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The Question

(Submitted November 24, 1997)

I'm a 13 year old student from Denmark, who wants to know how big the Universe is and how the size of it is measured.

The Answer

The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked * below).

We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.

(If any of these terms like 'parallax', 'Cepheid' and 'red shift' are unfamiliar, try entering them in the search window on our home page).

The determination of greater and greater distances is one of the great themes of astronomy. Most introductory books will give you an outline of the story, which you can then fill in to any level of detail with further reading.

Our website has a lot of material on recent developments. For instance, there are already several answers in the 'Ask an Astrophysicist' archive which deal with the size and age of the Universe. If you enter things like 'size of the Universe', 'age of the Universe', or 'distance scale' in our search window you will get lists of links to many of the most relevant discussions.

Paul Butterworth
for the Ask an Astrophysicist team

* Note: The observable Universe may be only a small part of the physical Universe. In some theories, the Universe may have expanded very fast just after the 'big bang', and only a little bit may have remained within range of detection.

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