Measuring the Universe: the Cosmological Distance Ladder

Book cover - Measuring the Universe

My first book Measuring the Universe was in production at roughly the same time astronomers were making one of the great discoveries in science: the accelerating expansion of the universe. The book was becoming out-dated even as it was being published! My treatment of the distance ladder remains valid, though, and this blog lets me discuss some of the new distance-measuring techniques – such as baryon acoustic oscillations as a standard ruler. Available from Springer Science.

Mapping the Galaxy

In Measuring the Universe I described the ESA Hipparcos mission in detail. Hipparcos was the first space-based mission dedicated to astrometry: it mapped the precise position of 100000 stars, and by doing so helped firm one of the first steps on the cosmological distance ladder. In the book I wrote that:

“the success of Hipparcos has led ESA … to consider further space astrometry missions. A proposed ESA mission, cclled the Global Astrometric Interferometer for Astrophysics (GAIA), would measure the parallaxes of one billion stars down to magnitude 20, with a precision 100 times better than that of Hipparcos.”

I gave two possible launch dates for GAIA: 2009 or 2014. When I wrote the book, both dates seemed impossibly distant. But time passes, and GAIA is on track for launch on board a Soyuz rocket in 2013.

Gaia mission will measure positions of one billion stars in the Galaxy

An artist's impression of the Gaia mission
Credit: ESA

In September 2011, the mission team took receipt of the last of ten state-of-the-art mirrors. These mirrors will form the heart of a system that will study a billion stars in our Galaxy, observing each star 70 times in order to pinpoint its location. The GAIA catalogue, which will be finalised some time in 2021, will provide us with a superb three-dimensional map of the Galaxy. The value of this map won’t be so much in developing our understanding of the distance ladder; rather, it will tell us volumes about how the Milky Way formed and subsequently evolved.

Keep an eye out for GAIA.