It now seems that our entire universe is living on borrowed time. How long it can survive depends on whether Stephen Hawking’s theory checks out.   Time is flying by on this busy, crowded planet... as life changes and evolves from second to second. And yet the arc of human lifespan is getting longer: 65 years is the global average ... way up from just More..More.. 20 in the Stone Age. Modern science, however, provides a humbling perspective. Our lives... indeed the life span of the human species... is just a blip compared to the age of the universe, at 13.7 billion years and counting. It now seems that our entire universe is living on borrowed time.. And that even it may be just a blip within the grand sweep of deep time. Scholars debate whether time is a property of the universe... or a human invention. What's certain is that we use the ticking of all kinds of clocks... from the decay of radioactive elements to the oscillation of light beams... to chart and measure a changing universe... to understand how it works and what drives it. Our own major reference for the passage of time is the 24-hour day... the time it takes the Earth to rotate once. Well, it's actually 23 hours, 56 minutes and 4.1 seconds... approximately... if you're judging by the stars, not the sun. Earth acquired its spin during its birth, from the bombardment of rocks and dust that formed it. But it's gradually losing that rotation to drag from the moon's gravity. That's why, in the time of the dinosaurs, a year was 370 days... and why we have to add a leap second to our clocks about every 18 months. In a few hundred million years, we'll gain a whole hour. The day-night cycle is so reliable that it has come to regulate our internal chemistry. The fading rays of the sun, picked up by the retinas in our eyes, set our so-called "circadian rhythms" in motion. That's when our brains begin to secrete melatonin, a hormone that tells our bodie...