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/H_o = 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. The Hubble 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 H_o. 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 holds.

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 additions.

Contributed by: Dr. Sandra Faber