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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. See, for instance:
http://epunix.biols.susx.ac.uk/Home/John_Gribbin/cosmo.htm
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