The expanding universehttps://www.livescience.com/hubble-constant.html.
In the 1920’s Edwin Hubble discovered that the universe is expanding by noticing that the vast majority of galaxies were receding from us (by measuring their distance using universal measuring sticks and their redshift) and that more distant galaxies were receding faster. The Hubble constant is a measurement of the rate of this expansion.
Traditional Measurement Methods
The Hubble constant has traditionally been measured in two ways: using supernovae in distant galaxies to measure their distance, or the fluctuations in the Cosmic Microwave Background Radiation (CMBR). Using the supernova method, calculations of the Hubble constant are about 68 km/s/Mpc (units: kilometres per second per mega-parsec) whilst the CMBR method produced a result of about 72 km/s/Mpc cWarren, S., 2022. The Hubble constant, explained. [online] News.uchicago.edu. Available at: [Accessed 22 July 2022]. - a significant discrepancy. Note that the units of the Hubble constant indicate a rate of expansion for a particular distance from an observer, the further the distance between two points, the greater the rate of expansion between them.
A New Light
In 2010, Wendy Freedman and Barry Madore proposed a new method of measuring the Hubble constant that makes use of red giants: bloated and dying stars. During their final death throes, red giants rapidly increase their core temperature to about 100 million degrees and reduce their overall brightness cWarren, S., 2022. The Hubble constant, explained. [online] News.uchicago.edu. Available at: [Accessed 22 July 2022].. Red giants at known distances can be used to calibrate this pattern to be applied to red giants at unknown distances - thus helping to measure the recession rate of a greater sample of galaxies in our universe. This method produced a Hubble constant value of about 69.8 km/s/Mpcd is improving confidence in the true value of the expansion rate of our universe at about 70 km/s/MpcFreedman, W. and Madore, B., 2010. The Hubble Constant. [online] Ned.ipac.caltech.edu. Available at: [Accessed 22 July 2022]. - almost splitting the middle between the two previous results.
A New Wave
Even more recently, in 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingstone, Louisiana, observed the gravitational wave and electromagnetic aftershock of a collision between two orbiting neutron stars about 120 million years ago/light-years away. Using both of these results, researchers were able to calculate a new value for the Hubble constant at about 70 km/s/MpcWarren, S., 2022. The Hubble constant, explained. [online] News.uchicago.edu. Available at: [Accessed 22 July 2022].. This result strongly agrees with the red giant method found by Freedman and Madore in 2010 and is improving confidence in the true value of the expansion rate of our universe at about 70 km/s/MpcFreedman, W. and Madore, B., 2010. The Hubble Constant. [online] Ned.ipac.caltech.edu. Available at: [Accessed 22 July 2022]..
