What the Heck is Hubble Tension? (aka The Hubble Wars)

Article October 14, 2023

Alright Space Enthusiasts, it’s time to get big picture. The biggest picture, really. Getting bigger all the time. Expanding, you might say.

Okay fine, I’m talking about the expansion of the Universe. We know it’s happening, and we even know it’s accelerating. But if you want to start a fight amongst a group of cosmologists, ask how fast it’s accelerating. You might get one of a couple of different answers. That’s because there are some very distinct camps amongst those who study universe expansion and the arguments between them make up the ongoing saga known as the Hubble Tension, or what I like to call the Hubble Wars.

To be clear, I’m pretty sure I’m the only one who calls them that. But it sounds a lot cooler than Hubble Tension, so let’s see just what these astronomers are arguing about.

Some Background

Astronomer Edwin Hubble. Credit: Johan Hagemeyer

Once upon a time everyone assumed the universe was completely static. Albert Einstein even infamously altered his original theory of general relativity in 1915 because it suggested that the universe should be changing size and that seemed ridiculous to him. Then in 1929 along comes a guy named Edwin Hubble. Even if you haven’t of Edwin, you’ve heard of the telescope named after him. Hubble wasn’t the first person to propose that the universe was actually expanding, but he is credited as being the one to prove through observations that it was happening when he was able to measure the redshift of a number of galaxies.

If you’ve ever heard a police siren change pitch as it moves towards or away from you, you’ve experienced the principle behind redshift in action. The pitch changes because the motion of the siren causes the frequency of the sound waves to change—as the siren moves towards you the sound waves pile up on top of each other and the frequency increases. As it moves away, the sound waves get more distant from each other and the frequency drops.

A diagram showing how light from objects moving away from an observer looks redder, while light from an object moving towards an observer looks bluer. Credit: Ales Tosovsky

The same thing happens with light. When something moves towards you the light waves it emits will pile up and the wavelength will shorten—the light will appear bluer. We call this blueshift. The opposite happens when something is moving away from us—the light wavelengths stretch and the light becomes redder. This is redshift. The faster something moves away from us, the more redshifted its light will be. Hubble (Edwin, not the Telescope) proved that the light of more distant galaxies was far more redshifted than closer galaxies. In essence, the farther away a galaxy is, the faster it appears to be moving away. We call this Hubble’s Law. So not only is the universe expanding, but that expansion isn’t steady—it’s accelerating.

Hubble determined that the rate of universal expansion was 500 km/s/Mpc. That means for every megaparsec (Mpc, about 3.26 million light years) you look out, the expansion rate will appear to increase by 500 kilometers per second (Americans might like to measure speed in miles per hour, but scientists prefer kilometers per second when they can get it). Also everyone agreed that “rate of universal expansion” was far too much of a mouthful, so we just refer to this as the Hubble constant. Laws, constants, telescopes…this dude got a lot of stuff named after him.

Modern Measurements

Edwin Hubble may have proven the universe is expanding at an accelerating rate (pushed on by the outward force of dark energy, whatever that turns out to be), but his measurements of the Hubble constant were way off. He just didn’t have the instrumentation to make a fine measurement. These days we have a few different methods that we use to measure the Hubble constant.

Some measurements are made using data from very early in the universe’s history. For instance, we can look at the Cosmic Microwave Background, which is the very low-level background radiation we can see in every direction that’s left over from the Big Bang (and it doesn’t get much earlier than that) using telescopes like WMAP and Planck. Looking at the size of fluctuations in this radiation can help us calculate a value for the Hubble constant.

The Cosmic Microwave Background. Credit: NASA

We can also look at very distant observations to determine the distribution of visible matter when the universe was very young through tools like the Sloan Digital Sky Survey. Seeing how things were distributed billions of years ago and comparing them to how they are distributed now can help us figure out how things moved in the time in between.

Alternatively, we can study things much closer, things like Type 1a supernovae or Cepheid variable stars. These are objects that have a very predictable brightness. And if you know how bright something actually is, you can measure how far away it is based on how bright it appears to be. Tackling this task was one of the reasons the Hubble Space Telescope was a major goal for astronomers in the early 1990s, and it’s one that other major observatories like Gaia and Webb have taken on lately as well.

So these days we have the instruments to make veryhighly accurate measurements of the Hubble constant. And that’s exactly where the problem begins. Our very highly accurate measurements don’t agree with each other.

The Hubble Wars

What we have these days are a few very distinct “camps”, depending on whether a Hubble constant value was calculated using something close by and recent or far away and long ago. Measurements based on things observed nearby (astronomically speaking) and therefore things that happened fairly recently (astronomically speaking), such as the supernovae and variable stars, produce a Hubble constant of about 73.5 km/s/Mpc. Measurements based on things very, very far away and representing the early universe produce a value of around 67.7 km/s/Mpc. 73.5 does not equal 67.7.

A graph showing measured values for the Hubble constant over time using Cepheid variables, red giant stars, and the Cosmic Microwave Background. Credit: Freedman et al./Astrophysical Journal

To throw more fuel on the fire, within the last couple of years a new technique was tried using red giant stars. Like the supernovae and variable stars, we have a good idea of the actual brightness of these stars, so if we see one in another galaxy we can use its appearance to figure out how far away it is and how fast it’s moving away from us. Measurements using this method give a Hubble constant of 69.8 km/s/Mpc. Right between the other two. Cue the frustrated facepalms.

So now we have three different Hubble constant values, each very precisely measured and calculated, and each having its own groups of diehard supporters insisting that there is nothing wrong with the measurements. A lot of scientific papers have been published on the topic and sometimes the arguments between the different camps have gotten…heated. But what is actually going on?

Resolving the Issue

An artist’s illustration outlining the expansion of the universe over time: Credit: NASA/WMAP Science Team/Dana Berry

For a long time the arguments between different groups of cosmologists raged because it was assumed that one camp or another had to simply be wrong. Each camp accused the others of making some sort of fundamental miscalculations, since their own were clearly correct.

As our observations get more and more accurate, however, these different numbers keep appearing, and it’s becoming apparent that something needs to be rethought. More and more astronomers are coming around to the idea that the issue is with the way we think the universe works, and a whole garden of theories has bloomed as astronomers try to come up with an explanation for the varying values of the Hubble constant (which, it turns out, may not be a constant at all).

Did an early abundance of dark energy result in an extra burst of expansion in the universe, causing the rate of expansion to change? Did dark matter interact with neutrinos when the universe was smaller and denser, causing alterations in the Cosmic Microwave Background, affecting our ability to measure it? Is our estimate for the age of the universe hilariously incorrect? Do we really just not understand how gravity works? Did bubbles of dark energy smack into each other in the early universe, releasing a bunch of energy and giving the universe a kick? Frankly, that last one sounds like the most fun, so I vote for that one.

Who Knows?

If you read all this way just so I could tell you what the grand answer to the Hubble Tension is, I am very sorry. People with a far greater understanding of the problem than I have are still tearing their hair out over it. It always seems like it will just take the building of the next big telescope and we’ll have our answer. The Hubble Telescope couldn’t solve the issue. There was a hope that Webb would be able to, but so far all it’s done is agree with its partner Hubble (the Telescope, not Edwin), which provided zero percent additional clarity.

Perhaps the recently launched Euclid Observatory can bring us the answer as it tries to map dark matter and dark energy. Or maybe it will be the future Nancy Grace Roman Space Observatory. Or the Thirty-Meter Telescope, if that ever gets built. Astronomers are quite determined to get to the bottom of this, and they’ll use whatever tools they can get their hands on.

And just maybe, along the way, they’ll find out that universe is a very different place than we thought was. Or that physics has been hiding something up its sleeve this whole time. Or that things operate just a little differently than we thought they did. Maybe we’ll discover something completely new that we weren’t expecting and didn’t even know to look for.

In the meantime I, for one, am going to savor the enigma, absorb the scientific drama, and think about the possibilities. If you can’t have a little fun contemplating a cosmic mystery, you are missing out my friend.