Dark energy is something that causes the universe to expand faster and faster as time goes by. The big puzzle facing cosmologists today is to understand what this "thing" is.
“We can tell a lot about the properties of dark energy and how it behaves,” says Tamara Davis, a professor of astrophysics at the University of Queensland in Australia. “But we still don't know what happened. That's the real question.”
How do we know dark energy exists?
Astronomers have known for a long time that the universe is expanding. In the early 1900s, Edwin Hubble observed that galaxies were in motion and proposed Hubble's Law. This law shows the relationship between the speed of a galaxy and its distance from us. However, the newly detected supernovae in distant galaxies towards the end of the 20th century revealed a dilemma: The universe is expanding not at a constant rate, but at an accelerating rate.
"We were all surprised by the acceleration of the universe," says astrophysicist Katherine Freese, who works at the University of Texas at Austin. Unlike the gravitational force of gravity, dark energy should “exhibit a kind of repulsive behavior and pull objects apart faster and faster,” she says.
Many observations conducted since the 1990s have confirmed that the universe is expanding at an accelerating rate. Exploding stars in distant galaxies appear fainter than they would in a steadily expanding universe. Even the cosmic microwave background, the remaining light from the first known moments in the history of the universe, shows signs of the effects wrought by dark energy. Dark energy needs to be included in our mathematical cosmology models to explain the observable universe.
The term dark energy was first coined by astrophysicist Michael Turner to be compatible with the term dark matter. The term also suggests that the accelerating expansion of the universe is a very important and unsolved problem. Many scientists at the time thought that Albert Einstein's cosmological constant was a perfect explanation for dark energy because it fit well with their model. (The cosmological constant is a “correction factor”, also known as lambda, that Albert Einstein included in his theory of general relativity to handle mathematical operations.
“My view was that it couldn't be that simple,” says Turner, who is now an adjunct professor at the University of California, Los Angeles (UCLA).
He sees the accelerating universe as “the deepest problem” and “the greatest mystery in all of science.”
Why is dark energy important?
Will Tyndall, an astrophysicist working at Yale University, says that the Lambda-CDM model, which states that we live in a universe consisting of only 5 percent normal matter and that the universe consists of 27 percent dark matter and 68 percent dark energy, is the "current approach in cosmology." This model, he notes
, “aims to unify (and explain) the entirety of cosmic history in a very ambitious way.” But the model still leaves much unexplained, including the nature of dark energy.
“After all, how can we understand so little of something that supposedly makes up 68 percent of the universe we live in?” adds Tyndall.
Dark energy is also one of the important factors that determine the ultimate fate of our universe. Will the universe fall apart in the Great Rupture, where everything is broken down into atoms? Or will it end with a whimper?
These scenarios depend on whether dark energy changes over time. If dark energy is a cosmological constant that never changes, our universe will expand and eventually become a very lonely place; In this scenario, we would not be able to see any stars beyond our local galaxy cluster, and these stars would be too red to be detected.
If dark energy becomes stronger, it can lead to what is known as the Great Rupture. Perhaps dark energy is weakening and our universe will contract again, starting the cycle over again with a new big bang. Physicists won't know which of these scenarios awaits us until we better understand the nature of dark energy.
What could dark energy actually be?
Dark energy appears in the mathematics of the universe as Einstein's cosmological constant. But this does not explain what is physically causing the accelerating expansion of the universe. One of the leading theories is a strange feature of quantum mechanics known as vacuum energy. This energy is created when pairs of particles and their antiparticles suddenly appear and disappear, and this happens almost always, everywhere.
That sounds like a great explanation for dark energy. But there is a big problem:
There is a huge and unexplained difference between the value of hole energy measured by scientists and predicted from theories. This is called the cosmological constant problem. In other words, particle physicists' models say that, according to Turner, what we think of as "nothingness" must have some weight. But measurements show that this weight, if any, is very small. “Maybe the weight of nothingness is also nothingness,” says Turner.
Cosmologists have come up with other explanations for dark energy over the years. One of these, string theory, suggests that the universe consists of small thread-like pieces and that the value we see in dark energy is only a possibility found in many different multiverses. Many physicists think this is quite anthropocentric in its logic; We could not exist in a universe with other cosmological constant values, so we existed with this even though it was an extreme value compared to the others.
Other physicists have considered changing Einstein's equations of general relativity entirely. But most of these attempts have been ruled out by measurements from LIGO's pioneering observations of gravitational waves. “In short, we need a bright new idea,” says Freese.
How can scientists solve this mystery?
New observations of the universe could help astrophysicists measure the properties of dark energy in more detail. For example, astronomers already knew that the universe was expanding at an accelerating rate; But was this acceleration always the same? If the answer to this question is no, then dark energy was not stable before, and this would disrupt the lives of all physics theorists who are struggling to find new explanations.
A project known as the Dark Energy Spectroscopic Instrument, or DESI, is currently being carried out at the Kitt Peak Observatory in Arizona, USA. In this study, cosmic cartography is carried out to look for signs of changing acceleration in the universe. “It's like laying out map method paper over the universe and measuring how it expands and accelerates over time,” Davis says.
Many more experiments are on the way, such as the European Euclid mission launching this month. Euclid will map galaxies up to 10 billion light-years away and look back 10 billion years in time. “During this entire period of time, dark energy has played an important role in accelerating the expansion of the universe,” the mission website says. Radio telescopes such as CHIME will map the universe in a slightly different way by tracking how hydrogen spreads throughout space.
But new observations won't solve everything. “Even if we measured the properties of dark energy with infinite precision, we wouldn't be able to find out what it is,” Davis adds. “The real revolution needed is a theoretical revolution.” New experiments are on the astronomers' calendar that will keep progress going and allow better and better measurements to be recorded. But theoretical revolutions are unpredictable; They can happen a year, a decade or even a hundred years later. “There are few serious puzzles in science. A serious riddle means you don't know the answer at all,” says Turner. “I think dark energy is one of them.”
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