Hubble-Lemaître Law

The Hubble-Lemaître Law is a physical law that states that most of the galaxies(except our closest ones because they are in our Local Group) in the universe are receding(moving away) from us at speeds proportional to their distances. To break this down, essentially, if a galaxy is very far from Earth, it will be moving faster away from us than a galaxy close to Earth. Equation 10 on the Science Reference Astronomy Equation Sheet represents this linear proportionality. Both images (a) and (b) represent this relationship visually.

Measuring Recessional Velocities

The alternative is using redshift to analyze the recessional velocities. The shift in the spectrum of these receding bodies is directly proportional to the speed at which they move away from us. However, the common redshift technique has one issue: The Hubble Constant.

The Hubble Constant

The Hubble Constant is the capital H in the Hubble-Lemaître Law. So, why would this give uncertainty in the redshift method? Well, here's the thing. The Hubble Constant, defined as on the Science Reference Astronomy Equation Sheet, is subject to noticeable changes. In fact, because of this, there is even currently some statistical uncertainty in its measurement. The reason for the change in this constant over time is because the universe is observed to be expanded at a growing rate, which means the universe's expansion is accelerating. While this has other cosmological implications like dark matter, which we won't delve into, it also means that the Hubble Constant must increase over time. However, at any given time, it can well be considered a constant, even if there is uncertainty about its value.

Citations/Attributions

Astronomy. Provided by: Openstax. Located at: https://openstax.org/books/astronomy/pages/1-introduction License: CC BY 4.0

Hubble's law. Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Hubble%27s_law. License: CC BY-SA: Attribution-ShareAlike

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