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The universe has been expanding since the Big Bang occurred 13.8 billion years ago. Determining the history of our Universe’s expansion is critical to solving its energetic composition or to testing the key ingredients of standard cosmology, such as the validity of large-scale General Relativity and the cosmic principle.
The Hubble constant is a unit that represents the rate at which the Universe is expanding at various distances from a specific point in space.
Currently, two different measurements are available for the Hubble constant. However, they both give different results.
Leiden physicist David Harvey adapted a third independent measurement method using the light deformation properties of galaxies predicted by Einstein.
Albert Einstein’s theory of general relativity predicts that a concentration of mass, such as a galaxy, can bend the path of light, just as a lens does. When a galaxy is faced with a bright light source, the light is swirled around it and can reach Earth through different paths, providing two, sometimes even four, images of the same source.
HoliCOW
In 1964, Norwegian astrophysicist Sjur Refsdal discovered that one path is longer than the other when the concentrating galaxy is a little off-center. It means the light takes longer on that path.
So when there is a change in the quasar’s brightness, this blip will be visible in one image before the other. The difference could be days, or even weeks or months.
This timing difference can be used to determine the distances between the quasar and the target. And comparing them to the redshift of quasars can give you a measure of the Hubble constant.
As part of the HoliCOW project, an examination group used six of these lenses to narrow the Hubble constant to about 73. In each case, there is a complication: apart from the difference in distance, the mass of the galaxy in the first place. plan also exerts a retarding impact, dependent on the exact distribution of the mass.
“We need to model that distribution, but a lot of questions remain Harvey says. “Uncertain, it limits the accuracy of this technique. “
Individually modeling these foreground galaxies is computationally impossible. Therefore, Harvey designed a method for calculating the average effect of a full distribution of up to 1,000 lenses.
Harvey said, “In that case, the individual peculiarities of the gravitational lenses are not that important and you don’t need to run simulations for all the lenses. You have to make sure you model the entire population.”
“In the paper, I show that with this approach, the error in the thresholds of the Hubble constant at 2% when approaching thousands of quasars.”
“This margin of error will allow for a meaningful comparison between the different candidates for the Hubble constant and will help understand the discrepancy. And if you want to go below 2%, you need to improve your model by doing better simulations. I imagine this would be possible. “
Journal reference:
- David Harvey, A 4% measurement of H0 using the cumulative distribution of strong lens time delays in double-image quasars. Royal Astronomical Society Monthly Notices (2020). DOI: 10.1093 / mnras / staa2522
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