 We now ask, what could be accelerating the expansion of the universe? In the How Small Is It? video book chapter on the Higgs Boson, we covered how so-called empty space is actually filled with matter and energy fields. We modeled the waves in these fields as quantum harmonic oscillators. And given the Heisenberg uncertainty principle, the zero-point energy for any wave in the field must be greater than zero. We have seen that radiation and matter in the universe are diluted as space expands. But zero-point vacuum energy does not dilute. In fact, the total amount of vacuum energy increases as the volume of the universe increases. In a small universe, it would have little impact. But today, it is estimated to be around 70% of the energy density of the entire universe. The zero-point quantum vacuum energy is called dark energy. And it is enough to force the cosmos into its accelerating expansion.