INTERLUDE 26-2 The Age Controversy Revisited
In Section 26.4 we note a potentially serious discrepancy in the estimates of the age of the universe yielded by two independent branches of astronomy. For H0 = 65 km/s/Mpc, cosmologists calculate that the likely range of cosmic ages is 10—15 billion years, with the upper limit (1/H0) corresponding to a uniformly expanding universe devoid of matter, and the lower limit (two-thirds of this value) to a marginally bound universe. In part because observations seem to indicate a cosmic density of at least a few tenths the critical value, and for theoretical reasons to be discussed in Section 27.4, a value closer to the lower limit is favored by many cosmologists. However, this estimate may be at odds with the ages of the oldest star clusters in our galaxy, as determined by analyses of the turn-off point for main-sequence stars on the H—R diagram. (Sec. 20.5) For example, the age of the globular cluster 47 Tucanae, according to this method, is at least 12 billion years. Hence the paradox at hand: some stars seem to be older than the universe itself.

This problem is not really a new one. Despite its notoriety in the media lately, the essence of the problem has been around for well over a hundred years. In the 1860s Lord Kelvin sought to assess the age of Earth by making two assumptions—that Earth probably formed at the same time as the Sun, and that the Sun shone probably because it burned some chemical, such as wood or gasoline. The first assumption was a good one (indeed, we make it today), but the second one was most definitely not—the Sun, most assuredly, is not made of wood! In any case, based on the amount of sunlight radiated by our star, Kelvin came up with an estimate for the age of the Sun, and hence Earth, of at most 100 million years. But this value conflicted with the amount of time needed by Charles Darwin to explain the fossil record in terms of evolution by natural selection; long-dead life forms seemed to be at least several hundred million years old. How could life on Earth be older than Earth itself?

This early age discrepancy eventually went away. Once radioactivity was discovered around the turn of the twentieth century, geologists learned to measure the ages of rocks directly. They found that Earth was several billion years old—old enough to explain Darwin's fossils. Much later, physicists unraveled the truth about solar energy generation and realized that it could sustain the Sun's output for many billions of years, and the paradox was fully resolved. (Sec. 16.5)

The problem resurfaced in the 1930s. At issue then was Edwin Hubble's first measurements of H0. (Sec. 24.5) Owing to observational uncertainties in the brightnesses of the galaxies, and also to calibration errors in the analyses of the acquired data, he found a very high value for H0 of around 500 km/s/Mpc, meaning that 1/H0 2 billion years—less than the age of Earth. Again, the discrepancy gradually went away. This time what helped most was painstaking work by many astronomers: improved observations of more galaxies, careful measurements of their brightnesses, and better analyses of the data. By

the 1950s the value of H0had diminished fivefold, and therefore 1/H0 increased fivefold, so the inferred age of the universe became around 10 billion years, safely older than Earth, and the problem dissipated—for a while.

The issue reemerged yet again in the 1980s. Some observers measured a value for H0of around 90 km/s/Mpc, implying 1/H0 11 billion years. At the same time, stellar evolution theorists maintained that the oldest globular clusters had to be at least 14 billion years old. Again, improved observations have partly alleviated this problem, although it has not yet been completely eliminated. Reanalysis of the abundance of helium in globular clusters, together with a general increase in estimates of their distances, suggest that the ages of these clusters may have been their ages overestimated by almost 20 percent, reducing the 14 billion year limit to the 12 billion quoted earlier. At the same time, a recalibration of the cosmic distance scale has resulted in an increase in galactic distance estimates, a corresponding reduction in H0, and hence an increase in the "cosmological" age of the universe. (Interlude 17-1)

The value of H0 is still being vigorously debated at observatories around the world. Some researchers continue to maintain that a high value, perhaps as much as 80 km/s/Mpc, is correct, in which case the age problem persists. Others, who have always favored a lower value—50 km/s/Mpc or less—point out that in their universe, the latest "age problem" never existed in the first place! The choice of H0 = 65 km/s/Mpc adopted in the text is a compromise value, favored by many moderates, between these extremes.

The accompanying image shows the latest battleground in the quest to determine H0, the barred spiral galaxy NGC 1365 in the Fornax Cluster, another rich cluster of galaxies like the Virgo Cluster, lying about 15 Mpc away. The chevron-shaped insert at the right shows a true-color magnification of the outskirts of this galaxy, where about 50 Cepheid variable stars are being monitored by the Hubble Space Telescope in an attempt to measure accurately the galaxy's distance and hence the value of H0.

This area of forefront research will continue, as will perhaps the controversy. Keep in mind, though, that both age estimates—of stars and of the universe—are very close to each other. Remarkably, we are trying to specify a couple of key cosmic numbers to well within a factor of 2, when many other cosmologically important numbers are known only to within a factor of 10. Don't lose sight of the larger significance of what's involved here: astronomers on Earth are today probing the universe and attempting to pin down one of the crucial parameters of the universe—its age. Our generation is likely to find the answer, and we are the ones who will then put this important number into the books for all our descendents to read. It will be one of the legacies that we leave to the future.