An example with sound

A simple explanation comes from the pitch of sound. When an ambulance is passing in front of you the pitch is going up. Imagine an ambulance is approaching close to you, the pitch of the siren will be going up. When it passes through and far away (receding) from you, the pitch will be going down. From these observations, we can draw some comments:

• Approaching object gives away high pitch sound
• Receding object gives away low pitch sound
• Receding star gives away red color

Similarly, if a star is moving far away from the Earth, the light that a telescope captures from the star turns red.

This is called Doppler effect, and now comes the calculation:

f’ = V / (V + v) x f (when the object letting the sound out is moving)

f’ = (V - v) / V x f (when the object is moving, relative to the air)

Where:

V is the speed of sound and v is the speed of the object perceiving the sound.

Consider the example: a police car is moving west at 20m/s toward a driver who is moving east at 25m/s. The police car emits a frequency of 900 Hz. What frequency is detected by the driver? (The speed of sound in air at 20C is 343m/s).

In case of light

f’ = square root of((V - v) / (V + v)) x f where V is the speed of light and v is the relative speed between the object producing light and receiving light.

Yes, the Universe is expanding

The Universe is not static, it rather exists in 3 possible states: warp, twist, and expand. Today, more and more evidence, especially redshift, verifies the expansion theory throughout the observable universe. As a result, the Universe as a whole is getting bigger and, at the same time, getting colder. When returning to the past (Big Bang), it was smaller and hotter.

E = hv = hc / lambda (h is Planck constant)