Eternity in common parlance is either an infinite or an indeterminately long period of time. In classical philosophy, however, eternity is defined as what exists outside time while sempiternity is the concept that corresponds to the colloquial definition of eternity. It is often symbolized by the image of a snake swallowing its own tail, known as the Ouroboros. The circle is also commonly used as a symbol for eternity, as is the mathematical symbol of infinity. While scientific predictions of the future can never be absolutely certain, present understanding in various fields allows for the prediction of far future events, if only in the broadest strokes. These fields include astrophysics, which has revealed how planets and stars form, interact, and die; particle physics, which has revealed how matter behaves at the smallest scales; evolutionary biology, which predicts how life will evolve over time; and plate tectonics, which shows how continents shift over millennia.
All projections of the future of the Earth, the Solar System, and the Universe must account for the second law of thermodynamics, which states that entropy, or a loss of the energy available to do work, must increase over time. Stars eventually must exhaust their supply of hydrogen fuel and burn out. Close encounters gravitationally fling planets from their star systems, and star systems from galaxies.
2 × 1036 – The estimated time for all nucleons in the observable Universe to decay, if the proton half-life takes its smallest possible value (8.2×1033 years).
3 × 1043 – Estimated time for all nucleons in the observable Universe to decay, if the proton half-life takes the largest possible value, 1041 years, assuming that the Big Bang was inflationary and that the same process that made baryons predominate over anti-baryons in the early Universe makes protons decay.
By this time, if protons do decay, the Black Hole Era, in which black holes are the only remaining celestial objects, begins.
1065 – Assuming that protons do not decay, estimated time for rigid objects like rocks to rearrange their atoms and molecules via quantum tunneling. On this timescale, all matter is liquid.
5.8 × 1068 – Estimated time until a stellar mass black hole with a mass of 3 solar masses decays into subatomic particles by the Hawking process.
1.342 × 1099 – Estimated time until the central black hole of S5 0014+81, as of 2015 the most massive known with the mass of 40 billion solar masses, dissipates by the emission of Hawking radiation, assuming zero angular momentum (non-rotating black hole).
1.7 × 10106 – Estimated time until a supermassive black hole with a mass of 20 trillion solar masses decays by the Hawking process. This marks the end of the Black Hole Era.
Beyond this time, if protons do decay, the Universe enters the Dark Era, in which all physical objects have decayed to subatomic particles, gradually winding down to their final energy state in the heat death of the universe.
10200 – Estimated high time for all nucleons in the observable universe to decay, if they don’t via the above process, through any one of many different mechanisms allowed in modern particle physics on time scales of 1046 to 10200 years.
101500 – Assuming protons do not decay, the estimated time until all baryonic matter has either fused together to form iron-56 or decayed from a higher mass element into iron-56.
1010(26) – Low estimate for the time until all objects exceeding the Planck mass collapse via quantum tunnelling into black holes, assuming no proton decay or virtual black holes. On this vast timescale, even ultra-stable iron stars are destroyed by quantum tunnelling events. First iron stars of sufficient mass will collapse via tunnelling into neutron stars.
Subsequently neutron stars and any remaining iron stars collapse via tunnelling into black holes. The subsequent evaporation of each resulting black hole into sub-atomic particles (a process lasting roughly 10100 years) is on these timescales instantaneous.
1010(50) – Estimated time for a Boltzmann brain to appear in the vacuum via a spontaneous entropy decrease.
1010(76) – Time until all matter collapses into neutron stars or black holes, assuming no proton decay or virtual black holes, which then (on these timescales) instantaneously evaporate into sub-atomic particles.
1010(120) – High estimate for the time for the Universe to reach its final energy state, even in the presence of a false vacuum.
1010(10)(56) – Estimated time for random quantum fluctuations to generate a new Big Bang.
1010(10)(58)– Estimated time for a quantum fluctuation-generated Big Bang to produce a new universe identical to our own, assuming that every new universe obeyed the same laws of physics and contained at least the same number of subatomic particles.
Essentially, by this time the entire life cycle of the universe from Big Bang to final energy state to rebirth will have repeated the same number of times as all possible combinations of subatomic particles in the observable universe.