Until well into this century, it was not understood whether the great groupings of
stars that were seen through telescopes were part of our own galaxy, or distant
galaxies in their own right. This puzzle was finally resolved by using Cepheid
variables to establish a distance to the objects like the "spiral nebula" in the
constellation Andromeda, and to determine the size of our own galaxy. By around
1925, Hubble, Leavitt, Hertsprung, Shapley, and others had established conclusively
that objects like the Andromeda "Nebula" were in fact much further away than objects
in our own galaxy and thus were themselves galaxies.
The Expansion of the Universe
Then, in the late 1920's, Hubble, building on results obtained earlier by Slipher,
combined Doppler shift measurements of radial velocities with distance measurements
to conclude that almost all galaxies were flying away from
the Milky Way, and that the velocity of recession was proportional to the distance
from us: the further the galaxy from us, the faster it was receding.
It later turned out that there were systematic errors in the earliest
measurements, associated primarily with a failure to realize that there were two
kinds of Cepheids, failure to disguish in some cases Cepheid and RR-Lyra variables,
and failure to account for the scattering of starlight by interstellar dust.
However, although corrections for these mistakes changed the distance scales by as
much as factors of two, they did not alter the fundamental conclusions:
- There are many galaxies outside of our own.
- These galaxies are all receding from us if we go to large enough distances.
- The velocity of recession is proportional to the distance from us.
That the Universe is not static but expanding helps solve a paradox that has been
known at least since the 1500s but that was popularized by Heinrich Olbers in 1826
and has come to be known as Olber's Paradox. Briefly, if the Universe is
static and uniformly filled with stars and galaxies, one can show that the night
sky should be as bright as the surface of a star. That this is not so constitutes
the paradox. The expansion of the Universe
solves this problem, as is discussed nicely in
The Distant Galaxies are Flying Away from Us
are all flying apart (on very large distance scales), with
the velocity of recession proportional to the distance between them.
The adjacent image, taken by the
Planetary Camera (WFPC2) of the
Hubble Space Telescope, shows many galaxies
billions of light years away. Most of the fuzzy patches are galaxies containing
billions of stars; click on the image to get a larger version.
The galaxies in this image are receding from us at high velocities.
The details of this expansion are
dictated by the value of the
Hubble Constant. The
objects furthest away from us appear to be receding at near the
This expansion of the universe is a result of the original
explosion that created the universe-the big bang.
The big bang did not happen in space and in time; our modern understanding
is that space and time as we presently experience them
are themselves created in the big bang.
it makes no more sense to ask what was before the big bang than to ask what is
north of the north pole.
Here is a Java applet
(written by someone else) that can be used to illustrate the Hubble Law. However,
the reader is warned that it is very slow loading and may not run properly if you do
not have the latest browser.
A Two Dimensional Analogy
The Universe has 3 spatial dimensions, but it is easier to visualize an analogy
to its expansion for the 2-dimensional surface of a balloon.
There is no center. If you stand on any galaxy, all the others will appear to
be moving away from you with a velocity proportional to the distance from you.
An analogy in 2 dimensions is to put dots on the surface of a balloon and blow
the balloon up. As it expands, there is no dot that is the "center", but if
you stand on any dot you will see all other dots moving away from you (and the
rate at which they move away will be proportional to the distance. Dots close
to you will be moving away slower than those further away). The expansion of
the Universe appears to be like this, but in 3 rather than 2 space dimensions,
which makes it much harder to visualize, but it is possible to describe it
It is often simpler to discuss things happening in the expansion of the Universe
if we adopt a vantage point that is moving uniformly with the expansion.
Astronomers call such a vantage point a comoving coordinate system.
The balloon analogy in 2 dimensions illustrates simply the idea of comoving
coordinates. If instead of viewing the expansion of the balloon from the outside
I place myself on one of the dots on the balloon's surface, I appear to be
stationary and I see the other dots moving away from me (and in my immediate area
I see the apparent curvature of the balloon's surface decreasing).
Astronomers will also often speak (in 3 dimensional space) of a comoving
volume. This means a volume of space that is moving uniformly with the
Illustration of Comoving Coordinates in 2 Dimensions
Here are two MPEG movies illustrating the
formation of structure in the Universe that illustrate
the difference between viewing the surface of a balloon from outside or from
comoving coordinates on its surface:
Here are some still frames from the corresponding movies:
Case 1: 2-dimensional expanding surface from outside.
Case 2: 2-dimensional expanding surface from comoving coordinates
Comoving coordinates in the real Universe correspond to extending this idea to 3