BANG!!!

“Ten to twenty billion years ago, something happened. The Big Bang, the event that started our Universe. Why it happened is the biggest mystery we know. That it happened is reasonably clear”. Thus wrote Carl Sagan in 1980.

Now nearly three decades later, we know that it happened not “ten to twenty billion years ago”, but more like 13.7 billion years give or take 200 million. And we’re beginning to home in on why it happened.

For the cosmologically uninitiated, the Big Bang is the colossal explosion which started our Universe. It’s not really your garden variety explosion - it happened everywhere at the same time and space itself began to expand and has been expanding ever since. (Expanding into what ? Wrong question! Space just expands. There need not be anything outside for it to expand into.). As a bonus, all the matter and energy in the Universe came into existence at the Big Bang. Must have been really something to see (for the picosecond before your retina evaporated.)

Ok then, so what caused the Big Bang? Answer: Gravity.

“Now hang on”, you say, “gravity is an attractive force. It makes everything collapse and clump together. The Big Bang was an explosion. Am I missing something here?”

We-e-ell, you see, there’s gravity and gravity. Our usual type of gravity, the type that foils your attempts at spontaneous levitation, is caused by our everyday type of matter. But make that matter sufficiently weird, and gravity can push things apart with enough OOOMPH to satisfy the most ardent anti-gravity aficionado.

Let me elaborate. Our best model of gravity today is Einstein’s General Theory of Relativity. What GTR says is that the gravitational force generated by an object is proportional to its mass/energy. This is pretty much what we know, the heavier the body, the more the gravity. But there’s also something else – the pressure of the system also contributes.

To illustrate, if you have a spring, then its mass will create a gravitational attraction. If you squeeze the spring, its pressure increases and so does the pull of its gravity. Now here’s the crux – if you stretch the spring, the pressure term becomes negative and the gravitational pull decreases. And if you stretch it enough, the negative pressure term can totally overwhelm the effect of the spring’s mass, and the gravitational field will repel rather than attract. A sufficiently stretched spring is an anti-gravity machine!

Unfortunately, any real spring will rip into pieces long before anti-gravity sets in. But there does exist a substance which can act like our hyper-stretched theoretical spring. What is it? Nothingness.

No, I haven’t suddenly gone Zen on you. If you remove all our usual matter and energy, the empty space that remains (which would qualify as nothing for most people) might still have an ‘energy’. The details are technical, but the gist is, this weird ‘vacuum energy’ generates a ferocious burst of antigravity, the Bang of the Big Bang. In the space of 10^(-35) seconds the nascent universe expands by a factor of at least 10^30, and the rest is history.

But wait, first let’s give a sense of those numbers. 10^30 is one followed by thirty zeroes. That’s like blowing up a virus to the size of our galaxy! And how fast did this happen? Well, if a blink of an eye was scaled down to 10^(-35) seconds, then the lifetime of our universe would be one hundred millionth of a nanosecond. Unimaginable doesn’t even begin to describe it.

As if this wasn’t enough, we have every reason to believe that the expansion went on way beyond this. A consequence is, our observable universe is miniscule compared to the actual universe. To picture this, imagine the entire Universe to be the surface of a giant balloon studded with galaxies. The observable universe, which is what we can see, from the Earth to the most distant galaxies spotted by our telescopes, would be like a patch drawn on the balloon. The diameter of that patch would be about 90 billion light years, which is about 10^27 metres. Now the question is, “What is the circumference of the entire balloon?” A possible estimate is, 10^(10^12) metres. Yes, you read that right. One followed by a trillion zeroes!!!

Usually astronomers like to convey the scale of things with analogies like, “Suppose the earth was the size of an orange, then the Sun would be...” etc. So, let me try to convey the size of the whole Universe. Shrink the Universe by a factor of 10^40. The observable universe, with all its hundreds of billions of galaxies is now the size of an atomic nucleus. Now repeat the procedure 25 billion times. At this point, the Universe has the same size as our observable universe... or an atomic nucleus if we’ve scaled down one too many times by accident! Does it really matter?

Now let’s go back to the beginning, before that miniscule speck of vacuum energy expanded to incomprehensible size. On the smallest of scales, quantum physics rules, which implies roughly that “nothing ever sits still”. So, even on the tiniest scales, the vacuum energy cannot be uniform- it is perpetually roiled by quantum fluctuations which ensure that its values are never quite the same everywhere. What do these elusive quantum fluctuations look like? Take a look below.

What you saw was a map of our universe on the largest scales we can see. The reddish foam-like structure represents clusters of galaxies strewn across space with vast voids in between. The gigantic cosmic expansion stretches the quantum fluctuations out from submicroscopic scales to cosmic ones - resulting in a froth-like clustering of galaxies as far as we can see. The largest structures we see in the visible universe are quantum fluctuations stretched across the heavens.

And finally the grand slam. The universe contains an enormous amount of matter and energy. At least a hundred billion galaxies with a hundred billion stars each. Where did all this come from? The traditional answer is, “From the Infinite and Eternal Mind of God”.

But we’ll try for something much humbler – by starting with a glass falling from your hand. The glass gains kinetic energy from the earth’s gravitational field, which then gets converted to sound, heat and a mess on the floor. A rather more spectacular example is a supernova. A giant star at the end of its life collapses under its own gravity. The energy gained from the gravitational field converts into a cataclysmic explosion which blows the star apart and temporarily outshines an entire galaxy. This is all with our familiar matter and energy.

A speck of vacuum will also gain a gigantic amount of energy from its own gravitational field. But instead of collapsing while doing so, it blows apart in a Big Bang. At the end of this blast of expansion, the energy gained at gravity’s expense is let loose in a flood of radiation and matter that makes up everything you see and much more. So, there we go - you don’t need a hundred billion galaxies worth of material to start off the universe. Just a tiny seed of vacuum energy and the rest will follow.

At this point, it’s a case of ‘almost, but not quite’. After all, where did that seed come from? Nobody knows, but here’s a possibility. Start off with truly empty space – so empty, it’s even devoid of vacuum energy. Now, remember those pesky quantum fluctuations? They’re always around, wriggling, wiggling, flickering and jittering. So, even if you assume that the vacuum energy of empty space is zero, it’s never quite so. The value keeps fluctuating up and down, here and there. Given enough time, somewhere, on some miniscule, submicroscopic domain, a fleeting fluctuation will exceed a critical threshold, and.... BANG!!!

So, at the end of things (or is it the beginning?), think of Space. From the endless voids between the galaxies and the yawning gulfs separating the stars therein to the emptiness within an atom, space pervades everything. Think of the fact that every bit of that space, every minute, infinitesimal bit – so small that an atom looks is a galaxy in comparison – has the potential to blossom into an entire Universe of stars and galaxies, life and mind. Maybe it is happening somewhere right now. Maybe, even as you read this sentence, a billion, trillion, zillion Universes are exploding into existence....