Whether I’m reading into astrological interpretations, obsessing over flat-earth conspiracy documentaries or getting deeply lost in YouTube vlogs from astrophysicists like Neil deGrasse Tyson, I’ve always had an interest in our Universe and how it affects life here on Earth.
This passion inspired me to take a course at the University of Michigan to learn more about the dark side of our universe- the more mysterious aspect that we can’t directly see. For those of you who share my interest in our world and want to know more, here’s a brief explanation ahout one of the more substantial astronomical discoveries in our time: the expansion of our Universe since the Big Bang.
A famous American astronomer, Edwin Hubble, first discovered that the Universe was expanding in 1932, by observing distant galaxies with his Hubble telescope. He was able to make this discovery based on the Doppler effect, which states that the pitch or frequency of either a sound or light wave changes as an object moves towards you or away from you. By studying Doppler’s effect and using his own observations, he was able to create his own law – Hubble’s law.
The law states that the redshifts in the spectra of distant galaxies are proportional to their distance. In other words, when a galaxy is moving away from us, the color of the light shifts toward the red end of the spectrum. Furthermore, Hubble demonstrated that there is a relationship between velocity, which is measured from redshift, and distance, which is measured mostly through Cepheid variables. This is now described as the Hubble Constant, a unit of measure used to describe the expansion of the Universe.
Through his measurements, Hubble was able to conclude that more distant galaxies are moving away from us faster than closer ones, and that this movement occurs because the Universe is expanding. Standard candles, or Cepheid variables, are any nearby object that we can know its true intrinsic luminosity. Therefore, they can be used as a kind of cosmic yardstick, allowing us to measure distances of tens of millions of light-years away. For example, when looking at lampposts lining the side of a highway, we can intuitively tell how far those lampposts are from us because we know that they all have the same light energy. This analogy works for stars too, except it would be unnatural to assume that every star has the same exact brightness. Compared to typical stars, which are unchanging, Cepheid variables are stars that brighten and dim periodically.
In 1912, Henrietta Leavitt discovered this pattern of variability through observing stars in the Magellanic Cloud. She was able to measure the period of each star by measuring the timing of its peaks and valleys of brightness. She discovered that brighter Cepheid variable stars have longer periods, or amount of time they take to brighten, dim and then brighten again. They are very important when attempting to measure the expansion of the Universe because their period is regular, or doesn’t change with time, so once that period is known, their brightness can be inferred and their distance from Earth can be calculated.
The Universe has been expanding ever since the Big Bang. This theory suggests that the Universe was extremely hot and dense early on, until it experienced a burst of expansion, which is referred to as inflation. After inflation, the Universe began to cool, allowing matter to form along with neutrons, protons and electrons. This is the very matter that makes up you and me. When the temperature dropped to around 109 Kelvin, protons and neutrons combined to form deuterium,an isotope of Hydrogen. The deuterium then combines to make Helium.
The era of recombination was the point at which matter cooled enough for atoms to form. This also created the conditions that allowed light to form in the Universe for the first time. This initial light flash is detectable today as the Cosmic Microwave Background, or the leftover radiation from when the Big Bang occurred. The gravitational waves associated with this radiation have allowed us to gain understanding about the expanding Universe. At this point, no stars had formed, so the Universe was still in a state of darkness until the period of reionization. During this time, gases collapsed to form stars and galaxies. This fromation now allows us to measure the expansion of the Universe. Compared to the rate of its initial expansion during the Big Bang, the Universe’s expansion rate has slowed due to gravity, but dark energy has started to speed up its expansion yet again.
The age of the Universe can be calculated by measuring the distances and velocities of other galaxies, which are usually moving away from us at speeds proportional to their distances. By using Hubble’s constant, or the expansion rate of the Universe, we can essentially “travel back in time.”
For example, one experiment was done using the Hubble telescope where it was pointed far away at an area of space that was mostly dark. The telescope was pointed at that same spot in space for 10 days and 10 nights, and after that time was up, the telescope showed thousands of galaxies and stars that had been there all along. We just couldn’t see them yet because of how time travels in space. By extrapolating Big Bang and examining the Cosmic Microwave Background, we can not only estimate the age of the Universe’s oldest stars and galaxies, but also the history of its expansion rate.
As I took this astronomy class and began to understand more about our Universe and its history, I realized just how complex it all is, and how there are many monumental things we still don’t have the answers to. Many of the dark aspects of the Universe are still unknown, even to the most brilliant astronomers and astrophysicists YouTube has to offer. I hope this piece helped break down some of these complicated phenomena for you, and quenched at least some of your thirst for knowledge about the vast, expanding and mysterious Universe we live in.