Doppler Effect- How this daily life phenomenon solved the greatest physics dilemma of all time.
Doppler Effect is the relationship between speed and wavelength which can be observed through everyday experiences. Listen to the music being played in an open ground in a car moving through a constant speed. As you approach near the ground where the music is being played, the music will sound with a higher pitch than it is originally. But as you move away from the ground, the music will sound with a lower pitch. The higher pitch corresponds to sound waves with a shorter wavelength (the distance between one wave crest and the next) and a higher frequency (the number of waves per second) and vice versa. This is because, as the car moves closer to the music source, you will be closer when the music source will emit the next wave crest, hence lessening the distance between wave crests.
This effect was actually used to examine that whether the universe was expanding or it was shrinking. The scientist who performed this phenomenal task was Edwin Hubble. It was previously thought that our universe is static, meaning its shape and characteristics in real time are stationary. Einstein's equations seemed to predict that space is not static, but it is either expanding or contracting. Einstein fudged up his equations and added the cosmological constant that seemed to make the state of universe static. This was one of the great missed opportunities of theoretical physics. If Einstein stuck to his original equations, he could have predicted that the universe is either expanding or contracting. This paved the way for Edwin Hubble who discovered that the universe was expanding. He looked at the stars of neighboring galaxies and measured their distances through their relative light spectrum. Later, he discovered with the help of Doppler Effect(as explained above) that all the galaxies where red shifted.
As per Hubble's situation, the galaxies which move away from earth is red shifted and the galaxies which move closer to earth is blue shifted (blue light wavelength is lesser than red light). This way Edwin Hubble through his telescope proved universe is expanding as all galaxies and stars he observed are red shifted. These telescope observations revealed that the farther other galaxies are from us, that faster they are moving away from us. The universe is expanding, with the distance between any two galaxies steadily increasing with time. This discovery removed the need for cosmological constant in the equation. Einstein called cosmological constant as the greatest mistake of his life. So, if the galaxies are moving faster with increase in the distance from us, this also proves that our universe is expanding with constant acceleration. Quite fascinating isnt it?