Universe Is Expanding
Edwin Hubble and Milton Humason discovered in the 1920's that the universe is
not static---it is expanding. This is enough to resolve the paradox. As the
universe expands, the light waves are stretched out and the energy is reduced.
Also, the time to receive the light is also lengthened over the time it took
to emit the photon. Because the luminosity = the energy/time, the apparent brightness
will be reduced enough by the expansion to make the sky dark.
The expansion of the universe means that galaxies were much closer
together long ago. This implies that there is a finite age to the universe,
it is not eternal. Even if the universe is infinite, the light from places very
far away will not have had enough time to reach us. This will make the sky dark.
The age of the universe can be easily estimated from the simple
relation of Time = Distance/Speed. The Hubble law, Speed = Ho × Distance,
can be rewritten 1/Ho = Distance/Speed, where Ho is the Hubble constant. Notice
that the expansion time interval = 1/Ho. The Hubble constant tells us the age
of the universe, i.e., how long the galaxies have been expanding away from each
other: Age = 1/Ho. This value for the age is an upper limit since the expansion
has been slowing down due to gravity. Taking the expansion slowdown into account,
we get an age closer to 2/(3 Ho). Still, the age looks like a number ×
(1/Ho), so if the Hubble constant is large, the age of the universe will be
How did Hubble figure out that the Universe is expanding?
Famed Austrian scientist, Doppler (a mentor to a young monk named Gregor Mendel
who discovered the genetic inheritance patterns of traits in peas) was the first
to explain the cause of changes in sound pitch as a train approaches or recedes.
In a little series of powerpoint slides, I hope to give you a simple explanation
of the Doppler Shift as it pertains to sound, and then as it pertains to light.
Since Ragnar the Viking mascot is a close personal friend, I have no trouble
poking fun of him in these slides :)
Slipher discovers that nebulae in the Milky Way are not clouds, but collections
In 1913 V.M. Slipher (I could not find his picture) at the Lowell
Observatory (same place where Lowell saw "canals" on Mars, and where
Clyde Tombaugh later discovered Pluto) reported on the spectra of faint, nebulous
objects in the sky. These were some of the Messier Deep Sky objects that you
will look at on the next page. They nebulae showed a mixture of stellar spectra,
meaning that they were composed of more than a single star ionizing the gas.
In fact, some of the nebulae had Doppler shifts that suggested rotation of the
nebulae. Even stranger, most had red shifts as if they were receding. The red-shift
is discussed in the slides above, and seen in stars within our Galaxy. The nebulae
that Slipher was photographing with his spectroscope showed a shift in location
of the absorption lines of Hydrogen, and the faintest of the nebulae had large
red shifts. His conclusion was that the nebulae might be objects OUTSIDE the
confines of our Galaxy, and that they were receding from us.
Slipher and Edwin Hubble collaborated after this date and began
a serious search of the so-called nebulae.
In 1929 Edwin Hubble published an extensive
study of galaxies, and this is the subject of the following
page, or go back to Olber's Paradox,
or even to the Syllabus
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