RCs: 71 to 80 Questions for CAT with Answers PDF

Passage

Barely a century ago, conventional wisdom held that our universe was static and eternal. Now, we can feel smug for having discovered the underlying expansion of the universe, the cosmic microwave background, dark matter and dark energy. But what will the future bring?
Poetry ... of a sort.
The domination of the expansion of our universe by dark energy was inferred from the fact that this expansion is speeding up. Our observable universe is at the threshold of expanding faster than the speed of light. And with time, because of the accelerated expansion, things will only get worse.
This means that, the longer we wait, the less we will be able to see. Galaxies that we can now see will one day in the future be receding away from us at faster-than lightspeed, which means that they will become invisible to us. The light they emit will not be able to make progress against the expansion of space, and it will never again reach us. These galaxies will have disappeared from our horizon.
The way this works is a little different than you might imagine. The galaxies will not suddenly disappear or twinkle out of existence in the night sky. Rather, as their recession speed approaches the speed of light, the light from these objects gets ever more redshifted. Eventually, all their visible light moves to infrared, microwave, radio wave, and so on, until the wavelength of light they emit ends up becoming larger than the size of the visible universe, at which point they become officially invisible.
We can calculate about how long this will take. Since the galaxies in our local cluster of galaxies are all bound together by their mutual gravitational attraction, they will not recede with the background expansion of the universe. Galaxies just outside our group are about 1/5000th the distance out to the point where the recession velocity of objects approaches the speed of light. By about 2 trillion years, their light will have become completely invisible, and the rest of the universe, from the perspective of our local cluster, will literally have disappeared.
Two trillion years may seem like a long time, and it is. In a cosmic sense, however, it is nowhere near an eternity. The longest living ‘main sequence’ stars have lifetimes far longer than our Sun and will still be shining in 2 trillion years (even as our own Sun dies out in about only 5 billion years). And so in the far future there may be civilizations on planets around those stars. And there may be astronomers with telescopes on those planets. But when they look out at the cosmos, essentially everything we can now see, all 400 billion galaxies currently inhabiting our visible universe, will have disappeared!
In any case, those astronomers in the far future would be in for a big surprise, if they had any idea what they were missing, which they won’t. Because not only will the rest of the universe have disappeared, but essentially all of the evidence that now tells us we live in an expanding universe that began in a Big Bang will also have disappeared, along with all evidence of the existence of the dark energy that will be responsible for this disappearance.
While less than a century ago conventional wisdom still held that the universe was static and eternal, in the far future, long after any remnants of our planet and civilization have likely receded into the dustbin of history, the illusion that sustained our civilization until1930 will be an illusion that will once again return, with a vengeance.