The Big Bang

August 31, 2008

Where did the Universe come from? How old is time? By the early twentieth century, these questions seemed beyond the scope of human knowledge. That the universe was eternal and unchanging seemed the only sensible viewpoint as there was no observational evidence to suggest otherwise. The Big Bang Theory offers a remarkably different view of an ever-expanding Universe. According to this bizarre theory, the entire universe, all matter, energy, space and time, were condensed in to a single point smaller than a pinhead, and, several billion years ago, it began a violent expansion that is still occurring today. This is truly an extraordinary claim and would be very difficult to believe without extraordinary evidence to support it.

In order to understand why the evidence for the Big Bang Theory is so compelling, we would do well to find what makes a strong scientific theory. The most well phrased definition of “theory” that I’ve ever heard or read is from Stephen Hawking’s best-selling book A Brief History of Time: “A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations.” Let’s move forward with an overview of some of the key evidence for the Big Bang Theory and see how it holds up to Hawking’s criteria.

The Expanding Universe

One of the revolutionary implications of Albert Einstein’s remarkably successful General Theory of Relativity was that the Universe was an extremely unstable place. According to the theory, gravity acts between every bit of matter in the cosmos and, if left unchecked, should cause the Universe collapse in on itself. While the rest of the theory had been vindicated time and again, this collapse was clearly not an accurate description of the Universe that we lived in. Einstein’s theory needed some force to counter gravity in order to fit observation. Rather than accept the possibility of an expanding Universe that his theory seemed to suggest, Einstein altered his equations with what would become known as the cosmological constant.

Mathematician Alexander Friedmann, however, posed three unique solutions to Einstein’s equations, each describing a Universe that was expanding from a primeval state. While Friedmann’s proposed expansion had the effect of counterbalancing gravity, his theory had two major setbacks: 1) It challenged the scientific orthodoxy of the time, which had always perceived the Universe as static, and 2) It did not agree with observation as expansion is not noticeable in “small” systems, such as solar systems and galaxies, and observational data of anything beyond the Milky Way was scarce.

Astronomer Edwin Hubble

Friedmann’s solutions would remain in obscurity until they were corroborated by the observations of astronomer Edwin Hubble. Using the method of spectroscopy, Hubble showed that the light from distant galaxies was being shifted toward the red (lower energy) end of the electromagnetic spectrum. Hubble interpreted this as the Doppler effect, which Wikipedia describes as “the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves.” In other words, high-energy light waves from galaxies millions of light years away from us were being “stretched” in to lower energy light waves on their way to us. Hubble had documented the expansion of the Universe!

This first observational evidence in support of the Big Bang Theory changed the way we thought of the Universe, inviting cosmologists to use the theory to explore questions that had previously escaped the grasp of science.

The Abundance of Elements

One of the cosmic mysteries that eluded scientists in the decades before the Big Bang Theory took foot was the uneven distribution of chemical elements throughout the Universe. Again by the method of spectroscopy, it had been established that hydrogen, the lightest of the elements, is by far the most abundant, consisting of over ninety percent of all of the atoms in the universe. Helium, the second lightest element, is a distant second consisting of about nine percent of all atoms. This means that all of the remaining chemical elements consisted of only one thousandth of a percent of all of the atoms in the universe.

Nuclear Physicist George Gamow

In the nineteen-forties, nuclear physicists George Gamow and Ralph Alpher attempted to explain the processes of nuclear synthesis during the early stages of the Universe within the framework of the Big Bang model. Gamow knew that the Friedman theory supported by Hubble’s observations implied an extremely hot and dense early Universe. Working forward from this primeval state using painstaking calculations, Gamow and Alpher found that the Big Bang Theory predicted that there should be approximately one helium nucleus for every ten hydrogen nuclei, mirroring observations of astronomers.

While their findings were nothing short of a triumph for the Big Bang Theory, Gamow and Alpher found that they could not explain the synthesis of heavier elements in the conditions of the early Universe. This discrepancy was later resolved when Fred Hoyle posed stellar nulceosynthesis as a mechanism for the formation of elements heavier than helium, which I’ll discuss in a future post. The Big Bang Theory was proving to be very good at describing existing astronomical observations, but had failed to make any scientific predictions that could be tested.

Cosmic Microwave Background Radiation

All of the nuclear synthesis that Gamow and Alpher described happened in the first few minutes of the Universe. Afterwards, the Universe was cooled down to a point where nuclear synthesis was no longer possible, but was still so hot that electrons could not attach themselves to the newly formed hydrogen and helium nuclei. During this period, which lasted more than 300,000 years, the abundance of free-floating electrons prevented the free travel of the gamma rays that permeated everything. The Universe continued to expand and cool, eventually to the point that electrons could join with the nuclei, forming hydrogen and helium atoms. With no haze of electrons to block their path, the gamma rays were free to travel across the Universe.

Based on this model, Gamow and Alpher along with fellow physicist Roger Herman, predicted that this abundance of gamma rays would still permeate the Universe in our time, but because the waves would seem to be “stretched” by the familiar phenomenon known as redshift, they would be microwaves when they reached us in the Twentieth Century. More specifically, Alpher and Herman predicted that the temperature of this background radiation should be approximately 5 Kelvin.

Fifteen years passed before anyone detected the Cosmic Microwave Background Radiation (CBR), but it was finally discovered in 1964 by two physicists working for Bell Labs, Arno Penzias and Robert Wilson. The reality of the CBR matched prediction almost exactly in that it was an all pervading radiation, equally measurable from any direction. Alpher and Herman had even gotten the approximate temperature right (the actual temperature of the CBR is 2.70 Kelvin as opposed to the 5 Kelvin predicted).

The fulfillment of the prediction of the CBR was a momentous triumph for the Big Bang Theory. Not only was the CBR a very precise prediction, but it is also yet another strange observation of the Cosmos that can be explained by the Big Bang and the Big Bang alone.

Microwave Imaging of the CBR from the WMAP Sattelite

Microwave Imaging of the CBR from the WMAP Sattelite

There are still quite a few holes in the Big Bang Theory. It cannot be used to predict what occurred in the first few moments of the Universe because at those extreme temperatures and pressures the laws of physics as we know them break down. Also, we are only in the speculative stage regarding the question of why there is an abundance of matter over antimatter in the Universe. While questions like these seem to be beyond the grasp of the Big Bang model, history has shown us that it is a mistake to consider anything to be outside the reach of science.

My knowledge on these topics relies heavily on a few resources not cited above including Big Bang by Simon Singh, and Origins by Neil Degrasse Tyson. I strongly recommend these books for further reading.


2 Responses to “The Big Bang”

  1. Mike Smith Says:

    What a beautifully written piece. Thank you for writing that.

  2. […] and Quantum Leap October 20, 2008 In a recent article on the Big Bang, I mentioned the method of spectroscopy twice without explaining what it is or how it works. […]

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