How Breaking One of the Most Basic Physical Laws Might Explain Dark Energy
Fudging the law of conservation of energy offers a new perspective on dark energy.
Orion's Nebula. Image: DES/Dark Energy Survey
A group of theoretical physicists from France and Mexico has offered a fun new what-if for dark energy, one of physics' most profound outstanding mysteries. As described in the current Physical Review Letters, this proposed solution involves violating a key principle in our most basic understanding of fundamental physics: the conservation of energy. In this new framework, dark energy just represents the sum total of many tiny leaks of non-conserved energy spread throughout the universe. It's a bit weird.
To recap, the law of conservation of energy states that energy in an isolated system can neither be created nor destroyed. It can only change forms. Chemical energy turns to electrical energy in a battery; kinetic energy turns to thermal energy via friction; potential energy turns to kinetic energy as the skydiver leaves the plane. Both old-school Newtonian physics and relatively new-school relativity depend on this law.
Meanwhile, dark energy is the vast something in the universe that adds up to about 68 percent of all energy in existence. Crucially, dark energy is a repulsive force—it pushes things apart. Because of dark energy, the universe is not only expanding, but is accelerating in that expansion. Space itself is being thrown apart, and the more "apart" it becomes, the faster the expansion occurs. Empty space begets dark energy begets empty space. Eventually, that's all there will be: cold, empty space.
Our awareness of dark energy is only about two decades old. In the 1990s, observations made with the Hubble Space Telescope tipped astronomers off that the universe seems to be accelerating in its expansion. The repulsive force responsible for this was dubbed dark energy. Incredibly, circa 1917, Einstein had battled with an explanation for dark energy without ever having been aware of its IRL existence. At the time, he just needed some force that would counteract gravity at cosmic scales to allow the universe to remain in a more or less static state and not immediately collapse in a big crunch.
This was Einstein's cosmological constant. In short time, new astronomical observations of cosmic expansion would allow him to abandon the idea, and so it remained tucked away until the 1990s discovery of dark energy. The accelerating universe and its implied dark energy happened to align with Einstein's shelved cosmological constant and so the idea was reborn. Einstein had imagined a repulsive force permeating empty space, and we now know that this force exists.
This repulsive force has a tempting explanation. We know that empty space breeds a curious fizz of "virtual particles" just because quantum physics forbids actual empty space. So, as the universe expands and more would-be empty space is created, the more repulsive energy exists. That's nice, but as it turns out, the predicted repulsive energy of these virtual particles is vastly smaller than what's required to explain the observed expansion of the universe. Mystery not-solved.
Finally, we return to the current paper. "Ever since the discovery of the acceleration in the Universe's expansion, almost two decades ago, there has been a puzzlement about the strange value of the corresponding cosmological constant Λ , the simplest, and so far most successful, theoretical model that could account for the observed behavior," the authors note. "The origin of this puzzle is that, within the usual framework, the only seemingly natural values that Λ could take are either zero or a value which is 120 orders of magnitude larger than the one indicated by observations."
In the framework proposed by the new paper the cosmological constant isn't so constant; Λ changes in accordance with the aforementioned tiny energy leaks as "a record of the energy-momentum nonconservation during the history of the Universe." It's when the universe reaches current scales that regular matter becomes so diluted by dark energy that Λ starts to look constant.
How these energy leaks are supposed to happen is more difficult to explain. The basic idea is to first imagine gravity as a continuous sheet, a field that has a value at every point all the way down to the infinitely small. Quantum physics is concerned with what happens when we take seemingly continuous fields like this and make them instead "grainy" or quantized—for example, light waves becoming the individualized packets of light known as photons. We don't really have a quantum theory of gravity yet, but if we can imagine at some point that continuous gravitational fields become individual particles, we can imagine that some energy may be lost in that translation.
So, if this energy is loss is real—and that's a big ol' if—we can come up with a pretty convenient place for it to go that helps explain dark energy. Unfortunately, this whole idea just throws us up against another mystery, which is quantum gravity itself. Proving or disproving dark matter-as-energy nonconservation is a long ways off.