How Quickly is the Universe Expanding? The Hunt for the True Hubble Constant

Microwave emission map that shows the Cosmic Microwave Background, a distribution of early universe temperatures. The blue spots are colder than the red spots. Via NASA’s Astronomy Picture of the Day/Planck Collaboration

It is now a well-known fact that the universe is expanding, but how quickly is it growing? Despite decades of cosmological research across the globe, no astronomer can definitively identify this rate of expansion, the Hubble Constant H0. Such uncertainty stems from Hubble Tension, the quantitative difference between two different methods used to calculate H0. Luckily, Dr. Bruce Partridge, one of the original collaborators on the Planck Satellite team, is an Emeritus Professor of Astronomy here at Haverford College — and he was excited to explain this puzzle.

As soon as we can determine the true value of H0, our understanding of the universe and how it has changed from its birth to now will become much more refined, allowing us to further develop our theories of dark matter, universe inflation and expansion, and measurements of light from distant objects. 

What is the Hubble Tension?

If we’ve been trying to measure the expansion of the universe for so long, what’s stopping us from landing on a definitive value? Partridge says the answer lies in a discrepancy in the way we measure expansion locally versus distantly. The basic equation to find H0  gives a measure of an object’s changing speed as its distance from Earth increases. However, this can be measured using either present-day values of distant objects and their velocities, which would give an H0 in this current space and time, or using values from the Cosmic Microwave Background (CMB). Since the CMB is a map of early universe temperatures, which can tell us about characteristics of that time and space, using its values will yield an H0 from the earliest stages of the universe.

Needless to say, the supernova [local value] folks think there are problems with the CMB measurement, and vice versa.

Dr. Bruce Partridge

Using both methods as well as the temperature of the CMB left over from the Big Bang (T0), astronomers have found that the H0 calculated from the CMB and its assumed T0 is roughly 10% larger than the value calculated from local measurements, which is far greater than any reasonable error. This disparity is called the Hubble Tension.

Shouldn’t these values be the same? How can this happen?

Partridge says there are a few posited explanations for the discrepancy: “[t]he simplest is that one of the values is wrong, because of some subtle error in measurement that we haven’t uncovered yet. Needless to say, the supernova [local value] folks think there are problems with the CMB measurement, and vice versa. That is why there is so much interest in some new, third way to measure [H0 ] — to break the tie.”

In addition to these theories, some say there is simply an inherent difference in the local and CMB values of H0, which would imply that there is something fundamentally wrong with our understanding of the universe—or alternatively, something is lacking. There have been several proposed solutions to this inherent difference, including a form of dark energy that fades after the creation of the CMB (see also: What is Dark Matter? By Hannah James).

More recently, and more contested between the local and CMB projects, is the suggestion that our measured and calculated value for T0 — the temperature of the CMB — is incorrect. Due to the influence of T0 in the calculation of the Hubble Constant, if T0 = 2.55K instead of the currently accepted value of T0 = 2.726K, the local and CMB values of H0  would match. However, as Partridge notes, “[s]o far, none of these alternative explanations involving new science have worked. Some can indeed explain away the tension in [H0 ], but they introduce other discrepancies with the data. One example is the suggestion that [T0] is wrong. There are several, independent ways of measuring [T0], and they agree on 2.726K, not a lower value.” Dr. Partridge’s own research has confirmed this value of T0 multiple times, one of which involved a pair of Haverford students, Benjamin Walter ’13 and Gerrit Farren ’20, who confirmed the calibration of the Planck satellite. Since the Planck satellite was Europe’s first mission solely to define and constrain the age of the universe and its early characteristics, including composing a full map of the CMB, this confirmation was an important step in ruling out errors in our estimates of T0. He also notes that several other methods of measuring T0 favor the CMB value, indicating that this discrepancy in T0 would, if anything, favor the CMB value of H0 .

Understanding the Hubble Tension is incredibly important for both understanding early cosmology and how our universe works today, including revising our theories of dark matter and the effects the expansion of the universe has on our measurements of light from distant sources. For now, as Partridge says, “we’ll keep working on other explanations — and on checking our observational results.”

This article was edited by Hedy Goodman and Emi Krishnamurthy.