We know the universe is expanding, and that the expansion is accelerating. However, an exciting new finding casts doubt on long-held beliefs about dark energy and how the universe evolves over time. Here’s what you should know.
How Did We Discover Dark Energy to Begin With?
Physicists and astronomers have accepted that the universe is expanding since the 1930s, when Edwin Hubble discovered that all galaxies are moving away from each other at a rate related to their distance. The farther apart two galaxies are, the greater their speed.
However, in the late 1990s, scientists discovered something even weirder: not only is the universe expanding, the rate of expansion is also accelerating over time. In simpler terms, the older the universe gets, the faster things move apart.
This has alternatingly excited and puzzled astronomers and physicists for decades, since the finding is so unintuitive. If you lift a weight up off the floor, you use chemical energy to do so. If you want to launch something into space, you must burn hundreds of thousands of pounds of rocket fuel to overcome Earth’s gravity. So how is it possible that all the galaxies are moving away from each other faster and faster? It’s a bit like letting go of a ball only to watch it fly off into the sky all by itself. There must be something providing energy that allows galaxies to overcome the attractive force of gravity.
This mysterious energy was dubbed dark energy, and to date, no one knows for sure exactly what it is or where it comes from. Because we can measure the expansion of the universe by looking at distant galaxies, we can even estimate how much energy should be required, and there is a ton of it—it is thought that about 68% of all energy in the universe is dark energy. All of this has created a tantalizing area of study for scientists.
One of the most exciting tools we have to study dark energy is the Dark Energy Spectroscopic Instrument (DESI), which measures how matter is distributed across the universe. New results from DESI released by the Berkley Lab are threatening to force us to rethink previous assumptions.
Researchers Discover Dark Energy Might Be Changing
For a long time, it was generally believed that dark energy has behaved the same way since the beginning of the universe. However, there have been occasional hints that suggest dark energy might actually change as the universe ages. Taken alone, they’re not conclusive. That is where the latest DESI results come in.
By studying the position and motion of nearly 15 million galaxies (and quasars), and combining their data with other experiments, the researchers found that the matter spread out across the universe isn’t quite where it should be. The leading explanation for the discrepancy is that dark energy—which moves entire galaxies—didn’t behave in the same way at the beginning of the universe as it does today. Specifically, it looks like the energy density of dark energy has decreased with time.
Energy density sounds complicated, but it isn’t. It just describes how much energy is in a certain amount of space. If you’ve ever looked at a can of Coca-Cola and noticed that there are 150 calories (a measure of energy) in 12 fluid ounces, you’ve thought about energy density.
Since dark energy drives the accelerating expansion of the universe, decreasing the amount of dark energy should result in a decrease in the rate of acceleration, too.
That’s a bit hard to parse, but if you’ve ever ridden or driven in a car, you’re familiar with changing rates of acceleration. When starting or stopping, sometimes the change feels smooth and gentle. That is continuous (or constant) acceleration. However, if the start or stop feels “jerky,” that is because the rate of acceleration is changing.
Variable dark energy invites all sorts of important questions. What causes the energy density dark energy to change? What will happen to the energy density of dark energy in the distant future? Is the change a result of an inherent property of dark energy, or is it a result of something else acting on dark energy?
It’ll likely take much more time to work out a complete explanation for dark energy, but whoever manages to will receive a Nobel Prize for their efforts.
Is the Universe Going to End in a Big Crunch?
The fact that the universe’s accelerating expansion seems to be slowly decreasing invites an immediate question: is it going to stop expanding eventually? Is it going to reverse its expansion and begin contracting? Will it eventually collapse back to a tiny point again?
The evidence so far still suggests “No, the universe will not end in a big crunch.” However, even if it did, it isn’t something to worry about for many tens of billions of years at least—it certainly won’t affect your vacation plans.
Despite the excitement around these results, and that they’re now close to the point where the physics community accepts it as fact, it is important to remember that nothing is set in stone. We’re just beginning to scratch the surface of what can be learned about dark energy and the universe as a whole.
