The Expansion of the Universe:
(The Hubble Law)
1. All galaxies beyond the Local
Group are moving AWAY from us.
Their spectra are all REDSHIFTED (c.f. the Doppler Effect).
2. More distant galaxies
are speeding away from us faster than nearby galaxies
3. Facts #1 and #2 imply that the Universe is expanding.
4. There is NO CENTER to the expansion: the Universe is
infinite, AND is expanding everywhere.
5. These facts imply that the
expansion started a finite time ago -- roughly 14 billion years ago,
in what has
been dubbed THE BIG BANG.
6. Asking what happened BEFORE
the Big Bang is kind of like asking what is south of the south pole:
time as we
know it was not a linearly increasing quantity then.
7. Given the Hubble Law, we can derive the distance to any
galaxy, if we have a spectrum of it.
Still, astronomers ask the questions:
"Where did we come from? How did the universe begin? Where are we
going? Why is the universe the way it is? The questions are
clear and deceptively simple, but the answers have always seemed well
our reach -- until now."
--- Stephen Hawking, see Stephen
1. The distances to galaxies can be found using "Standard
A Standard Candle is any object we can identify whose
MAGNITUDE or LUMINOSITY is known.
So, if we can measure it's APPARENT MAGNITUDE, we can
We've already talked about two
(1) Cepheids (pulsating stars, period of
pulsation --> luminosity)
(2) Type Ia Supernova (except for a small
correction easily made from measuring how fast they dim, they are
The book calls Type Ia Supernovae "white dwarf supernovae" since they
are not the kind of
supernovae that ressult directly from massive star evolution, they are
the type where a white dwarf accretes material from a stellar companion.
There are a few other techniques, described in the text,
the Tully-Fisher Relation, where you measure the velocity
the disk of a spiral galaxy and from that derive the luminosity of the
2. The HUBBLE LAW states that the Doppler Shift of a galaxy is
to its distance.
Virtually all galaxies have spectra which are redshifted, that all the
are moving away from the Milky Way.
Hubble made the following plot, now known as the Hubble Law:
v = Ho d
where v = the speed of the galaxy moving away from us
= the distance to the galaxy
Ho = a constant
number, called the
Here's the best version of this plot, Published in The
Astrophysical Journal, Volume 553,
Issue 1, pp. 47-72, 2001.
Note how much further in Distance the X-axis goes.
3. Hubble's Law means that the Universe is Expanding.
(Image from www.astronomynotes.com/galaxy/s7.htm)
See Interactive Figure 1.16
The distant galaxies are all moving away from us.
However, we are NOT at the center of expansion.
Every galaxy sees all other galaxies moving away.
The expansion of the Universe has no center.
The galaxies are not really moving, the Doppler Shifts are the result
the expansion of space. Sometimes the spectral shifts are
referred to as "cosmological redshifts" since they result from space
We assume that the Universe is infinite, it has no edge. However,
YOU are not expanding, however, since you are held together by
The Milky Way, Local Group, Clusters of Galaxies -- are not expanding,
are bound by gravity.
I like to visualize this with M. C. Escher's drawing:
The galaxies are at the cubes, and the rods between them are expanding.
4. If you play the movie backwards, space was infinitely
a point in the past. The expansion started in an event called THE
The Big Bang occured 13.7 billion years ago.
What came before the Big Bang? That's like asking what's south
the South Pole. At the time of the Big Bang, time was not the
linear quantity we know it to be today.
5. Since the Universe has a finite age, there is a
We can't see farther than 13.7 billion light years, since light from a
farther than that has not had enough time to reach us.
However, we assume that the Universe is infinite, and that what we see
a fair sample of the Universe.
6. Having trouble believing the expansion? You're
Einstein's Theory of General Relativity predicts
should expand. Einstein recognized this, long before Hubble
the Hubble law. Expansion of the Universe seemed so implausible
Einstein that he added an arbitrary constant, called the cosmological
constant, to his equations
so that the Universe would not expand.
Hubble found that the Universe is expanding, Einstein called the
constant, "the biggest blunder of my life".
Einstein at Mt. Wilson, visiting the telescope used to find the