User:Wjs64/cosmic age problem

The cosmic age problem is a historical problem, now believed solved, that various objects in the Universe appeared to be older than the time since the Big Bang, as estimated from measurements of the expansion rate of the Universe known as the Hubble constant (more correctly called the Hubble parameter, since it generally varies with time).

Following theoretical developments of the Friedmann equations by Alexander Friedmann and Georges Lemaitre in the 1920s, and the discovery of the expanding Universe by Edwin Hubble in 1929, it was immediately clear that tracing this expansion backwards in time predicts that the Universe had almost zero size at a finite time in the past. This concept later became known as the Big Bang theory. If the Universe had expanded at a constant rate in the past, the age of the Universe now (i.e. time since the Big Bang) is simply the inverse of the Hubble constant, often known as the Hubble time; for models with zero cosmological constant and positive matter density, the actual age must be slightly younger than this Hubble time.

Hubble's early estimate of his constant was 550 km/s/Mpc, and the inverse of that is 1.8 billion years. It was believed in the 1920's that the Earth was probably over 2 billion years old, but with large uncertainty.

The possible discrepancy between the ages of the Earth and the Universe was probably one motivation for the development of the steady state theory in 1948; in this (now obsolete) theory the Universe is infinitely old and on average unchanging. The steady state theory requires spontaneous creation of matter, and thus most galaxies have an age less than 1/H_0. However, if H_0were 550 km/s/Mpc, our Milky Way galaxy would be exceptionally large compared to most other galaxies, so could well be much older than an average galaxy.

In the 1950s, two substantial errors were discovered in Hubble's extragalactic distance scale: first in 1952, Walter Baade discovered there were two classes of Cepheid variable star. Hubble's sample comprised different classes nearby and in other galaxies, and correcting this error made all other galaxies twice as distant. A second error was discovered by Allan Sandage and coworkers: for galaxies beyond the Local Group, Cepheids were too faint so Hubble used the brightest stars as distance indicators. Many of Hubble's "brightest stars" were actually HII regions or clusters containing many stars, which caused another underestimation of distances for these more distant galaxies. Thus, in 1958 Sandage published the first reasonably accurate measurement of the Hubble constant, at 75 km/s/Mpc, which is close to modern estimates of 68-74 km/s/Mpc.

The age of the Earth (actually the solar system) was first accurately measured around 1955 by Clair_Patterson, at 4.55 billion years. For H_0 ~ 75 km/s/Mpc, the inverse is 13.0 billion years, so after 1958 the Big Bang universe was comfortably older than the Earth.

However, in the 1960s and onwards, age measurements for globular clusters were estimated from stellar evolution theory, and generally gave age estimates of around 15 billion years, with substantial scatter. Further revisions of the Hubble constant by Sandage and Tammann in the 1970s gave values around 50-60 km/s/Mpc, and an inverse of 16-20 billion years, consistent with globular cluster ages.

However, in the late 1970s to early 1990s, the age problem re-appeared: new estimates of the Hubble constant gave higher values, with Gerard de Vaucouleurs estimating values 90-100 km/s/Mpc, while Marc Aaronson and co-workers gave values near 80 km/s/Mpc. Sandage and Tammann continued to argue for values 50-60, leading to a period of controversy called the "Hubble wars". The higher values appeared to conflict with globular cluster ages, and gave rise to significant speculation in the 1980s that the Big Bang model was incorrect.

The age problem was eventually resolved by various developments after 1995: firstly, a large program with the Hubble space telescope measured the Hubble constant at 72 km/s/Mpc with 10 percent uncertainty. Secondly, the Hipparcos spacecraft in 1995 revised globular cluster distances upwards by 10 percent; this makes their stars brighter than previously assumed and therefore younger, shifting their age estimates down to around 12-13 billion years. Finally, from 1998-2003 a number of cosmological observations including supernovae, CMB observations and large galaxy redshift surveys led to the acceptance of dark energy and the establishment of the Lambda-CDM model as the standard model of cosmology. This dark energy implies the universe was expanding more slowly at around half its present age, and makes the Universe older for a given value of the Hubble constant. The combination of the three results above essentially solved the cosmic age problem.

More recent measurements from WMAP and the Planck spacecraft have established a very accurate age of the universe of 13.80 billion years with 0.3 percent error (based on the standard model), and all direct age measurements for globular clusters and other objects are currently smaller than this value. A large majority of cosmologists therefore believe the age problem is now resolved.