We have known for a good while that the universe is expanding. Since its creation at the Big Bang, it has constantly moved away from its primordial point of existence.
What happens from here is still a debate among the scientific community. Some suggest that the universe will keep expanding indefinitely. Since the rate at which the universe is expanding, known as the Hubble constant, is slowing down, others have proposed that at some point the universe will stop expanding and start contracting back, becoming so dense again that a new Big Bang will occur.
Those questions are most likely to remain in the hypothesising phase, but before delving into what may happen billions or tens of billions of years from now, calculating the expansion rate of the universe is more achievable, albeit not without its own mysteries.
1. The Hubble constant
Using data collected by NASA’s James Webb Space Telescope (JWST), astronomers have recently confirmed the expansion rate previously measured with the Hubble Space Telescope (HST). To measure this rate, scientists observe pulsating stars called Cepheid variables and Type Ia supernovae, a special class of exploding stars.
These Cepheid variable stars are found in crowded star fields, over a hundred million light years away. Their brightness and interval at which they are pulsating, combined with the redshift of the galaxies they are in, are used to accurately measure distances between objects that far away, where the expansion rate is more noticeable.
“They pulsate (that is, expand and contract in size) over a period of weeks that indicates their relative luminosity. The longer the period, the intrinsically brighter they are. They are the gold standard tool for the purpose of measuring the distances of galaxies a hundred million or more light years away, a crucial step to determine the Hubble constant”, said Physics Nobel Laureate Adam Riess, from the Johns Hopkins University.
However, light contamination from surrounding stars made Hubble’s measurement of the brightness of a Cepheid less reliable. “We can account for the average amount of blending, statistically, the same way a doctor figures out your weight by subtracting the average weight of clothes from the scale reading, but doing so adds noise to the measurements. Some people’s clothes are heavier than others”, Riess explained. Webb’s sharper infrared vision allows for a Cepheid target to be more clearly isolated from surrounding stars, thus offering a more accurate measurement.
Webb’s measurements coincide with Hubble’s, confirming that earlier measurements were accurate despite the added “noise”. This however also confirms a mystery that scientists have been unable to resolve for decades.
2. The Hubble Tension
The current expansion rate of the universe, measured by Hubble and confirmed by Webb, is approximately 72 km/s per megaparsec, where a megaparsec (Mpc) is a measurement for distance equal to 3.26 million light-years. On the other hand, measurements of the cosmic microwave background, the radiative energy filling the universe that is believed to be the radiation remaining from the big bang, predict that the current expansion rate should only be about 68 km/s/Mpc. This difference is called the Hubble Tension.
One possible explanation for the Hubble Tension is errors made in measurement, however, scientists have ruled out this option since all of the steps are conducted independently. Moreover, since Webb’s measurements coincide with Hubble’s, any error that might have occurred from HST’s lower quality observations are insignificant for the Hubble Tension.
“It may indicate the presence of exotic dark energy, exotic dark matter, a revision to our understanding of gravity, or the presence of a unique particle or field”, Riess suggested.
Regardless of the explanation, the existence of the Hubble Tension further proves just how little our understanding of the universe is. The more we find out, the more the mystery deepens.
