What is the evidence for Big Bang?
The Universe hasn’t existed forever. It was born. About 13.82 billion years ago, matter, energy, space & time, erupted into being in a fireball called Big Bang.
A universe popping into existence out-of-nothing is so bonkers that scientists was to be dragged kicking & screaming to the idea. But evidence is compelling.
The galaxies are flying-apart like pieces of cosmic shrapnel.
The heat of the Big Bang remains around us. Greatly cooled-by cosmic expansion, this afterglow appears not as visible light, but principally as microwave radiation, the cosmic background radiation, which was discovered-by radio astronomers in 1965.
Where did the Big Bang happen?
When a stick of dynamite explodes, the detonation occurs in one place & shrapnel flies into the void.
In the Big Bang, there was no center & no pre-existing void, so it did not happen at any location. Space itself popped-into existence & began expanding everywhere at once.
Astronomy books often-liken the Universe to a rising cake, with raisins representing galaxies.
As the cake grows, raisins recede from one another, with no center of expansion, like the Big Bang. But a cake has an edge, unlike the Universe, which go on forever.
Was the Big Bang a one-off?
In the beginning of the Big Bang, there was the inflationary vacuum.
When its volume doubled, its energy doubled; when its volume tripled, its energy tripled. If bank notes were like this and you pulled apart a stack, ever-more would appear.
Physicists call inflation the ultimate free lunch.
The inflationary vacuum expanded, ever faster. But it had been a quantum thing. And quantum things are actually fundamentally unpredictable. Randomly, all over the inflationary vacuum, parts of it decayed into ordinary, everyday vacuum.
Think, tiny bubbles forming in a vast ocean. In each & every bubble, the inflationary vacuum disappeared, but its huge energy had to go somewhere.
It went into creating matter & heating it. It went-into creating a Big Bang. Our Big Bang Universe is just one such bubble among a possible infinity of other Big Bang universes in the ever-expanding inflationary vacuum.
To start all this, a piece of inflationary vacuum of only a kilogram was required. Incredibly, the laws of quantum theory permit this to pop into existence out-of-nothing.
What are the problems with the Big Bang theory?
The basic idea that the Universe began hot & dense & has been expanding & cooling ever since, is incontrovertible. But cosmologists have to make tweaks to the theory, to account particular observations.
First, in the standard Big Bang model, galaxies grow-by gravitationally pulling in matter. But if this were the sole thing going-on, it could take much longer than 13.82 billion years for-them to form.
Second, the Big Bang predicts, the gravitational attraction between the galaxies acts like web of elastic & slowing cosmic expansion.
However, in 1998, astronomers discovered, the Universe’s expansion is speeding-up. They fix this, by postulating the existence of dark energy, which is invisible, fills space & has repulsive gravity.
A final tweak to the basic theory is required to elucidate, why the Universe has the same temperature everywhere.
To account this, astronomers think, the Universe early was smaller than expected, then underwent a superfast expansion in its first split-second, an inflation.
This was driven-by an inflationary vacuum, a high-energy version of the vacuum that exists-in space today.
What happened before the Big Bang?
The twin pillars of modern physics are Einstein’s General Relativity & quantum theory. The former–reigns supreme in the large-scale Universe, while the latter orchestrates the small-scale world of atoms & their constituents.
They resisted a merger, which is a problem because in the Big Bang, the Universe was small.
To understand, how it emerged, it’s essential to unite Einstein’s theory with quantum theory. The best candidate is string theory, which views the basic building blocks of reality as tiny strings of mass-energy vibrating in 10D space-time.
Only if we obtain such a theory: will we be able to answer the ultimate questions: what is space? What is time? What is the Universe? Where did it come from?