Why The Universe is Losing Energy
Violations of conservation of energy are only a matter of time.
Energy is one of the most familiar concepts in physics. We think about it routinely in our lives, such as when considering the calories in a meal or the cost of running an air-conditioner. Yet it is surprisingly difficult to pin down what energy actually is. What we call energy takes many forms. Some forms are obvious, but others are hidden. You can see the energy of a fire in its light and feel it as heat. Yet, this same energy was hidden in dull, cold logs in the fireplace before the fire was lit. The energy of a boulder rolling down a hill can be seen by how it destroys everything in its path. Yet, this same energy was already there without doing any harm as the boulder sat at the top of the hill. Energy takes many forms that do not seem to share any characteristic features.
To understand what unifies these forms into a singular concept of energy, it is helpful to consider an analogous concept — wealth. A person’s wealth can manifest in many different assets — cash, shares, real estate, artworks, cars, antique vases and so on. These assets are related only by the fact that they are all exchangeable through the medium of money. This means they can all be assigned a value in dollars, which can be combined to give a single quantity — the total wealth. Similarly, all of the different forms of energy are unified by the fact that they are interconvertible; any one can be transformed into any other. This means that whatever form energy takes, we can assign it a value in a shared unit such as joules. The combination of all of these values is the total energy. Energy is therefore not a material thing like an atom or planet; it is best understood as an accounting concept.
The reason energy is an important quantity worth accounting for is that it is a finite resource that cannot be created or destroyed. This is encapsulated by the law of conservation of energy, which states that the total energy stays the same over time. Conservation of energy is a foundational pillar of science that we expect to hold true even when all else is open to question. But ninety years ago, a startling discovery was made that seems to undermine this pillar — energy is disappearing from the universe! To understand how this can be, we must go deeper than just accepting the law. We must ask why it is that energy should be conserved. Then we will be able to see why it is not.
Why is Energy Conserved?
Two profoundly important concepts in physics are conservation laws and symmetries. A conservation law is a principle dictating that a certain physical quantity stays constant over time. For example, conservation of energy is a conservation law that states that the total amount of energy stays the same over time. This is just one of a number of conservation laws (such as conservation of momentum, of angular momentum and of electric charge) in physics. These laws are important because they constrain what is physically possible. This can help us to predict what a physical system will do.
Symmetries are things you can do to a physical system without changing them. For example, rotation of a circle is a symmetry because the circle looks identical however you turn it. Symmetries are essential to science because they are what allow us to apply the conclusions of one experiment to other physical systems. In particular, one fundamental symmetry of physical systems is spatial symmetry. This states that physics works the same way in different places; the same experiment done in Australia and Germany under otherwise identical conditions will give the same results. Another is temporal symmetry. This means that physics does not change over time; if everything else remains unchanged it does not affect the results of an experiment whether we conduct it on Monday or Tuesday. These symmetries are what allow the laws that Newton discovered in 17th century England or that Einstein discovered in 20th century Germany to be applicable in 21st century Australia, or in outer space. Without them, scientific discoveries would simply be observations of how things happened at one particular place and time with no broader applicability. Science would be futile.
In 1918, Emmy Noether showed that conservation laws are deeply connected to symmetries. Specifically, every conservation law has a corresponding symmetry. The conservation law only holds true as long as the symmetry does, and vice versa. The symmetry corresponding to the law of conservation of energy is temporal symmetry. This means that if a physical system changes over time, then it will not conserve energy, and vice versa.
To understand this connection, imagine a world in which the laws of physics did not obey temporal symmetry. For example, imagine that gravity was only half as strong on Monday as on Tuesday. It turns out we could use this effect to create energy! For example, on Monday we could pump a big tank of water to the top of a cliff. Then, on Tuesday we could let the water flow off the cliff and turn a turbine to generate electrical energy at the bottom. Because the gravity is only half as strong on Monday as Tuesday, the energy we need to pump the water to the top on Monday is only half as much as the amount we get back out on Tuesday. So, we get more energy out of our system than we put in! This is generally possible for systems that are not symmetric in time — we can create energy by doing work against forces when they are at their weakest and then reap more energy than we put in by allowing the forces to push back when they are at their strongest. If time symmetry does not hold, then neither does the conservation of energy.
The Universe is Losing Energy
In 1929, Edwin Hubble discovered that the universe is expanding. This was a profoundly important finding that has shaped our understanding of the universe ever since. However, it also has a curious implication — that the universe is losing energy.
To see this, imagine drawing a wavy line on a deflated balloon. We call the distance between consecutive peaks of this pattern the wavelength. Now inflate the balloon. This stretches the wavy line, making the wavelength larger than it was originally. Light travels through space as a wave, similar to the pattern on the balloon. The expansion of the universe is like the inflation of the balloon, and so increases the wavelength of light. Importantly, in 1900, Max Planck showed that the energy of light is related to its wavelength. Specifically, the longer the wavelength, the lower its energy. This means that the expansion of the universe, by increasing the wavelength of light, also decreases its energy. The universe is permeated by an immense amount of such light. This light is called the cosmic microwave background because it now consists of mostly low-energy microwaves. However, when this light was produced soon after the Big Bang, it was much higher energy visible light. The expansion of the universe has caused the amount of energy in the cosmic microwave background to decrease. Energy has gone missing!
Where did this energy go? It cannot have gone anywhere — there is no place outside the universe for it to go! Conservation of energy has been violated, and we can see why. The expansion of the universe violates temporal symmetry. It means that the universe is different now from how it was ten billion years ago, and from how it will be in ten billion years’ time. The decreasing energy of the cosmic microwave background is a manifestation of this asymmetry. In 2009, the temperature of the cosmic microwave background (which is a measure of its energy) was observed to be 2.73 kelvins. If the same experiment had been performed ten billion years ago, it would have found a different result — about 8 kelvins. The expansion of the universe causes the same experiment to give different results at different times and so breaks the temporal symmetry of the universe. As Noether taught us, if temporal symmetry does not hold, then neither should conservation of energy.
The Limits of Conservation of Energy
The conservation of energy is a central pillar of science that we learn at school and that is used by scientists every day. Without it, science would break down. But the universe as a whole violates it! Does this mean the end of science?
Certainly not! While conservation of energy does not hold absolutely, it still essentially works in all areas except cosmology. This is because at human scales of time and space, the cosmic violation of conservation of energy is so small as to be irrelevant. The amount the universe changes over periods of days, weeks, years, or millennia is so small that any breaking of temporal symmetry on these scales is imperceptible. This means that conservation of energy also holds well enough on these timescales that any violation is undetectable.
However, what we have seen is that the conservation of energy is not an absolute truth. This teaches us a valuable lesson. In science, we must never forget to keep questioning our assumptions. If we believe that a law must hold true, we should always be prepared to ask why. If our reason no longer holds, then we must also be ready to put aside the law. Statistician George Box once said, “all models are wrong, but some are useful.” This is the approach that a physicist must take; they can depend on physical theories and laws for as long as they work but they must also always remember that these are only imperfect approximations to reality. So, science can continue to make use of the law of conservation of energy. However, we must remember to be careful with it, because ultimately: violations of conservation of energy are only a matter of time.