Hubble's Law & Hubble's Constant

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  • 0:01 Edwin Hubble
  • 0:33 The Hubble Law and…
  • 2:39 Practice Calculation
  • 3:21 Understanding the Hubble Law
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will go over and define the Hubble Law and Hubble constant. We'll not only do a practice problem but also define conceptually what these two things mean to our cosmos.

Edwin Hubble

Edwin Hubble was a famed American astronomer of the 20th century. He contributed so much that all sorts of things are named after him.

You've likely heard of at least one, probably the most famous one, the Hubble Space Telescope. Although it was launched many years after Hubble's death, the telescope, like its namesake, led to many breakthroughs in the understanding of our universe.

Other things that were named after Hubble are the Hubble Law and Hubble constant, the subjects of our discussion.

The Hubble Law & Hubble Constant

In the early 20th century, Edwin Hubble and Milton Humason observed and measured the distances to a lot of different galaxies. The graph they plotted had a horizontal axis of distance to a galaxy in Mpc, megaparsecs, and a vertical axis of apparent velocity of the galaxy's recession in km/s (kilometers/second).

When they plotted their observations, they noticed that their points fell along a straight line. Thus, their observations pointed out a direct (or linear) relationship between a galaxy's velocity of recession and its distance, something known as the Hubble Law.

Graph of the Hubble Law
Graph of the Hubble Law

The Hubble Law is telling us that galaxies farther away from us are also moving away faster from us. A galaxy twice as far away is moving away twice as fast.

This relationship, the Hubble Law, is shown below as an equation. Note how Vr stands for the recessional velocity of a galaxy, d is the distance to the galaxy, and H is the Hubble constant.

The equation for the Hubble Law
Equation of the Hubble Law

The average value of the velocity of recession divided by distance is the Hubble Constant I just mentioned, and it is now believed to be about 70-74 km/s/Mpc (but we'll use 70 km/s/Mpc for our lesson). By the way, the Hubble constant is the slope of the line from the graph you saw before.

Now, the Hubble constant of 70 kilometers per second per megaparsec may be a very confusing thing to hear out loud and understand. What it's telling us in simple terms is that for every megaparsec to a galaxy, the galaxy's speed away from us will increase by 70 kilometers/second.

Working off of this notion you should realize that the Hubble constant helps measure and show us the rate of expansion of the universe and thus, lends credence to the belief that our universe is expanding at a rate of 70 km/s/Mpc.

Practice Calculation

Let's do a practice calculation of the Hubble Law to find the distance to a galaxy. Let's pretend the galaxy's velocity is 1050 km/s. We know that the Hubble constant is set at 70 km/s/Mpc. To figure out the distance, we need to set the distance d equal to the velocity divided by the Hubble constant as shown on your screen. From there, the calculation is simple.

First, you divide 1050 km/s by 70 km/s/Mpc and then you cancel you the units of km/s. This leaves you with a distance of 15 Mpc to the galaxy in question.

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