Expanding Universe Lab Instructions
Class Copy
Purpose: Create a model to explore the expansion of the Universe and Hubble’s law.
Background and Theory
The Hubble Law tells us that our Universe is expanding. We observe galaxies, find their distances and their velocities, and find that they are all moving away from us. The more distant the galaxy, the faster it is moving away. From this information, we can estimate the age of our Universe. In our model, we will assume that the Universe has always been expanding at the same rate. If we assume this is true, then we know how long distant galaxies have been traveling in order to get where they are today! Hubble looked at the relationship between a galaxy’s distance from us and the speed at which it is moving away from us. When he graphed his results, the galaxies seemed to lie on a straight line – in other words, the velocity of each galaxy was proportional to its distance! This discovery led to what is now called Hubble’s Law. This rate of expansion is called Hubble’s constant (H0). It is written as v = H0 * d.
Question: What evidence do we have that demonstrates the galaxies are moving away from us?
Procedure:
1) Blow up the balloon a little bit. DO NOT TIE IT SHUT!
2) Draw a dot on the end of the balloon furthest from the opening. Label this dot “US”
3) Draw and number 5 galaxies (dots) on the balloon in a straight line.
4) Measure the distance between the reference galaxy and each of the numbered galaxies. The easiest way to do this is to use a piece of string. Stretch it the shortest distance between the two points on the balloon, then measure the string. Record these data in the table. Be sure to indicate the units you are using.
5) Starting with the partially inflated balloon, time yourself as you blow up the balloon until full. Record the time below your data table.
6) Tie the balloon shut.
7) Measure the distance between “US” and each of the numbered galaxies now that the balloon is fully inflated. Record these data in the table in the second column.
8) Subtract the first measurement from the second measurement, record the difference in the data table. This is the distance each galaxy traveled as the balloon expanded.
9) Divide the distance traveled (difference) by the time it took to inflate the balloon to get a velocity. This is how fast the galaxy moved away from “Us” as the balloon expanded.
10) Using your minimum and maximum values, determine the scale you should use on the graph. You will plot the velocity on the y-axis and the distance to each galaxy on the x-axis.
Expanding Universe Lab Name ______Block __
Partially inflated balloon / Distance (cm)
Fully inflated balloon / Difference (cm)
Distance the galaxy
traveled as the
universe expanded / Velocity (cm/s)
How quickly the galaxy is moving away from us
From Us to Us / 0 cm / 0 cm / 0 cm / 0 cm/s
From Us to 1
From Us to 2
From Us to 3
From Us to 4
From Us to 5
Time to inflate from the partially inflated balloon to the fully inflated balloon: ______seconds
Analysis Questions:
1) Draw a best-fit straight line through your data points using a straight edge. A best fit line does not go through every point, but represents the general trend or relationship between the variables. The line should go through (0,0) on your graph. Why?
2) What would happen to the distance between any two galaxies if you inflated the balloon to twice its original size?
3) What would happen to the distance between any two galaxies if you deflated the balloon to half its original size?
4) Find the slope of your best-fit line. To find the slope, identify any two points that lie on the line. These points do not have to be actual data points, but must lie on the line. Calculate the slope by dividing the difference in the y-coordinates by the difference in the x-coordinates. Show your work and include the correct units! This slope is called the Hubble Constant.
Hints:
Coordinates of the 1st point on the line: ( ___, ___ )
Coordinates of the 2nd point on the line: ( ___, ___ )
Since slope is “change in y over change in x”, find the difference in the x and y values!
Slope = (Rise in y) ______= ______
(Run in x)
5) What relationship exists between the speed of the galaxies moving apart and their initial distance from one another?
As ______increases, ______increases/decreases.
6) What evidence do we have that demonstrates that galaxies are moving away from us?