The never-ending story: H₀

The Hubble constant has a long history of measurements. It is also one of the hardest cosmological parameters to measure since high accuracy is required.

• Hubble himself obtained the value for H₀ of 500 km/s/Mpc. That was way too large, and such a universe would be younger than the Earth.
• In 1952 Walter Baade showed that there were two types of Cepheid variables, and that Hubble mistook one type for another. That reduced the Hubble constant to 200, still too large.
• In 1958 Alan Sandage found another error in original Hubble's analysis, and found H₀ between 50 and 100 km/s/Mpc.
• Until 1996 the Hubble constant floated between 50 and 100, depending who measured it and what method was used.

• In 1996 the Hubble ... found Cepheid variables in a distant galaxy M100, and using them found $H_0\approx70$ km/s/Mpc.
• About at the same time supernovae Ia were used as standard candles, and the Hubble constant measured using SN Ia was found to be also about 70.

Thus, at the moment the best guess we have (also supported by a large set of indirect clues):

$$H_0 \approx 70 {{\rm\,km}/{\rm\,s}\over{\rm\,Mpc}}$$

This number is not very accurate, so it can also be 60 or 80, but not 50 or 90.

The age of the universe

We cannot directly measure the age of the universe, but we can measure the age of various astronomical objects. Hopefully, a mother is older than a daughter.

The age of the Earth and the solar system can be measured by means of radioactive dating: 4.6 billion years.

The age of white dwarfs can be measured rather accurately by studying their cooling. The oldest white dwarfs are 10 billion years.

The age of stars can also be determined using theoretical models. The oldest stars are found to be about 13 billion years. This ``measurement'' however is partly based on theoretical modeling, and thus may contain a systematic error.

Thus, the conclusion:
The universe is probably 13 billion years old, but certainly not younger than 10 billion years.