1100 1150 1200 1250 Emitted wavelength (Â)

Fig 19.18.

Spectrum of 3C273, showing the Lyman alpha line in emission and a number of other lines in absorption.The lower spectrum is of a quasar with z = 3.48, in which there are many absorption lines at lower redshift, the Lyman-alpha forest, corresponding to intervening material. [William Keel, University of Alabama, NASA, and L. Lu, Keck Observatory]

is not proper to think of the redshift as being a Doppler shift. We will see in Chapter 20 that a wavelength shift of 15% means that, in the time it takes light to get from 3C273 to us, the separations between all pairs of objects have increased by 15%. If we approximate the expansion rate as constant, then this means that the light has had to travel for 15% of the Hubble time to reach us. If the distance is d, then the travel time is d/c, so d/c = 0.15 (1/H0)

Ha line, whose rest wavelength is 656.3 nm, was observed at about 760.0 nm. This is a 15% shift in wavelength.

In Chapter 20 we will see that Hubble's law, as presented in Chapter 18, is not a useful description for finding the distance modulus to an object with a very large redshift. This is because it

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