A Ladder to the Universe: Time and Distance in Space

Figure 1. Ladder of calculating distances in astronomy. The three major rungs are parallax, standard candle, and redshift. Only parallax and redshift will be discussed in this blog post.

The key to understanding how astronomers calculate the distances to the most distant objects begins by taking the first step on the cosmic distance ladder. The first rung of this ladder is called parallax. To visualize this technique, extend your hand and raise your thumb. Now close one eye and take note on the background surrounding your thumb. Open the eye that was previously closed and close the eye that was open, but keep your hand extended. What has happened to your thumb? Does it appear to be moving between closing and opening one eye? Your thumb isn’t moving but the position you are viewing your thumb is changing. This phenomenon is parallax. We take advantage of this property in order to calculate the distances to stars. For Earth’s case, scientists point their telescope at a star, record its location and six months later record the same star’s position. From these measurements, scientist can calculate the angle that the observed star has moved and use that angle and the known distance between the Sun and Earth to calculate the distance to the star.  The diagram below shows how parallax works for Earth. Parallax is limited by the angle that can be measured by scientists. The smaller the angle becomes, the less reliable the measurements become.

Figure 2. Trigonometry of Parallax

The final step to take when finding the distance of the furthest objects in our universe is to find the redshift of the object. Redshift occurs when an object is moving away or towards us. You have experienced this every time you hear an ambulance. Have you ever wondered why when an ambulance is speeding towards you it sounds very loud but once it moves past you the volume decreases? The siren isn’t lowering its value! In fact, this is an example of redshift but with sound. When a star, for example moves away from you, the light it originally released gets stretched, so that it looks redder than it originally is. The opposite occurs when a star is moving towards you; the star appears bluer! This effect becomes more noticeable with objects that are really far away. The value that is given to redshift is related to the amount of time it takes light to travel to your eyes or telescope. By knowing how long it takes the light to reach our eye we can calculate the distance to that object because we know how fast light travels.

Figure 3. Redshift of an object

But why are these distance measurements so important? These values help us understand time in space. When we are looking at the night sky, we are looking at the past. Light travels at incredible speeds but our universe is so large that light becomes delayed. Let’s take our sun for example. If the sun were to explode right now, it would take 8 minutes for us to notice because light takes 8 minutes to travel from the Sun to the Earth. Thus, the objects that are the furthest away from us are the ones that help us peek into the early beginnings of our universe. Once we know the distances to objects, we are able to create a timeline of when things occurred in our universe. Without our ladder to the universe our grasp of our universe would be unbelievably limited.