Immediately after universe beginning, space itself expanded equally at all points in all directions at highest rate (the "Big Bang"). Why did space expand at universe origin {space-expansion, causes} {Big Bang, causes}?
beginning-universe properties
By observation and calculation, at universe origin, space had smallest volume, and universe had same energy as now, so space had highest energy density. Because space had shortest distances, universe had lowest potential energy and highest kinetic energy and temperature. At highest temperature, forces unify and have greatest strength, so beginning universe had only radiation of one unified type, with zero rest mass, light speed, highest wave frequency, shortest wavelength, and highest radiation internal and external pressure. Because mass-energy density and internal pressure were highest, universe had highest gravity and space curvature, consistent with smallest 3-sphere volume. Because entropy varies directly with volume and inversely with gravity, beginning universe had lowest entropy.
space filling
Space is where matter and radiation are. Radiation-wave random reflections, refractions, and diffractions, and particle random motions and collisions, send waves and particles in all directions, so particles and waves fill space homogeneously and isotropically.
expanding space
In systems, most particle random motions and collisions, and most radiation-wave reflections, refractions, and diffractions, move particles and radiation apart, where they are less likely to encounter other particles and radiation and so less likely to change direction, so they continue moving apart. As particles and radiation move apart, space volume increases, mass-energy density decreases, potential energy increases, kinetic energy decreases, temperature decreases, internal pressure decreases, gravity decreases, and space curvature decreases.
Space expansion stretches lengths between particles and radiation. Space expansion adds quantum lengths between particles and waves.
expanding space: waves and particles remain intact
If particles are points, they have no insides, so space-expansion forces do not change point-particles. If particles have internal forces, those forces are much greater than space-expansion forces and particle-collision forces, so space-expansion forces do not change particles. Wave electromagnetic forces are very much stronger than gravity, and waves do not have collisions, so space-expansion forces do not change wave internal structure.
forces
Gravity is attractive and has infinite range, so it always opposes particle and radiation separation. Electromagnetism has infinite range but is attractive and repulsive and so has no overall effect on particle and radiation separation. Strong and weak nuclear forces have short range and so do not affect particle and radiation separation.
general relativity
Object random motions are mostly outward and so separate objects, increasing potential energy and decreasing kinetic energy, temperature, mass-energy density, and space curvature. Outward force varies directly with temperature (and so with mass and speed). If object kinetic energy overcomes gravity, kinetic energy decreases, and gravitational potential energy increases, by the same amounts. Total energy stays constant.
Matter, antimatter, and radiation have positive mass, positive energy, positive mass-energy density, and positive internal pressure, so gravity is positive and attractive, and gravitational energy is positive energy. Attractive gravity makes positive space curvature. Masses gravitationally attract each other at any distance less than infinite distance, so masses tend to move relatively closer, decreasing potential energy and increasing kinetic energy, temperature, mass-energy density, and space-time curvature. Gravitational force varies directly with mass and varies inversely with distance squared. General relativity adds internal pressure as a relativistic source of gravity, so objects with higher temperature have more gravity.
Objects with higher temperature also have greater outward motion. Because increased gravity is a relativistic effect of higher temperature and increased outward motion is a direct effect of higher temperature, higher temperature increases outward motion more than it increases gravity, and so increases space expansion. At universe origin, temperature was highest, and radiation external pressure was highest, so space expansion rate was highest.
Gravity and temperature vary directly with total energy at universe origin, but in different ratios. Because space expansion decreases temperature and gravity unequally, space expansion rate decreases over time.
Because gravity and temperature vary with energy at different rates, space has zero probability of neither expanding nor contracting. Universe is always changing.
quantum mechanics and vacuum intrinsic energy
By observation, space has a constant negative intrinsic-energy (dark-energy) density that causes repulsion (antigravity) and uniform space expansion. Dark energy can do negative work. Negative work decreases negative kinetic energy as it pushes space apart, and increases negative potential energy in the added space. (Positive work decreases positive kinetic energy and increases positive potential energy.) Dark energy adds intrinsic energy and adds space in exact proportion, so space always has constant dark-energy density.
Dark-energy strength was much lower than object random motions at universe origin and contributed little to original rapid space expansion.
quantum mechanics and vacuum intrinsic energy: expansion rate
Dark-energy repulsion also pushes particles and radiation apart, increasing positive potential energy and decreasing positive kinetic energy, decreasing mass-energy density and space curvature. Total energy stays constant, as required for closed systems.
For closed universes, because positive energy changes and negative energy changes always offset, universe positive energy and negative energy can be any amount. Mass-energy density and intrinsic-energy density can be any value, so space expansion rate can be any value for closed universes. By observation, universe has had periods of greatly differing space-expansion rates, such as initial expansion rate, the very high cosmic-inflation rate, and ever-slower expansion rates.
gravity at very short distances
Perhaps, gravity is repulsive at very short distances and very high temperatures.
string theory at very short distances
String theory theorizes that, when universe diameter was Planck length or less, strings repulsed, making gravity repulsive and space curvature negative, so space expanded rapidly.
string-theory inflatons
Perhaps, spontaneous symmetry breaking disunified forces, phase transition made string/brane scalar (inflaton) field, and inflaton-field repulsions caused space expansion.
space-expansion rate
At universe origin, unified force was greatest, and space expansion decreased at highest rate.
At any instant, space-expansion rate depends on initial radiation-velocity-driven space-expansion rate, initial unified-force/gravity space-contraction rate, and constant dark-energy antigravity space-expansion rate.
As space expands over time, distances between objects increase, so gravitational potential energy increases and object kinetic energy decreases, so object average speed becomes less, temperature decreases, and space-expansion rate decreases.
Gravity space-contraction rate depends on mass-energy density and on internal pressure, so it decreases as space volume increases. As space expands over time, gravity always decreases space-expansion rate, but ever more slowly.
Universe dark-energy density is constant, so it constantly increases space-expansion rate.
After initial expansion, object kinetic-energy expansion force decreases, gravity contraction force decreases, and dark-energy expansion force is small compared to them, so space-expansion rate decreases.
Eventually, dark-energy expansion force, plus object kinetic-energy expansion force, overcome gravity contraction force, and space expands ever faster.
entropy
Space expansion makes more volume, more states, more information bits, fewer force symmetries (as forces separated from other forces), more matter, same energy, lower temperature, and higher entropy, and so more entropy.
Physical Sciences>Astronomy>Universe>Cosmology
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Date Modified: 2022.0224