Evrim Yazgin
Cosmos science journalist
Most of the universe still isn’t accounted for – the solution might be in black holes.
Ordinary matter makes up only about 5% of the universe. This includes everything we can see and interact with, from stars and planets to dust, gas and you.
About 25% of the universe is made up of dark matter. This matter can’t be seen and barely interacts at all with ordinary matter. We only know it should be there because of observations of how gravitational fields operate in galaxies.
But a whopping 70% of the universe is made up of an even more elusive substance called “dark energy”.
Dark energy is what cosmologists believe is driving the accelerating expansion of the universe. It is thought to have emerged 13.8 billion years ago when the cosmos began its inflation.
Scientists are trying to run the clock back to the Big Bang to understand dark energy.
“If you ask yourself the question, ‘Where in the later universe do we see gravity as strong as it was at the beginning of the universe?’ the answer is at the centre of black holes,” said Gregory Tarlé, professor of physics at the University of Michigan in the US.
Tarlé is co-author of a paper published in the Journal of Cosmology which suggests that studying black holes could reveal the conditions which led to the emergence of dark energy.
“It’s possible that what happened during inflation runs in reverse, the matter of a massive star becomes dark energy again during gravitational collapse – like a little Big Bang played in reverse,” Tarlé says.
The theory is strengthened by observations from the Dark Energy Spectroscopic Instrument (DESI). The instrument is made up of 5,000 robotic eyes mounted on the Mayall telescope at the Kitt Peak National Observatory in southern Arizona.
Recent studies have put a pin in the idea that black holes make up a significant proportion of dark matter. But the new research suggests that they may have a lot to do with dark energy.
“If black holes contain dark energy, they can couple to and grow with the expanding universe, causing its growth to accelerate,” says lead author Kevin Croker from Arizona State University. “We can’t get the details of how this is happening, but we can see evidence that it is happening.”
DESI data of tens of millions of distant galaxies can be used to calculate how fast the universe is expanding at different points in its history. This is used to infer how the amount of dark energy is changing through time.
Croker’s team compared this with the number of black holes created by the deaths of large stars over time.
“The two phenomena were consistent with each other – as new black holes were made in the deaths of massive stars, the amount of dark energy in the universe increased in the right way,” adds co-author Duncan Farrah from the University of Hawai‘i. “This makes it more plausible that black holes are the source of dark energy.”
Unlike previous studies looking at the link between black holes and dark energy, the new research doesn’t just look at black hole growth rate, but their birth. It also investigates black holes that are younger than in previous studies.
“This will only bring more depth and clarity to our understanding of dark energy, whether that continues to support the black hole hypothesis or not,” Ahlen says. “I think as an experimental endeavour, it’s wonderful. You can have preconceived notions or not, but we’re driven by data and observations.”
“Fundamentally, whether black holes are dark energy, coupled to the universe they inhabit, has ceased to be just a theoretical question,” Tarlé said. “This is an experimental question now.”
