Another more up-to-date theory?
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."
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.
By Dominic Basulto
With many thanks to The SMH
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