DARK MATTER IS IT REAL?
Does the elusive substance really make up most of the mass in our universe? All About Space discovers that some physicists and astronomers are starting to have doubts
For almost a century, astronomers have believed that our universe is dominated by something we cannot see – a type of matter that neither emits nor absorbs light and other forms of electromagnetic radiation, and which reveals itself only through the influence of its gravity on objects we can directly observe. This ‘dark matter' is thought to vastly outweigh the 'normal matter' that emits and interacts with light – it makes galaxies and larger cosmic objects behave as if they have, on average, about six-times the mass suggested by their visible stars, gas and dust. And yet, decades of attempts to directly detect this mysterious matter have failed to produce any
“The stars were moving faster than predicted by Newtonian gravity” Margot Brouwer
evidence of their existence.
So is the ‘dark matter paradigm’ an accurate way of looking at our universe? Support for alternative theories, which instead rewrite long-standing laws of physics, is growing – and a recent astronomical study has offered the first observational evidence that one of these rebel theories is a remarkably good match for reality. Is dark matter living on borrowed time?
“The story started decades ago,” explains Margot Brouwer of the University of Leiden, whose recent research has helped fuel the debate. “In the early 1930s, Dutch astronomer Jan Oort was studying how fast the stars in our Milky Way galaxy were moving. When he compared that to calculations based on Newton’s laws of gravity, he found that they weren’t moving in the way he expected – they’re actually moving too fast.”
Isaac Newton’s laws of motion and gravity, published in 1687, describe how objects move under the influence of forces, including gravity. One of their most important consequences is that the speed of a small object orbiting a more massive one is related to both the distance between them, and the mass at the centre. The heavier the central mass, the faster the orbiting object must move at a given distance, so faster orbits for stars in the outer reaches of the galaxy implied more mass in the centre. The only problem was that the measurable amount of stars, gas and dust in the galactic core was only a small fraction of that required to produce such orbits.
“Later, lots of other scientists started to see the same thing,” continues Brouwer. “In 1933, Swiss astronomer Fritz Zwicky was studying the speed of galaxies moving in large clusters, and he found they were also moving too fast. And then, in the 1970s, US astronomer Vera Rubin measured how fast galaxies were rotating around their centre, and she also found that the stars were moving faster than predicted by Newtonian gravity.”
Rubin’s work on what became known as the ‘galaxy rotation problem' finally convinced most astronomers to take the issue seriously. Most agreed that the unexpected orbits really were due to the gravity of undetected matter (which Zwicky had named ‘dark matter’ as early as 1933). “If you look at the entire universe,” explains Brouwer, “there has to be about five times as much of this ‘dark matter’ as
“Of course there’s still a hope that one day they’ll track down dark matter particles” Margot Brouwer
the normal matter we can see, in order to make our calculations work using the framework of Newton and Einstein’s gravity.”
But what exactly was this matter? Many astronomers hoped it would turn out to be unseen normal matter – compact objects such as rogue planets and burnt-out stellar cores that could hold a lot of mass while rarely interfering with the light from other objects. But prolonged searches in and around our own galaxy found very few of these objects, and instead, most scientists reluctantly concluded that dark matter must be something very strange indeed – vast clouds of particles that permeate the universe but are somehow immune from all interactions with light.
The search for dark matter particles has continued for decades, but so far no suitable candidate has been directly detected. Astronomers have found supporting evidence for the influence of particle dark matter in collisions between galaxy clusters, and even the formation of large-scale cosmic structure in the early universe, but the particles themselves remain elusive.
Even as early as the 1970s, however, there was a minority view that missing forms of matter were not the only answer to the galaxy rotation problem.