Hubble's Constant

Hello!

Has been a while isn't it? Today I will tell you about the "Hubble's Constant" and how it sparked one of the biggest crises in cosmology!

But before we learn about it, we should know a few basic facts that will help us:

• The universe is currently expanding and the expansion is accelerating.
• The galaxies not gravitationally bound to us are receding away from us.
• A megaparsec is defined as 1 million parsecs, where 1 parsec is roughly 3.46 light years.

Now we can move on to understand Hubble's Constant.

If you know about cosmology, you probably know about this equation, which is the Hubble's Law:

\(v=H_{0}D\)

where \(v\) is the recessional velocity (basically how far away a non-gravitationally bound galaxy is moving away from us in kilometers per second), \(H_{0}\) is the Hubble's Constant (denoted in kilometers per second per megaparsec) and \(D\) is the distance between us and the galaxy (in megaparsecs).

The Hubble's Constant has two main ways to measure it which are:

• Determining the redshifts of galaxies using the Doppler effect formula for light to find an answer. Longer wavelengths of EM waves indicate that the galaxy is further away.
• Measuring the light left over from the Cosmic Microwave Background to find an answer (basically light left from the Big Bang).

The crisis stems from the fact that both these measurements give radically different results (the values obtained are around 67.4 and 73.2 respectively). Future observations have made it clear that the values they have gotten disagree on the 4.4-sigma level (which means it has an incredibly low chance of the different numbers popping up from statistical deviations). 

This sparked a crisis "The Crisis In Cosmology" that challenges the current science that we use to understand the universe and it hints to a newly undiscovered concept.

It is really exciting to know that there is still much more to discover and we have just started to understand the universe!

Thanks for reading!

Read more about Hubble's Constant here: https://news.uchicago.edu/explainer/hubble-constant-explained