First there was an explosion. A big bang so to speak. This massive explosion created the universe and filled it with the seeds of all the matter and energy present today.
But with this kind of turbo start, growth isn’t something that ends after the initial burst. The universe continues to expand—even accelerate—at a rate called the “Hubble constant.”
The Hubble constant is an incredibly important number in cosmology, but it’s not easy to figure out. So hard to guess, in fact, that the “Hubble tension” caused by our inability to come up with a single number has become as famous as the Hubble constant itself. And recent results from the James Webb Space Telescope (JWST) have just reaffirmed this tension as a very real problem to overcome.
If a researcher wants to estimate the Hubble constant, there are two main ways. There are others, of course, but the two most common are Cepheid variables and the cosmic microwave background.
The Cosmic Microwave Background (CMB) is a remnant of radiation from the Big Bang that still permeates the universe. Scientists often compare using the CMB to using a photograph of a child to predict what a person will look like as an adult. To some extent, knowledge of the universe’s infancy should allow researchers to predict its current state. Researchers have long used it to make inferences about the origins of our universe and used it to predict what the Hubble constant should be. Fairly recent CMB calculations set the Hubble constant at about 68 km s-¹ Mpc-¹.
But the thing about theories is that they need to be tested. And when the Hubble telescope was launched, it became possible. Freed from the limitations of ground-based telescopes clouded by the atmosphere, Hubble was able to make extremely accurate measurements of the distance of distant objects. And this is where cepheid variables come into play.
Cepheid variables are a type of pulsating star that belongs to a category of objects called “standard candles,” that is, objects that have a measurable and constant brightness, allowing us to measure their distance relatively easily. Measuring distances isn’t easy in space, but it’s crucial to being able to measure expansion.
While it would be convenient to stop there, the Cepheid method has one more step. Once we guess its luminosity and know how bright a Cepheid is blinking at a given rate, we use that luminosity to calculate the luminosity of nearby supernovae. Then, using a quality called “redshift,” we use these supernovae to calculate the luminosity of other supernovae that are even further away.
Because they are much brighter, we can see supernovae at much greater distances than Cepheids. And since the expansion of the universe is better appreciated the further you look, we look to distant supernovae to do the final calculation. This chain of brilliance is called the “distance ladder” and you cannot advance a step without first counting all the steps in front of it.
Over time, the Hubble telescope recorded the distances of several Cepheid variables (the first rung of this distance scale), allowing researchers to experimentally measure the Hubble constant. But instead of fulfilling the prediction, it turned out quite differently: 73 km s-¹ Mpc-¹.
When experiment and theory diverge, it can mean one of two things: either the measurement is wrong or the theory is wrong. And that’s the Hubble tension in a nutshell. What is wrong, theory or experiment?
This new round of JWST measurements seems to reinforce the idea that something is wrong with the theory. JWST, which is able to repeat the Hubble observations with higher resolution at wavelengths where Hubble had difficulty getting clear images, virtually confirmed that Hubble did not see the ghosts. In fact, it produces data that allows the exact calculation of the Hubble constant of the current universe.
So what’s wrong with the theory? If you can answer that, you are light years ahead of the best astrophysicists in the field. It could have something to do with dark energy, dark matter or gravity. We may be looking at all the data correctly, but somehow we are wrong about the fundamental nature of Cepheid variables. We don’t know right now.
But we will undoubtedly continue to search. In astrophysics, tension is just another word for mystery.
Associate News Editor
Jackie is a writer and editor from Pennsylvania. She especially enjoys writing about space and physics, and enjoys sharing the strange wonders of the universe with anyone who will listen. She is watched over in her home office by her two cats.
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