Mysterious dark matter still mapped in minute detail

The ACT map covers a quarter of the sky

In the mapped areas, more matter is colored orange; less matter is colored magenta

It is being described as the most detailed map ever of the influence of dark matter throughout cosmic history.

A telescope in Chile has traced the distribution of this mysterious matter over a quarter of the sky and across nearly 14 billion years of time.

The result is once again a spectacular confirmation of Einstein’s ideas.

Although dark matter makes up about 85% of all mass in the Universe, it is extremely difficult to detect and defies ready description.

But dark matter affects the large-scale structure of everything we see – where all the galaxies are, where the voids in space are. It is the scaffolding on which the visible structure of the Universe hangs.

It neither emits nor absorbs light. The only way you can obviously infer its presence is through its interaction with gravity.

Large rotating galaxies of stars would separate were it not for the inclusion of some invisible mass that attracts them and keeps them intact.

But the dark matter will bend, or slow down, the background light, which is how its location has been mapped by the Atacama Cosmology Telescope (ACT).

Atacama Cosmological Telescope

The telescope was positioned at an altitude of 5,200 m in the Atacama

The Chile facility observed the Cosmic Microwave Background, or CMB, a pervasive but faint glow of long-wavelength radiation that reaches us from the far edge of the observable Universe.

ACT mapped the subtle distortions in this ancient light that were introduced as it passed by all intervening matter.

You can liken it to the way light is bent as it passes through the bulges and bumps of an old glass window.

If you know what you’re looking at outside, you can use distortions to say something about the glass.

Likewise, the CMB can be decoded to reveal all the intermediate structure on its way to us.

There have been detections of “gravity lensing” similar to this one in the past, most notably by the European Space Agency’s Planck Observatory a decade ago. But ACT surpasses all in terms of resolution and sensitivity.

Composition of the Universe

Later experiments indicate that 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 sky studied by the telescope.

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 gray/white areas show where light contaminating dust in our Milky Way galaxy has obscured deeper vision.

The distribution of matter agrees very well with scientific predictions.

The ACT observations indicate that the “bumpiness” of the Universe and the rate at which it has expanded after 14 billion years of evolution are just what you would expect from the Standard Model of cosmology, which has Einstein’s theory of gravity (relativity general) at its foundation.

Recent measurements using an alternative background light, that emitted by stars in galaxies rather than the CMB, had suggested that the Universe lacked sufficient clutter.

“It’s one of the ‘cosmic tensions’ we all talk about,” said Prof. Jo Dunkley of Princeton University, USA. “But with this new result, we find exactly the right amount of irregularities – no strain! So if there’s a strain, it’s something that shows up in the galaxy’s data – not ours,” he told BBC News.

Warped window graphics

Warped window graphics

Another tension concerns the rate at which the Universe is expanding, a number called the Hubble constant.

When Planck observed the temperature fluctuations across the CMB, he determined the velocity was about 67 kilometers per second per megaparsec (one megaparsec is 3.26 million light-years).

In other words, the expansion increases by 67 km per second for every 3.26 million light-years we look further into space.

A tension arises because measurements of the expansion in the nearby Universe, made using our own recession of variable stars, arrive at about 73 km/s per megaparsec.

It is a difference that is not easily explained.

ACT, employing its lensing technique to nail down the rate of expansion, produces a number similar to Planck’s. ‘It’s very close – about 68 km/s per megaparsec,’ said Dr Mathew Madhavacheril of the University of Pennsylvania.

ACT team member Blake Sherwin, from the University of Cambridge, UK, added: ‘We, Planck and many other probes are getting on the lower side. Obviously, you could have a scenario where both measurements are correct and there’s new physics that explains the discrepancy. But we’re using independent techniques, and I think now we’re starting to close the loophole where we could all be riding this new physics and one of the measurements has to be wrong.”

Papers describing the new results were submitted to The Astrophysical Journal and posted on the ACT website.

The telescope, which operated from 2007 to 2022 before being decommissioned, was funded by the US National Science Foundation. The scientific collaboration has yet to finish analyzing all of its data.

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