![]() The Hubble relation is a (locally) linear correlation between the redshift of a galaxy and its distance from the Milky Way. The next step is using the Hubble relation. Measuring the galaxy's redshift is one step toward determining its distance. The cosmological redshift is due to the expansion of space itself (or more correctly, in terms of the theory of general relativity, the expansion of space- time). The Doppler effect is due to the relative motions of objects traveling through space or another material medium. Remember that the conceptual basis for galaxy redshifts is quite distinct from the Doppler effect. The uncertainty in velocity corresponds to a redshift uncertainty of 120 / 3 x 10 5 = 0.0004, so we would quote the complete measurement as z = 0.0418 ± 0.0004. A typical velocity measurement for a galaxy might be 12,540 ± 120 km/s, which corresponds to a redshift of z = v / c = 12,540 / 3x 10 5 = 0.0418. Redshifts can be measured very accurately. In general, cosmological redshifts are not the same as Doppler shifts. Z = v / cThis equation is actually an approximation that is only valid to describe the redshift of galaxies when the recession velocity v is much smaller than the velocity of light c. ![]() Now we can use the Doppler effect (Δλ/ λ 0 = v/c) to define the redshift in terms of the recession velocity of the galaxy (v) and the speed of light (c): The difference in the two wavelengths (λ-λ 0), which is a positive number, represents how much the wavelengths of the galaxy's light have been shifted to longer wavelengths (Δλ). The wavelength of that same spectral feature observed in a gas in the laboratory is λ 0. The spectral feature will typically be an absorption line of hydrogen, calcium, or magnesium. Suppose we observe a galaxy and label the wavelength of any spectral feature as λ (lambda). The only exceptions are a few very nearby galaxies that are bound by gravity to the Local Group. ![]() Astronomers observe the light from almost every galaxy to be redshifted. We start with the way that redshift is defined. Let us look at the implications of the Hubble relation in a bit more detail. There is no need to convert nanometres to metres as units cancel top and bottom.This graph gives us the Hubble Constant.Hubble showed that the redshift of a galaxy is correlated with its distance from the Milky Way. So if a distant galaxy emits a characteristic spectral line of 91 nm ( ultraviolet light at the 'Lyman limit') but when observed on Earth it appears to be 640 nm (red) we can calculate the red shift using this equation: The following equation is used to calculate redshift: ![]() This is because galaxies are receding (moving away) at such high speeds that relativistic effects need to be considered in calculations. The Doppler equation used for sound calculations cannot be used in this situation. Redshift is also the name of the factor z indicating the relative change in wavelength due to the Doppler shift for a receding galaxy. The upper diagram shows the absorption spectrum from a stationary galaxy with one wavelength of light and no redshift. Lower diagram shows spectrum of light redshifted from a distant galaxy moving away from the Earth This indicated the stars were moving away from Earth (just as the sound of a siren moving away from you has a decreased frequency and increased wavelength).Īs the light was shifted towards the red end of the spectrum (lower frequency/longer wavelength) this phenomenon was termed 'redshift'. The same shifts in frequency and wavelength are also observed for light coming from stars in distant galaxies.īy comparing the light from distant stars with the spectrum of light from our Sun it was noticed that the spectra from distant stars had a slightly decreased frequency and slightly increased wavelength.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |