How do astronomers know the universe is expanding?
All known elements emit and absorb particular wavelengths of light, which is part of the electromagnetic spectrum. By studying the wavelengths of light (as indicated by ‘lines’ within the electromagnetic spectrum) emitted by an object in space, astronomers can get a range of information. One thing they examine is the change in position of lines in the spectrum from a star—this can tell astronomers how far away the star is, whether it is moving towards or away from us and how fast it is moving.
When looking at the radiation emitted by distant stars or galaxies, scientists see emission spectra ‘shifted’ towards the red end of the electromagnetic spectrum—the observed wavelengths are longer than expected. Something causes the wavelength of the radiation to ‘stretch’. But rather than an actual change in the wavelength, this phenomenon was something similar to the Doppler effect—they only appear stretched relative to the observer. The further away an object is, the greater the shift.
The Doppler effect
The noise of a siren or a car speeding past sounds higher in pitch the closer it gets to you and lower as it moves away. This is called the Doppler effect, where waves, in this case sound waves, change in frequency and wavelength as the source moves towards you (higher frequency, shorter wavelength) or away from you (lower frequency, longer wavelength). There is no actual change in sound; the car isn’t making a different noise. It just sounds different due to the car’s movement relative to you.
This apparent change in wavelength can also be observed for the visible light emitted by stars or galaxies. So, if a star is moving towards Earth, it appears to emit light that is shorter in wavelength compared to a source of light that isn’t moving. Because shorter wavelengths correspond to a shift towards the blue end of the spectrum, this is called blueshift. In contrast, the light from a star moving away from us seems to shift towards longer wavelengths. As this is towards the red end of the spectrum, astronomers call it redshift.
The degree of shift can also give astronomers information about how fast the object is moving relative to us. A faster-moving object has a greater shift in wavelength.
Using various measures to establish how far away the galaxies were, Edwin Hubble (and those that followed him) found that their velocity was always proportional to their distance. The ratio of the two became the famous ‘Hubble constant’ and represents the expansion rate of the universe. But is the expansion rate really constant? Apparently not … and that’s where dark energy comes in.