The mystery of Dark Energy is really the mystery of why the expansion of the Universe does not match the predictions of General Relativity, our current theory of gravity. Calling it the mystery of Dark Energy is to assume that Dark Energy is the solution, which might not be the case. In fact, if antimatter has antigravity, no Dark Energy is needed!

So let’s take a step back and see what observations tell us there is a mystery here. In our current theory of gravity, everything attracts everything else, so the expansion of the Universe *must *be slowing down. But recent observations of Type 1a supernovae [1] indicate that the expansion of the Universe is not slowing down, but may actually be accelerating. In order for the theory of General Relativity to be consistent with this observation, it is necessary to add a “cosmological constant” to the equations. There is no physical motivation for adding this constant except to make the predictions match the observations. In order to provide a physical basis for the cosmological constant, physicists have postulated the existence of Dark Energy, but the nature of Dark Energy is a complete mystery.

It is worth noting that Einstein himself originally included the cosmological constant in General Relativity, but for a different reason. At the time the theory was developed, physicists believed the Universe was eternal and static. Einstein recognized that his equations described a universe that was either expanding or contracting, so he added the cosmological constant in order for his equations to be consistent with a static universe. Once Edwin Hubble discovered the expansion of the Universe, Einstein removed the cosmological constant and called it his greatest mistake.

#### Antigravity for Antimatter can solve this Mystery!

If gravity between matter and antimatter is repulsive, then it is likely that putting the cosmological constant back in the equations will once again be seen as a mistake. This is because if the Universe is comprised of equal amounts of matter and antimatter that repel each other with gravity, then the net “gravitational charge” of the Universe would be zero, so the expansion of the Universe would not be slowing down.

It is not obvious if the expansion would just be coasting, or if it would be accelerating. We can use an analogy from the electric force to see that there could be a net repulsive force. The analogy comes from the fact that ionic solids, which are electrically neutral but with alternating regions of positive and negative electrical charge. Ionic solids are stable because the net force is attractive when all the attractive and repulsive forces are added. However, with electric forces, opposite charges attract and like charges repel. If we were to change the sign of these forces, making opposite charges repel and like charges attract, then the sum of the forces would be repulsive and ionic solids would not be stable. This might be the case for the Universe, with gravity supplying the net repulsive force we label Dark Energy.

**The Age Problem**

General Relativity makes a clear prediction about the age of the Universe. Without a cosmological constant, the age of the Universe should be given by the inverse of the Hubble constant (the expansion rate of the Universe as a function of distance). The problem with this is that with the current measurement of the Hubble constant (70 …), General Relativity predicts the age of the Universe should be 9 billion years. However, we observe stars that are older than this! This is known as the Age problem. We do not hear much about this mystery these days because Dark Energy modifies the prediction for the age of the Universe so that the predicted age is older than the oldest stars. Basically, the initial expansion of the Universe does not need to be as fast to overcome the universal attraction of gravity.

#### Antigravity for Antimatter can solve this Mystery!

Antigravity for antimatter solves the Age problem a different way. A universe with no net gravitational charge would be expanding linearly; the calculated age of a linearly expanding Universe is very close to the age of the oldest known objects in the Universe.

#### References

- Riess, A. G., Filippenko, A. V., Challis, P.,
*et al., “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant,”*1998, AJ, 116, 1009; Perlmutter, S., Aldering, G., Goldhaber, G.,*et al*. “Measurements of Ω and Λ from 42 High-Redshift Supernovae,” 1999, ApJ, 517, 565. - Chaboyer, B., Demarque, P., Kernan, P. J., & Krauss, L. M., “The Age of Globular Clusters in Light of Hipparcos: Resolving the Age Problem?” 1998, ApJ, 494, 96