The Expanding Universe
What does the Big Bang Theory really tell us? This mini lecture explores the evolution of our universe, explaining Hubble’s historic discovery of its expansion and how it uncovered a look back in time.
For more details on the science behind studying the universe, including the three pillars of expansion, check out the transcript below from Professor Tarlé's interview.
Implications of an Expanding Universe
One of the most astounding discoveries of the 20th century was when Edwin Hubble found that our Universe was expanding.
In the 1920s, Hubble turned the mirror of the newly completed 100-inch Hooker telescope at Mount Wilson towards fuzzy objects called nebula and found that these were actually entire galaxies separated from our own Milky Way, each with many billions of stars.
Using a type of variable star that puts out a known amount of light, he was able to measure the distance to these other galaxies by how dim they appeared. Then using the shift in the color of the spectral lines of these galaxies, he was able to determine how fast they were moving.
He discovered something very startling. All the galaxies were moving away from us. Their spectrum was shifted towards the red. Moreover, the further away they were, the faster they were receding. This linear relationship between velocity and distance, now known as “Hubble’s Law” provided a new extragalactic distance scale and proved conclusively that these galaxies were far outside the Milky Way.
How was one to interpret what Hubble found? It was as though there was a giant explosion in the past hurling galaxies through space. The ones that were moving faster were now further away from us. The problem with this view is that there would be a center to the universe and there never was and has never been any evidence for that!
A more likely explanation was arrived at but ignored by Albert Einstein over a decade earlier when he applied his new theory of gravity to the universe. Einstein found that a space-time filled only with matter and energy cannot be static. It must be either expanding or contracting. At the time, everyone knew that the universe was unchanging, and so Einstein put into his equations a fudge factor called the cosmological constant, a type of antigravity that was tuned to keep a static space-time from collapsing. Years later, when Hubble discovered that the universe was expanding, Einstein called the cosmological constant his “biggest blunder.”
So, the expansion of the universe is not an explosion with galaxies moving away from us through space. It is an expansion of space itself. Except for local motion due to nearby gravitating objects, the galaxies are just “hanging out.” As the space between the galaxies expands, we see this as the Hubble expansion. Every galaxy sees the same thing; everything appears to be moving away and those galaxies further away will have had its light stretched (redshifted) by a larger and larger amount as it travels through the expanding space.
The Hubble expansion has profound implications for the history and evolution of the universe. If we were to follow the Hubble expansion in reverse, we would come to a time 13.7 billion years ago when the clock of the universe started ticking—a beginning to what we now call time. This Big Bang somehow launched the universe into an expansion that continues to this day.
At earlier times, the universe was very hot and dense. It was dominated by radiation and began to cool as the universe expanded.
By the first few minutes, it was cool enough for nuclei to form (billions of degrees). The abundances for the light nuclei such as protons, deuterium, He, Li was set at this time and can be observed in the un-burnt outer layers of old stars today. The excellent agreement between measurements and the predictions of this Big Bang Nucleosynthesis is considered to be one of the most important experimental tests of the Big Bang Theory.
380,000 years after the launch of the universe, the temperature had dropped to about 3,000 degrees; cool enough for atoms to form. Before this time, the universe was ionized and opaque, just like you see in a gas discharge tube (like a brightly lit neon sign). After this time, the universe was relatively transparent, like you see when you turn off the discharge. Free electrons falling into atoms produce ultraviolet light, which could now stream freely through the newly transparent universe. Since this time, the universe has expanded by a factor of 1,000, stretching the wavelength of this light into the microwaves. The observation of these microwaves with a spectrum predicted by the Big Bang Theory is considered to be an important confirmation of the Big Bang Theory.
The expansion of the universe, the Cosmic Microwave Background Radiation and Big Bang Nucleosynthesis form the “three pillars” that has convinced cosmologists that the Big Bang Theory provides a good basis for our understanding of the history and evolution of the universe. More recent observations have led to refinements of the Big Bang theory but the basic idea still stands.
As we look out into the universe we are looking further and further back in time. Since light travels at 300 million meters per second and nothing travels faster than light, the furthest back we can possibly probe is the distance that light can travel in the age of the universe. Thus there is a horizon to the universe, beyond which we cannot see. We assume that the part of the universe outside of our horizon is just like the part inside but we can never know.
The microwave background radiation was released shortly after the launch of the universe, 13.3 billion years ago. The most distant objects that we can see (quasars) are billions of years old, older than the Earth. Hubble observed galaxies, as they were millions of years in the past, before humans walked on the Earth. It takes light four years to reach us from the nearest star. When we look at the sun, we are seeing it as it was eight minutes ago.
So the next time you look up at night, contemplate the giant time machine that lies before you and appreciate how Hubble’s discovery has put this all within the reach of our minds.