I answered that same question in your other thread. Here it is again.
To understand how the universe came from nothing you need to understand three things, which were verified by experiment time and again.
1. T-symmetry
Elementary particle (quarks, electrons, pions, etc.) can move forward and backward in time. That is called time reversal symmetry. Larger objects composed of these particles arguably also could do the same, but the statistics are astronomically huge against ever seeing time reversal macroscopically.
2. Uncertainty principle
ΔE x Δt ≥ tiny constant
What this means is that in any event, the uncertainty in energy multiplied by the uncertainty in time is greater than a tiny Planck's constant h = 6.62607015 × 10^−34 joule second. In short, the more exactly time is known, the less exactly energy is known, and vice versa.
3. Vacuum fluctuations - a very simplified description
Because of the above two principles, any elementary particle can be spontaneously created by borrowing energy from the future and then annihilated by returning that energy to the past. This means that vacuum is "not nothing", but it is thriving with activity at the sub-microscopic level.
The theory of the creation of the universe is that a vacuum fluctuation was so huge in an extremely short time that it exceeded a critical limit which caused space-time to explode in a "big bang." There many astronomical observations that support this.
One upshot is that the total energy of the universe is zero because the energy of the mass of the universe is equal to the negative gravitational potential energy. D'souza mocked that energy could be negative. Every physics 101 student knows that potential energy is negative. Hitchens probably didn't know that either, but also didn't make a fool of himself.
A well known story in physics circles: One day in the 1940's Einstein and Gamow were crossing in the middle of a street in Princeton. Gamow mentioned that one of his students had calculated that it was possible to make a star from nothing; its gravitational energy is equal and opposite to rest energy. Einstein, realizing that this could apply to the whole Universe, stopped in his tracks causing several cars to stop.
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