Mona Cromb, of Green Island, asks :-

How can astronomers look 13 billion years into the past?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

The universe is currently believed to be 13.7 billion years old. Astronomers can't see things quite that far back, but most of the way.

The currently accepted idea for the origin of the universe is the Big Bang theory. In it the universe began as a very small and very hot blob of matter and radiation. The blob quickly expanded. As it expanded, the matter and radiation cooled. Around 300,000 years after the Big Bang, the temperature of the universe had cooled to 3000 C. This allowed atoms to form. Atoms don't interact with light and heat radiation. So the radiation that filled the universe at the time continued to cool.

Space has expanded by a thousand times since atoms formed. So the Big Bang radiation has now cooled to 3 degrees above absolute zero or 3 degrees Kelvin. The left-over radiation can be detected by radio telescopes at microwave frequencies. Tiny variations in the temperature measured over the sky tell us a lot about the hot gas at the time when atoms formed. Other analysis of this radiation tells us that the universe is 13.7 billion years old. Light travels at about three hundred million metres per second so the further the object is away from us the longer ago that light was emitted. Hence we are looking into the past history of the universe.

Light gets stretched, becomes redder, as space expands. The reddening is called the 'red shift'. By measuring the red shift of a distant galaxy we can tell how much the universe has expanded since the light left it. Certain kinds of stars have a particular brightness. By seeing how bright one appears in a distant galaxy, we can calculate the galaxy's distance. By comparing the distance and the red shift we can then tell how fast the universe is expanding. From that we can also estimate the age of the universe.

Using giant telescopes and sensitive digital cameras, astronomers can currently see galaxies whose light has been travelling for 13 billion years.