Energy Conservation in an Expanding Universe

The principle of energy conservation states that energy can neither be created nor destroyed, it can only change from one type to another.

The question about energy conservation in an expanding Universe is one instance of how our knowledge of modern physics start to get challenged as we explore more extreme environments and distances. And the answer to this question is: there is no violation of energy conservation during expansion of the Universe.

The solution to our problem lies in the fact that although the redshifting of light is usually attributed to the Universe’s expansion, it can also be interpreted as an effect due to the receding motion of the galaxies with respect to the observer.

This is called Doppler effect (see the animation above from University of Queensland in Australia). We can observe this, for example, in a moving car or ambulance. As it passes by, the pitch of the sound coming from it becomes high when it approaches us, and the pitch goes down as it goes away from us.

What happens during this event is that the sound waves get clustered on the approaching side of the vehicle, and so the wavelength of the sound waves in front of the car is shorter (we call this blueshifted). On the other hand, the sound waves on the side moving away from the vehicle have their wavelengths longer (or in this case, redshifted). In this case, energy is conserved. In addition, Doppler effect arises from relative motion; that is, the driver of the vehicle can hear the sound from it just fine, but when it is heard by an observer standing outside, the sound produces a changing pitch. In short, Doppler effect is a matter of relative frames of reference. With some math, we can calculate the relative velocity of the vehicle by just knowing the detected wavelength of the redshifted or blueshifted waves.

The same effect happens to stars and galaxies, but instead of sound waves, they emit light, and so the wavelength of photons are also affected by Doppler effect. This light becomes redshifted because they recede from us. The fact that we can observe “redshifted light” is not because the photon loses energy, but it is just a result of observing the with respect to the observer’s point of view compared to the galaxies’ point of view. Relative motions of galaxies and stars can also be calculated using the same concept. No energy is being involved in this process, and so we can also say that the redshifting light caused by the expansion of the Universe is outside the jurisdiction of energy conservation law.