by Mark Aragona
You don’t feel it, you don’t see it, but the universe is slowly growing colder and darker.
Using powerful Mopra radio telescopes, scientists in Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) have discovered that about a third of the molecular hydrogen that stars use as fuel has already been used up. By comparing light densities now to how they were 5 billion years ago, they have concluded that less stars are being born now than before. Apparently, the universe had reached its star-birthing peak after a few billion years and is now in the decline.
“We’ve seen a decline in the amount of stars being formed by more than a factor of 10, probably closer to 20 or even 30,” says Robert Braun, lead scientist of CSIRO. “It turns out that these galaxies actually had 10 times more gas with which to form the stars than they do today. We just aren’t seeing as much gas fall in to form the new stars.”
Braun’s team came to this conclusion by comparing older, more distant galaxies with the ones nearby. Galaxies burn interstellar gases they attract from the space in between them, but over time they tend to lose gas, particularly during events such as supernovae.
What’s more, the universe continues to rapidly expand due to the presence of dark energy, which has taken over the cosmos a few billion years back. Dark energy counteracts gravitational force and causes galaxies to accelerate away from each other, making it harder for them to find much needed molecular hydrogen to refuel their stars.
So what are the likely scenarios for an ever-dimming universe? It depends on whether the universe’s rate of expansion will remain as is or if it will accelerate.
Scientists say that the most likely event will be the Big Freeze, stated to happen if the universe maintains its rate of expansion. At around 1014 years after the Big Bang, no more stars will be born. For many more billions of years after that, all other existing stars will burn out, and even black holes dry up as the universe reaches the point of entropy. The universe will be nearly empty, save for electrons, positrons and dark matter.
Another possible scenario, described by a paper called “Phantom Energy and Cosmic Doomsday,” depends on the kind of dark energy existing in the universe. If it happens to be phantom energy, where the sum of the energy pressure and density is negative, the universe will expand at an exponential rate until it reaches singularity, opposite that of the center of a black hole. Phantom energy’s expansion will be so great that it will tear apart everything in the universe. First galaxies will pull away from each other, then the Milky Way will dissipate, followed by our own solar system. In the last few moments, the rate of expansion will reach infinity, pulling apart atoms, nuclei, and subatomic particles. This scenario is aptly called the Big Rip.
It’s strange and humbling to think that something as vast as the universe still has a lifespan. Perhaps humans won’t be around to witness it, perhaps they will. Perhaps we will even find some way to endure even when the last star fizzles out, as in Asimov’s short story, “The Last Question.” Or maybe another version of the universe will come into being. We may never know, but we can always dream.