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The Hubble constant might break our understanding of the universe

The Hubble tension is making us question everything we know

There is something wrong with our universe. Or at least, our interpretation of it.

This week, news came out that a discrepancy within one of the fundamental laws of our universe, the Hubble constant, isn’t just an observational error. That means that somehow, we seriously misunderstand our universe.

To be clear, we’ve known about this discrepancy — called the Hubble tension — for awhile. But this week, JWST confirmed that the Hubble Space Telescope’s measurements of this discrepancy were correct. This isn’t just a problem with observations. 

JWST deep field, Credit: NASA/STScI

Let’s break down what the Hubble constant is, and what all of it means.

The Hubble constant is less constant than it should be

The Hubble constant is, basically, the rate of expansion of our universe. Ever since the Big Bang, our universe has been expanding at an increasing rate. In 1929, Edwin Hubble realized that the further a galaxy was from us, the faster it appeared to be moving away from us. This meant that the universe wasn’t just expanding, but the rate of expansion was increasing — this is thanks to dark energy.

v = H0D

where v is the recessional velocity of an object, H0 is the Hubble constant, and D is the distance

Understanding the Hubble constant is important for many reasons, one of which is that it gives us a good idea of the age of the universe, which is currently calculated to be 13.8 billion years old. 

So figuring out the value of the Hubble constant, something that is so important to understanding our universe, is probably simple, right?


This is actually a huge challenge for science. There are three different ways the Hubble constant can be calculated:

  1. Taking a look at objects around us to see how fast they’re moving away from us

  2. Measuring small deviations in the CMB, or cosmic microwave background, which is light left over from the Big Bang

  3. Looking at gravitational waves that are produced by the collision of massive objects like black holes and neutron stars

Measuring the Hubble constant any one of these three ways should lead to the same result because it’s a constant. But we don’t get the same values from these different measurements. And that’s where this discrepancy, the Hubble tension, is.

The Space Shuttle Discovery with Hubble in its payload bay, credit: NASA

Part of the justification for building and deploying the Hubble Space Telescope is that scientists hoped it could give us a precise measurement of the Hubble constant, but instead it has muddied the waters significantly. 

Hubble constant, meet the Hubble tension

The ESA’s Planck satellite is mapping the universe’s CMB. Cosmic microwave background is a relic of the first light in the universe after the Big Bang, that dates from around 380,000 years after the Big Bang. As the universe has expanded, it has stretched light wavelength of CMB which is why it’s all the way at the microwave end of the spectrum. The longer the wavelength of light, the lower its energy.

Credit: STScI

Now, Planck has measured the Hubble constant as 67.4 km/s/Mpc. A parsec is about 3.26 light years, and a megaparsec is a million parsecs. It’s complicated, but this basically means that a space of 3.26 million light years should be expanding 67.4 km, or 48.1 miles, every second. The uncertainty here is less than one percent. 

CMB, credit: ESA

But another method of measurement, visible observation of Cepheid variable stars, gives us a different number: 73 km/s/Mpc.

Credit: ESA

Cepheid variables are very reliable stars that pulse in brightness on a regular schedule, between 1 and 70 days. This makes them excellent for measuring distance from Earth, because their brightness is directly connected to their duration of pulsation. Basically, the brighter the star, the longer it takes to pulse.

Cepheid variable star RS Puppis, Credit: NASA/Hubble

The Hubble Space Telescope observed Cepheid variable stars very soon after its launch, and the scientists used the observatory to continue to refine that Hubble constant measurement over the years. There’s certainly a discrepancy between the Planck and Hubble measurements, but they could have been for various reasons: dust obscuring the brightness of Cepheid variables, Cepheid variables looking brighter than they actually were because of an inability to distinguish them from other stars — this could have just been some sort of measurement error.

But in 2022, a team used Hubble to examine multiple different distance markers, including Type Ia supernovae. Cepheid variable stars are great distance markers for our local universe, but for further distances, these supernova work better. They form when white dwarf stars explode; because white dwarfs have to gain a precise amount of mass to collapse, that means that the peak brightness of any Type Ia supernova is the same. That makes these kind of stellar events a reliable standard candle to measure distance.

Type Ia supernova G299.2-2.9, credit: Chandra X-Ray Observatory

They found that, once again, the Hubble constant was 73 km/s/Mpc, with an extremely small margin of error. 

JWST checks the Hubble Space Telescope’s work

And now we come to today. We have precisely one observatory that can improve on Hubble’s measurements, and tell us once and for all what’s going on. This week, scientists using JWST confirmed that the Hubble Space Telescope’s measurements are accurate.

This isn’t the first time that JWST has double checked Hubble’s work. Back in 2023, JWST confirmed that Hubble’s measurement of Cepheid variable stars were accurate. Because it’s an infrared-optimized telescope, JWST can see through gas and dust that might obscure these stars, and its precision and accuracy make it easer to distinguish Cepheid variables from the light of surrounding stars.

Credit: STScI

But this new survey, which was published in The Astrophysical Journal Letters, is huge. Scientists used JWST to survey over 1,000 Cepheid variable stars and eight Type Ia supernovae. They’ve looked at every single measurement Hubble took. And the Hubble Space Telescope’s measure of the Hubble constant stands. The Hubble tension is alive and well.

What does it all mean??

So, what’s next? Honestly, we have barely scratched the surface. We’ve finally established beyond doubt that the problem exists, and isn’t some sort of measurement or observation error. Now we can start looking at the whys. (And if you’re wondering what that third method, the gravitational wave method, tells us about the Hubble constant, that reading lies in between the CMB and supernova observations, at around 69(+16-8) km/s/Mpc, and could be reconciled with either given the error range.)

The key to the Hubble tension that CMB is left over from the Big Bang, whereas the observations of astronomical phenomena are from the more recent universe. Maybe there’s something about that early period of the universe we don’t yet understand (and to be clear, there is a LOT about the early post-Big Bang universe we don’t understand). Or there’s a theory that the rate of expansion of the universe is actually constant, and any changes within it are due to an unequal distribution of matter (it’s important to note this goes against a fundamental principle of the standard model of cosmology).

Whatever it is, scientists will continue to study the Hubble tension and if we find the answer, you can bet it’ll be big news.