Back to the Future: Cosmic distance measurements just got better!

Driven by a mysterious dark energy, the expansion of the Universe is accelerating. This increased rate of expansion is one of the most puzzling issues in astronomy in the last two decades. The most promising way to understand the nature of dark energy is to measure the expansion history of the Universe by determining distances to when the Universe was younger. To measure these distances, the Universe provides us with a cosmic yardstick, 500 million light years long, known as baryon acoustic oscillations (BAO). These are sound waves formed in the very early Universe, whose wavelength has been imprinted in the distribution of galaxies. By measuring the BAO on the sky at various cosmic epochs we are measuring the distance to those epochs and therefore mapping the expansion history of the Universe.

The SDSS survey used a sample of luminous red galaxies to detect for the first time the presence of the BAO and measure its angular extent on the sky. The specific size of the BAO is imprinted in the separation of the overdense regions of the Universe in Figure 1. As the Universe evolves with time, these overdense regions will form galaxies. However, the positions of the galaxies are distorted by gravitational interactions between the large overdensities over time as shown in Figure 2. In order to accurately measure the BAO extent at different epochs, we would need to measure the undistorted positions of the galaxies, washed away by gravity.

In a recent series of three papers, a team of SDSS astronomers has applied for the first time a novel technique to estimate the displacement of each galaxy due to gravitational effects, shown as blue arrows in Figure 3. Subtracting these displacements effectively moves the galaxies back in time, thereby undoing a majority of the gravitational distortions and reconstructing the original matter distribution in Figure 4. This reconstruction technique sharpens the focus on our standard ruler and increases the precision of cosmic distance measurements. These results will pave the way for even more precise measurements from the Baryon Oscillation Spectroscopic Survey.

More details can be found at:

The three papers submitted to Monthly Notices of the Royal Astronomical Society are available at: (Padmanabhan et al. 2012 – Methods and application to SDSS) (Xu et al. 2012 – Fitting techniques) (Mehta et al. 2012 – Cosmological measurements and interpretation)

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