Will it be Heat Death, the Big Crunch, or the Big Rip?
Long, long ago, in a galaxy far, far, away, a young star was paying the price for a life led too fast and too furious. The mighty sun had ripped through her precious store of hydrogen and briefly worked through the heavier elements until her nuclear furnace went dry; then she blew her starry guts out. The light of her destruction, now part funeral pyre and part grave marker, would thread its way past stellar nurseries and gaudy nebulae for over a hundred thousand years, until a small portion fell on the alien shores of a distant, blue-green planet circling a modest yellow star. There it would come to the attention of a recently evolved denizen that walked on two legs, known by the name of Colin Henshaw.
Prior to the day that violent supernova appeared in the sky, many mysteries of space and time were wrapped up in a pretty box called the Hot Inflationary Big Bang Model. The Big Bang explained why our cosmos is expanding; the Hot Inflationary Model covered how ripples of matter and energy arose in the infant universe to form the first galaxies and stars.
The looming question remaining in cosmology was how fast the universe is expanding, and whether it will end in fire or ice. If the mass of the universe is below a critical magnitude, it will keep expanding forever, and our cosmos will end in Heat Death: a perpetual state of utter black emptiness and cold. The background temperature will gradually approach absolute zero and such will be the fate of the cosmos to the grim end of time. If the mass of the universe is above that critical magnitude, one day it will stop expanding and slowly begin to contract. As the universe collapses, it will grow hotter and denser until time itself ends, and everything is contained in a single point known as a singularity: the Big Crunch. Both pictures are simple and compelling-and, as it turns out, wrong.
In the 1980s, cosmologists measured the universe's rate of expansion to a higher degree of accuracy than ever before. When they extrapolated the rate backward in time they ran into one hell of a paradox: The universe was younger than the oldest stars within it! It would be another decade before science would began to unravel this unwelcome twist. To understand how astronomers eventually made the biggest discovery since the Big Bang itself, let's return to that supernova.
On a balmy South African evening in 1987, our amateur astronomer noticed a tiny brilliant point in the Large Magellanic Cloud; one of two puffs of stars hanging high above the blazing disk of our own Milky Way galaxy. It hadn't been there before. He called up a few observatories and asked them to check it out, certain that the professionals were already aware of the oddity. In that, he was wrong. Within hours of his report, though, every major observatory on Earth locked onto that region in the sky, to witness one of the most beautiful and destructive shows nature can put on. Supernova 1987A had arrived, the nearest to earth in a millennium.
Its legacy would shake the scientific community to its core.
One immediate benefit was that SN1987A demolished Young Earth Creationism, a belief that the universe was created only six thousand to ten thousand years ago. As seen with the eye of the mighty Hubble Space Telescope, the remnant of SN1987A is a single bright dot, surrounded by double offset rings of incandescent debris and a smaller primary ring centered on the core of what had once been the star. Because the apparent width of the ring can be measured, and because the actual diameter can be obtained using basic astrophysics, astronomers can directly calculate the distance to the supernova using simple trigonometry. That distance is 168,000 light-years. And scientists can categorically state that the light from SN1987A has not changed velocity during the transit. The conclusion is straightforward: She blew up 168,000 years ago, or about 160,000 years before Young Earth Creationists claim the universe existed.
But a more significant legacy of SN1987A would leave astronomers picking their collective jaw up off the floor. Observations of SN1987A led cosmologists to a new standard candle (an astronomical object with a known luminosity used to calculate distance) in a certain type of stellar remnant. The new technique allowed them to measure with unprecedented accuracy how fast galaxies are separating from one another. The results were astounding.
After meticulous observation to measure how fast the expansion of the universe was slowing down, the stunning conclusion was that the rate wasn't decreasing at all. The universe was expanding all right, but the rate of expansion was increasing. The universe was accelerating outward! The key to making the equations balance was a mysterious force dubbed "dark energy," which accounts for more than two-thirds of the mass of the entire cosmos. What we think of as "the universe"-stars, planets, light, atoms, and energy-is but a light frothing of what physicists call baryonic matter floating in an invisible sea of dark energy. And since this mysterious force is increasing in magnitude, if unchecked it will grow and grow, until galaxies, stars, planets, atoms, and even black holes are torn asunder: The Big Bang will end in the Big Rip!
Which brings us back to the puzzle of the universe being younger than the oldest stars within it. The formerly accepted estimate for the age of the universe was based on the false assumption that the expansion was slowing down. That age is a bit less than the new figure arrived at by assuming the rate of expansion is increasing. This explains the discrepancy between the age of the universe and the oldest stars within it. And although astronomers and physicists are now at an absolute loss to explain dark energy, at least the conundrum of old stars in a younger universe is cleared up.
Serendipity is waiting to strike again: The tantalizing clues into the nature of the dark-energy phenomena hint that, once resolved, the results will be as significant as when Isaac Newton was conked on the head with an apple.
References: Fact-checked with Dr. Sean Carroll, Assistant Professor of Physics, University of Chicago
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