Hubble Space Telescope astronomers have confirmed that dark energy a mysterious repulsive force that is causing cosmic expansion to accelerate existed in the early universe. The discovery extends evidence of dark energy to 9 billion years ago, a time when most galaxies had formed and the universe was undergoing an intense period of star formation.
Astronomers have long known that galaxies are receding from each other. In the last decade, however, scientists were surprised to discover that the rate of expansion of the universe is accelerating, not slowing due to gravitational attraction, as everyone had expected.
Adam Riess (Space Telescope Science Insitute and Johns Hopkins University) and the members of the High-z Supernova Team and the Supernova Cosmology Project first detected this acceleration from observations of a small handful of distant type-Ia supernovae in 1998. This kind of stellar explosion involves white-dwarf stars, and it's an excellent "standard candle" because all type-Ia supernovae are believed to have nearly identical intrinsic brightness. Therefore, the apparent brightness of a type-Ia supernova is a reliable indicator of its distance.
What on Earth (or in the universe) could cause galaxies to speed up as they move away from each other? If the universe is undergoing a tug-of-war, with gravity on one end of the rope, what’s tugging on the other end? Astronomers have dubbed this mysterious repellent force dark energy.. By most calculations, it makes up 70% of the energy-mass budget for the universe.
To study how dark energy behaved long ago, a team led by Riess has compiled properties of 23 of the furthest known type-Ia supernovae. The researchers used these supernovae, which exploded between 3.5 and 10 billion years ago, to measure the expansion rate of the early universe and how dark energy affected it.
A Cosmic Jerk
The dark-energy theory has only recently become the popular “golden child” in cosmology, though Einstein theorized its possibility nearly 100 years ago in his general theory of relativity. Team member Mario Livio (Space Telescope Science Institute) explained the significance of the findings: “We know that dark energy dominates today's universe, and that gravity dominated up to about 5 billion years ago, but we didn’t know if dark energy even existed 9 billion years ago, or if its properties have changed. This work shows that dark energy did exist then, and its properties are still similar today.” In addition, the scientists have confirmed beyond the shadow of a doubt that type-Ia supernovae existed in the early universe, and that their properties are also still the same today. This solidifies their usefulness as the key tools for measuring cosmic expansion.
The team’s observations confirm that the early universe was dominated by matter whose gravity was slowing down the universe’s expansion rate. The observations also confirm that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy’s repulsive force overtook gravity’s attractive grip. In a “cosmic jerk,” the universe transitioned from deceleration to acceleration.
While the latest results do not allow scientists to rule out specific cosmological theories, the new data is consistent with Einstein’s theory of a cosmological constant, a measure of the energy density in the universe over time. The value of the cosmological constant in Einstein’s equations determines whether the universe has enough mass for gravity to cause it to fall back in upon itself, continue expanding forever, or eventually come to a stand still. “We can’t discount any alternate theories yet,” says Riess, “but these findings are consistent with a negative equation of state parameter equal to minus 1, which means the gravity would be repulsive.”
Much more research on space-based telescopes is needed before scientists can understand what dark energy actually is. Said Riess, “Our latest clue is that the stuff we call dark energy was just starting to make its presence felt nine billion years ago. But each clue we gather helps put together the puzzle and look at what scenarios still remain.”