by S. Prasad Ganti

Edwin Hubble was the legendary astronomer of the early twentieth century. Credited with the discoveries of the Andromeda galaxy and the expansion of the universe. With these, he brought the concept of galaxies to our lexicon. And that the size of our universe is ever expanding. This eventually led to the postulation of the Big Bang theory and the birth of the universe from such a humble beginning. 

Hubble used the 100 inch Hooker telescope on Mount Wilson in California to come up with his historic observations. He measured distances to different stars using standard candles – stars whose absolute brightness can be determined and whose apparent brightness as seen from Earth can be measured. First standard candle is a star known as the Cepheid variable which was discovered by Henrietta Leavitt. These stars vary cyclically in brightness over an interval of time. This interval is proportional to its absolute brightness. Leavitt observed the brightness of different Cepheid variables and their periods. By determining the distance of few such stars using other methods like parallax (which work for stars at smaller distances), she was able to plot the brightness cycle vs. distances.   

Hubble observed more Cepheid variables in what appeared to be distant nebulae. He found that the distances  of such nebulae are much more than what was the known size of our galaxy, the Milky Way. That is how he discovered the Andromeda galaxy, our nearest neighbor. He then discovered several galaxies distinct from the Milky Way. He found their distances by making an assumption that the brightest star in every galaxy is equally bright.

Along with different galaxies, he also found something surprising. That the galaxies are moving away from each other. He found this using the redshift of the light coming from that galaxy. The color of light changes if the source is moving towards us or away from us. The color change is apparent only at high speeds, not like a few miles per hour. Light from sources moving away from us will be shifted towards the red end of the spectrum. While light from any source moving towards us will be shifted towards the blue end of the spectrum. Hubble concluded that the further a galaxy is away from us, the faster it is moving away from us. 

The expansion of the universe was formulated into an equation which had “Hubble’s constant” as one of the parameters. He attempted to measure the value of this constant given the distance of the galaxies and the red shift they were producing. He went through several iterations before settling on a value. More recent observations resulted in more precise calculations and a better value. 

A second way of calculating the Hubble’s constant is by looking at the CMB (Cosmic Microwave Background) pattern of the universe produced by several space based observatories – COBE (Cosmic Background Explorer), WMAP (Wilkinson Microwave Anisotropy Probe), and Planck. The results from the two methods differ by a non-trivial amount. This discrepancy is known as “Hubble tension”. 

The discrepancy was resolved in 1998 by postulating that the expansion of the universe is accelerating. And that dark energy is responsible for such an accelerated expansion. Now dark energy is accepted as a part of the “standard model” of our universe. Yet some discrepancy still exists between the values calculated using two different methods.

If the two methods  yield values close enough, then the standard model is validated again. Work continues in this area. More progress is expected when two observatories start getting the first light. Vera Rubin observatory is being built in Chile. It is expected to see the first light in 2025. A space based telescope Nancy Grace Roman is expected to be launched towards the end of 2026. 

With newer discoveries, hope is that the “Hubble tension” will be eased if not erased.