The great dispute over the rate of expansion of the universe deepens with the James Webb Space Telescope. The Hubble tension is a unique conundrum that is the subject of one of the greatest and most contentious cosmic debates of our time.
This phrase describes the fact that although scientists are aware that the cosmos is expanding outward in all directions, they cannot determine exactly how fast it is expanding. As a result, there is a pretty big gap in our knowledge about the universe.
Researchers reported on Tuesday (September 12) that this was the first time JWST had looked into the situation, but it was unable to shed light on the puzzle. In fact, JWST has made this even more concrete.
What is the Problem with Determining the Expansion Rate of the Universe?
In essence, finding the true value of the Hubble constant, a key factor in determining the expansion rate of the universe, is necessary to precisely reduce the Hubble tension. However, for some reason, the constant does not seem to behave as predicted by our theoretical models.
According to most models, the Hubble constant should be about 68 km/s per megaparsec (km/s/Mpc). For context, a megaparsec is 1,000 parsecs, or about 3,260 light-years.
However, after looking at stars and galaxies across the universe, some scientists estimate the constant to be 69,8 km/s/Mpc, while others have found it to be as high as 74 km/s/Mpc, depending on the measurement technique. Others have offered suggestions that fall somewhere in between.
This mismatch may indicate that our tools are not smart enough or that we are significantly wrong in our theoretical predictions. In other words, could the models that underpin our current understanding of the world be missing?
The Kavli Institute for Theoretical Physics in California hosted a famous meeting of leading physicists in 2019 to try to formally solve this problem. The result was a headache.
“We wouldn't call it a stress or a problem, we would call it a crisis,” said particle physicist David Gross. Since then, scientists have worked diligently to determine where they might have gone wrong and have ticked off a list of potential causes of the Hubble strain, which you can view here.
Coming back to JWST's findings: The space observatory has added another item to this list. In summary, it showed that the alleged crisis was probably not a result of technical problems with the readings provided by its telescope sibling, the aptly named Hubble Space Telescope.
This is important because Hubble observations, or more specifically Hubble observations of Cepheid stars, are one of the features most frequently used by scientists to solve for the Hubble constant.
“The Webb measurements provide the strongest evidence yet that systematic errors in Hubble's Cepheid photometry do not play a significant role in the current Hubble voltage,” Adam Riess of Johns Hopkins University and the Space Telescope Science Institute said in a statement.
Hubble's ability to measure stellar brightnesses with incredible precision makes it a crucial tool in the effort to resolve the Hubble tension. It achieves this because it sits above the planet's hazy atmosphere, unlike ground-based observatories, which are hampered by our planet's hazy shield.
Since we are aware of the invariant speed of light, such brightnesses can help us determine the distance between these stars and how long it takes for their light to reach us. After certain calculations, the researchers concluded that such data collected from a large number of stars could be useful in determining the Hubble constant.
According to Riess, “Before Hubble was launched in 1990, the rate of expansion of the universe was so uncertain that astronomers were not sure whether the universe had been expanding for 10 billion or 20 billion years.”
To further elucidate the rate of expansion of the universe, scientists often use Hubble to focus on Cepheid stars. These supergiant stars are almost 100.000 times luminous than the Sun.
Stating that they are the "gold standard tool" for calculating the distances of galaxies of 100 million light years or more, Riess stated that these observations are "a very important step to determine the Hubble constant."
Riess also emphasized the pulsations, or expansions and contractions in size, of the Cepheids that reveal their changing brightness. Because they are naturally brighter over longer periods of time, longer periods provide baseline brightnesses and ultimately more accurate observations, he said.
As a result, the telescope can recognize certain Cepheids in galaxies more than 100 million light-years away due to Hubble's location above our atmosphere. This allows us to calculate the time interval of the brightness changes of these galaxies. However, Hubble has its limitations.
Infrared light wavelengths that lie beyond the red end of the electromagnetic spectrum and are still invisible to human vision cannot be fully detected by it. The Cepheid brightness we see here is mixed in with other stars in Hubble's field of view because, unfortunately, red light vision is not as sharp as blue light vision.
When looking at distant things, infrared vision is very important because initially, the light from these sources is stretched as it moves towards our location on Earth. The previously short, bluish wavelengths change to longer, red ones. In fact, this is where the name “redshifted galaxies” comes from, referring to regions further away from this end of the spectrum from our vantage point on Earth.
If a Cepheid were covered by an interstellar cloak, it would appear dimmer to us because only infrared light can pass through the dust unharmed. This runs the risk of the sample's light mixing with light from another nearby Cepheid, or giving the impression that a star is farther away than it actually is.
According to Riess, we can take the average amount of blending into account statistically, just as a doctor determines your weight by subtracting the average weight of your clothes from the scale reading. “But doing so contaminates the measurements. Some people's clothes vary in weight.
The infrared universe will be revealed thanks to this $1 billion telescope, located about 1,6 million miles (10 million kilometers) from Earth.
According to Riess, with our 1685 General Observers program, we collected observations of Hubble-identified Cepheids in the first year of Webb operations at two levels known as the cosmic distance ladder.
According to the team, the first phase involved calibrating Cepheid observations in a galaxy with a known geometric distance. This galaxy was NGC 4258. To essentially double-check Hubble's observations, the next step was to study Cepheids in host galaxies of Type 1a supernovae, which are recent, bright star explosions.
If Hubble is wrong, then perhaps we can finally understand why there is a discrepancy. But Hubble's observations were correct.
“In my view, JWST effectively put an end to the question of whether Hubble's Cepheid measurements were accurate.” Riess made the statement while presenting the study at JWST's First Years of Science conference on Tuesday.
However, it should be noted that the Nobel Prize-winning researcher does not see this situation as a problem as it is beginning to be perceived.
During the conference he said, “I don't care what the Hubble constant is. “I want to know why our best tools, our highest standard cars, don't match,” he says.
source: space.com
📩 14/09/2023 12:24