Word: cosmically
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...increasingly uncomfortable, and eventually become unlivable. Calculating how and when this will happen is a genuinely dismal science, but not without a certain grim fascination. The classic Big Bang theory, refined over the decades since the astronomer Edwin Hubble discovered the expanding universe in 1929, suggests that cosmic destiny will be decided through a tug-of-war between two opposing forces. One is the expansion of space, which for more than 10 billion years has been carrying galaxies ever farther apart from one another. The other is the mutual gravitational force exerted by those galaxies and all the other stuff...
...into heavier ones. When the hydrogen and helium run low, old stars will sputter out without any new ones to take their place, and the universe will gradually fade to black. Such were the gloomy alternatives that Robert Frost wrote about after being briefed on the theory of the cosmic endgame by the astronomer Harlow Shapley...
...found another possible mechanism for time travel using cosmic strings, thin strands of energy millions of light-years long, predicted by some theories of particle physics (but not yet observed in the universe). You could try to construct a cosmic-string time machine by finding a large loop of cosmic string and somehow manipulating it so it would contract rapidly under its own tension, like a rubber band. The extraordinary energy density of the string curves space-time sharply, and by flying a spaceship around the two sides of the loop as they pass each other at nearly the speed...
...back in time by one year, unfortunately, you'd need a loop containing about half the mass-energy of an entire galaxy. Worse yet, the contracting cosmic-string loop would probably trigger the formation of a rotating black hole, trapping any time-travel regions inside. You would almost certainly be torn apart by near infinite space curvature before you could travel anywhere...
...Earthlings are newcomers to cosmology, the study of the universe as a whole, and there is no cosmological question about which we have more to learn than the riddle of where it's all ultimately headed. But we have glimpsed at least a few clues to cosmic destiny, some of them hopeful and others bleak...