SPACE AND TIME
Measuring the Age of the Universe
by Wendy Freedman
Knowing the age of the Universe and realising that it has a beginning is one of the most profound discoveries in modern science. This discovery began in the 1920s when the astronomer, Edwin Hubble measured the distance and speed of our neighbouring galaxies, to discover that they are speeding away from us.
Using the 100 inch telescope at Mount Wilson Observatory, Hubble also discovered that the furthest galaxies are travelling at a faster rate than closer galaxies. This observation is known as Hubble's Law. Hubble had discovered that the Universe was expanding, speeding apart with everything in it.
This revolutionised cosmology and lead to a question that has been debated for 70 years: how old is the Universe?
Hubble's observation, coupled with Einstein's General Theory of Relativity, meant that in the past the Universe was more compact. Once, all known objects in the Universe were tightly compressed together at unimaginable temperatures and densities. It is at that point that the Universe, as we know it, is thought to have originated.
The idea of the Big Bang - or creation of the Universe - is a direct result of Edwin Hubble's work. Since his time, hundreds of astronomers have spent years trying to date the Big Bang, but only recently has the quest become realistic.
Dating the Universe
To measure the age of the Universe the distance and speed at which galaxies or 'Cosmological Distance Indicators' [CDIs] are moving away from us must be established. CDIs include bright young, pulsating stars [Cepheid variables], exploding white dwarf stars [Type 1a Supernova], and even entire galaxies.
The Hubble Space Telescope, named after the pioneering astronomer, was launched in 1993, and enabled astronomers to examine distant galaxies billions of light years away. With its 2.4 metre mirror, the Hubble Telescope focused on galaxies and other ancient sources of light in the Universe.
It looked at Cosmological Distance Indicators, objects whose size or brightness is sufficiently large [and yet at the same time well understood and calibrated] that they are seen from great distances. Using the apparent dimming and/or decrease in size with increased distance, the Hubble Telescope was used to observe these CDIs and gauge their distances.
Measuring the speeds at which CDIs move apart is also required to determine the age of the Universe. We can calculate when in the past the CDIs were touching [even though at the very beginning there was only matter - not fully formed galaxies], to date the Universe. We measure their speed using light frequency, known as 'redshift'.
Because galaxies are moving away, their light frequency decreases and the wavelength of the light received by us increases. This shifts the spectrum of colours from blue [for galaxies close to us], towards the red end of the spectrum. The further away a galaxy, the redder its light.
Edwin Hubble's observations led to a mathematical quantity to measure the speed that galaxies travel apart. Known as the Hubble constant, this measurement provides the conversion factor between cosmological distances and cosmological redshift of galaxies. Using the Hubble constant and information about the expansion of the Universe from the Hubble Space Telescope, we have been able to measure the age of the Universe to between 12 and 14 billion years old.
Dr. Wendy Freedman is one of the leaders of an international team of 27 astronomers at the Observatories of the Carnegie Institution of Washington, America.
