## Saturday, 19 November 2011

### 2. The Big Bang

How and when did our universe begin?

According to the most acceptable scientific model at present, our universe was born ~13.7 billion years ago. A so-called 'Big Bang' occurred at the moment of the birth of the universe, and this moment was also the Time Zero for our universe.

But how can something emerge out of nothing? Is that not a violation of the law of conservation of mass and energy? Was there not a Creator involved? No.

A proper explanation requires reference to what is called quantum field theory. But I shall attempt a simple explanation here, just to convey the point that there was no net creation or annihilation of mass/energy involved.

We all know about the force of gravity that the Earth exerts on all objects. If your mass is m, then your weight is mg, where g is 'acceleration due to gravity'. The mass is a measure of the quantity of matter. The weight is a force; the force the Earth exerts to pull you towards its centre. [Newton's second law of motion says that force is equal to mass multiplied by acceleration: F = m x a; and a is the same as g in our example.]

Your weight on the Moon would be one-sixth what it is on Earth because the Moon has a lower mass than the Earth, with a correspondingly lower value for its g.

The gravitational force or interaction is one of the four fundamental interactions of Nature. It is an attractive interaction (rather than a repulsive interaction). There is a force of gravitational attraction between any two objects. It is directly proportional to the product of their masses, and inversely proportional to the square of the distance between them. For example, if the distance is doubled, the gravitational force of attraction becomes one-fourth.

Consider an object (say of mass m1) that is so far from everything that there is no gravitational force on it. This is really not possible, but just imagine it as an idealization. We say that it is a 'free' object, free from any gravitational force acting on it.

Next, consider another object, of mass m2, at a certain finite distance from the first object. The two attract each other. Physicists speak of this as a negative-energy configuration, because (positive) energy needs to be spent to take the two objects so far away from each other that they are free of each other's force of attraction. By contrast, a positive–energy configuration would be one in which the two objects repelled each other, rather than attracting.

So, the gravitational interaction makes a negative contribution to the overall energy.

The other big idea I have to introduce here is that mass and energy are inter-convertible. This fact is embodied in the famous Einstein equation: E = mc2. Here c is the speed of light in vacuum. The equation says that if a mass m disappears, an equivalent amount of energy E would be produced or released.

This is how energy is produced in our Sun (by thermonuclear reactions). The mass of five atoms of hydrogen (or rather an atom of 'deuterium' (which is twice as heavy as an atom of hydrogen) and an atom of 'tritium' (thrice as heavy as an atom of hydrogen)) is a little more than that of an atom of helium and a charge-neutral particle called the 'neutron'. In the interior of the Sun, conditions are right for the fusing of deuterium and tritium atoms, and the creation of an atom of helium plus a free neutron; the balance mass appears as energy. We receive some of this life-sustaining solar energy on Earth.

Similarly, in a fission-based nuclear reactor we produce energy by losing a little mass of, say, uranium. The nucleus of an atom of uranium captures a neutron to form a 'compound nucleus'. This then splits (fissions) to two different nuclei the sum total of the masses of which is a little less than the mass of the compound nucleus. The balance mass appears as kinetic energy (same as heat energy) of the particles involved.

What happened at the Big Bang was that there was a simultaneous emergence of the gravitational interaction. There was an explosion of sorts, in which radiation (and, a little latter, matter) emerged. The universe has been expanding ever since then. Expansion means an increase in the distances between the celestial bodies. Such ever-increasing distances mean a build-up of negative energy, which gets compensated by the creation of an equivalent amount of matter.

This is how mass gets created out of 'nothing', and there is no violation of the law of mass/energy conservation. This 'nothing' is actually a vacuum, which has some remarkable properties which can be described in the language of quantum mechanics only. Further, according to Einstein's theory of gravitation (to be described in a future post), the existence of a certain 'cosmological constant' can endow empty space with mass/energy.

But let us not get into those details yet. For the time being, suffice it to say that our universe emerged out of nothing, and this did not require the intervention of a Creator. The book by Hawking & Mlodinow explains that in some detail.

Of course, there are other models which compete with the Big Bang model, and the final word has not been said yet. Science is open to all ideas, subject to rational analysis and experimental verification. One such model postulates a cyclical set of events, involving an endless series of Big Bangs and Big Crunches. But then, in such a model, there is no need to argue how matter can emerge out of nothing.

1. I must applaud your efforts to explain science. The following lines in your blog made me ponder.

//Such ever-increasing distances mean a build-up of negative energy, which gets compensated by the creation of an equivalent amount of matter.//

Does it mean that the creation of matter has not stopped and therefore we could expect more matter created to compensate the accelerating expansion of the universe ?

2. Thank you for your comment and question. As you can see from the level of presentation, I am glossing over many important details. My sole aim here was to give a plausibility argument for how something can emerge out of 'nothing'.
The emergence of matter from the early radiation field was a symmetry-breaking phase transition. A field called the Higgs field has been introduced in cosmology to understand this. This field results in the existence of a cosmological constant, which turns 'empty' space into a space that has an energy content. The problem at present is that the predicted cosmological constant has too large a value for a correct understanding of the observed cosmic evolution. It is believed that perhaps the Higgs cosmological constant had a large value right after the Big Bang, resulting in a violent and very rapid expansion (or inflation) of the universe. At a certain stage of this inflation, a cosmic phase transition occurred, which freed enormous amounts of energy. After this prelude of inflation and cosmic phase transition, the normal (much slower) expansion of the universe set in, and has continued ever since.
But there are gaps in our understanding. Even as early as in the 1930s, it was known that gravitational effects in large galactic clusters are much higher than what can be expected from the known amount of matter there. Apparently, there is another, unknown, form of matter that is a full 90% of all matter, as indicated indirectly by the gravitational effects. It is called dark matter.
There is not only dark matter, but also dark energy. Till the early 1990s the generally accepted belief in cosmology was that the universe cannot go on expanding at the current rate; the gravitational pull of all matter must at least slow down the rate of expansion. Two possibilities were conceivable: (i) The average energy density of the universe may be high enough to ultimately stop its expansion, and then cause its recollapse. (ii) The energy density may be so low that it may never stop expanding, but gravity would still slow down the expansion.
Then something unexpected happened. In 1998 the Hubble Space Telescope observations seemed to show that the expansion of the universe is actually accelerating, rather than slowing down. Three explanations have been offered for this: (i) Perhaps the acceleration can be explained in terms of a long-discarded version of Einstein theory of gravitation, the one that contained a cosmological constant. (ii) Perhaps there is some unknown energy-fluid that fills all space. (iii) Perhaps Einstein's theory is wrong, and a new theory is needed that would include a new field that can explain the acceleration.
No matter what the true explanation is, the phrase 'dark energy' was introduced to account for the mystery. It turns out that ~70% of the universe is dark energy. Dark matter makes up ~25%. The rest (including normal matter) adds up to less than 5% of the universe.
Further experiments and their interpretations have conjured up the following scenario: Till ~5 billion years ago, the universe was not having an accelerated rate of expansion. Both dark matter and normal matter pull the universe together. But dark energy does the opposite: It pushes the universe apart. Dark matter dominated the early universe, but dark energy overtook the influence of dark matter ~5 billion years ago. As the universe expands, the domination of dark energy over the effect of dark matter is getting stronger and stronger. Why should that be so? One explanation can come from Einstein's general-relativity theory mentioned above, with cosmological constant included. According to this theory, it is possible for more space to come into existence, and that 'empty space' possesses energy. Being an intrinsic property of space, this ENERGY CONTENT increases as the universe expands. This is what made me make the (admittedly speculative and simplistic) statement you have focused on. The phrase 'energy content' may include mass content also, but I am not sure.