The Expansion of the Universe

From the Hubble Law to the Hubble Space Telescope, and Beyond!

About Hubble’s Law

Hubble’s Law shows our Universe to be ever-changing, expanding its limits in space and time simultaneously.  In 1929, the American Astronomer Edwin Hubble made a simple graph showing that galaxies farther from each other move away from each other systematically faster.  He drew a simple straight line on the velocity vs. distance plot shown below, summarizing the trend that now bears his name as Hubble’s Law, represented today as: Velocity = (Hubble’s Constant) x Distance
This law implies that the Universe is expanding, and Hubble’s Constant measures how fast. These findings were revolutionary in the 1920s.

How do Hubble’s galaxies look through the Hubble Space Telescope? 

In the interactive version of Hubble’s graph below*, yellow circles mark individual galaxies, and green labels show identifiying NGC numbers (or names) for each galaxy.  Click on a yellow circle below to learn more about each galaxy and see a Hubble Space Telescope image of it superimposed on all-sky images.  Online graph URL goes to:

 *This graph is based on his original published figure.

How correct is this graph?   

A1. Hubble’s mis-labeling of velocity units as “KM” instead of “KM/S” has not been corrected here.

A2. The actual “Hubble constant” (slope) one would derive from this plot is about 500 km/s per million pc (Mpc), but the modern value is about 70 km/s/Mpc. 3.) Hubble’s measurements of  “velocity” did not use the same system as in modern times, so a couple of the galaxies may be mis-labeled here.  Please let us know if you figure out which they are!

Where can I find more information about Hubble’s graph and Law?

The original version of Hubble’s plot appeared in his 1929 paper, A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae. An explanatory article about Hubble’s Diagram and Cosmic Expansion was published by Robert Kirshner in 2004 in PNAS, and its a great resource for learning more about Hubble’s diagram and its significance.

For reference, the expert-level caption for Hubble’s original graph given in Kirshner’s PNAS article reads: 

“Radial velocities, corrected for solar motion (but labeled in the wrong units), are plotted against distances estimated from involved stars and mean luminosities of nebulae in a cluster. The black discs [encircled in yellow in this site’s interactive plot] and full line represent the solution for solar motion by using the nebulae individually; the circles and broken line represent the solution combining the nebulae into groups; the cross represents the mean velocity corresponding to the mean distance of 22 nebulae whose distances could not be estimated individually.” 

Caught your Interest?  KEEP READING about Hubble's Law!


What's next for this project?

We are planning to showcase Hubble’s story in an introductory data-science curriculum, using a simplified interface of NSF & NASA-supported glue and WWT software.  Educators with an interest in data science who would like to know more, please contact