How do scientists measure the speed of the universe's expansion?

Scientists measure the universe's expansion speed using redshift data and the Hubble Constant.

The expansion of the universe is a fundamental concept in cosmology, the study of the universe's origin, structure, and evolution. Scientists measure this expansion using a method that involves the observation of distant galaxies and their 'redshift'.

Redshift is a phenomenon where light from an object moving away from the observer is increased in wavelength, or shifted towards the red end of the spectrum. This is similar to the Doppler effect, where a sound's frequency changes if the source is moving relative to the observer. In the case of the universe, the 'stretching' of light to longer, redder wavelengths is due to the expansion of space itself.

By observing the redshift of distant galaxies, scientists can determine how fast these galaxies are moving away from us. The greater the redshift, the faster the galaxy is receding. This relationship between the speed of a galaxy's recession and its distance from us is described by the Hubble Constant, named after the American astronomer Edwin Hubble who first proposed this concept.

The Hubble Constant is currently estimated to be about 70 kilometres per second per megaparsec. This means that for every megaparsec (a unit of astronomical distance equivalent to about 3.26 million light-years) a galaxy is from us, it is moving away at a speed of 70 kilometres per second due to the expansion of the universe.

However, measuring the Hubble Constant and thus the speed of the universe's expansion is not straightforward. It involves complex observations and calculations, and there is ongoing debate among scientists about the exact value of the Hubble Constant. Despite these challenges, this method remains the most effective way we have to measure the universe's expansion speed.