The Big Bang Theory
"The evolution of the world can be compared to a display of fireworks that has just ended; some few red wisps, ashes and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanishing brilliance of the origin of the worlds." Lemaitre
An overwhelming weight of evidence has convinced cosmologists that the universe came into existence at a definite moment in time, some 13 billion years ago, in the form of a superhot, superdense fireball of energetic radiation. This is known as the Big Bang theory. Until the arrival of the Big Bang theory the universe was believed to be essentially eternal and unchanging, represented by the Steady State model. The first clear hint that the universe might change as time passes came in 1917 when Albert Einstein developed his General Theory of Relativity. Einstein realised that his equations said that the universe must be either expanding or contracting, but it could not be standing still, because if it were then gravity would attract all the galaxies towards one another. This was, at the time, a revolutionary concept, so revolutionary that Einstein refused to believe it and introduced his infamous 'cosmological constant' into the equations so that the sums agreed that the universe could be static. He later claimed it was the biggest blunder of his career. It was in 1920 that Edwin Hubble discovered that the universe was expanding by measuring the light from distant galaxies. This discovery was followed in 1927 by Georges Lemaitre, a Belgian astronomer, who was the first person to produce a version of what is now known as the Big Bang model.
It is necessary to understand that the Big Bang did not begin as a huge explosion within the universe, the Big Bang created the universe. A popular misconception is that it happened within the universe and that it is expanding through it. This causes people to wonder where in the universe it started, as if by running the clock backwards we would reach the point where all the galaxies come together in the centre of the universe. The universe does not have a centre, any more than the surface of a sphere has a centre, there is no preferred place that could be termed the centre. I know this sounds odd, it must have a centre, mustn't it? The problem we have here is we are trying to visualise the universe in the standard 3 dimensions that we are familiar with and therefore expect to find a centre to an expanding sphere. The universe, however, is not an expanding 3 dimensional sphere, it contains also the dimension of time (see 'What is Time?') and many other dimensions as well. By way of an illustration imagine a balloon with dots painted on the surface to represent the galaxies. If the balloon is now inflated we can see that all the dots are moving away from one another, just as the galaxies are in the real universe, and we can also see that on the surface of the balloon there is no centre point from which all the galaxies are moving away from. I am not suggesting that we are existing on the 'outside' of an expanding bubble, only that we cannot visualise the entire expanding universe.
Let's begin with a brief look at how the Big Bang describes the creation and evolution of the universe before moving on to some of the evidence to support the theory and the problems associated with the theory.
The Big Bang theory
The standard model of the Big Bang theory proposes that the universe emerged from a singularity, at time zero, and describes all that has happened since 0.0001 (10-4) of a second after this moment of creation. The temperature of the universe at that time was 1,000 billion degrees Kelvin (1012) and had a density that of nuclear matter, 1014 grams per cubic centimetre (the density of water is 1 gram per cubic centimetre). Under these extreme conditions, the photons of the 'background' radiation carry so much energy that they are interchangeable with particles. Photons create pairs of particles and antiparticles which annihilate one another to make energetic photons in a constant interchange of energy in line with Einstein's equation E = mc2. Because of a small asymmetry in the way the fundamental interactions work, slightly more particles were produced than antiparticles - about one in a billion more particles than antiparticles.
When the universe had cooled to the point that photons no longer had the energy required to make particles, all the paired particles and antiparticles annihilated, and the one in a billion particles left over settled down to become stable matter.
One-hundredth of a second after time zero the temperature had fallen 90% to 100 billion K. By one-tenth of a second after time zero the temperature was down to 30 billion K. The temperature after 13.8 seconds was down to 3 billion K, and by three minutes and two seconds had cooled to 1 billion K, only 70 times hotter than the centre of the Sun today. At this temperature nuclei of deuterium and helium could be formed and stick together despite collisions with other particles.
During the fourth minute after time zero reactions took place that locked up the remaining neutrons in helium nuclei, as described by Gammow et al in 1940 and Fred Hoyle and others in the 1960's. This epoch ended with just under 25% of the nuclear material converted into helium, and the rest left behind as lone protons - hydrogen nuclei.
By just over 30 minutes after time zero, all of the positrons had annihilated with almost all of the electrons - with again one in a billion left over - to produce the background radiation proper, and the temperature had dropped to 300 million K, and the density was only 10% of that of water. At this temperature stable atoms were still not able to form.
The interactions between electrons and photons continued for 300,000 years, until the universe had cooled to 6000 K, roughly the temperature of the surface of the Sun, and the photons were becoming too weak to knock electrons off atoms.
Over the next 500,000 years the background radiation decoupled, and had no more significant interaction with matter. The Big Bang was in effect over, and the universe left to expand and cool. About 1 million years after time zero, stars and galaxies could begin to form. Nucleosynthesis inside stars convert hydrogen and helium to make heavier elements, eventually giving rise to our Sun, the Earth and ourselves.
This is only a very brief overview of the main points describing the evolution of the universe, a number of books have been published that describe just the first four minutes or less!
So how does it all stack up? How much evidence do we actually have to support the Big Bang model of the universe?
Einstein's Theory of Relativity. This is a theory of spacetime, offering a complete mathematical description of the universe. Relativity, along with Quantum Mechanics, (see "What is Quantum Mechanics?") is considered to be the most complete and accurate theory ever devised, mathematically describing such diverse phenomenon as the constant speed of light and the formation of black holes. Einstein's equations tell us - apart from many other things - that the universe is expanding, and that by going back in time there must have been a time when all the galaxies were very close together. And further back when all the stars must have been touching each another, merging to make one great fireball as hot as the inside of a star at 15 million degrees Kelvin (Kelvin is absolute zero temperature). Einstein's equations actually go further back than that, to a time when all the matter and energy of the universe emerged from a single point of zero size, a singularity. This is how the Big Bang theory describes the birth of the universe.
Expansion of the universe. One of the reasons the universe is believed to be expanding is because of the phenomenon known as 'red shift'. Light, or other electromagnetic radiation from an astronomical object may be stretched, (due to a number of reasons) making its wavelength longer. Because red light has a longer wavelength than blue light, the effect of this stretching on features in the optical spectrum is to move them towards the red end of the spectrum. If then the optical spectrum of a distant galaxy shows features that are shifted towards the red end of the spectrum (red shifted), it can be due to one or more of the following three reasons:
1) Motion. The galaxy is moving away from us, this is known as the Doppler effect. The same effect can be detected in sound. When a police car is speeding towards us the sound waves made by its siren are 'squashed' and the pitch sounds higher. As it passes us and starts to move away the sound waves are 'stretched' and the pitch sounds lower. In the 1920's Edwin Hubble observed that all galaxies (apart from a few local ones attracted towards our own and showing blueshift) show red shift. This indicates that the galaxies are all flying away from us, as in a Big Bang explosion.
2) Expansion of the universe. Einstein's famous equations show that the universe should be expanding, not because the galaxies were moving through space, but because the 'empty' space between them (spacetime) is expanding. This cosmological redshift results because the light from the distant galaxies is stretched by the amount that space expands while the light is en route to us. This also reveals that the Earth is not at the centre of the universe with all the galaxies moving away from us, but that due to the expansion of the universe, all the galaxies are moving away from each other, like painted dots on a balloon moving apart as it is inflated.
3) Gravity. This is also explained by Einstein's general theory. Light moving outwards from a star is moving 'uphill' in the star's gravitational field, and loses energy as a result. Because light cannot slow down - it always travels at the same speed - when it loses energy its wavelength increases, in other words, it is redshifted. It does however, require a very powerful gravitational field for this effect to be measurable, such as created by a white dwarf star.
All three kinds of redshift can be at work at the same time. If we had telescopes sensitive enough to see light from a white dwarf star in a distant galaxy, the overall redshift in that light would be due to a combination of Doppler, cosmological and gravitational redshifts.
The fact that we can measure redshift in the light from distant galaxies tells us that the galaxies are receding from us, and from each other. It only takes a little logical deduction to conclude that as they are now all receding from one another, then at some finite point in the past (believed to be around 13 billion years or so) they must have all been at the same point.
Microwave Background Radiation. The universe is filled with a sea of radiation at a temperature of just over 2.7 degrees Kelvin, detectable at microwave radio frequencies both by Earth based radio telescopes and by instruments onboard artificial satellites. This is interpreted as direct evidence of the Big Bang fireball in which the universe was born, being the remnant of the superhot radiation from the fireball that has cooled down as the universe expanded. The discovery of the background radiation is therefore the most important observation made in cosmology since the discovery by Edwin Hubble that the universe was expanding. The existence of the background radiation, and its temperature, was accurately predicted by the Big Bang theory. When it was later discovered, by chance as it happens, this was yet another confirmation of the theory.
Nucleosynthesis of the light elements.
As the universe expanded and cooled, so the process of matter building began that led to the formation of stars, planets, galaxies etc. The process began with the simplest element, hydrogen, then helium, and then eventually onto more complex elements. The observed abundance of hydrogen, the simplest element and the most common in the universe, followed by helium, is yet further confirmation of the Big Bang theory.
The study of stars reveals how their internal nuclear interactions cause simple atoms, such as hydrogen and helium, to create more complex elements. Stars are in fact gigantic matter producing factories, converting hydrogen and helium into carbon and heavier elements by the process of nucleosynthesis deep within their interiors. If stars did not exist, we would not be here, our own atoms that make us, were formed by the stars, and by the supernovas at the death of certain size stars. All as described by the Big Bang theory.
Formation of galaxies and large-scale structure
the Big Bang model provides a framework in which to understand the collapse of matter to form galaxies and other large-scale structures observed in the universe today. At about 10,000 years after the Big Bang, the temperature had fallen to such an extent that the average density of the universe began to be dominated by massive particles, rather than by light and other radiation. This change meant that the gravitational forces between the particles could begin to take effect, so that any small perturbations in their density would grow. These small perturbations led to the formation of galaxies.
These are the principle observed phenomenon that go to support the Big Bang theory. Is it proof enough? Do we have a universe that was born out of a singularity, that is expanding and cooling, and is therefore finite in both age and size? See Can anything 'real' be infinite? for an explanation of a finite universe.
What do I think?
The Big Bang theory clearly still has a long way to go in order to be able to explain the origin of the universe. That is not to say that the theory as it stands is in any way wrong.
The staggering amount of evidence in support of the Big Bang theory is simply overwhelming. So much so that the theory simply cannot now be overturned. What is known to agree with the theory today cannot be changed tomorrow, by any theory, to make it disagree.
The situation is such that any new theory, far from displacing the Big Bang, would have to incorporate it. In other words, it can only be improved upon in much the same way that Einstein incorporated Newton's theory of gravity into his own theory of relativity. Relativity did not overthrow Newton's theory, it incorporated and developed it.
I realise that some people do not support the Big Bang theory, and a few have contacted me to inform me of this. Okay, agreed, it is 'only a theory.' It is also 'only a theory' that the Earth orbits the Sun, and not the other way around. But eventually, the mass of accumulated evidence for 'only a theory' becomes so powerful and persuasive it becomes impossible to ignore, whether or not you happen to like it.
For more information on the Big Bang and related topics, the following is an excellent site: In the Beginning
POLL & MESSAGE BOARD
Is the Big Bang theory correct?
Yes No Don't know
Current Results
In a nut shell, here are the reasons why the Big Bang theory is believed to be correct. Taken from Ask the Space Scientist
1.... The universe is expanding.
2.... There exists a cosmic background radiation field detectable at microwave frequencies.
3.... The cosmic microwave background field is measurably isotropic to better than a few parts in 100,000 after compensation is made for the relativistic Doppler effect caused by Earth/Sun/Milky Way motion.
4.... The cosmic microwave background radiation field is precisely that of a black body.
5.... The cosmic microwave background radiation field has a temperature of 2.7 K.
6.... There does exist a universal abundance ratio of helium to hydrogen consistent with the current expansion rate and cosmic background temperature.
7.... The cosmological abundance of deuterium relative to hydrogen and helium is consistent with the levels expected given the current expansion rate and density.
8.... There are only three families of neutrinos.
9.... The night sky is not as bright as the surface of the Sun.
10... The cosmic background radiation field is slightly lumpy at a level of one part in 100,000 to 1,000,000.
11... There are no objects that have ages indisputably greater than the expansion age of the universe.
12... There are about 10,000,000,000 photons in the cosmic background radiation field for every proton and neutron of matter.
13... The degree of galaxy clustering observed is consistent with an expanding universe with a finite age less than 20 billion years.
14... There are no elements heavier than lithium which have a universal abundance ratio.
15... The universe was once opaque to its own radiation.
16... The universe is now dominated exclusively by matter and not a mixture of matter and anti-matter.
I should point out that, although some rival theories have proposed alternate explanations to a few of these observations, there are no rivals that provide a simple explanation for ALL of these remarkable observations. What could be simpler than a universe expanding from a hot, dense state to the present cool, rarified one with all of the above features emerging! No new physics is required.
I think that is pretty convincing, don't you?
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