brightness. This lets us know how far away they are.

Edwin Hubble first used Cepheid variable stars to measure the size and expansion rate of the universe in 1929, although he got the scale wrong.

As you get beyond 100 million light years, we need something much brighter so we use Type 1a Supernovae which are also a standard candle of brightness.

They can also use other brightness measurements including the Tip of the Red-Giant Branch method. The idea is to use the brightness of the brightest Red giant stars in a galaxy as a measure of distance.

Below you can see the Red Giant Branch as the plot of bright red points. The Hertzsprung-Russell diagram below is a scatter plot showing the relationship between absolute brightness and stellar classification (temperature). Brighter stars are towards the top while hotter stars are towards the left.

The brightness of red giants maxes out at a predictable magnitude because of how stars evolve.

Take our Sun as an example: