How do we calculate the Hubble constant?
The problem causing the discrepancy could lie somewhere in these methods
1 Parallax
Parallax is an ancient method for measuring the distance to relatively nearby stars up to approximately 65 light years away using Earthbased measurements – or 326 light years if having used the ESA’s Hipparcos mission. This method uses trigonometry to measure the displacement of a star’s background, having observed it from the opposite side of the Sun. This method is commonly used as the first step on the ‘cosmic distance ladder’ which astronomers use to calibrate other results.
2 Cepheids
Cepheids are unstable stars that are coming to the end of their lives. During this period of imbalance, there is a periodic pulsation to the star. From Earth this pulsation appears as a timely dimming and brightening. Astronomy changed when Henrietta
Swan Leavitt discovered the periodic pulsating could mathematically infer the star’s distance up to 20 million light years using what is now known as the ‘period luminosity relationship’.
3 Type Ia supernovae
Another standard candle that helped provide a more recent Hubble constant figure are Type Ia supernovae. These are not stars by definition, but are instead the explosive events that mark the end of a white dwarf star. Specifically one more than 1.44 times the mass of our Sun. The peak luminosity when they explode is consistent across the universe, and therefore astronomers can take the observed magnitude and use a similar distance-luminosity relationship equation to determine the distance to galaxies tens of millions of light years away.
4 Redshift
This is the ultimate ruler when it comes to working out cosmic distances. In the past galactic redshifting revealed the distance to the farthest known galaxy from Earth, GN-z11, which is 13.4 billion light years away. This method requires spectroscopic data of a galaxy, then astronomers determine how far the emission or absorption lines for elements have shifted to the red end of the electromagnetic spectrum.
5 Cosmic microwave background
The CMB is leftover heat radiation from the Big
Bang that can only be seen when observing the universe through microwave wavelengths. This provides visible and accessible information about what the universe was like over 13 billion years ago. In order to paint the best picture, astronomers have produced cosmological models that fit the CMB data. It just so happens that these models predict a different figure for H0 than what is calculated using standard candles and megamaser geometry.
6 Megamaser geometry
Masers are radio analogues for visible-light lasers, and can therefore be easily picked up by Earth-based observatories. By measuring a maser’s centripetal acceleration and radial velocity, the Megamaser Cosmology Project then applies geometry to calculate the distance between
Earth and the host galaxy. This is unique because it is independent of standard candles and the CMB, and therefore the results carry more weight and provide a new outlook on the discrepancy.