The universe is expanding at an accelerated rate, however, the nature of this expansion – why it's expanding and why it's doing so at such an accelerated rate – is uncertain. One culprit has been dubbed "dark energy", a hypothetical form of energy that could be seen as anti-gravity.  While what dark energy is precisely remains unknown, as a cosmological constant astronomers could roughly agree on its effects to measure the expansion rate of the universe, known as the Hubble constant. But not anymore.

Over the last few months, research has been published and news has followed about new measurements for the expansion rate of the universe. This is a Hot Topic in cosmology. Over the last several years, that earlier agreement has disappeared in the face of a gulf between the value depending on which method you use to measure it. The issue, dubbed the Hubble tension, took center stage, with cosmologists and astrophysicists arguing for one method of measurement – and its resulting number – over the other. But there is no clear winner. 

How could it not become a core issue? The standard model of cosmology – the universe started with a Big Bang, inflated exponentially, and has been expanding ever since (made up of regular matter, dark matter, and dark energy in a 5:25:70 proportion) – is possibly at stake. The most advanced observatories humans have ever put into orbit to measure extraordinary phenomena can't agree on a single value for the expansion rate. 

One, the Planck Observatory, looked at the cosmic microwave background (CMB). That’s the first light ever released in the universe. It is considered the light echo of the Big Bang, emitted once the universe was cool enough for neutral atoms to form, the light no longer shackled to them.

With the data from the CMB, scientists worked out that the universe is expanding at a rate of 67.4 kilometers per second per megaparsec, with 1 megaparsec being 3.26 million light-years. This means that if two galaxies are 1 megaparsec apart, the expansion of the universe would make them look like they are moving away from each other at a speed of 67.4 kilometers (42 miles) per second.

Astronomers approach the measurements of distances in the universe in different ways. One is to use standard candles, objects that have the same intrinsic luminosity so by measuring how dim they appear, we can work out how far away they are. The Hubble Space Telescope has measured several of these standard candles. One in particular, the Cepheid variable star has been particularly useful. That is what astronomers used to work out the expansion rate of the universe. But – it is a different number: 73 kilometers per second per megaparsec.

The uncertainty around each number is small and they do not overlap. It's like there are two expansion rates in the universe, one for the very early universe and one for the last several billion years or so. However, the standard model of cosmology, our current best theory for what the universe is like, says that it should be the same. So either our model is wrong or our measurements are wrong. Thus we have the Hubble Tension.

Multiple observatories have thrown their hat at measuring the expansion rate using standard candles. Even JWST, the most powerful space telescope ever launched into space, was found to be in agreement with Hubble when it comes to Cepheid variables. 

A new solution?

Given the inherent uncertainties, the value of the Hubble constant is consistent with that obtained from the cosmic microwave background. But it cannot rule out new physics.

Professor Wendy Freedman

At present, we do not have an answer to what is the correct solution, but back in April, research presented just days apart indicated some exciting new insights. The Dark Energy Spectroscopic Instrument (DESI) released the largest 3D map of the universe, allowing researchers to estimate independently the expansion rate of the cosmos between 8 and 11 billion years ago. Turns out, it agrees with the cosmic microwave background.

Another challenge has recently appeared. A team led by Professor Wendy Freedman of the University of Chicago also used JWST to better estimate the Hubble constant. Their work uses three independent ways to estimate distances in the same galaxies, with different standard candles (including Cepheids variables) measured.  

This approach was optimized to make sure that each method for the distance measurements was as accurate as it could be. Professor Freedman and her team found a value for the expansion rate of 69.96 kilometers (43.5 miles) per second per megaparsec. Including its uncertainty, the value is consistent with the cosmic microwave background, but Freedman is not ready to claim that the Hubble tension is now just gone. More observations are still needed, she noted. 

"The galaxies that are most distant give a different result than those nearby. The measurements for the more distant galaxies have lower resolution and are less accurate. Thus it remains for JWST to determine if there is a problem with those most distant objects. The more distant objects do not yet have JWST data," Professor Freedman told IFLScience.

Is physics as we know it safe?

In the uncertainties, new physics might be lurking. A very recent proposal considers a new type of dark energy existing at the dawn of the universe. Unlike the dark energy that continues to exist today (despite the fact we don't know what it is) this version, dubbed "early dark energy", would only stay around for a brief period. Its presence though would fix the Hubble tension. 

The presence of this dark energy made the number of early bright galaxies in the simulation increase. A result that kills two birds with one stone. If this is correct though, it will depend on future observations. The Hubble tension might melt away and the paradigm for galaxy evolution might change by other means.

"Given the inherent uncertainties, the value of the Hubble constant is consistent with that obtained from the cosmic microwave background. But it cannot rule out new physics. This work makes clear that more data are needed before additions to the standard cosmological model are required," Professor Freedman told IFLScience. 

The DESI finding as well as the work from Professor Freedman and her team are an exciting development in the Hubble tension saga. They tease a solution in sight toward a number closer to the CMB-measured one. Still, for now, the uncertainties remain and they will stay there until astronomers can agree on exactly why the tension appeared and where corrections are necessary.

The Hubble tension might indeed die but we will have to wait and see if it is from the bang of destroying our current model of the universe or the whimper of correcting our observations.