Why do physicists think dark energy exists? Physicists think dark energy exists because something must be overcoming the pull of gravity to be speeding up the expansion of the universe.
13.8 billion years ago, the Big Bang happened and the universe began to expand. This is often described as an explosion into space, but it wasn’t an explosion. It was space itself expanding. In roughly the first 10 to the power of -36 seconds, the universe underwent incredibly rapid expansion. This is called Cosmic inflation. Distances increased exponentially. Then, in the first few minutes, the universe cooled enough for protons and neutrons to combine, producing mostly hydrogen and helium nuclei. About 380,000 years later, it had cooled enough for electrons to join those nuclei and form neutral atoms. At this point, the universe became transparent. After hundreds of millions of years, the first stars and galaxies start to form. Matter has gradually been gathering under gravity and it starts to concentrate in clumps. The universe is still expanding, but the rate of expansion is decelerating. Just like if you throw a ball straight up in the sky, it starts to slow down as gravity pulls on it. At some point, it stops rising, and starts to fall back. Oddly, and this is where dark energy comes in, that doesn’t happen to the universe.
After several billion years in the life of the universe, galaxies and galaxy clusters have grown. Gravity is still slowing down the rate at which the universe expands. And this is where things start to get interesting. Up to now, the universe has been matter dominated, but from this point on that changes. The entire amount of the matter in the universe hasn’t changed since the big bang, but, as the universe expands, it spreads out and the density of the universe drops. About 5 to 6 billion years ago, when the universe is roughly 8 billion years old, the expansion stops decelerating and starts accelerating again. And this is where we find ourselves today. The universe is still accelerating, which means the rate of expansion is increasing. Something therefore has to account for this acceleration. Physicists call the unknown cause dark energy.
So, what is dark energy? Nobody knows. However, whatever dark energy is, its effect appears to be causing the expansion of the universe to accelerate. Whatever it is, it was most likely there from the very start of the universe, but it is an extremely low source of energy. It doesn’t have enough power to counteract gravity. But that changes as the density of the universe falls and when the universe reaches a certain level of density, which it hit about 5 billion years ago, the dark energy does have enough power to overcome gravity. And, if that is true, as matter becomes increasingly diluted, dark energy will dominate the evolution of the universe more and more.
What is dark energy Nobody has directly identified what dark energy is, but its apparent effects on the expansion of the universe can be measured, just like dark matter. Physicists know that dark matter is there because they can see far more gravity than the visible matter can possibly produce. There must be five times more dark matter than visible matter for gravity to make sense. The same is true for dark energy. The universe is expanding in a way that is different to the way matter and gravity alone could predict. Something must be pushing it.
How do we know that the expansion of the universe is accelerating? In 1998, two independent teams of astronomers were studying distant Type Ia supernovae. These exploding stars are extremely bright and can be used to estimate enormous cosmic distances. The astronomers expected their observations to show that the expansion of the universe had been gradually slowing because of gravity. Instead, the distant supernovae appeared dimmer than expected, meaning they were farther away than the astronomers’ models predicted. The best explanation was astonishing: the expansion of the universe had actually been accelerating. The two teams announced their results in 1998, providing the first strong evidence for what we now call dark energy. The discovery was so important that Saul Perlmutter, Brian Schmidt, and Adam Riess were awarded the 2011 Nobel Prize in Physics for it.
There are three possible theories for what dark energy might be. The first, and simplest, is that it is a cosmological constant, sometimes associated with the energy of empty space. The strange thing about this energy is that its density would remain constant as the universe expands. In the early universe, matter and radiation were incredibly dense, so the tiny amount of dark energy would have been insignificant. As the universe expanded, matter became more spread out, while the density of dark energy stayed the same. Eventually, dark energy became dominant and the expansion of the universe began to accelerate. The problem is that physicists have no idea why the energy of empty space should have the tiny value that observations suggest. Quantum physics actually predicts that empty space should have energy, but attempts to calculate how much can give values enormously greater than the amount needed to explain the expansion of the universe. This enormous disagreement between theory and observation is known as the cosmological constant problem.
Another theory is something called quintessence, which is a fluid or field that is slowly filling space. If that is true, then nobody knows where it could be coming from or why it could be forming.
And the third theory is that there is no dark energy and our theory of gravity is incomplete. Out of all of these theories, the cosmological constant seems the most likely, but it probably won’t be solved in my lifetime, And this is what I learned today.
Sources
https://science.nasa.gov/dark-energy
https://news.uchicago.edu/explainer/dark-energy-explained
https://en.wikipedia.org/wiki/Dark_energy
https://en.wikipedia.org/wiki/Expansion_of_the_universe
Photo by Francis Glenn Marciano: https://www.pexels.com/photo/the-milky-way-galaxy-in-the-sky-12940327/