What’s ‘dark’ about dark energy?

Nearly a century ago, Edwin Hubble, an American astronomer, made the surprising observation that galaxies are moving apart, and that galaxies farther away from us are moving faster than nearby galaxies. With this, he showed us that the universe is expanding.

Towards the end of the last century, scientists discovered something even more surprising. When they more carefully studied how fast nearer and farther galaxies are moving away from us, they found that the universe is not just expanding, but that the rate of expansion is also increasing. This discovery led to Saul Perlmutter, Adam Reiss and Brian Schmidt winning the Nobel Prize for Physics in 2011.

This finding is as surprising as tossing a ball upwards only to discover that instead of slowing down, the ball is speeding up! So what could be causing the expansion of the universe to accelerate? “Normal matter cannot cause that. So you need to introduce this hypothetical substance called ‘Dark Energy’ with the necessary properties to cause the expansion of the Universe to accelerate,” explains Prof Bharat Ratra, a theoretical cosmologist and astroparticle physicist from Kansas State University, USA.

Prof Bharat Ratra, who was born in Mumbai, is today one of the leading experts on dark energy. Much of his academic work involves studying the structure and evolution of the universe. Along with Prof James Peebles of Princeton University, he proposed the first ‘dynamical dark energy’ model to help explain the accelerated expansion of the universe.

While we do not yet understand what dark energy is at a fundamental level, we know some of its properties by inference. We know that it is something that uniformly fills the otherwise ‘empty’ space and causes the expansion of the universe to accelerate. Since the density of dark energy is relatively insignificant on laboratory length scales, it is difficult to detect in laboratory experiments, and we can only observe its effects on the scales of clusters of galaxies or larger.

Cosmological constant

The simplest type of dark energy is the ‘cosmological constant’ introduced by Albert Einstein in 1917 to make his equations of general relativity agree with a static or non-evolving universe — a belief held at that time. However, after the discovery of an expanding universe, the cosmological constant proved no longer necessary. Since then, it has been revived in the light of new observations of the accelerating expansion of the universe. As the universe expands, the density of ordinary matter decreases because there is only so much of it occupying more and more space. However, the density of cosmological constant dark energy remains constant in space and time.

Quantum theories that describe physics on small length scales suggest that there might be such a cosmological constant energy density associated with the vacuum. “People hope that is what might be going on in cosmology on very large length scales,’’ remarks Prof Ratra. However, it turns out that a naive application of quantum theory on small length scales suggests an amount of cosmological constant energy density much larger than the measured dark energy density and it is also “very difficult to reconcile a static cosmological constant with other parts of physics,” he says.

Alternatively, the density of dark energy may slowly vary over the course of the evolution of the universe. “If its density changes with time, that is dynamical dark energy,” explains Prof Ratra. Dynamical dark energy models suggest that dark energy can be produced by an independent field, detached from intrinsic properties of space. “So, the hope is that this energy slowly changes with time and gets less and less dense. In those cases, you could build models of particle physics with very low mass particles, nearly zero, to help explain this,” he suggests.

Is dark energy constant or dynamic? We do not yet know. There are several ongoing and future sophisticated experiments which seek to map out the positions of distant objects such as galaxies and quasars to better understand their distribution across the universe and infer its expansion history from it.

Inflation in the early universe

Before working on dark energy, Prof Ratra’s work focused on a much earlier phase in the evolution of the universe, when it underwent rapid expansion, known as inflation.

“In an expanding universe with a finite life, if you look in two different directions, the farthest you can see is the distance light could have travelled in that time. If you go back and look at what those regions were doing when the universe was younger, you find that they were not able to communicate with each other because there wasn’t enough time for light to go back and forth. However, surprisingly, they look very much the same,” he says, setting out the problem inflation solves.

Consider adding cold milk to hot coffee. Initially, both have very different temperatures, but once the milk is added to the coffee, the particles begin to mix, and given enough time, the whole mixture reaches the same temperature. But, in the universe, the milk and coffee seemingly contrive to achieve the same temperature although there wasn’t enough time for them to mix! So how is this possible?

“One rather unsatisfactory answer is that the initial conditions were the same all around,” says Prof Ratra, suggesting that the coffee and milk, in the above example, were somehow at the same temperature, to begin with. Since we cannot establish this fact, scientists have ruled out this possibility.

“A more satisfactory solution is what is called inflation. At the outset, different parts of small regions in the universe could communicate with each other and the particles could mix. So each of these small regions could become uniform. At a later stage, the universe underwent a period of rapid expansion known as inflation that greatly expanded these small regions, with one of them eventually growing large enough to encompass the whole of the universe that we can currently observe,” he explains.

Inflation also predicts that the universe is nearly spatially flat, that is, parallel light rays, over long distances, continue to remain parallel and do not converge or diverge. This is consistent with the results from high precision observations of the cosmic microwave background, relic radiation from the early universe, although slightly spatially curved universes cannot yet be ruled out. Current measurements lack the precision needed to properly test the idea, but future experiments, including the possible detection of gravitational waves from early universe, will shed more light on this.

(The writer is with Gubbi Labs,
Bengaluru)