The Degenerate Era: 1014 to 1040 Years
When stellar evolution comes to an end, we have about equal numbers of brown dwarfs and white dwarfs, and as many as about 1 in every 1000 will be black holes or neutron stars. Using the expansion rate defined by [H] as a function of time, the universe will be, by the end of the stelliferous era, some 100,000 times larger than at the present age. However, during the Degenerate era, the universe still will expand, albeit at a slower rate from 1015 to 1030 light-years in radius.
A brown dwarf is a potential fledgling star that had insufficient mass to create the pressure and temperature required for hydrogen fusion. As a result, by the degenerate era, brown dwarfs end up holding most of the unburned hydrogen in the universe. If these objects collide then new stars could be created, although in the context of the totality of the universe, this will be a relatively rare event. Since we can make an estimate of the number of brown dwarfs, per galaxy, and their potential collision rate, we can also make a ballpark estimate as to how many of these aggregated brown dwarf stars might be shining in a galaxy in the Degenerate Era. From a galaxy that originally had 100 billion shining stars, by the degenerate era, this would have reduce to 2-3 stars and typically they will be about 10,000 times dimmer than our Sun.
Since the white dwarfs are quite a bit larger than the brown dwarfs, by a factor of 10 in mass, the vast majority of the baryonic mass, i.e. protons, are contained within the white dwarfs. These objects will be the most important stellar objects in the universe for the next 1037 years. A white dwarf is the core of a small and medium-size star after all the hydrogen it contained has fused into helium. During a red giant phase, most of its outer material was expelled until only the core remained. Although this core was incredibly hot, by the degenerate era it has cooled to the frigid temperature of liquid nitrogen. Occasionally, as measured on this astronomical scale, white dwarfs may also collide, which may result in the gloom being punctuated by a spectacular supernova. However, white dwarfs would also collect dark matter particles and, over time, these particles annihilate within the star and generate radiation that would become the dominant energy source in the universe. The power generated by such a white dwarf is about quadrillion watts, which is quite small compared to the total output of the Sun; in fact it is more analogous to the energy that our Earth currently receives from the sun.
Over a longer timeframe, the white dwarfs and neutron stars will evaporate energy via a process called neutron and proton decay. At this stage in the life of the universe, most of the protons and electrons are contained in the white dwarfs. When a proton decays into a positron, this positron will very quickly collide with an electron and annihilate, thus mass-energy begins to be converted back into radiation-energy. At this point, we can summarise the complete life cycle of our Sun. It was born in the spiral arm of a galaxy and shines as a normal star for some 10-12 billion years. After which it becomes a red giant and then a hot white dwarf, which then radiates its heat into the universe for trillions of years. However, proton decay ultimately takes over as the white dwarf continues to radiate its mass and energy away. During this phase, a white dwarf may only be generating 400 watts of power, i.e. the energy of about 4 light bulbs. Eventually the star is reduced to a block of hydrogen ice, with a dwindling store of mass and internal radiation energy. Eventually, the block of hydrogen ice will simply evaporate out of existence. This is the projected fate that awaits 99% of all stars.
At the beginning of the Degenerate Era, there was an inventory of brown dwarfs, white dwarfs, neutron stars and black holes. As the era draws to a close, all that remains are black holes that have continued to capture stars and gas and anything that falls into their event horizons. It has been estimated that the degenerate era may last for as long as 10,000 trillion, trillion, trillion (1040) years.