World Science Scholars
2.2 Negative Energy Density Review I
summary
Inflation proposes that a 'patch' of repulsive gravity material existed in the early universe.

• If the inflationary expansion took place at the Grand Unified Theory scale, then the required patch could have been as tiny as 10-28 cm across.
• Such a patch would have been more than a billion times smaller than a proton.
• Inflation could stretch this patch large enough to become the visible universe.

Gravitational repulsion was the driving force behind the Big Bang.

• The inflationary patch would have been driven into exponential expansion with a doubling time of about 10-38 seconds.
• This means that every 10-38 seconds, the patch increased in size by a factor of $e$ (2.718).
• The patch expanded by a factor of at least 1028 (100 doubling times), but it could have expanded much more.
• That factor is the minimum expansion necessary to ensure that the visible universe has the features revealed by observations.

When inflation ended, the acceleration 'turned off.'

• At the end of inflation, the region destined to become the presently observable universe was only about the size of a marble.
• This marble-sized universe continued to expand, but began to slow due to the ordinary force of attractive gravity.

Why did inflation end?

• The repulsive gravity material was unstable and decayed, similar to a radioactive substance.
• This decay process released energy, which then produced ordinary particles and became the primordial soup.
• The patch continued to coast and cool, eventually encompassing the region we know to be the visible universe.

Surprisingly, during the exponential expansion, the energy density in the source of the repulsive gravity did not thin out.

• Typically, if a region expands to a larger volume, the mass density it contains will decrease.
• Think of a gas in a bottle being released into a room. There is a known number of particles that then fill up the volume of the room, driving the density down.
• Although this might suggest that energy was not conserved during inflation, there’s a sense in which it is— this relies on the “miracle” that the energy of a gravitational field is negative!

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