Astronomers have known for nearly a century that the cosmos is expanding. Space-time is expanding out over billions of light-years, separating the galaxies within it like raisins in a rising loaf of bread. This continuous expansion, along with the universe’s will to collapse under its own gravity, means there are two basic options for how the universe will end.
These situations are known as the Big Crunch, which occurs when gravity overcomes expansion and the Big Bang occurs in reverse, and the Big Freeze, which occurs when gravity loses out to expansion and all matter is isolated by unfathomable distances. (See “The Big Crunch vs. the Big Freeze,” page 50.)
A cosmological puzzle
For a while, scientists thought the universe’s fate was leaning toward the end scenario. However, astronomers discovered something surprising in the late 1990s that revolutionized our idea of the universe’s future: the most distant galaxies were not just drifting away from us. They were accelerating.
Two teams of astronomers independently found this phenomenon when measuring distant supernovae to calculate the precise rate at which the universe was expanding, expecting to see it slowing down. Three of these scientists, Saul Perlmutter, Adam Riess, and Brian Schmidt, were awarded the Nobel Prize in Physics in 2011 for their research.
The observations were made throughout a survey of distant type Ia supernovae. Astronomers believe these explosions occur when a white dwarf — a dense remnant of a Sun-like star — accumulates enough matter to drive it above a physical mass limit. Since this limit is the same for all white dwarfs, all type Ia supernovae have the same true brightness. In the mid-1990s, this trait made these supernovae ideal standard distance markers, or standard candles.
The two teams were actually seeking back in time for the beginning of cosmic deceleration: they were looking for the point in time when gravity overcame the cosmos’ rapid acceleration following the Big Bang. This moment would mark a turnaround, as gravity finally started to slow the rate at which galaxies and clusters of galaxies are pulled away from one another by the expansion of the universe.
Because scientists know the true brightness of standard candles, they can predict how brilliant these distant supernovae will be if expansion stops. Instead, they discovered that the observed type Ia supernovae were 25% fainter than expected, demonstrating that the universe’s expansion is speeding up instead of slowing down.
By the end of 1998, both teams had submitted papers detailing their findings to academic journals. Riess and Schmidt’s team published in The Astrophysical Journal, whereas Perlmutter’s team published in The Astrophysical Journal.
The conclusion of both: a large percent of the universe is made up of something previously undiscovered and unexpected. And this so-called dark energy is overcoming gravity and separating space-time from within.
A lot of missing pieces
The composition of the universe is surprisingly tricky to pin down. Aside from dark energy, space is also filled with dark matter, an invisible kind of material. Astronomers now know that conventional, visible matter accounts for only 5% of the cosmos, whereas enigmatic dark matter and dark energy account for 26% and 69%, respectively. In other words, astronomers don’t really understand what about 95 percent of the universe is really made of.
Even decades after their discovery, scientists still know shockingly little about our universe’s “dark” forces. “Understanding and measuring dark matter and dark energy is hard,” says Riess. “Imagine bumping around in a dark room, occasionally touching an elephant, having never seen one, and [trying to understand] what it is, what it looks like.”
But the dark room is the size of the universe and instead of touching the elephant, astronomers can only see the effects it has on other objects. Astronomers can detect that dark matter interacts with visible matter gravitationally, so they assume it is made up of one or more unknown particles. Dark energy could be the universe’s fifth fundamental force. (The known four are: the weak force, the strong force, gravity, and electromagnetism.)
However, its precise properties remain unknown, especially as dark energy appears to have randomly activated. According to Riess, the most recent data reveals that dark energy started this acceleration some 5 billion to 6 billion years ago, and it’s been the dominant force ever since.
The most simple explanation for dark energy is that it is the intrinsic energy of space itself. When laying out his theory of relativity, Albert Einstein first developed such a concept to allow for a flat universe.
Einstein’s cosmological constant is a repulsive force that opposes gravity’s attraction force, allowing for an universe that neither collapses nor expands. However, after Edwin Hubble observed the universe expanding, Einstein rejected his hypothesis. The Nobel-winning supernovae work in the 1990s resurrected the cosmological constant and related it to dark energy.
Though astronomers cannot directly observe dark matter, they can infer its position from observations. This image reveals the distribution of dark matter (magenta) in supercluster Abell 901/902 by combining a visible light image of the supercluster and a dark matter map of the area. ESO, C. Wolf (Oxford University, U.K.), K. Meisenheimer (Max-Planck Institute for Astronomy, Heidelberg), and the COMBO-17 team created VISIBLE LIGHT. NASA, ESA, C. Heymans (University of British Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M. Barden (Innsbruck), and the STAGES cooperation created the DARK MATTER MAP.
What lies ahead
To ultimately resolve this dark energy puzzle, Riess says scientists will need more than just measurements. The world’s best theoretical physicists have tried to work out a grand unified theory of physics that fully explains all aspects of the universe. However, despite the fact that theorists believe their unification is fundamental to any theory that will also explain dark energy, gravity and quantum physics do not seem to mesh so far.
One thing scientists have determined is the devastating influence dark energy will have on the universe in the distant future.
If the contribution of dark energy increases as the universe ages, the universe will expand at a quicker rate. Other galaxies beyond our Local Group, which will have merged into a single enormous galaxy called as Milkomeda, will eventually be whisked out to such great distances that any future occupants of our solar system will be unable to see them.
In fact, Alexei Filippenko, an astronomer at University of California, Berkeley, who has worked with both teams that discovered dark energy, says, “If all records are lost, future civilizations might not ever know about other galaxies.” For them, he says, “[The universe] will be a cold, dark, lonely place.”