Guide to the Cosmos

 Making the Wonders of our Universe Accessible to Everyone


  Did Dark Matter Doom the Dinosaurs?


In 1905, an uncredentialed outcast made the outlandish claim that trains got shorter as they went faster. “What a Fool” people said. But after his crazy ideas were proven, Albert Einstein became the most famous scientist in history.


Inspired by his success, countless physicists and fools (not entirely disjoint groups) have made their own outlandish claims. Few are remembered.


You be the judge of this intriguing proposal.


Lisa Randall says the comet that killed the dinosaurs 65 million years ago, along with most life on Earth, was launched on its course with destiny by dark matter.


The demise of the dinosaurs led to the ascension of mammals. Do we owe our position at the top of the food chain to far out physics?



Randall is no uncredentialed outcast. She’s been a Full Professor at Princeton, MIT, and Harvard, and is one of the brightest and most creative living physicists. And in her signature black leather motorcycle jacket, no physicist looks as sharp as Lisa does.


In her new book, Dark Matter and the Dinosaurs, Randall skillfully argues that our galaxy harbors a concentrated disk of dark matter. As the Sun periodically passes through that disk, she says comets are sent careening toward the inner Solar System, and us.


Here’s a 300-word summary of her 400-page book:


A thorough yet accessible discussion of dark matter (“DM”) is in Our Universe 3: CMB, Inflation & Dark Matter. The highlights are:


(1) Stars, planets, and we are made of normal matter, but 85% of all the matter in our universe is something else: DM.

(2) DM exerts gravitational forces, but is invisible because it does not emit or absorb light.

(3) Normal matter cools by emitting light, allowing it to condense into stars. DM seems unable to do that: DM can’t clump.


Randall’s clever idea is that not all DM is created equal. Perhaps, like normal matter, DM is varied, with different types of dark particles interacting with different dark forces. Just as the Milky Way’s stars (made of normal matter) condensed into a flat disk, perhaps some few dark particles can clump, forming a dark disk. The dark disk would be very thin but very massive.


As our Solar System circles the center of the Milky Way, it also bobs up and down through the galactic disk. Passing through Randall’s massive, thin dark disk would shock our Solar System with rapidly changing gravitational forces.


At the periphery of our Solar System lie countless comets (the Oort Cloud) that are weakly held in their orbits. Randall’s periodic gravity shock could dislodge some Oort comets that might then fall toward the Sun, possibly colliding with a wonderful blue planet that just happens to be in the wrong place at the wrong time.


These opportunities for global devastation, Randall says, occur every 32 million years. She claims the five major extinctions of life on Earth roughly match this periodicity. If so, we are now near the end of the latest Doom Time.




Most outlandish claims invoke forces, particles, dimensions, or even entire universes that are undetectable by design. Job security is enhanced if you can’t be proven wrong.


But Randall says her theory is testable, even without a catastrophe. Her dark matter disk would alter the orbits of stars near the galactic plane in a way that our newest satellites will soon precisely measure and provide us with proof.


When judging Randall’s theory, consider the words of Nobel Laureate Wolfgang Pauli: “This idea is crazy. But is it crazy enough to be true?”





Best Regards,



April 5, 2016


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