A PARADIGM SHIFT 241
Doppler effect (a change in the
wavelength of light due to relative
motion between source and
observer), and therefore indicated
that the nebulae were moving away
from us at very high speeds—far
too fast for the Milky Way’s gravity
to keep hold of them.
Measuring the universe
By 1922–23, Edwin Hubble and
Milton Humason of California’s
Mount Wilson Observatory were in
a position to end the mystery once
and for all. Using the observatory’s
new 100 in (2.5 m) Hooker Telescope
(the largest in the world at that
time), they set out to identify
Cepheid variables shining within
the spiral nebulae, and this
time they were successful in
finding Cepheids in many of the
largest and brightest nebulae.
Hubble then plotted their
periods of variability and therefore
their absolute magnitude. From
this, a simple comparison to a
star’s apparent magnitude revealed
its distance, producing figures that
were typically millions of light
years. This proved conclusively
that the spiral nebulae were really
huge, independent star systems, far
beyond the Milky Way and rivaling
it in size. Spiral nebulae are now
correctly called spiral galaxies.
As if this revolution in the way
we see the universe were not
enough, Hubble then went on to
look at how galaxy distances
related to the redshifts already
discovered by Slipher—and here
he found a remarkable relationship.
By plotting the distances for more
than 40 galaxies against their
redshifts, he showed a roughly
linear pattern: the farther away a
galaxy is, on average, the greater
its redshift and therefore the faster
it is receding from Earth. Hubble
immediately realized that this
could not be because our galaxy is
uniquely unpopular, but must beIn 1842, Christian
Doppler (p.127)
showed that if a light
source is moving
toward us or away
from us, the light
waves arrive at
different rates. If
the light source is
moving toward us,
we see a bluer color
as waves bunch
together at the blue
end of the light
spectrum; if it is
moving away, we
see a redder color.
Hubble guessed that
sodium light was
the same color in
far galaxies as it is
on Earth, but the
Doppler effect meant
that it would be
blueshifted or
redshifted if moving
toward or away
from us.the result of a general cosmic
expansion—in other words, space
itself is expanding and carrying
every single galaxy with it. The
wider the separation between
two galaxies, the faster the space
between them will expand. The
rate of expansion of space soon
became known as the “Hubble
Constant.” It was conclusively
measured in 2001 by the space
telescope bearing Hubble’s name.
Long before then, Hubble’s
discovery of the expanding
universe had given rise to one
of the most famous ideas in the
history of science—the Big Bang
theory (pp.242–45). ■Equipped with his five
senses, man explores the
universe around him and calls
the adventure science.
Edwin Hubble