**Title**: New Limits on Early Dark Energy From the South Pole Telescope

**Authors**: Christian L. Reichardt, Roland de Putter, Oliver Zahn, Zhen Hou

**First Author’s Institution:** University of California, Berkeley

*power spectrum*. The power spectrum characterizes the size of temperature fluctuations as a function of multipole , where large (small) corresponds to small (large) scales on the sky. The multipole number is similar to the frequency of a wave, in that a larger multipole number corresponds to fluctuations of a smaller physical scale. For a great introduction to the CMB power spectrum, check out this tutorial by Wayne Hu. The CMB’s power spectrum as measured by SPT is shown in the figure below. The addition of EDE increases the expansion rate in the early universe, which suppresses the growth of matter perturbations. This suppression in turn drives an increase in the amplitude of the temperature anisotropies, most strongly on small scales. Thus, the addition of EDE enhances the peaks in the CMB power spectrum at high . Until recently, the CMB power spectrum had been measured with small errors only at low by the

*WMAP*satellite (shown on the plot by the open diamonds), but as the effects of EDE are strongest at high , the

*WMAP*measurements are not sufficient for strong constraints on its existence. However, SPT has a higher spatial resolution than

*WMAP*and is able to measure the small-scale CMB temperature anisotropies with much greater precision. The figure above shows the

*WMAP*and SPT measurements of the CMB power spectrum as well as six different best-fit models with varying EDE density, denoted by , from 0% (black) to 5% (red). Each of the EDE models is consistent with the

*WMAP*data at large scales, but they differ significantly from each other and from the data at the smaller scales to which SPT is sensitive. Using the combination of

*WMAP*and SPT measurements, the authors place a strong upper limit on the density of early dark energy. With a confidence level of 95%, the authors find an upper limit . If dark energy existed in the early universe, it did not account for more than 1.8% of the total density of the universe. This is roughly a factor of 3 improvement over the upper limit derived solely from the

*WMAP*data, . The probability distribution for derived from the data is shown in the figure to the left. The authors also point out that in the next year, order of magnitude improvements in the measurements of the small-scale CMB power spectrum are expected from surveys like SPT, the Atacama Cosmology Telescope, and the

*Planck*satellite. These improvements promise to further our understanding of the nature of dark energy and its importance in the early universe.

very nice

Good article