The observed wavelength of a wave depends upon the relative motion of
the source and the observer. If either is moving toward the other, the
observed wavelength is shorter; if either is moving away, the wavelength
is longer. Because the light seen from almost all distant galaxies has
longer wavelengths than comparable light here on earth, astronomers believe
that the whole universe is expanding.
| NSES Content Standard D
Earth and Space Science: Origin and evolution of the universe Grades 9-12, page 190 The origin of the universe remains one of the greatest questions in science. The "big bang" theory places the origin between 10 and 20 billion years ago, when the universe began in a hot dense state; according to this theory, the universe has been expanding ever since. |