An international scientific collaboration, led by the University of Nagoya (Japan), investigated the nature of dark matter that surrounds observed galaxies as they were 12,000 million years ago, that is, billions of years more than what has been achieved so far.
Their findings, published in the journal Physical Review Lettersopen the possibility that, going back this far and examining the early history of the universe, some fundamental rules of cosmology are not so widely believed.
Observing and analyzing something that happened so long ago is difficult. Due to the finite speed of light, we see distant galaxies not as they are today, but as they were billions of years ago. But even more complex is looking at dark matter, which emits no light.
To investigate a galaxy’s dark matter, another, even more distant source galaxy is often used. The gravitational pull of the foreground, including its dark matter, warps space and time around it, as predicted by Einstein’s theory of general relativity.
As light from the source galaxy travels through this distortion, it bends, changing the galaxy’s apparent shape. The greater the amount of dark matter, the greater the distortion. By analyzing this, scientists can measure the amount of dark matter around the galaxy ‘lens’ what is in the foreground.
However, after a certain point, a problem arises. Galaxies in the deepest parts of the universe are incredibly faint. Therefore, the further away from Earth we look, the less effective this technique will be. Lens distortion is subtle and difficult to detect in most of these cases, so many background galaxies are needed to detect the signal.
Most of the previous studies stopped there. Unable to detect source galaxies far enough away to measure distortion, they could only analyze dark matter from no more than 8 to 10 billion years ago. These limitations left open the question of the distribution of dark matter between that time and 13.7 billion years ago, around the beginning of our universe.
The ‘echo’ or radiation residue from the Big Bang
To overcome these problems and observe dark matter from the far reaches of the universe, the team led by Hironao Miyatake from Nagoya University, in collaboration with the University of Tokyo, the National Astronomical Observatory of Japan and Princeton University (USA), used a different source of backlight: the cosmic microwave background (CMB), the ‘echo’ of the Big Bang, the microwaves released shortly after the beginning of the universe.
First, with the help of subaru telescope in Hawaii (specifically, data from the Subaru Hyper Suprime-Cam Survey), the authors identified 1.5 million lensing galaxiesselected for visualization 12 billion years ago.
Then, to overcome the lack of light from even more distant galaxies, they used the WBCthat radiation residue from the Big Bang recorded, in this case, by the European Space Agency (ESA) Planck satellite. With the observed data or microwaves, it was possible to measure how dark matter around the target galaxies distorted these microwaves.
A distant galaxy and its surrounding dark matter halo (blue) can deflect light rays (white arrows) from the cosmic microwave background through gravitational lensing. Miyatake and his colleagues used measurements of this lens by the Planck satellite and measurements of optical wavelength light (orange arrow) from these distant galaxies by the Subaru Telescope on Earth to determine the distribution of dark matter around the galaxies. /APS/Alan Stonebraker
“The main result is that it is the first measurement of the distribution of dark matter in galaxies from 12 billion years ago”, confirms Miyatake to SINC. This is just 1.7 billion years after the beginning of the universewhich also means that these galaxies are being observed soon after their formation.
“I’m glad we opened a new window to that time, where things were very different,” he adds. More galaxies are seen in the formation process than at present. They also begin to form the first galaxy clusters”. These comprise between 100 and 1,000 galaxies held together by gravity with large amounts of dark matter.
Discrepancy with the cosmological model
Precisely one of the most interesting findings of the study is related to the agglomeration of dark matter. According to the standard theory of cosmology, the Lambda-CDM Model, subtle fluctuations in the CMB form clumps of densely packed matter, attracting surrounding matter through gravity. This creates inhomogeneous clusters that form stars and galaxies in these dense regions. The new findings suggest that the agglomeration measure is lower than expected by the Lambda-CDM model.
Miyatake acknowledges that there are still some uncertainties in his result and that more data will be needed to confirm it, “but if true, it would suggest that the entire model is flawed as you go back in time. This is exciting because it could suggest an improvement in the model and provide insights into the nature of dark matter itself.”
The Japanese scientist is also aware that his result is quite new, so there’s still not much discussion of alternative theories to explain it, “and honestly, theorists might not be interested in explaining it yet due to the uncertainties I mentioned. ”.
“However,” he emphasizes, “we are starting to see other evidence of Lambda-CDM collapse (specifically, discrepancies in cosmological values called the Hubble parameter and sigma8), between the measurements of the late universe (8 billion years ago to the present) and measurements of the ‘very’ primitive universe (about 400,000 years after the Big Bang).
“There are many candidates, such as extending the dark energy model and modifying general relativities, but there is no theory that can perfectly explain these discrepancies. Our result falls somewhere between the measurement of the late universe and that of the early universe, and I hope it helps to discover what fundamental rules of cosmology can best explain our universe.”
the next step is reduce uncertainty with more information. The group has only reviewed a third of the data from the Subaru Hyper Suprime-Cam Survey and will now focus on the rest, which will offer an even more accurate measurement of the distribution of dark matter.
In the future, the team hopes to use other data sources, such as Legacy Space and Time Survey (LSST) of the Vera C. Rubin Observatory, which is currently being built in northern Chile, to explore more areas of space and try to see the distribution of dark matter even further back in time, up to 13 billion years ago.
Reference:
Hironao Miyatake et al. “First identification of a CMB lens signal produced by 1.5 million galaxies at z∼4z∼4: Constraints on high redshift matter density fluctuations”. Physical Review Letters2022