Big-Bang: Cosmic Evolution

As we've discussed in a previous article Quantum Cosmology: Birth of a Universe, the Big-Bang was not the ACTUAL beginning of the Universe, but rather described its exponential development since its creation (if it was indeed created). But in this article, I plan to talk more about what the Big-Bang actually was and clear up any misconceptions about it.

First of all, the Big-Bang was not an actual “Bang”, or explosion (that’s actually quite a misleading name). Rather, it was actually an expansion of space-time.

Interesting: The name “Big-Bang” was actually first coined by a critic of the Big-Bang, astronomer Fred Hoyle. Although he used it in a mocking sense, the name was so memorable it stuck!

Another misconception (or rather misunderstanding) you may have learnt about the Big-Bang from high-school is that it started out from a “singularity”; a point with infinite density and 0 volume. It is important not to take this definition literally though, as it’s actually physically-impossible for such an object to exist. General Relativity says no point can have infinite density or energy. And if you've read the previous article, you might remember that the currently most-accepted hypothesis by cosmologists is that the Universe started out as a virtual particle, and quantum physics doesn’t allow a particle to inhabit a space smaller than its wavelength (i.e. 0 volume).

Singularities are a sign that current general relativity and quantum mechanics aren't suitable to adequately explain points of high gravity. Developing a quantum theory of gravity to solve the problem of this mathematical quirk has become the goal of modern cosmology.

One of the main pieces of evidence and main sources of information about the Big-Bang is the fact that galaxies are travelling away from us, a prediction of the Big-Bang Theory. With the Hubble space-telescope, we were able to first see that the light from distant galaxies was red-shifted (this meant that their wavelengths were being stretched out by movement away from us), which provided the first real evidence of the Big-Bang Theory. In fact, this was the case with all the galaxies we saw around us.



Waves are stretched out in the direction the object emitting them is moving away from and is cramped in the direction the object is moving towards, according to the Doppler Effect.

Interesting: Although similar, the cosmological red shift is not EXACTLY the Doppler Effect, as the waves don’t travel through a medium (space is a vacuum). Rather, it is the expanding space between galaxies that causes the light to stretch out and become red-shifted.

Now, that does seem to have interesting implications. If all the galaxies we saw were moving away from Earth, wouldn't that mean that the Milky Way (our galaxy) is the centre of the Universe? Not exactly. See, from our point of view, it may look like we’re the centre. But imagine (for example) how an alien would view us from another galaxy. As the distance between our galaxies grows, they would also think they are the centre! So unless you can think about the Universe objectively from the outside, than any (and every) point in space can look like the centre.



Depending on how you think about it, you can therefore say that the Universe has either no centre... or eveywhere is a centre!

Another interesting fact about the expansion is that recent observations indicate that, opposite to common-sense, the expansion isn't slowing down. In fact, it’s actually speeding up! So galaxies are moving at a faster and faster speed away from each other.

Now, that, again, would have some interesting implications. If the speed of galaxies is continually accelerating, what will happen once it reaches the speed of light? Does it stop? No. In fact, it will surpass the speed of light.

The reason that may seem strange is because of a common physics misconception. Most people have the idea that, relativity says that “nothing can travel faster than the speed of light”. This is NOT what relativity states. What it actually states is that “information within space-time can’t travel faster than the speed of light”. And you have to remember that the galaxies aren't moving of their own accord; they are being pushed by the expanding space around them (kind of like how a person travelling in a car can go 80km/h, but the person themselves isn't able to travel 80km/h). So information within space-time may not be able to travel faster than light, but space-time itself can.

We can calculate, based on the current rate and speed galaxies are travelling, how long ago the actual Big-Bang was. Our current best estimates place it at 13.72 billion years ago.

Another one of the most convincing pieces of evidence for the Big-Bang Theory is the CMB (Cosmic Microwave Background) radiation, a predicted “afterglow” (radiation) left by the Big-Bang. It used to be one of the biggest problems facing the Big-Bang Theory until it was finally discovered, effectively solidifying the Big-Bang’s status as the Standard Model of Cosmology.It was first detected by the Prognoz 9 satellite in 1983, confirmed by COBE (Cosmic Microwave Background Explorer) in 1989-1992, and since studied extensively and in great detail by other satellites, including the WMAP (Wilkinson Microwave Anisotropy Probe), Planck satellite and, most recently, the BICEP2 satellite.

A picture of the radio-wave distribution of the CMB, collected by the Planck satellite.

Interesting: Have you ever tried to turn on an old, analogue TV set and tune in to a channel but get nothing but static? Approximately 1% of that static is the CMB… left over from the Big-Bang.

Well, there are some of the basics about the Big-Bang. In the next 2 articles we’ll look at the 2 main hypotheses devised to try and explain what caused the Big-Bang; inflationary and cyclic cosmology.