These notes deal with space and time. The first thing we notice about
the universe around us is how big it is. In order to quantify things in
cosmology very large numbers are required and the endless writing of
zeroes quickly becomes tedious. Thus people invented what is called *
the scientific notation* which is a way of
avoiding writing many zeroes. For example the
quantity `one million' can be written as 1,000,000 which is a one followed by six zeroes, this is abbreviated as 10^{6} (the little number above the zero is called the *exponent* and denotes
the number of zeroes after the one). In this way we have

one billion=1,000,000,000=10^{9}

one trillion=1,000,000,000,000=10^{12}, etc.

So much for large numbers. There is a similar short-hand for small numbers, the only difference is that the exponent has a minus sign in front:

one thousandth=0.001=10^{-3}

one millionth=0.000001=10^{-6}, etc.

In order to get several times the above quantities one multiplies by
ordinary numbers, so, for example, 8×10^{6}=
eight millions, 4×10^{-12}=four trillionths, etc.

This notation is a vast improvement also on the one devised by the Romans,
and which was used up until the Renaissance. For example, our galaxy,
the Milky Way, has a diameter of about 10^{5} light years (a light year
is the *distance* light travels in one year),
in Roman numerals

The Andromeda galaxy is about 2×10