One of the things that excited me so much about quantum chromodynamics after the work of Gross and Wilczek and Politzer was that it seemed to provide… - Steven Weinberg

The last thirty years of Einstein's life were largely devoted to a search for a so-called unified field theory that would unify James Clerk Maxwell's theory of electromagnetism with the general theory of relativity, Einstein's theory of gravitation. Einstein's attempt was not successful, and with hindsight we can now see that it was misconceived. Not only did Einstein reject quantum mechanics; the scope of his effort was too narrow. ... Nevertheless Einstein's struggle is our struggle today. It is the search for a final theory.

In the case where the universe does not recollapse, the proper distance to the is...<math>d_{MAX}(t) = a(t) \int_{0}^{r_{MAX}(t)} \frac{dr}{\sqrt{1-Kr^2}} = a(t)\int_{0}^{\infty} \frac{dt'}{a(t')}</math>... In the absence of a cosmological constant, <math>a(t)</math> grows like <math>t^{\frac{2}{3}}</math>, and the integral diverges, so there is no event horizon. But with a cosmological constant, <math>a(t)</math> will eventually grow as exp(<math>Ht</math>) with <math>H = H_0 \Omega^{1/2}_\Lambda</math> constant and... an event horizon... approaches... <math>d_{MAX}(\infty) = 1/H</math>. As time passes all sources of light outside our gravitationally bound will move beyond this... and become unobservable. The same is true for the quintessence theory... In that case <math>a(t)</math> eventually grows as exp(const <math>\times\, t^{2/{(2+\frac{\alpha}{2})}}</math>), so for any <math>\alpha \ge 0</math> the integral... [<math>d_{MAX}(t)</math>] converges.

As a special case of Newton's Second Law, a body... when acted on by zero force, will experience zero acceleration—that is, it will move with constant velocity. Newton listed this... as the First Law... The Third Law... action equals reaction: If one body exerts a force on another... the second... exerts an equal force in the opposite direction on the first.