The following is a quote from the Wikipedia article on the Cosmological Constant:
" Einstein included the cosmological constant as a term in his field equations for general relativity because he was dissatisfied that otherwise his equations did not allow, apparently, for a static universe: gravity would cause a universe that was initially at dynamic equilibrium to contract. To counteract this possibility, Einstein added the cosmological constant. However, soon after Einstein developed his static theory, observations by Edwin Hubble indicated that the universe appears to be expanding; this was consistent with a cosmological solution to the original general relativity equations that had been found by the mathematician Friedmann, working on the Einstein equations of general relativity. Einstein reportedly referred to his failure to accept the validation of his equations—when they had predicted the expansion of the universe in theory, before it was demonstrated in observation of the cosmological redshift—as his "biggest blunder"."
The first sentence says that without the cosmological constant, Einstein found that his equations lead to a contraction of the universe due to gravity. Fair enough. But, it then goes on to say that Friedmann constructed a model which led to expansion, and was coherent with Hubble's findings. Einstein latched on to this as a theoretical prediction that was made before Hubble's observational confirmation. Hence, the cosmological coefficient was his "biggest blunder." So which is it? Do the Einstein equations predict contraction, as he initially found, or do they predict expansion as Friedmann found? Or can it be both, so the introduction of a cosmological constant was superfluous from the start? and Einstein did not do his homework properly.
That wouldn't be so bad had the cosmological constant remained dead and buried as it did until its re-emergence in the 1990's. Modern conventional wisdom calls the cosmological constant a "dark energy". Surely, it is dark since a positive cosmological constant corresponds to a negative energy density in the Einstein energy-stress tensor. That is, it must be subtracted from the energy-stress tensor. Not only does a positive cosmological constant, which is denoted by Lambda, correspond to repulsion, it violates the conservation laws, and can be made to fit in to Hoyle's continuous creation that avoids the initial singularity in favor of an infinite sequence of bounded oscillations. Such is the absurdity of modern day cosmology which builds upon a structure laid down by Einstein, and subsequently retracted.
However, the cosmological constant is a convenient trash bin to put in all the embarrassing conclusions that result from the standard theory. When 68% of the mass in the universe went missing, it was simple and expedient to blame it on Lambda. Again quoting from the Wikipedia article:
"Since the 1990s, studies have shown that around 68% of the mass–energy density of the universe can be attributed to so-called dark energy. The cosmological constant Λ is the simplest possible explanation for dark energy, and is used in the current standard model of cosmology known as the ΛCDM model. While dark energy is poorly understood at a fundamental level, the main required properties of dark energy are that it functions as a type of anti-gravity, it dilutes much more slowly than matter as the universe expands, and it clusters much more weakly than matter, or perhaps not at all. "
In fact, a citation is definitely required for such nonsense. So if the dark energy density is positive, the associated pressure is negative which will "drive an accelerated expansion of the universe." Try filling your tires with a negative pressure and see what happens!
Nonsense inevitably breeds more nonsense. Further in the same article we read:
As was only recently seen, by works of 't Hooft, Susskind and others, a positive cosmological constant has surprising consequences, such as a finite maximum entropy of the observable universe (see the holographic principle)."
The "entropy" these authors are talking about is not an entropy at all. The only surprise there is in their nonexistent holographic principle. It is well-known that the Friedmann equations lead to an analogy with the first and second laws of thermodynamics. And these laws clearly show that the expansion of the universe is adiabatic. Hence, there is no change in the entropy because the universe is expanding into nothing, and that "nothing" does not allow for an exchange of heat.
The time component of Einstein's equations can be used to predict a critical density separating expanding and contracting universes. For this to happen, Lambda must vanish. Expressing the critical density in terms of a critical mass, M_c, results in
where v is the velocity of expansion, and R the radius of the expanding universe. This would be Kepler's law were it not for the factor of 2, whereas if the velocity was that of the speed of light, the expression would define the Schwarzschild radius of a black hole. So why is the critical mass, or critical density related to the escape velocity? Is it that for speeds less than the one given in the formula, the universe will contract, while for speeds greater it will expand? Is the fate of the universe so easily determined?