Since the dawn of humanity, we have thought that the heavens are huge, but of constant size. We first knew that the stars move around, then that it is actually the Earth rotating on its axis. In the 18th century, the French astronomer Messier observed fuzzy objects that he didn’t know were galaxies. Only in 1923, Hubble conclusively proved that the Milky Way was not the entire universe. So while our understanding of the size of the universe expanded, we still thought it is constant or perhaps infinite. It took until the 1920s and 30s and again the brilliant observations of Hubble to show that the universe is expanding based on the red shift of galaxies. In 1998, observations made by the Hubble Space Telescope showed that this expansion is accelerating. Let’s attempt to understand Hubble’s experimental process and what the expansion of the universe means.
From 1912 and 1922, Vesto Slipher showed that the light from many astronomical objects was red shifted, which was soon interpreted as galaxies moving away from the observer on Earth. The light emitted by stars lies on the electromagnetic spectrum and composed of waves. Due to the Doppler effect, when an object is moving away from you, the wavelengths you observe appear longer and stretched. Longer wavelengths appear more red, so that is why the term redshift. The amount of redshift is a function of velocity. In 1922, Alexander Friedmann developed a theoretical model of the expanding universe based on Einstein’s field equations. This model was confirmed in 1929 when Hubble measured the redshifts of various distant galaxies and their distances based on the apparent brightness of Cepheid stars in them. He found a groundbreaking linear relationship, which confirmed that the universe was expanding. This data was further improved upon by Hubble and Humason in 1931, after the determination of distances improved. This process showed the powerful achievements that parallel theoretical and experimental work can produce.
Hubble’s equation for the redshift is v = H x r, where v is the velocity of recession, r is the distance and H is Hubble’s constant, which is the slope of the graph. The speed of expansion has been the center of debate in astrophysics, as more and more accurate measurements are made for H. The value is placed at around 67.8-74, with the consensus being at around 73. These values are still calculated by looking at Cepheids, but recently Freedman from the University of Chicago used a new technique and got a value of 69.8. Knowing a precise value for H can also help accurately determine the distance to distant galaxies based on how fast they are moving. Therefore, this debate is still raging and we will probably see the accepted value shift a little bit in future years.
The expanding universe has three possible shapes based on the Friedmann equations. The types are open, flat and closed. The open universe has a negative curvature, the closed one has a positive curvature and the flat one, as the name suggests, has zero curvature. These shapes are important as they determine the fate of the universe. With current technology we are able to see around 80% of the observable universe, and they agree with the flat universe. I will come back to the shape of the universe in another post.
This was not the end of the story for the expansion of the universe. In 1998, two independent research teams observed distant supernovas using the Hubble space telescope. The hypothesis was that the universe should decelerate due to gravity, however they found that actually it is accelerating. This was later confirmed by analysing baryon acoustic oscillations. An explanation for this was dark energy, a new form of energy which acts against gravity and does not interact with electromagnetic forces. This is still a theoretical model and experimental confirmation is still yet to come. But the question of expansion is still as potent as it was in the 1920s.
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