The universe just isn't what it used to be. Billions of years ago, when the universe was still in its infancy, the casual observer would have seen stars forming all around him, and at a pretty rapid clip. In recent years, however (i.e. the last few billion), the rate of star formation has slowed to a fraction of its former pace. Now, a team of Australian astronomers believes it knows why.
Astrophysicists have known for some time now that the rate of star formation has been steadily declining ever since it peaked sometime early on in the universe's roughly 13 billion-year existence. They know this because even though light moves pretty fast, the Universe is also extraordinarily huge, and when light from a distant star or galaxy has to travel vast distances, it can still take a while (like five billion years) to get where it's going.
In other words, when astronomers observe the light and radio waves arriving on Earth from distant galaxies, they're actually getting a glimpse at these galaxies as they existed billions of years ago; and, in looking to the past, astronomers see significantly more stars.
Using the ability to spy on galaxies past to their advantage, a team of scientists at CSIRO Astronomy and Space Science in Australia measured the chemical makeup of distant galaxies by observing shifts in the wavelength of their ancient (but recently arriving) light.
The astronomers discovered that the light from galaxies 3—5 billion years ago suggests those galaxies contained about 10 times as much hydrogen gas as comparable galaxies in the present day universe. Because stars develop out of clouds of molecular hydrogen, the researchers believe the relative lack of hydrogen in present-day galaxies has, in turn, led to a decline in star formation.
"Our result helps us understand why the lights are going out," said Dr. Robert Braun, who led the study that is published in the most recent issue of Monthly Notices of the Royal Astronomical Society. "Star formation has used up most of the available molecular hydrogen gas."
But stars don't stay stars forever, right? Over the course of many years, all stars will eventually run their course, often going out with a bang in the form of a supernova. In doing so, they return vast quantities of gaseous elements (including hydrogen) to the interstellar medium — shouldn't this be enough to sustain the future formation of stars?
Unfortunately, no. As Braun explains, stars tend to take from the interstellar medium more than they give back:
Most of the original gas — about 70% — remains locked up, having been turned into things such as white dwarfs, neutron stars and planets...So the molecular gas is used up over time. We find that the decline in the molecular gas is similar to the pattern of decline in star formation.
Braun believes that the relative scarcity of hydrogen (and, by extension, stars) in recent galaxies is merely a symptom of a larger Universal event, namely the effects of something known as dark energy.
Astronomers believe that as much as two thirds of the universe's gas supply actually remains untapped by star formation, but this gas is thought to lie in the regions of space separating galaxies from one another — a region known as the intergalactic medium. Dark energy, in turn, is hypothesized to be a form of energy that increases the rate of expansion of the universe. As the universe expands more and more rapidly, less and less of the free gas in the intergalactic medium is available for capture by galaxies in need of a re-up on their star fuel — hence the decline in star formation.
So what does the future of the universe hold? If the dynamics of gas distribution throughout the universe continue on their current course — and there's no reason to suspect that they won't — the universe's oldest stars will begin dying out, while the formation of new stars will grind to a halt.
In Braun's own words, "twelve billion years from now, the universe will be a dark, dark place."