In this excellent seminar, Marina told us about her group’s attempt to solve the coincidence problem of dark energy by allowing for multiple phases of acceleration. The coincidence problem is that it seems we live in a special time where (according to the ΛCDM model) the energy density of matter and dark energy is comparable. Since the energy density of matter decreases as a^-3 (where a is the scale factor) whereas that of dark energy is constant, the period of time where the two are comparable is very small when compared with the age of the universe, thus making the composition of the universe that we currently observe, where dark energy makes up about 70% and matter 30%, quite unlikely.
Although the Anthropic Principle has been suggested as a solution to this problem, Marina proposes another possible solution: that there were multiple phases of acceleration (MPA) in the history of the universe and we just happen to be experiencing one right now. Marina’s group have looked at the simplest possibility: the dark energy equation of state was equal to -1, as in ΛCDM, and at some point, its energy density dominated over matter, causing a period of acceleration. But the acceleration ceases when, for a period w becomes greater than zero, causing dark energy to dilute and allow matter to dominate again. Then it switches back to w=-1 and expansion accelerates again once dark energy dominates over matter. Thus there could have been another period of acceleration before this one, meaning we do not occupy a special point in time.
Marina told us that it is difficult to “hide” this period of acceleration and deceleration in the data we have, especially the CMB power spectrum, the shape of which is sensitive to the behaviour of dark energy. It is not the short period of acceleration which has the most effect but the period of dilution, which necessarily has to be quite long to allow matter to dominate again, which strongly affects the CMB through the integrated Sachs-Wolfe (ISW) effect.
Through Markov Chain Monte Carlo methods, Marina and her team investigated at what possible redshifts this could happen. The effect on the CMB by the ISW effect and the fact that extra periods of acceleration affect the distance to the surface of last scattering, seem to rule out low redshifts as candidates for a period of extra acceleration. They did, however, find that it was possible to “hide” an extra period of acceleration in the CMB during the radiation dominated era (z~300 to 3000).
It has been shown that the universe could have undergone another period of accelerated expansion and the CMB would not be altered. However, it seems that the only period during which this is possible is in the radiation dominated epoch.