# If one object has a large redshift and another object has a tiny redshift what can we say about the two objects?

Nov 28, 2015

The two objects are moving away from the observer at different relative velocities.

#### Explanation:

Redshift and blueshift are phenomena that occur when an object is moving relative to an observer. It is called the Doppler effect. If an object moves towards an observer, it will have a blueshift. The frequency of light, sound and any other information sent by the object and picked up by the observer will start to increase. If the object moves away from the observer, it will have a redshift and the frequencies of light, sound or any other information sent by it will decrease.

The magnitude of redshift and blueshift depend on the velocity at which the object is moving relative to the observer. The magnitude is greater for a greater velocity. So if an object has a greater redshift, it implies the object is moving away from the observer with a greater relative velocity. Likewise, an object with a tiny redshift means it is moving with a smaller relative velocity.

Hence, if an object has a large redshift, it can be said that the object is moving away from the observer with a greater relative velocity while the object with a tiny redshift is moving away from the observer with a smaller relative velocity.

However, the above conclusion can only be made if it is assumed that both objects are moving through the same medium. Doppler effect is also affected by the gravitational field strength of the medium through which the object is moving.

Although two objects might be moving away from, or towards, an observer at the same relative velocity, they will have different amounts of redshift, or blueshift, if one of them is travelling through a region of high gravitational field strength, like near a supermassive star or a black hole, while the other is travelling through a region of low gravitational field strength, like in empty space.