It's painful to say this, but our universe is fading, not that any of us would notice. Looking up at the sky, the stars are not disappearing from sight (in spite of regular apocalyptic predictions rippling through our culture). But the rate at which the cosmos makes new stars has been declining sharply over the past few billion years. The age of splendor, of countless lights born anew in the heavens, is long gone.
In a wide-ranging study published this November in the British journal Monthly Notices of the Royal Astronomical Society, an international team of astronomers led by David Sobral from the University of Leiden, used three huge telescopes to measure the rate of star formation at different times, from very far in the past to more recent times.
To do this, and since telescopes can't resolve individual stars in galaxies billions of light-years away, the astronomers focused on particular kinds of radiation emissions that are abundant when stars are born. If galaxies are actively making stars, the emissions intensify accordingly. They sampled the same types of galaxies at four different epochs: at 4.2, 7.0, 9.2 and 10.6 billion years ago. (Recall that our universe is 13.7-billion-years old, the time elapsed since the big bang.)
What they found is discouraging. Equating the rate of star formation to our cosmic GDP, "the cosmic GDP output is now only 3 percent of what it used to be at the peak in star production," said Sobral. "If the measured decline continues, then no more than 5 percent more stars will form over the remaining history of the cosmos, even if we wait forever. The research suggests that we live in a universe dominated by old stars." In other words, 95 percent of the stars that our universe will make in its history have already been made.
Stars started forming only about 300 million years after the big bang. Those first stars were giants, hundreds of times bigger than our sun. Overweight stars live short lives, and these progenitors exploded within millions of years, spreading their guts into interstellar space. These stellar cataclysms spurred the birth of new stars and sprinkled space with heavier chemical elements, including carbon, oxygen, sulfur and others vital for life. As the late cosmologist Edward Harrison said, "people is what happens to hydrogen if you wait long enough." We are, in a very definite sense, children of these stars. And we are lucky to be able to witness our cosmos in its glory. Another 10 billion years from now, the sky would not be as inspiring.
The peak of star formation occurred about 11 to nine billion years ago, when about half of the stars were made. It took five times longer to make the other stars. Now, only five percent are left to be made. The great gravitational engine that compresses hydrogen clouds into energy-churning stars has lost most of its steam.
Unless, of course, the universe reverses its expanding trend and starts recollapsing upon itself due to its own attraction. In this case, matter will be compressed again and much can happen.
Unfortunately, current measurements indicate that the universe will continue to expand, possibly into endless time, picking up speed as it goes. This being the case, as Robert Frost speculated in his poem Fire and Ice, ice will win.
But I take inspiration from another poet, Dylan Thomas, who declared war on decay:
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
If only the universe would listen.
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