Lab for Jul. 14 (The Hubble Deep Field) - SFSU Physics & Astronomy

Part 3: The Mass and Density of the Universe
(i) The Mass of the Universe
Now, let’s estimate the mass of one galaxy, in terms of the mass of the sun (solar units).
We’re going to assume that the Milky Way galaxy, our galaxy, is pretty typical. There are
about 400,000,000,000 (400 billion) stars in the Milky Way. For this exercise, let’s assume
each star has a mass equal to the mass of the Sun, 2 x 10 33 grams.
Q7: Calculate the mass of the Milky Way galaxy in grams, using the information in the above
paragraph.
Q8: Now multiply your result from Q7 by the total number of galaxies in the universe to find
the total mass of all galaxies in the universe.
Now we know how much observable matter there is in the universe. We will use this number
to say something about the fate of the universe. For example, is there enough matter in the
universe for it to eventually collapse? Another way to ask the question is: Is the gravitational
force between all the galaxies large enough for the universe to eventually pull itself back
together? Well, the gravitational force depends on the mass AND the distance between the
masses.
(ii) The Size of the Universe
We do not know the size of our current universe very accurately, but we can estimate it. We
know that light travels at 3.0 x 10 10 centimeters per second (cm/s). If we assume that the
light from the most distant galaxies was emitted roughly ten billion years ago, we can
calculate the distance.
Q9: How many centimeters are there in one light year? Another way to think about this
question is: how far (in centimeters) can light travel in one year if the speed of light is 3 x 10 10
cm/s?
Q10: The farthest galaxies are 10 billion (10,000,000,000) light years away. Express this
distance in centimeters.
(iii) The Density of the Universe
The formula for the volume of a sphere in terms of its radius, r, is: V = (4πr 3 )/3.
Q11: Assuming that the universe is roughly spherical, and its radius is the distance to the
farthest galaxies, calculate the volume of the entire observable universe in units of
centimeters-cubed (cm 3 ).
Q12: Finally, determine the density of the universe in grams per centimeter-cubed (g/cm 3 ).
The density is the total mass (which we figured out in Q8) divided by the volume (from Q11).