The Expansion of the Universe

“ Physics is nothing less than

the study of existence itself.”

Astronomy, in the plainest sense is looking up at the night sky and wondering. Wondering how far it goes: does it ever stop? What if it doesn’t?

There have been discoveries in the history of astronomy that have completely changed the way we think about the universe. The expansion of the universe is certainly one of them.

We already know for a fact that the universe is expanding. But how did we come to this conclusion? How is it expanding? What does it imply? What are its consequences? In this blog, we are going to dive deep into these questions.

The name which comes up first for this groundbreaking discovery is not a name you would expect. Decades before Hubble’s observations, Vesto M. Slipher, an American astronomer published a paper in 1913 where he measured the blueshift of Andromeda and concluded, “…we have at present no other interpretation for it. Hence, we may conclude that the Andromeda Nebula is approaching the solar system...”. In the 1915 paper, Slipher showed the redshifting of 11 out of 15 galaxies. In his 1917 paper, the redshift:blueshift ratio has increased to 21:4 and Slipher interprets as “For us to have such motion and the stars not show it means that our whole stellar system moves and carries us with it. It has for a long time been suggested that the spiral

nebulae are stellar systems seen at great distances ... This theory, it seems to me, gains favor in the present observations".

In 1924, Swedish astronomer Knut Lundmark provided the first observational evidence for expansion.

But…. What is Red Shift and why is it relevant here? Red shift is a phenomenon in which the wavelength of light travelling through large distances gets shifted to the red part of the electromagnetic spectrum. It is similar to the Doppler Effect observed in sound. It is used to study the movements of cosmological objects with respect to us. The more redshifted the light is, the farther the object is. Additionally, an increase in red shift shows us that the distance is increasing due to expansion. Conversely the more blueshifted it is the closer it is to us and if the blue shift increases, it means that the object is moving closer.

The theory was further expanded on by Belgium astronomer Georges Lemaitre in his 1927 report. He proposed the idea that the universe was expanding, derived from General Relativity. He was also the first to estimate the Hubble constant by measuring how much each galaxy was redshifting based on its distance. He was the father of The Big Bang Theory.

Now what is Hubble Constant?

It is one of the most important constants in astronomy. It stems from Hubble’s Law which states that cosmological objects are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The constant that determines the speed at which they are moving, i.e., it determines the rate at which the universe is expanding.

In 1929, Edwin Hubble, an American astronomer observed the light emitted from various galaxies and discovered that the farther the galaxy is, the faster its receding away and the light was increasingly redshifted. This meant that the universe was expanding in all directions.

But the universe is not only expanding, but its expansion is also accelerating! This observation was made by Saul Perlmutter, Brian Schmidt and Adam Riess by observing distant supernovae. In 2011, they were awarded the Nobel Prize in Physics for the same.

A general misconception is that expansion means cosmological objects get farther apart but this is incorrect. In reality, the fabric of spacetime itself stretches, seperating the cosmological objects.

But the big question still remains…. why?

Why was the expansion accelerating?

There have been quite a few models proposed to provide explanations and we are going to go over some of them.

We start off with the most popular model, The Dark Energy model. Not much is known about these mysterious terms Dark Energy and Dark Matter except that it effects the expansion of the universe. Astonishingly, around 68 % of the universe consists of Dark Energy, 27% is Dark Matter and the remaining 5% makes up everything that’s observable, i.e., visible matter. Dark energy is theorized to be vacuum energy i.e., a cosmological constant, contributing in the acceleration.

Another model that’s talked about is the Modified Gravity model. This model suggests that we can explain the increase in acceleration by simply modifying gravitational physics by introducing modifications to existing equations of Einstein and Newton.

There are many such alternative models such as the Unmodified Gravity model and modifications to the dark energy model but none have given a complete explanation as they are yet to answer all the questions raised.

This major discovery bore large consequences, challenging our understanding of the universe. As every great discovery, this shed light of many questions but gave birth to even more. It led to the proposal of one of the most widely accepted theory about the beginning of the universe, The Big Bang Theory. It also led to numerous theories about the end of the universe including Heat Death, The Big Rip and so on. It also paved the way to potentially one of the biggest mysteries of the universe, The Dark Matter and Dark Energy. Suffice to say this discovery was one of the most crucial discoveries in the history of humanity bringing us closer to understanding our universe.

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