Researchers have successfully mapped the presence of dark matter around some of the earliest and most distant galaxies discovered so far. These 1.5 million galaxies, as observed, date back to about 12 billion years ago – approximately 2 billion years post-Big Bang. The light from these galaxies distorts the cosmic microwave background, which is the light from an even earlier period of the universe. This distortion, known as gravitational lensing, allows scientists to determine the distribution of dark matter around the galaxies, as described in the August 5 issue of Physical Review Letters.
By understanding how dark matter gathered around galaxies during the early stages of the universe, scientists hope to gain insights into this enigmatic substance. Moreover, this gravitational lensing method might also help solve the puzzle of how matter groups together in the universe.
Dark matter, a massive yet unidentified substance, envelops galaxies, exerting gravitational effects observed in the cosmos. Although it hasn't been directly detected, one observable effect is gravitational lensing, where light from a galaxy is bent by its mass like a lens. The degree of bending provides clues about the galaxy's mass, including its dark matter.
Cosmologist Hironao Miyatake from Nagoya University in Japan notes that mapping dark matter around galaxies at such great distances is challenging because it requires a light source more distant than the galaxy acting as a lens. Scientists usually use even more distant galaxies as light sources, but these are rare at such depths in space.
To overcome this, Miyatake and his team used the cosmic microwave background, the universe's oldest light. Combining measurements of its lensing made by the Planck satellite with observations from the Subaru Telescope in Hawaii, they succeeded in mapping these distant galaxies. The gravitational lensing effect is subtle, requiring many lens galaxies for accurate mapping, which matched the expected dark matter distribution, according to the researchers.
They also calculated sigma-8, a parameter indicating the "clumpiness" of matter in the universe. Previously, different methods of measuring sigma-8 have shown discrepancies, suggesting potential gaps in current cosmic theories. However, the evidence remains inconclusive.
Cosmologist Risa Wechsler of Stanford University, who was not part of the study, finds it intriguing to determine if the observed tension in measurements is genuine. The techniques employed in this study exemplify approaches that could clarify this issue.
By measuring sigma-8 from various cosmic eras, researchers aim to understand the cosmos better. Hendrik Hildebrandt, a cosmologist at Ruhr University Bochum in Germany who wasn’t involved in the research, emphasizes the importance of diverse measurements of sigma-8. If disparities arise between estimates from different universe periods, they may guide physicists to formulate a new theory explaining the cosmos more accurately. Although the recent sigma-8 measurement isn't definitive enough to resolve the debate, future projects, like the Rubin Observatory in Chile, hold promise for refining these estimates.
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