Although dark matter makes up about 85% of the entire mass of the Universe, it is extremely difficult to detect and defies ready description.
It neither emits nor absorbs light. The only way you can very obviously infer its presence is through its interaction with gravity.
Large galaxies of rotating stars would break apart if they didn’t include an unseen mass that pulls them together and keeps them intact.
But dark matter will bend, or lens, the background light, and so its location has been mapped by the Atacama Cosmological Telescope (ACT).
The facility in Chile observed the cosmic microwave background, or CMB, a ubiquitous but faint glow of long-wavelength radiation that comes to us from the edge of the observable Universe.
ACT mapped the subtle distortions in this ancient light that were introduced as it passed through all the intervening matter.
In the same way, the CMB can be decoded to reveal all the structures involved in its journey to us.
There have been similar detections in the past, most notably by the European Space Agency’s Planck Observatory a decade ago. But the ACT surpasses everything in terms of resolution and sensitivity.
Composition of the Universe
Successive experiments indicate that the cosmic content includes:
- 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 the cosmic expansion.
The universe is calculated to be 13.8 billion years old.
Representative gray line
In the image at the top of this page, the colored areas are the portions of the 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 in diameter.
The gray/white areas show where contaminating 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 “world” of the Universe and the rate at which it has expanded after 14 billion years of evolution are exactly what you would expect from the standard model of cosmology, which is based on Einstein’s theory of gravity (general relativity ).
Recent measurements using an alternative background light, one emitted by stars in galaxies rather than the CMB, have suggested that the Universe does not have enough light.
„It’s one of the ‘cosmic tensions’ we’re all talking about”, said Professor JO Dunkley from Princeton University, USA. “But with this new result, we find just the right amount of light – no tension! So if there’s a tension, it’s something that shows up in the galaxy data—not ours“, she told BBC News.
Another tension relates to the rate at which the Universe is expanding, a number called the Hubble constant.
When Planck analyzed temperature fluctuations along the CMB, he determined the rate to be about 67 kilometers per second per megaparsec. A megaparsec is 3.26 million light years.
In other words, the expansion increases by 67km per second for every 3.26 million light-years we look further into space.
A tension arises because of measurements of the expansion in the nearby Universe, made using our recession of variable stars, which amounts to about 73km/s per megaparsec.
It is a difference that cannot be easily explained.
ACT, using its detection technique to reduce the rate of expansion, produces a number similar to Planck’s. “It’s very close – about 68km/s per megaparsec” said Mathew Madhavacheril of the University of Pennsylvania.
TEAM ACT member Professor Blake Sherwin, from the University of Cambridge, UK, added: “We and Planck and a few other probes come on the bottom. Obviously, you could have a scenario where both measurements are correct and there is new physics that explains the discrepancy. But we’re using independent techniques, and I think now we’re starting to close the gate where we could all ride this new physics, and one of the measurements must be wrong.”
Papers describing the new results have been submitted to the Astrophysical Journal and published on the ACT website.
The telescope, which operated from 2007 to 2022 before being dismantled, was funded by the US National Science Foundation. The scientific collaboration has not yet finished analyzing all its data.
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