### is slowly increasing due to the universal expansion.

Watch Video:

Teachable Topics:
• Cosmology
• Hubble's Law
• Hubble Constant

Theory:

While it was merely just a theory at the beginning of the 20th century, it is now commonly accepted that the universe is expanding. This theory was supported by the discovery that almost all galaxies observable from Earth have their spectral lines redshifted, meaning that they are moving away from us. Not only that, but the further away a galaxy is the faster it is moving away. This relationship is Hubble's Law, and is expressed as:

#### v = H0d

where v is the velocity of the galaxy, d is the distance to the galaxy, and H0 is the Hubble constant which is currently estimated to be between 50 hand 100 km/s/Mpc (Mpc referring to mega parsecs).

The velocity of the galaxy according to Hubble's law is also known as the recessional velocity, the velocity of the galaxy due to the universe expanding. As such, it can be said that space itself is carrying galaxies along as the universe is expanding. As the space between the galaxies stretches, the galaxies move further apart. This effect is what the video aims to show.

Apparatus:

• Black light
• Elastic band, cut into a string
• Bungee cord
• Balloon
• Small paper pieces (drawing galaxies on the paper is optional); two pieces for the elastic band, four for the bungee cord, and three for the balloon
• Tape (Scotch tape glows well under a black light)
• Backdrop (for scale); here a black piece of construction paper with a grid made out of tape was used

Procedure:

• Set up the apparatus by positioning the black light and backdrop for the demo.
• Elastic Band Demo: Attach two pieces of paper to the elastic band using the tape. Hold the rubber band under the black light and pull the ends in opposite directions. Observe how the space between the two 'galaxies' is increasing.
• Bungee Cord Demo: Attach four pieces of paper to the bungee cord with the tape, making sure space out all the pieces evenly. Now hold the cord under the backlight and stretch it. The space between all four galaxies should increase.
• Balloon Demo: Attach the three paper galaxies to the un-inflated balloon, and make sure the three are relatively close together. Inflate the balloon, and notice how far the galaxies have moved apart.