Big Bang Theory
The Big Bang Theory is the currently accepted model of the creation and evolution of the universe. The discovery that the universe is uniform in all directions and that all the observable galaxies are moving away from one another led to the conclusion that the universe is expanding. Logically then at some point in the distant past, the matter of the universe must have been more condensed. Taken to its logical conclusion the universe must at its beginning been in one place. A super dense mass known as a singularity.
At some point and for reasons still being investigated the matter and time underwent a sudden expansion, the so-called Big Bang. Before this point, there was no universe; no matter and no time, since then the universe has been expanding.
Current estimates place this event at 13.8 billion years ago.
In 1948 an alternative explanation for the structure and evolution of the universe was proposed, called the Steady State Model. In this model, the universe is still expanding but maintains a steady state of density by the creation of new matter in the form of stars and galaxies as the universe expands. In this model, there is no beginning to Space-Time, it has and will always exist.
As both models accept the universe is expanding, they both allow for the observation of Redshift and the observation that all observable galaxies are moving away from us.
- The Big Bang Theory and the Steady State model are two theories about the origins and evolution of the universe. What are the key differences between these two competing views of the universe?
- Your answer should include: beginning / 13.8 / billion years / constant density
Explanation: Steady State model has no beginning to Space-Time, whereas the Big Bang Theory has a definite start to Space-Time about 13.8 billion years ago. Both accept the expansion of the universe, but in the Steady State model, the density of the universe is constant as new matter is being generated by the creation of new stars.
- Explain what Redshift is and how it supports the hypothesis that the universe is expanding.
- Your answer should include: Elongated waves / doppler effect
Explanation: Light is affected by the Doppler Effect, as are all waves, this predicts that if a galaxy is moving relative to us then the light will be shifted to one end of the spectrum as the waves are either elongated or compressed. Light from a galaxy moving away from us will be elongated and thus shifted to the red end of the spectrum. Hubble and others observed that this is true for all observable galaxies, thus all galaxies are moving away from us and each other, so the universe must be expanding.
- How did the observation that cosmic microwave background radiation is evenly distributed across the sky provide evidence in support of the Big Bang Theory over the Steady State Model?
- Your answer should include: no initial creation / microwaves / equal distance
Explanation: In the steady state model there is no initial creation of the microwaves, as there is no start to Space-Time, instead the observation is explained by suggesting the microwaves are produced by stars and then scattered. This would not produce the even spread of microwaves across the sky that we observe. Whereas this is what is predicted by the Big Bang Theory, as all part of an expanding universe are equally distant from the initial point of the big bang.
Redshift refers to the observation that light coming from other galaxies has more red than blue light. This is a result of an effect called the Doppler Effect which explains how waves are altered as they travel from source to observer if the source and the observer are moving relative to each other. The most common example of this is the change in pitch of a car as it travels towards and then away from you as you stand beside the road. As the car comes towards you the wavelength of the sound wave is compressed giving a higher pitch, as the car moves away from you the waves are elongated and the pitch decreases.
The same thing happens to light waves from the galaxies. If a galaxy was moving towards us the wave would be shortened giving more blue light (Blueshift), if it is moving away from us the wave is stretched and gives a shift to the red end of the visible spectrum, or Redshift.
This was first noted in 1848 by French physicist Fizeau. It was the work of Edwin Hubble that established that all observable galaxies have light that is Reshifted and therefore are moving away from us. He also established Hubble’s Law to compare the degree of Redshift to the speed at which the galaxies are moving away and their distance from us. The further away a galaxy is the faster it is moving away from us and therefore the more shifted to red is its light. This provided proof of the expansion of the universe as all galaxies are moving away from each other.
Evidence to Support the Big Bang Theory
The observation of the redshift of the light from other galaxies provides evidence for the expansion of the universe. This does not, however, prove that the universe began as a Big Bang. Both the Big Bang Theory and the Steady State Theory accept that the universe is expanding and therefore both accept the Redshift evidence as support.
The most important evidence in support of the Big Bang Theory over the Steady State model came in the 1960’s when scientists in Russia and the USA first detected the Cosmic Microwave Background Radiation (CMBR). This was predicted by the expansion model of the Big Bang Theory, the microwaves are an ‘echo’ of the Big Bang itself and in accordance with the Theory the intensity of the CMBR is the same in all directions. Which means that the level of background microwaves coming from space is the same no matter which direction you aim the detector at in the sky.
Supporters of the Steady State model tried to explain this observation by suggesting that the microwaves were produced by point sources like stars, and then the microwaves are scattered by cosmic dust. However, this model would not create the uniformity of microwaves as observed, it would be more patchy with some areas of the sky having more radiation if it was in the direction of a star that is emitting these microwaves.
The image is a map of the intensity of the CMB radiation over a 9 year period.
It is as a result of the detection of the even distribution of CMBR that the Big Bang Theory is the accepted theory and the Steady State model is no longer accepted as fitting the observed evidence.