A Bare-Bones Big Bang Model
In motivating the Big Bang (BB), I start
simply with a bare-bones bang emerging out of a moment of very high density at
some finite time in the past. This
picture has two inescapable implications.
First, the Universe has a finite age, and therefore all objects in it should be younger
than it is. The time since
the BB is the expansion age of the Universe, which is simply how long the
Universe has had to expand in order to get to its present size. Mathematically, a rough estimate of the
expansion age is given by the inverse of Hubble's constant, 1/Ho =
d/v, where d is the distance of a galaxy and v is its recessional velocity.
Getting the age this way is precisely like estimating the time it would take
you to drive from Washington, D.C., to New York City based on the distance to
New York and your present speed. It turns out that galaxies are rather like
cars - it's easy to measure their speeds using a speedometer that astronomers
have at their telescopes called the “Doppler effect.” But getting their
distances is harder. In your case,
you'd look up the distance to New York in some table of inter-city distances,
but there is no such table for galaxies.
Space Telescope (HST) is making great contributions here by allowing
us to measure distances to nearby galaxies to an accuracy of about +/-10%.
The latest expansion-age measurements from HST
are converging on an expansion age of about 11-14 billion years. How does this compare to the ages of the
oldest stars? For technical reasons,
the ages of stars can be accurately measured only in groups - star clusters or
galaxies. The oldest star clusters in
our Galaxy are turning out to be almost 14 billion years old, near the upper
edge of the allowed expansion-age window.
Most astronomers consider this to be a promising agreement, others an
age that is uncomfortably large for Ho. The point here, however, is
that no very
old stars have been found - the upper limit of roughly 14 billion years for
stellar ages seems to be universal.
This is in contradiction to an infinitely old steady state universe (see
below), which would contain stars over a wide range of ages including some
exceedingly old ones. The statement
that there are no very old stars in the Universe could have been criticized a
short while ago, as accurate ages were limited to star clusters in just our own
Milky Way, which might happen to be a relatively young galaxy in an old
universe. However, HST and a new wave of very
large ground-based telescopes have succeeded in age-dating a wide array of
stellar populations in many galaxies, and the 14 billion year limit still
The second implication of the bare-bones Big
Bang model is that the Universe should look different at large
distances. This is because
looking out into space is also looking back in time, due to the finite travel
speed of light. The most distant
objects visible in our telescopes carry us back in time to within a couple of
billion years of the Big Bang, when the Universe was much smaller and all the
galaxies within it were much younger.
It is reasonable to expect that galaxies and their contents have changed
greatly over such a span of time.
The first clue that the Universe really looks
different far away came in the 1950's and 60's, when it was found that radio
galaxies and quasi-stellar objects (QSOs) seemed to be more frequent as one
looked farther away. The prevailing
picture now is
that QSOs and radio galaxies are both formed when gas rains down onto a massive
central black hole at the center of a galaxy, surrounding it with a swirling
disk of glowing white-hot gas on its way into the hole. Plausibly there was more loose gas early in
the Universe, before it condensed into stars.
This loose gas fell more copiously into black holes at that time, making
them shine more brightly. The same holes still exist at the centers of galaxies
today but are dim now because their gas supply has dried up.
Another even clearer piece of evidence for a
different Universe in the past again comes from HST, whose 250-hour “Hubble
Deep Field” exposure is the deepest picture of the Universe ever taken. As Joel Primack discussed, Ken Lanzetta's beautiful video takes us on a fly-through down the
Hubble Deep Field “core-drilling” through space-time. Large, mature galaxies
are present in the foreground, giving way roughly 10 billion years ago to
small, unformed lumpy proto-galaxies in the process of formation. Finally space
becomes empty as we fly past the earliest galaxies and into the cosmic “Dark
Ages,” the pre-galactic era before any galaxies had formed.
My arguments so far do not yet capture the
deep appeal that the Big Bang has for cosmologists. The real attraction of the bare-bones BB is that it can so easily
be embellished with several other ideas which can explain a great many more
phenomena beyond those already mentioned. The Big Bang is like a Christmas tree
on which many theorists have hung ornaments.
Over time the tree has grown steadily more beautiful until now it is
well nigh irresistible. The next three
sections explore these important
Contributed by: Dr. Sandra Faber