[Rosemary Nocera]

Rosemary Nocera  asked the Naked Scientists:
Hi - I was just listening to your podcast of Nov 2005 "Stars, Cosmology and the Beginning of the Universe" and you're saying the age of the universe is 14.9 billion. Were we live it's 13.7 billion - what's the deal?
What do you think?

[blakestyger]

You have to start with measuring distances and distances within the Solar System are known to extreme accuracy by a variety of methods, including the motions of the planets in the sky, radar, and timing of signals from interplanetary probes. Distances to stars within a couple of thousand light-years come from various geometrical methods; the most accurate values are those based on measurements of the annual parallax of about 10,000 nearby stars made by the Hipparcos satellite. The moving cluster method can be applied over a similar range, while main-sequence fitting works with open clusters out to a distance of about 60,000 light-years.

Beyond the Milky Way Galaxy, distances can be established most reliably using the period-luminosity relation of Cepheid variables, backed by similar observations of other bright stars whose intrinsic brightness is reasonably well known, including RR Lyrae stars and novae. This method can be applied out to the limit at which Cepheids and other individual stars can be distinguished inside their host galaxies - up to about 100 million light-years.

For more distant galaxies, standard candles brighter than Cepheids are needed. These include globular clusters and Type Ia supernovae, which can be calibrated as distance indica-tors using Cepheids in relatively nearby galaxies and then applied further afield-up to about 200 million light-years for globular clusters and out to at least 3 billion light-years for super-novae.

At the furthest limits, only whole galaxies are detectable, so other methods are used - which link measurable properties of galaxies, or clusters of galaxies, to their luminosity. Extragalactic distance indicators enable estimates to be made of the Hubble constant, a measure of the rate at which the universe as a whole is expanding. Observation of the redshift of a remote galaxy or quasar then supplies the object’s distance. This method, however, carries a lot of uncertainty, because not only do estimates of the Hubble constant vary widely (between about 50 and 100 km/s/Mpc) but it has been suggested that the constant itself may vary over time.

It should be remembered that the chain of overlapping methods used to establish distance scales for objects in the Universe has errors and uncertainties that can creep in at every step. Crucially, each step inherits all of the problems of the one below and the errors intrinsic to each step tend to get larger for the more distant objects. Real precision at base of the ladder (Solar system) degenerates into much greater uncertainty at the top.

Sorry it it's a bit wordy but the point I'm coming to is that the age of the universe (the Hubble Time) is derived from the Hubble Constant which is subject to all the errors mentioned above. The accepted value today is 13.2 billion years.