Baryon Acoustic Oscillation Intensity Mapping of Dark Energy

The expansion of the Universe appears to be accelerating, and the mysterious antigravity agent of this acceleration has been called “dark energy.” To measure the dynamics of dark energy, baryon acoustic oscillations (BAO) can be used. Previous discussions of the BAO dark energy test have focused on direct measurements of redshifts of as many as ${10}^9$ individual galaxies, by observing the 21 cm line or by detecting optical emission. Here we show how the study of acoustic oscillation in the 21 cm brightness can be accomplished by economical three-dimensional intensity mapping. If our estimates gain acceptance they may be the starting point for a new class of dark energy experiments dedicated to large angular scale mapping of the radio sky, shedding light on dark energy.

Figure 1

The power spectrum of neutral hydrogen hyperfine emission at $z=1.5$ is shown by the solid line. The dashed line indicates the map noise from a 200-by-200-meter radio cylinder telescope operated in transit mode for 100 days, assuming a telescope system temperature of 50 K. The horizontal dotted line indicates the nonlinear scale at $z=1.5$. The vertical lines indicate the location of peaks in Fig. 2.

Figure 2

Baryon acoustic oscillations. To show BAO we plot the ratio of matter power spectrum that contains contributions from baryons, calculated using a fitting formula [14], over a power spectrum with no baryons. To single out the oscillation, we further divide the ratio by a smooth fitting curve to the overall spectrum. The error bars represent projections of the sensitivity possible with a 200-by-200-meter cylinder telescope.

Figure 3

The observable parameter space in redshift and in scale ($k$) for BAO. The shaded regions are observationally inaccessible (see text). The horizontal lines indicate the scale of the first three BAO wiggles, and the dashed lines show contours of constant spherical harmonic order $\ell$.

Figure 4

The $1-\sigma$ contour for IM combined with Planck (inner thick solid for baseline model, outer thin solid for worst case), the Dark Energy Task Force stage I projects with Planck (outer dotted), the stage I and III projects with Planck (intermediate dotted), the stage I, III, and IV projects with Planck (inner dotted), and all above experiments combined (dashed, again thick for baseline, thin for worst case; the two contours are nearly indistinguishable).