Category:Dark Energy

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The observed accelerated expansion of the Universe requires either modification of General Relativity or existence within the framework of the latter of a smooth energy component with negative pressure, called the dark energy. This component is usually described with the help of the state equation $p=w\rho$. As it follows from Friedman equation, \[\frac{\ddot a}{a}=-\frac{4\pi G}{3}(\rho+3p).\] For cosmological acceleration it is needed that $w<-1/3$. The allowed range of values of $w$ can be split into three intervals. The first interval $-1<w<-1/3$ includes scalar fields named the quintessence. The substance with the state equation $p=-\rho\ (w=-1)$ was named the cosmological constant, because $\rho=const$: energy density does not depend on time and is spacially homogeneous in this case. Finally, scalar fields with $w<-1$ were called the phantom fields. Presently there is no evidence for dynamical evolution of the dark energy. All available data agree with the simplest possibility---the cosmological constant. However the situation can change in future with improved accuracy of observations. That is why one should consider other cases of dark energy alternative to the cosmological constant.