The issue of absolute ages of cosmic systems as well as their relative timescales is an important one in cosmic evolution. For the cosmic-evolutionary narrative to match a reasonable approximation of reality, all those ages must be consistent, each arranged sequentially along the arrow of time. Here, we intentionally become a bit more technical, mainly to give those readers who wish it a slightly deeper treatment of the central topic of time.

The age of the Universe has been a particularly vexing quandary for decades. Teams of researchers joust in heated argument, and not with just a little acrimony. Clear-cut biases are evident and reputations of some astronomers are on the line. The media, too, has caught on, viewing this issue as another kind of "Hubble wars" while claiming, often wrongly, all sorts of dire consequences for big-bang cosmology. Yet long-standing age problems among principal systems—cosmos, stars, Earth, life, humanity—have plagued science off and on for well more than a century.

In two paragraphs, here is a statement of today's concern: The simplest analysis of a uniformly expanding Universe implies an age of some 14 billion years. This is based on a Hubble constant of 22 km/s/ million light-years, our best current value specifying the rate at which the Universe expands. (Again, in units more commonly used by astronomers, this number is closer to ~70 km/s/Mpc.) That is, for each additional million light-years of distance, the galaxies seem to recede with an added 22 km/s, in accord with the established finding that distances and velocities of galaxies are well correlated. However, this age is correct only if the cosmic density is much lower than the “critical density” of a marginally bound Universe whose space balloons to infinity—namely, a Universe containing little or no matter. (Mathematicians say that such a Universe has a “trajectory” that will then stop at infinity, but since nothing can actually reach infinity, this is tantamount to the Universe expanding forever, much like the analogy of a rocket completely escaping its parent planet.) If the Universe does have matter (and of course it does) and if its total density does equal the critical value (which many astronomers favor), then the Universe might ordinarily be expected to decelerate with time, owing to the mutual gravitational attraction of matter everywhere, making the true age less than 14 billion years. This is one of the more famous Einstein solutions to his field equations, for which the Universe’s age, using today’s value of Hubble’s constant, would be more like 10 billion years.

By contrast, key parts of the Universe—namely, some of its stars—seem older than 10 billion years. These are the ancient stars of the globular clusters (which we shall meet later in the STELLAR EPOCH), tight-knit groups of typically hundreds of thousands of stars strewn throughout the halos of galaxies and which are probably as old as the galaxies themselves (a topic soon discussed in the GALACTIC EPOCH). Astronomers estimate such stellar ages based on the rates at which stars undergo nuclear fusion—in particular, according to the theory of stellar evolution that specifies when mature, normal stars begin changing into swollen red-giant stars. Many globular clusters were examined for this color change during the past few decades and most of them imply ages in the range of 12-16 billion years. Hence, the paradox at hand: At face value, some stars seem older than the Universe itself—a possible inconsistency of timescales and a clear embarrassment to astronomy if not resolved.

Actually, this problem is not really a new one. Debate has swirled around it in one form or another for well more than a century. For example, in the mid-19^th century, when the pioneers of geochronology sought to assess the age of the Earth on grounds other than religion or philosophy, they essentially made two assumptions: The Earth probably formed at the same time as the Sun and the Sun shone by the burning of some known chemical, like the wood or coal commonly used during the Industrial Revolution. The answer they got for the age of the Sun, and hence the age of the Earth, was a few thousand years, a value less than that of recorded history. So an age controversy developed, not so much heated than merely amusing to most theologians of the time who thought unwell of science: How could Earth be younger than the duration of human existence?

The first assumption of early Victorian science was a good one—we do now judge the Earth-Sun birth to have been coterminous, as noted later in the PLANETARY EPOCH. But the second one was most definitely not—the Sun is assuredly not made of wood or coal! Physicists such as Lord Kelvin of Britain and Hermann von Helmholtz of Germany later revised these calculations in the late-19^th century, taking the Sun to be made of an incandescent liquid mass (such as gasoline or kerosene) and allowing for some energy generation via gravitational infall (including meteors crashing into the Sun). Yet they were unable to increase the age estimate for the Sun to much more than a hundred million years—a value surely older than recorded history, but still much less than that then needed by the British naturalist Charles Darwin to explain the fossil record in terms of biological evolution. Long-dead life forms seemed at the time to be at least several hundred million years old, and we now realize they are even older as noted in the BIOLOGICAL EPOCH. Kelvin got similarly low values for Earth’s age when trying to estimate the rate at which our planet cooled, largely because he overlooked the poor thermal conductivity of the rocky interior—all of which put geological evolution into conflict with biological evolution. Thus, the age controversy continued, dominating scientific circles ~100 years ago, some of the debate (then as now) being quite vehement: How could life on Earth be older than the planet itself?

These early age discrepancies eventually went away. As radioactivity became better understood mainly by French scientists Henri Becquerel and Pierre and Marie Curie around the turn of the 20^th century, geologists could then measure the age of Earth directly. And what they found was a planet of a few billion years, fully enough to provide the long timescales needed to explain Darwin's fossils. Scientists now know that biological evolution has occurred for more than 3 billion years, yet there is no problem here since modern radioactive methods currently date Earth at nearly 5 billion years.

Alas, in the 1930s, a version of this “age-old” problem resurfaced. At issue then were some of the first measurements of the Hubble constant by Edwin Hubble himself and some of his colleagues. Owing to observational uncertainties in the brightness of the galaxies and especially to calibration errors in the analysis of the acquired data, they found a Hubble constant of more than 100 km/s/million light-years. This then implied that the Universe expands very much faster than we now know it does, meaning that the galaxies would have gotten out to where they are now observed much quicker. In fact, Hubble’s original analysis implied a Universe age of less than a few billion years, and suddenly the general problem was back: How could Earth be older than the Universe?

In turn, this problem gradually faded away as many astronomers undertook, over the course of several decades in the mid-20^th century, better observations and data analyses of the brightness and distances of the galaxies. By the 1950s, the value of the Hubble constant had decreased five-fold and the Universe age consequently lengthened to at least 10 billion years. Hence, the Universe was then safely older than Earth and the age problem went away again . . . for a while. To be sure, it has returned in more recent years given the claimed ages of some globular clusters. By the 1980s and into the 90s, we had the modern version of a recurring age discrepancy, to wit: How could some stars within the Milky Way be older than the Universe itself? Well, they can't be; you can't be older than your mother. It's as simple as that. Something is awry—again.

Fortunately, this lingering age controversy seems to be fading away again, just as better observations and improved models caused similar glaring contradictions to evaporate throughout the past century. Indeed, several recent developments favor the dissolution of this problem altogether. For example, today's astronomers are converging on a model Universe that is decidedly “open”—again, like the escaping rocket whose open-ended geometric path extends forevermore. In particular, our currently best value of the Hubble constant seems to be pointing at a somewhat older cosmic age of about 13.5 billion years (Figure 1.23). What's more, recent reanalyses of the globular star clusters, especially based on data acquired by Europe’s Hipparcos satellite, imply that the globulars have had their ages previously overestimated by nearly 20%. As a result, the average age of the oldest stars needs to be readjusted downward to 10-12 billion years, making them safely younger than the Universe.

FIGURE 1.23 – Cosmic ages determined from the Hubble recession of the galaxies, from analysis of the cosmic background radiation, and from the evolutionary nature of the oldest stars seem to be converging at ~13.5 billion years for the age of the Universe. (Dana Berry)

We need not be overly concerned about this periodic age controversy, other than to note it as an active area of forefront research that seeks to specify a number (mainly the value of Hubble’s constant) to an accuracy of a few percent, when many other cosmologically significant numbers (such as the cosmic density) are known only to within a factor of 5-10. As things now stand, the universal age will likely turn out to be 12-15 billion years; all arguments presented in this Web site aim toward a best current estimate of nearly 14 billion years. The specific number is not that important and will not likely be pinned down for many years, if ever. What is most remarkable is that the ages of the cosmos, stars, Earth, life, and humanity now do stack up so well chronologically along the arrow of time, indeed consistently so with increasing order and rising complexity over the course of all natural history.

As for the cosmic-evolution scenario presented here, our narrative is hardly affected by this on-again, off-again age controversy—even if it re-emerges. The arrow of time itself can be contracted or expanded, a little like an adjustable accordion, in order to match whatever is the true age of the Universe. The historical sequence of events along the arrow of time is more important than the magnitude of the arrow itself.