The Milky Way galaxy: the blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter. The blue sphere centered on the Sun's position shows the approximate size of the newly surveyed volume, but not its precise shape.
The Milky Way galaxy: the blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter. The blue sphere centered on the Sun's position shows the approximate size of the newly surveyed volume, but not its precise shape. L. Calçada/ESO
There are theories with a small "t" and theories with a capital "T." For the last two decades the concept that most stuff in the universe comes in an invisible form called Dark Matter has grown into the second kind. It's a Theory or a paradigm with lots of overlapping strands of evidence coming together to form a tightly woven fabric. Much of modern astronomy relies on the fabric of Dark Matter for a coherent account of the universe and its history.
It's not easy to overturn Theories because their reach is so wide. It does happen however and, when it does, there are usually key experiments that get the ball rolling. For Dark Matter, it just may be possible that one of those experiments has made its first public appearance.
This week a team of astronomers from the European Southern Observatory (ESO) released a study of stellar motions in the local volume of the Milky Way, our galaxy. Based on the way these stars moved (the way gravity pulled them into motion), the scientists hoped to infer the presence of Dark Matter. They did not find any.
None. Nada. Zip. Zero.
As the ESO press release puts it:
"The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighborhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful."
As team leader Christian Moni Bidin puts it:
"The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun. But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!"
This really appears to be a reasonable study by a reasonable group of scientists. So does that mean Dark Matter is dead? Not by a long shot. Remember, there are many other lines of evidence for Dark Matter. No single paper can knock it out of the ring so easily. But, as a Theory, Dark Matter is nowhere as solid as, say, Relativity, which was challenged by the faster-than-light neutrino results of a few months ago. I would never have bet against relativity when those results came out. With Dark Matter I am more wary. We have, after all, never directly detected even a single particle of dark matter. That means as a Theory it remains more vulnerable than Einstein's remarkable achievement.
The ESO result may eventually be explained away. But if the result stands up to scrutiny — and others like it begin to appear — then the Dark Side (matter at least) may find itself in deep trouble.
You can keep up with more of what Adam Frank is thinking on Facebook and Twitter. His latest book is About Time: Cosmology and Culture at the Twilight of the Big Bang.