The rate at which the Universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. However, there is a persistent difference between the value of the Hubble constant measured by a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang, called the “Hubble intensity.”
The James Webb Space Telescope confirmed that the Hubble’s sharp eye was right all along.
The age of the Universe and errors
One of the scientific rationales for the construction of the Hubble Space Telescope was to use its observational power to obtain an accurate value of the expansion rate of the Universe. Prior to the launch of Hubble in 1990, observations from ground-based telescopes produced huge errors. Depending on the obtained values of the expansion rate, the age of the Universe could be from 10 to 20 billion years.
Over the past 34 years, Hubble has reduced this measurement to an accuracy of less than one percent, splitting the difference with an age value of 13.8 billion years. This was achieved by refining the so-called “ladder of cosmic distances” by measuring important milestones known as Cepheid variable stars.
However, the Hubble value is not consistent with other measurements that indicate that the Universe expanded faster after the Big Bang. These observations were made by mapping the cosmic microwave background radiation of ESA’s Planck satellite, a blueprint for how the structure of the Universe will evolve after it cools down following the Big Bang.
Confirmation from Webb
A simple solution to the dilemma would be to say that perhaps Hubble’s observations are erroneous due to some inaccuracy that crept into his measurements of the dimensions of deep space.
Then came the James Webb Space Telescope, which allowed astronomers to double-check Hubble’s results. Webb’s infrared images of the cepheids matched Hubble’s optical-light data. Webb confirmed that the Hubble telescope’s keen eye was correct all along, dispelling any doubts about Hubble’s measurements.
The mystery of cosmology
The bottom line is that the so-called “Hubble tension” between what is happening in the nearby Universe and what is happening in the early Universe remains a vexing mystery for cosmologists. Perhaps there is something woven into the fabric of the cosmos that we do not yet understand.
Does this discrepancy require new physics to resolve? Or is it the result of measurement errors between the two different methods used to determine the rate of expansion of space?
Now, Hubble and Webb have teamed up to make the final measurements, which confirms the thesis that something other than measurement errors is affecting the expansion rate.
Dark energy
“If we set aside measurement errors, there is still a real and exciting possibility that we have misunderstood the Universe,” says Adam Riess, a physicist at Johns Hopkins University in Baltimore. Riess is a Nobel Prize winner for co-discovering that the expansion of the Universe is accelerating due to a mysterious phenomenon now called “dark energy.”
As a cross-check, Webb’s first observation in 2023 confirmed that the measurements of the expanding Universe were accurate. However, in the hope of relieving the Hubble “tension,” some scientists have suggested that invisible errors in the measurements may be growing and becoming visible as we look deeper into the Universe. In particular, a star cluster may systematically affect the brightness measurements of more distant stars.
SH0ES team
The SH0ES (Supernova H0 for the Equation of State of Dark Energy) team, led by Riess, has obtained additional observations with Webb of objects that are important cosmic milestones known as cepheid variable stars, which can now be correlated with Hubble data.
“We have now covered the full range of what Hubble observed and can rule out measurement error as the cause of Hubble’s intensity with very high confidence,” says Riess.
Webb’s first few observations in 2023 were successful and showed that Hubble was on the right track, firmly establishing the validity of the first rungs of the so-called cosmic distance ladder.
Measurement steps
Astronomers use a variety of methods to measure relative distances in the universe, depending on the object of observation. Collectively, these methods are known as a ladder of cosmic distances – each rung or measurement method relies on the previous rung for calibration.
But some astronomers have suggested that as we move outward along the “second rung,” the ladder of cosmic distances could become unsteady if Cepheid measurements become less accurate with distance. Such inaccuracies may arise because the light from the Cepheid may be mixed with that of a neighboring star, an effect that may become more pronounced with distance as stars crowd the sky and become harder to distinguish from one another.
The difficulty of observations lies in the fact that in previous Hubble images, these distant variable cepheids appear more crowded and overlapped with neighboring stars at increasingly greater distances between us and the host galaxies, requiring careful consideration of this effect. Interfering dust further complicates the accuracy of measurements in visible light. Webb cuts through the dust and naturally isolates cepheids from neighboring stars because its vision is sharper than Hubble’s in the infrared wavelength range.
Combining Webb and Hubble
“The combination of Webb and Hubble gives us the best of both worlds. We can see that Hubble’s measurements remain reliable as we climb further up the cosmic distance ladder,” says Riess.
Webb’s new observations include five host galaxies for eight Type Ia supernovae, containing a total of 1000 cepheids, and reach the most distant galaxy where cepheids have been well measured, NGC 5468, 130 million light-years away.
“This covers the entire range where we have made measurements with Hubble. So we’ve reached the end of the second rung of the cosmic distance ladder,” said study co-author Gagandeep Anand of the Space Telescope Science Institute in Baltimore.
Hubble and Webb’s confirmation of the existence of the Hubble Bump sets up other observatories to possibly solve this mystery, including the upcoming Nancy Grace Roman Space Telescope and the recently launched ESA Euclid mission.
For now, it looks as if the ladder of distances observed by Hubble and Webb is firmly entrenched on one side of the river, and the glare of the Big Bang observed by Planck from the beginning of the Universe is firmly entrenched on the other side. How the expansion of the Universe has changed over the billions of years between these two endpoints is something we have not yet observed directly.
“We have to figure out if we’re missing something in how to connect the beginning of the universe to today,” adds Riess.
