In the early 1900's (1910's), an important discovery was made by V. M. Slipher. Slipher found that all distant galaxies were receding from the Earth. Actually, he found that all distant galaxies showed redshifts which he inferred to mean that the galaxies were moving away from us. A redshift is a shift in the measured wavelength ( spectrum, spectral formation, Solar spectrum) of an object from its wavelength in a laboratory, i.e., the color of the object changes.
The redshift, z, is defined as
- Redshift ===> z = [W(observed)-W(rest)] / W(rest)
So the redshift is the relative change in the wavelength of the spectral feature. Slipher interpreted the redshift as due to motion through the Doppler shift. Using this, we can re-state Hubble's Law in its more familiar form. For small speeds, we have
- z = speed / speed of light = v/c
so that, if the distance between the observer and the source is increasing (they are moving apart), z will be > 0. If there is no relative motion then, z = 0.
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Hubble demonstrated that the larger the redshift, z, the greater the distance to the object. This can be easily seen in the above where the sources with the smaller redshifts present larger images on the sky. Since we know that angles (apparent sizes of objects on the sky) decrease as you move an object farther away roughly as angle ~ size/distance, smaller appearing objects must be farther away. Hubble found what is referred to as Hubble's Law. Algebraically, we have that
and
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Hubble's Law is more properly interpreted as caused by the expansion of the Universe (other possibilities?) as we discuss later. The expansion of the Universe is one of the fundamental observational results for the Universe and the determination of Ho an important mission. Currently, Ho is found to fall over a range 45 - 90 km/s per Mpc, where Mpc = 1 million parsecs and 1 parsec = 3.1x1013 km.
