We are all aware of the Big Bang Theory, but how much is known about the strength of the theory. For some the Big Bang is a vague and far-out idea, for others it is a T. V. sitcom. Nonetheless, it requires some background to appreciate how the Big Bang Theory became what it is today.
The Story Begins
Isaac Newton is credited for saying: “If I have seen further than others, it is by standing upon the shoulders of giants.” Newton was implying that his discoveries would not have been possible, without the brilliant people (giants) which preceded him. The Big Bang is such a theory, it was pieced together by several individuals spanning decades of work. Or perhaps a few centuries of work, it all depends on when one chooses to begin the story.
I will arbitrarily begin in 1687 when Newton published his Principia Mathematica unveiling his law of universal gravitation and his three laws of motion. Newton was the first to provide a mathematical framework to account for the effects of gravity, thus he could calculate the motion of the moon and planets. Gravity was also the force responsible for keeping objects firmly on the Earth or causing an object (such as an apple) to fall to the ground.
Newton’s laws have stood the test of time; however, they are not 100% exact and serve as a very close approximation. They are, however, practically exact for our experience of everyday events. Only in some extreme situations do they fall short. Also, Newton was forced to concede that he did not know the mechanism behind the force of gravity. In simple terms, Newton was able to calculate the effects of gravity even thought he was unable to provide a complete explanation for how gravity worked. Nevertheless, Newton’s laws were a major scientific breakthrough for its time and started the ball rolling in the right direction.
It wasn’t until 1915 when Albert Einstein came up with his Theory of General Relativity, which addressed some of the gaps in Newton’s understanding of gravity. Einstein was able to explain gravity in detail, as a consequence of curved space. The mass of bodies (such as planets and stars) bend the fabric of space, thus generating the attraction. The fact that space has dynamic qualities, which can expand and curve would later become important to the big bang concept.
Also significant, is that General Relativity predicts that the universe should be either contracting or expanding. However, in Einstein’s time the prevailing wisdom was that the universe was static and eternal. Einstein gave way to convention, and after the fact, arbitrarily added a figure in his equations known as the cosmological constant. This was a repulsive force with just the right value to counter the effects of gravity, thus keeping the universe stable. As it turned out, Einstein’s original prediction of a non-static universe was later proven correct. He then dropped the cosmological constant from his theory.
Measuring the Night Sky
After Einstein’s General Relativity it was left to astronomer Vesto Slipher, who worked at the Lowell Observatory in Arizona. Slipher took spectrograph readings of distant stars and discovered that the light emitted was moving away from us. The starlight was shifted to the red end of the spectrum. Slipher was the first to realize that receding light is red shifted and in coming light is blue shifted. This was an indication that the universe was not static after all; however, his work went unnoticed at the time. Slipher was not aware of General Relativity and his findings would only have an impact a few years later.
Another breakthrough came from a woman named Henrietta Swan Leavitt. She worked at the Harvard College Observatory as a computer, as they were known in those days. These women studied photographic plates of stars and made computations. Leavitt was able to establish Cepheid variables as standard candles; a method to determine the intrinsic brightness of a star. Cepheids are elderly stars which pulsate at regular intervals; these stars brighten and dime in a very reliable pattern. Leavitt worked out that these stars could be used to calculate distances. For the first time, there was a method of measuring the large-scale universe. Today, Type 1A supernovae are also used as standard candles. Similar to Cepheids, Type 1A supernovae are said to have intrinsic brightness, making them reliable measuring tools.
Building a Case
The story now shifts to the Mount Wilson Observatory in California. Equipped with a new telescope Edwin Hubble was able to make use of Slipher’s red shifts and Leavitt’s standard candles. In the early 1920s Hubble discovered that some of the starlight he observed was coming from distant galaxies. Before this finding the only known galaxy was our own. Today we know that there are well over 100 billion galaxies in the visible universe alone. In an instant, Hubble had shown that the universe was much bigger than anyone had theorized.
Roughly a decade later, Hubble made an equally stunning discovery. By observing distant galaxies, he determined that they were all moving away from us. The only exception to this was our own local cluster (close enough in proximity to be held together by gravity). All galaxies were moving away from us on average. In short, the universe was expanding in all directions! Furthermore, the distance between galaxies and the speed at which they were moving were proportional. For instance, galaxies twice as far away were moving twice as fast, three times as far away, three times as fast. Interestingly, debris from an explosion shares a similar signature. This is because the further away from the epicenter the debris comes to rest, the faster it has to travel.
Just as Slipher before him, Hubble had little understanding of General Relativity and failed to recognize the full significance of his discovery. It took a Belgian priest and scholar named Georges Lemaitre to put it all together. He applied General Relativity to Hubble’s findings, wound the clock backwards, and in 1931 he suggested that the universe began in a single geometric point. This was the original idea, which later became known as the Big Bang. Nevertheless, the world was not yet ready for Lemaitre’s bold idea. It took a few more decades before Lemaitre’s idea became an established scientific theory.
In 1964 the Big Bang Theory was finally confirmed by observation. Two Bell Laboratory scientists named Arno Penzias and Robert Wilson were testing a microwave detector. They were receiving interference coming from all directions. After ruling out a number of possibilities, it was determined that the signal was coming from outer space. In fact, they had discovered the cosmic microwave background radiation. They had accidentally stumbled upon the echo of the Big Bang.
The cosmic microwave background (CMB) is the remnant of light from the Big Bang. It had been predicted earlier, but now it was confirmed by observation. Due to the expansion of space, this light has been stretched to the microwave part of the spectrum. From its extremely hot beginning, the temperature of the CMB has now cooled to 2.7 degrees above absolute zero (nothing can be colder than absolute zero). No matter where we look the temperature of the CMB varies by less than a thousandth of a degree. These temperature measurements imply a common origin. How else could microwave radiation, separated by vast distances, have practically the same temperature (everywhere) unless it originated from a common event?
Evidence for the Big Bang (Recap)
- Receding Starlight- By measuring the red shift of distant stars Vesto Slipher discovers that distant stars are moving away from us, suggesting that the universe is not static.
- Establishing Cepheid Variables- Henrietta Swan Leavitt finds a way to make use of pulsating stars to measure distances in the large-scale universe.
- Expanding Universe- Edwin Hubble discovers that the universe is expanding proportionally in all directions.
- Compatible with General Relativity- Albert Einstein’s famous theory predicts a non-static universe and allows for space to bend and expand (necessary for the big bang concept).
- The Smoking Gun- Arno Penzias and Robert Wilson stumble upon the cosmic microwave background radiation (the echo of the big bang). The Temperature of the CMB varies by less than a thousandth of a degree.
Note: There are also other pieces of evidence which point to a Big Bang that requires a background in particle physics to appreciate (which I do not have), so I have left it out here.
Interesting Facts About the Big Bang
- The Theory begins a tiny fraction of a second after the bang. The known laws of physics cannot be applied prior to the theoretical beginning of time. What happened before is uncertain.
- The Big Bang created time and space as we know it, calling into question the idea of a before.
- At the Big Bang the universe was at its hottest; it has been cooling ever since.
- At the beginning, the universe was in its most orderly state. From the moment of its origin, it has been moving towards higher disorder.
- The universe was in its simplest form at the Big Bang; it has been growing in greater complexity since its birth.
- There is no such thing as the center of the universe. From any given galaxy an observer would see the same thing; all galaxies would be moving away on average.
- Galaxies don’t move through space, it is the space itself which is expanding and carrying the galaxies along.
- The term ‘Big Bang’ was coined by astrophysicist Fred Hoyle. It was meant as a put down for a theory he never accepted and the term stuck.
- Arno Penzias and Robert Wilson won a Nobel Prize for their discovery of the CMB; something they were not even looking for.
- If you tune a T. V. to a channel that is not broadcasting, 1% of the snow on the screen is due to the cosmic microwave background. So if you ever complain that there is nothing to watch on T. V., you can always disconnect the cable and watch the Big Bang.
References: Bill Bryson, A Short History of Nearly Everything (London: Black Swan, 2004).
Mark Henderson, Joanne Baker, Tony Crilly, 100 Most Important Science Ideas (United States: Firefly Books, 2011).
Dec. 30, 1924: Hubble Reveals We Are Not Alone, Randy Alfred, 12.30.09, https://www.wired.com/2009/12/1230hubble-first-galaxy-outside-milky-way/
Scientific America, What is the Cosmic Microwave Background Radiation? October 13, 2003, https://www.scientificamerican.com/article/what-is-the-cosmic-microw/