From Einstein's cosmological constant to dark energy:

In 1917 Einstein applied his theory of general relativity in the universe, and suggested a model of a homogeneous, static, spatially curved universe.

In 1917, Albert Einstein inserted a term called the cosmological constant into his theory of general relativity to force the equations to predict a stationary universe in keeping with physicists' thinking at the time.

Alexander Friedmann was a Russian mathematician and meteorologist who lived a short but eventful life. During the revolution of 1917 whilst besieged by White Russian forces in Petrograd (now St. Petersburg) he heard about Einstein's work on general relativity. He started to derive solutions, publishing his findings in 1922. His key insight was to realise that there was no unique solution to Einstein's equations, rather there was a whole family of solutions possible. This family of solutions thus allowed for different cosmological models of the Universe.

In Friedmann's models the only force that is considered is gravitation. His model universes are homogeneous (the same everywhere on a large enough scale) and isotropic (look the same in every direction). Most importantly they incorporate the concept of expansion and in some cases, contraction. Einstein himself had viewed the Universe as static. Friedmann thus provided the theoretical framework for an expanding Universe within the spacetime and mathematics of general relativity. Unfortunately he contracted typhoid and died in 1925 during the Russian civil war before his work became widely known.

In 1922 Friedmann introduced the idea of an expanding universe that contained moving matter. Correspondence with Einstein suggests that Einstein was unwilling to accept the idea of an evolving Universe and worked instead to modify his equations to ensure a static eternal Universe as believed from Newton's time.

Some years later, in 1926 Hubble published the redshift vs. distance relationship, namely, all the galaxies in the neighbourhood seemed to be receding at a rate proportional to their distance, formalising an observation made earlier by Carl Wilhelm Wirtz. It may be noted that in 1927 Belgian astronomer Georges Lemaître also independently reached the conclusion of an evolving Universe

Until 1931, physicist Albert Einstein believed that the universe was static.  Albert Einstein accepted the modern cosmological view that the universe is expanding long after many of his contemporaries. An urban legend attributes this change of perspective to when American astronomer Edwin Hubble showed Einstein his observations of redshift in the light emitted by far away nebulae -- today known as galaxies. But the reality is more complex. The change in Einstein’s viewpoint, in fact, resulted from a tortuous thought process. Now researchers explain how Einstein changed his mind following many encounters with some of the most influential astrophysicists of his generation.

However, the universe was not static. It was expanding. This observation, and those preceding Hubble's paper, led Belgian priest Georges Lemaître to propose in 1931 that the universe originated from a small and compact state, what he called a "Cosmic Egg" and what is now called the Big Bang.

When it became clear that the universe wasn't actually static, but was expanding instead, Einstein abandoned the constant, calling it the '"biggest blunder" of his life.

In 1932 Albert Einstein teamed up with the Dutch theoretical physicist and astronomer, Willem de Sitter, to propose an eternally expanding universe which became the cosmological model generally accepted until the middle of the 1990s. To Einstein's relief these two models no longer needed the cosmological constant.

The majority of the energy in our Universe went completely undiscovered until the late 1990s, and scientists still don't know what it is. Only 5% of the Universe, in terms of energy, is made out of things we're familiar with and understand: protons, neutrons, electrons, photons, neutrinos, black holes and even gravitational waves. Of the remainder, 27% is dark matter and 68%, the largest amount, is in the form of a new, mysterious substance called dark energy.

Dark energy was first revealed observationally by examining the light from ultra-distant signals like supernovae. With measurements of both distance and redshift, scientists concluded that the Universe couldn't just be made of matter and radiation, but needed a new form of energy that would change the fate of our Universe. Here's why, more than 20 years later, it's still the biggest unsolved problem of them all.

The effective mass of dark energy is <0 and its gravitating mass is more than matter mass that results a strong effect at large scale as such, the cosmological expansion accelerate.

Antigravity exerted by dark energy affects a cosmic structure strongly at large scale. The dark energy background produces antigravity which is stronger than the matter gravity in the present Universe as a whole. This makes the cosmological expansion accelerated.

The cosmic antigravity can be stronger than gravity globally and also locally in the scale between 3.26 lightyear and 3.262×10⁷ lightyear. The local weakfield dynamical effects of dark energy adequately described in term of Newtonian mechanics, and its effective gravitating density is negative, producing antigravity.

Gravity dominates at distances, while antigravity is stronger than gravity, therefore, a gravitationally bound system with its mass can exist only inside the zero gravity sphere of its radius [circumference of a sphere where, (antigravity - gravity = 0)], while dark energy is effective in the outer region of the domination of gravitating mass and practically have no effect within the strong domination of gravitating mass.

===================Alexander Friedmann ====================

The father of the Big Bang....

Alexander Friedmann corrected Einstein and found solutions to the field equations describing expanding, contracting and oscillating universes. He was a pure mathematician with interests in application of mathematics in the physical sciences and he originated the theory of the expanding universe.

The idea of a static universe goes back to Newton's absolute space and time, but even Newton was worried about the stability of the cosmos: The entire universe would collapse according to the universal attractions of all pieces of matter. When Einstein applied his improved theory of gravity to the whole universe, he could not find a distribution of the mass which would give a stable universe. Later he introduced a cosmological term, representing a repulsive force, which would save the universe from collapse. He proved that the universe must necessarily be stable and unchanging in time. When Friedmann wrote to Einstein that he had made an error in his proof which opened a new class of expanding, collapsing and pulsating world models and there was no need of changing Einstein's original gravity equations, Einstein did not respond. Many years later when Gamow discussed cosmology with Einstein he remarked that, the cosmological term was the biggest blunder he ever made in his life".

Source: Aug 09, 1997, Michael Cramer Andersen, Lecturer in Physics and Astronomy

Friedmann is seen as a profound, independent-minded, and daring thinker who destroys scientific prejudices, myths and dogmas; his intellect sees what others do not see, and will not see what others believe to be obvious but for which there are no grounds in reality. He rejects the centuries-old tradition which chose, prior to any experience, to consider the Universe eternal and eternally immutable. He accomplishes a genuine revolution in science. As Copernicus made the Earth go round the Sun, so Friedmann made the Universe expand.

In 1922 Alexander Friedmann published his famous paper on cosmology where he discovered non-stationary solution of Einstein's equations, describing closed expanding space with beginning and end of the Universe in time-closed Friedmann model.
In 1924 he published a paper on expanding Universe with negative curvature, infinite in space but having beginning in time. At that time he also published a popular book "The world as space and time" where discussed a new vision of the expanding Universe with possible beginning and end in time.

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