It is described as the most detailed map of the influence of dark matter throughout cosmic history. A telescope in Chile has traced the distribution of this mysterious material over a quarter of the sky and over nearly 14 billion years. The result is once again a spectacular confirmation of Einstein’s ideas. Although dark matter makes up about 85% of all the mass in the Universe, it is extremely difficult to detect and defies easy description.
But dark matter influences the large-scale structure of everything we see: where are all the galaxies, where are the voids in space. It is the scaffolding from which the visible structure of the Universe hangs. It does not emit or absorb light. The only way you can very obviously infer its presence is through its interaction with gravity. Large rotating galaxies of stars would drift apart if it weren’t for the inclusion of some invisible mass pulling on them and holding them together. But dark matter will bend, or lens, backlight, and so its whereabouts were mapped by the Atacama Cosmology Telescope (ACT). Webb telescope ‘fingerprints’ of the first galaxies Gaia registers a rapid cosmic expansion Atacama Cosmology Telescope The telescope was positioned at a height in the Atacama of 5,200m The Chile facility observed the Cosmic Microwave Background, or CMB, a penetrating but faint glow of long-wavelength radiation reaching us from the very edge of the observable Universe. ACT mapped the subtle distortions in this ancient light that were introduced as it passed through all the matter in between. You can compare it to the way light bends as it passes through the bumps and bulges in an old glass window pane. If you know what you’re looking at outside, you can use the distortions to say something about the glass. In the same way, the CMB can be decoded to reveal all the structure involved in its journey to us. There have been detections of “gravitational lensing” similar to this in the past, most notably by the European Space Agency’s Planck observatory a decade ago. But ACT beats everyone in terms of resolution and sensitivity.
Composition of the Universe Successive experiments indicate that the cosmic contents include: about 5% normal matter: atoms, the stuff we are all made of about 27% dark matter, hitherto unseen directly and defying description about 68% dark energy: the mysterious component that accelerates cosmic expansion The Universe is estimated to be 13.8 billion years old. In the image at the top of this page, the colored areas are the portions of the sky studied by the telescope. The orange regions show where there is more mass or matter along the line of sight; purple where there is less. Typical features are hundreds of millions of light-years across. The grey/white areas show where polluting light from dust in our Milky Way galaxy has obscured a deeper view. The distribution of matter agrees very well with scientific predictions.
The ACT observations indicate that the “lumpiness” of the Universe and the rate at which it has been expanding after 14 billion years of evolution are just what you would expect from the Standard Model of Cosmology, which has the theory of gravity. of Einstein (general relativity) at its founding. Recent measurements using an alternate background light, one emitted by stars in galaxies rather than the CMB, suggested that the Universe lacked enough clumps. “It’s one of the ‘cosmic stresses’ we all talk about,” said Professor Jo Dunkley of Princeton University, USA. “But with this new result, we found exactly the right number of bulges – without stress! So if there is stress, it is something that shows up in the galaxy data, not ours,” he told BBC News.
Another stress concerns the rate at which the Universe is expanding, a number called the Hubble constant. When Planck observed temperature fluctuations in the CMB, he determined the rate to be about 67 kilometers per second per megaparsec (one megaparsec is 3.26 million light-years). Or put another way: the expansion increases by 67 km per second for every 3.26 million light-years we look into space. A tension arises because measurements of the expansion in the nearby Universe, made using the recession of variable stars from us, register around 73 km/s per megaparsec. It is a difference that cannot be easily explained.
ACT, using its lensing technique to pin down the expansion rate, generates a number similar to Planck’s. “It’s very close, around 68 km/s per megaparsec,” said Dr. Mathew Madhavacheril of the University of Pennsylvania. ACT team member Professor Blake Sherwin from the University of Cambridge, UK added: “We, Planck and several other probes are entering the lower side. Obviously you could have a scenario where both measurements are correct and there is some new physics that explains the discrepancy. But we’re using independent techniques, and I think now we’re starting to close the gap where we could all be driving this new physics and one of the measurements has to be wrong.” Papers describing the new results have been submitted to The Astrophysical Journal and posted on the ACT website.