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The expansion of the universe can be understood as resulting from an initial condition in which the contents of the universe are flying apart. The mutual gravitational attraction of the matter and radiation within the universe gradually slows this expansion over time, but their density is too low to prevent continued expansion.
The timeline in this schematic diagram extends from the Big Bang/inflation era 13.8 billion years ago to the present cosmological time. Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time.
In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and ...
English: Expansion of the universe, proper distances diagram (Animation). Horizontal axis: Proper distance in billion light years. Vertical axes: time since Big Bang in billions of years Shown are the Particle Horizon rP (green), Event Horizon rE (magenta), Hubble Radius rH (blue). Past and future light cones rL (orange) are animated.
The observable universe is a spherical region of the universe consisting of all matter that can be observed from Earth; the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the ...
where t i is an initial time (τ i is the proper initial time), t f a final time (τ f is the proper final time), and H is the expansion parameter, also called the Hubble parameter. The BGV theorem states that for any spacetime where >, then the spacetime is geodesically past-incomplete.
The thinning of matter over time reduces the ability of the matter to gravitationally decelerate the expansion of the universe; in contrast, dark energy is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago when the universe entered the ...
Note we use conformal time in the following diagrams. Conformal time describes the amount of time it would take a photon to travel from the location of the observer to the farthest observable distance (if the universe stopped expanding right now). The blue circle is the CMB surface which we observe at the time of last scattering.