The universe is expanding, and that expansion is accelerating. But what caused it to happen? The main hypothesis is the repulsive force that astrophysicists refer to as “dark energy,” which pushes everything away from everything else. So how does it work? What does that mean for our future? And how sure are we that energy exists?
The word “dark” in his name does not mean literally dark. It’s more of a term suggesting that we really don’t know much about it, in the same vein as dark matter is just as mysterious. The main thing we know is that somehow, this energy is causing the expansion of the universe to accelerate.
The true nature of dark energy is still unknown, but there are some characteristics that “need” to explain the observations. Dark energy will have a very low density, spread evenly over all space, and emit a strong negative pressure that effectively pushes everything away.
Although it is nearly impossible to detect directly, physicists have several reasons to suspect the existence of dark energy. Measurements of the microwave background and observations of the large-scale structure of the universe show that matter, both orderly and dark together, can only make up about 30 percent of the universe. Some other form of energy must make up the remaining 70 percent or more.
But if dark energy is so difficult to study, how do we find it?
Dark History
The concept of dark energy can be traced back to Albert Einstein, although he did not recognize it at the time. In 1917, while working on his theory of general relativity, Einstein encountered a problem: his equations continued to state that the universe was dynamic, which contradicted the accepted view when the universe should be static.
To make his model consistent with contemporary beliefs, Einstein included a new number which he called the “cosmological constant”. It works to stabilize the static universe by saying that empty space itself has intrinsic energy, which exerts a negative pressure that pushes it outward, perfectly balancing against the pull of gravity inward.
However, the idea did not hold up for careful research. In 1929, Edwin Hubble discovered that the farther a galaxy is from us, the faster it is moving away from us, a clear indication that the universe is expanding. Einstein went on to say his “biggest mistake” was his refusal to accept what his own data said about a dynamic universe.
With the universe expanding into an accepted science, physicists largely abandoned the idea of a cosmological constant for about the next 70 years. But the concept became relevant again in 1998, with the groundbreaking discovery that this expansion was accelerating.
Accelerating Expansion
As light waves travel through space, the expansion of the universe stretches them out. That means its wavelength becomes longer, changing the color of this light redder, a phenomenon known as redshift. And in the late 1990s, two separate groups of researchers used this effect to measure the rate at which the universe was expanding. Basically, the redshift of an object reveals how much the universe has expanded since light left the object.
Both groups studied type Ia supernovae, very reliable light sources that have very close brightness at all times. Therefore, it is often referred to as the “standard candle”, and astronomers use it to calculate the distance to an object by measuring its brightness in the sky. Astronomers then compare the distance and redshift of these supernovae, to measure the speed of their recession, how fast the object is moving away from us.
Scientists hoped to confirm the main hypothesis of the time, that gravity would slow down the expansion of the universe, so that objects farther away from us would recede more slowly than objects closer. But to their surprise, they find the reality is the opposite. Since then, this finding has been supported by many independent studies.
And if the universe is expanding at an accelerating rate, what does that mean for the ultimate fate of everything? There are several possibilities.
Projecting various theories of dark energy into the future, the universe may be at its corresponding dark end.
If the expansion continued to increase, eventually all the galaxies in the sky would disappear from view. That’s because at some point, they will move away from us faster than the light they emit towards us, ensuring that this light can never reach us.
Dark energy will continue to push the universe apart long after every last star has collapsed into a black hole, and every last black hole has evaporated into nothingness. Eventually all the particles will spread so far apart that they rarely meet. The universe would cool to the point of devoid of thermodynamic free energy at all. This is known as the Big Freeze.
Different models have different ends. “Shadow energy” is a variation on quintessence, where the density actually increases as the universe expands (as opposed to most dark energy models where the density remains constant). This means that, in the end, this shadow energy will overpower all other fundamental forces in physics.
In this scenario, at some point gravity will become too weak to hold the galaxy together. Several dozen million years later and the strong and weak nuclear forces began to fail, destroying stars and planets, and then atoms themselves. Eventually, even the fabric of spacetime will be torn apart, which is ultimately called the Big Tear.
Or, if dark energy isn’t constant over time, there’s a possibility in the distant future that gravity could win the tug-of-war and start pulling things back. Over billions of years, the universe will contract until it reaches a point of infinity density, like the “upside down Big Bang”. This scenario is called “Big Crunch”.
Or perhaps, it is not the end but a new beginning, as another universe explodes out of the ashes in what is called the Big Bounce.
So far, in all the experiments scientists have gotten no results. It could rule out a certain mass of dark energy particles or force interactions, or it could mean that we are on an entirely wrong track.
Dark energy will likely remain one of the deepest mysteries of cosmology for quite some time, but we may get new clues soon. The Dark Energy Survey has been scanning the sky since 2013, while the Dark Energy Spectroscopy Instrument (DESI) recently joined it. That, as well as other research groups such as the Sloan Digital Sky Survey, will help produce 3D maps of the cosmos to investigate the physics of dark energy. SB/newatlas/I-1
