What killed the dinosaurs? Dark matter, says theoretical physicist Lisa Randall

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A number of theories have been advanced for what killed off the dinosaurs more than 66 million years ago, but until 2014 none involved dark matter and meteors.

In her book, Dark Matter and the Dinosaurs, acclaimed Harvard theoretical physicist Lisa Randall outlines a complex – and radical theory – that goes something like this: about 66 million years ago, gravitational perturbations caused by a thin pancake-shaped disc of dark matter in the Milky Way galaxy dislodged icy comets in the Oort cloud at the very edge of the known solar system, resulting in the fiery meteoroid that eventually crash-landed in the Yucatan, leading to the mass extinction of more than 75 per cent of life on the planet in the process.

The key to this theory, of course, is dark matter, which remains one of the mysterious components of the known universe, despite accounting for 85 per cent of all matter in the universe. Ordinary matter – the stars, the planets and the chair you’re sitting in right now – accounts for only 15 per cent of all known matter in the universe. That’s why solving the mystery of dark matter is so important.

The problem is that dark matter does not interact with ordinary matter except via gravity. You can’t “see” dark matter because light passes right through it: dark matter neither absorbs nor emits light. And as Randall points out, you can’t measure dark matter directly, you can’t feel it and you can’t smell it. Yet billions of dark matter particles pass through us every second and dark matter appears to permeate the entire cosmos.

As Randall points out in Dark Matter and the Dinosaurs, we can glimpse dark matter only indirectly. That’s because dark matter does have measurable gravitational effects, which results in some interesting phenomena: the expansion rate of the universe, the bending of the path of light rays and irregularities in the orbits of stars.

In short, dark matter must exist because some astronomical observations just don’t make sense if it doesn’t. One example that Randall cites as evidence of dark matter is the Bullet Cluster, which was formed by the merging of at least two galaxy clusters. The problem is that the Bullet Cluster doesn’t look the way it should. That’s because dark matter appears to have trapped gases in bulbous clusters, giving the Bullet Cluster the appearance of having giant Mickey Mouse ears.

In making the case for dark matter, Randall often uses pop culture references to make her point. In describing how we know that dark matter must exist even if we can’t see it, she invokes the example of what happens when a famous person walks down the footpaths of Manhattan.

“Even if you don’t see George Clooney directly, the disruptive traffic generated by the waiting crowd armed with cellphones and cameras suffices to alert you to a celebrity’s proximity,” Randall writes. “Though you detect the presence only indirectly, through George’s substantial influence on everyone else around, you can nonetheless be confident that someone special is near.”

That’s what it’s like when researchers try to get to the heart of the mystery surrounding dark matter. The celebrity-on-the-footpaths-of-Manhattan example is actually an important analogy, because it sets up the rest of the book – the story of dark matter’s ability to interact with ordinary matter in ways that can be felt, but not seen.

In this case, it’s a “dark disc” of densely packed dark matter embedded within the plane of the Milky Way that occasionally exerts outsized gravitational tugs on objects. Every now and then (every 30 million years, a brief moment in time for astrophysicists), the combined gravitational tug is so great that it dislodges big icy objects from their orbits in the Oort cloud, which is located at the very edge of the solar system, as far as 50,000 astronomical units (AU) away from the sun.

The rest, as they say, is history. Sooner or later, one of those big, icy objects rumbling through the solar system is bound to hit our planet. In fact, Randall acknowledges in her book that one such event accelerated her search for a dark matter theory of dinosaur extinction – the flaming meteoroid that exploded over Siberia in February 2013. That was a 18-metre-wide chunk of space rock weighing 10,000 metric tons. By way of comparison, the meteor that hit the Yucatan 66 million years ago was more than 14 kilometres long.

That may not sound like a big difference in size, but you’re forgetting about Einstein’s famous equation for translating mass into energy. The Chelyabinsk meteor generated an explosion equivalent to 500 kilotons of TNT. A big enough object travelling at a speed of 18 kilometres per second (about 60 times the speed of sound) hitting the Earth would be big enough to trigger a massive extinction event.

What makes the theory of dark matter and the dinosaurs so compelling, Randall says, is the remarkable periodicity of meteor strikes hitting Earth. If meteor strikes were just a one-and-done event, the idea of a “dark disc” in the Milky Way would be suspect. But as Randall outlines in chapter 14, there’s a remarkable pattern to meteor strikes and extinction events. About every 30 million years – about the time it takes the solar system to oscillate through the plane of the Milky Way and experience the gravitational pull of the “dark disc” – you can expect another meteor hitting Earth and a potential extinction event.

That’s a remarkable coincidence, but is it just that – a coincidence? Even Randall admits that, “Dark matter and dinosaurs are words you rarely hear together except perhaps in the playground, a fantasy gaming club, or some not-yet-released Spielberg movie.” But she hopes to change the way we think about dark matter. She jokingly refers to “ordinary-matter chauvinists” who think the entire known universe is all about ordinary matter. It’s hard, she says, to ignore dark matter when it accounts for 85 per cent of all matter.

If all goes according to plan, that means more innovation in the field of dark matter research. Randall notes that there are several different approaches for figuring out the mysteries of dark matter, with the most common ones – such as the Large Underground Xeon Detector (LUX) – involving dark matter detection facilities located deep underground. And new space satellites such as Gaia could offer new clues to the ultimate composition of the universe, including dark matter.

For now, Dark Matter and the Dinosaurs may be more conjecture than theory, but the book could be valuable in spurring further innovation in the field of dark matter research. Having “dinosaurs” in the title certainly helps. If all really goes according to plan, Randall suggests, Dark Matter and the Dinosaurs might be a way to unite many different disciplines – including particle physics and astrophysics – and prove the interconnectedness of the universe at both very large and very small scales.

Source:smh.com.au

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