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Big Bang

The 'Big Bang' is one of the most misunderstood and abused concepts in modern physics. Much of this is a result the media (magazines, TV, radio, etc) perpetuating the same description and the same questions, e.g. 'What was there before the Big Bang?'

OK, so let's take a little look at the history of the concept.

In 1912, Vesto Slipher was the first to observe a red-shift in the spectral lines of light from faraway galaxies. The red-shift is a form of the Doppler Effect. With sound, if you listen to a vehicle approaching you at speed, the noise it makes is of a higher pitch than when it's travelling away from you because the sound waves get bunched up. With light, the effect causes a similar change of the wavelength, and hence of the colour. One interpretation of a shift to the red end of the spectrum (i.e. a lengthening of the waves) is that those galaxies are moving away from us.

In 1929, Edwin Hubble discovered that the distances to such galaxies were generally proportional to their red-shifts. His interpretation was that the further away they are, the higher their apparent velocity relative to ourselves. This idea had already been proposed by Georges Lemaître in 1927 in his "hypothesis of the primeval atom". In conjunction with Milton Humason, Hubble formulated Hubble's Law to describe this observed effect.

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast, although not as a serious description. Unfortunately, it has stuck! It's unfortunate because it's a misnomer: there was no bang (or any noise at all) and it wasn't big (it was infinitesimally small). There wasn't even an explosion. With an explosion, particles are thrown outward into pre-existing space. However, in the case of the universe, it is space that is appears to be expanding between the particles. This is sometimes referred to as a 'metric expansion of space'.


Note that there is no place we can point to and say that's where the expansion started. The observed expansion is uniform in all directions (i.e. isotropic) and would be observed to be the same from all galaxies. What we can say is that space was smaller the further back in time we consider. The point at which the size would be zero is called the origin and is estimated to be about 13.7 billion years ago. However, mathematics describes this simply as a "singularity", as with the centre of a circle, rather than any causal event. Our desire to attach concepts such as 'explosions' or 'creation' to it is more to do with our perception of the world around us.

One of the most asked questions is 'What was before the Big Bang?' but the question doesn't mean anything because there was no time before the origin. Stephen Hawking compared it to asking 'What lies north of the North Pole?', although a closer analogy might be to ask 'What's before the centre of a sphere?'. I am suggesting a (hyper-)sphere analogy is closer because Hubble's Law already implies that same geometrical interpretation: space is bigger as time increases.


Let me expand on this, so to speak. In order to explain why galaxies further away appear to be travelling faster, science has invented crazy notions such as 'dark energy'. Imagine for a moment, though, that you're blowing up a balloon. The expanding membrane of the balloon is a fair analogy with the expanding space mentioned above, and the radius of the balloon for the measured time. If you draw a couple of dots on the surface then their separation increases as the radius of the balloon increases. Not only that, the increased separation is greater for two dots that were initially further apart, simply because there is more membrane expanding between them. In other words, the apparently accelerating expansion of the universe is merely a geometrical effect.



In the diagram, the separation of x1 and x3 has increased more than that of x1 to x2 between the two times t1 and t2. If the separation is expressed as:


x = aθt


where a is a constant and θ is an expression of the angular separation, then the speed of separation (dx/dt) is just aθ. In other words, the speed is greater for points with a greater angular separation, but there is no actual acceleration. The speed does not increase as the spatial separation increases for two points with a fixed angular separation.


Let's consider this membrane view of the universe a little more. The idea is not completely new but it has always been considered in the context of an 'expanding membrane'. Our section on 'Time' proposes a geometrical, non-dynamic view of time, though, so what impact does this have here? Well, instead of an expanding membrane, we're now considering concentric shells, each being all of space at a different time. Associating measured time with the radius of this hypersphere means we have an implicit 'arrow of time' by virtue of the geometry (i.e. time has an 'origin' at only one end). As the 'Time' section explains, there are only a few phenomena that imply time has a direction: the so-called expanding universe, the increase of entropy, and our memory of the past rather than of the future. I would also add gravitational attraction as we have yet to observe any equivalent repulsion. We have described how the expanding universe can be explained through this geometry. Stephen Hawking has already suggested that our mind's recording of the past (or by any recording device) is linked to the direction of increasing entropy. If increasing entropy is a natural consequence of the same content (matter, etc) being in a bigger volume of space then all of these arrows can be explained together.


So what does the universe look like in this static, geometrical view? How does it affect our other notions? Well, an important casualty is cause-and-effect, and hence the laws of physics themselves which are simply trying to predict cause-and-effect. Unfortunately, cause-and-effect is now just the description of the contents of space at a particular time from the contents at a time just before it. It is no more fundamental than trying to explain them from the contents at a time just after it. What we're observing is a pattern amongst the 'world lines' (i.e. paths of particles in space and time) and describing one part of it from another part of it. If all particles in the universe were persistent then all their world lines would be tethered at the origin of the universe. We're only looking at a relatively small period of time, though, and so we cannot guarantee that our observed pattern in those world lines would be the same at other points in time. In effect, the laws of physics may be slightly different as we consider more distant times, and we really have no way of knowing what they were at the origin.

Tony Proctor,
24 Nov 2011, 09:13