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| Cosmic Microwave Background Radiation NASA-WMAP |
The Hubble constant, which measures the speed of expansion of space in the universe, has
very curious properties. For instance, although we call it constant,
it turns out that it is not a constant, as it varies over time. That is why its
current value is represented by the symbol H0, but since its value
was different at other times, it can be represented by other symbols, such as HCMBR,
which refers to its value at the time when the cosmic microwave
background radiation originated, about 13.7 billion years ago.
