The end of the world as we know itAstronomer Chris Impey, in The Independent: What kind of catastrophe would it take to end the world?
Astronomical intruders provide a potentially serious threat... Every century or so, a 10-meter meteor slams into the Earth with the force of a small nuclear device... Every few thousand years Earth can pass through unusually thick parts of the debris trail of comets, turning the familiar light show of a meteor shower into a deadly firestorm. Roughly every 100,000 years, a projectile hundreds of meters across unleashes power equal to the world's nuclear arsenals. The result is devastation over an area the size of England, global tidal waves (if the impact is in the ocean), and enough dust flung into the atmosphere to dim the Sun and kill off vegetation. That could ruin your day.
Then there's the "Big One.' About every 100 million years, a rock the size of a small asteroid slams into the Earth, causing global earthquakes, kilometre-high tidal waves, and immediately killing all large land animals. Creatures in the sea soon follow, as trillions of tons of vaporised rock cause drastic cooling and the destruction of the food chain based on photosynthesis...
When massive stars exhaust their nuclear fuel, the result is a titanic explosion called a supernova. The dying star brightens to rival an entire galaxy and emits high-energy particles that can destroy the ozone layer of a planet like Earth if it occurs within 30 light years...
A supernova is a small squib compared to a hypernova. In this dramatic and rare event, the violent collapse of a very massive star ejects jets of gas and high-energy particles at close to the speed of light, and for a few moments the star outshines the entire universe in gamma rays. If a hypernova went off within 1,000 light years, and Earth was within the narrow cone of high-energy radiation, we'd experience an immediate global conflagration.
On longer time scales, attention turns to the sheltering Sun. Our constant companion is midway through its conversion of hydrogen into helium. In about 5 billion years, its guttering flame will be extinguished. The Sun's diffuse envelope will engulf the Earth and turn it into a lifeless cinder... The biosphere will actually die much sooner. The Sun burns hot as it gets older, and in 500 million years a turbocharged version of global warming will turn the Earth into a global desert...
The end of the Milky Way will come slowly, in a stellar lockdown... In galaxies across the universe the lights will gradually go out, and after tens of trillions of years the universe will have faded to black...
Fifteen years ago, it was discovered that the cosmic expansion is getting faster. The cause is inferred to be dark energy -- a manifestation of the pure vacuum of space that has an effect opposite to gravity... If dark energy grows, it will cause the universe to unravel in about 20 billion years in a crescendo called the "Big Rip.' First galaxies, then stars, and finally atoms will be torn asunder... Nothing can survive; it's an outcome of crushing finality.
Absent the big rip, cosmic acceleration will steadily remove galaxies from view... On even longer time scales, familiar gravitational structures become unglued... Planets detach from their dead stars and drift through interstellar space... The proton is not stable and will decay... The decay of protons heralds a final drawn-out phase of disintegration... as everything falls apart...
We imagine the last inhabitants of the universe huddled around the evaporative glow of gamma rays from the last black hole, telling timeless stories about time. It was fun while it lasted.
Chris Empey, How it Ends: From You to the Universe (Norton)
