Cosmology 
Studied objects and processes 

Universe History 

Observed processes 

Theoretical studies 

Equation of state of cosmological model  dependence$p(\epsilon )$ pressure from the mass energy density of the medium in this model.
In Friedmann's theory , not only is the density of matter created, but also the pressure of the medium: the density of the effective gravitating energy${\epsilon}_{G}=\epsilon +3\cdot p,$ Where$p$  pressure of the medium, and$\epsilon$  the energy density of the medium$\epsilon ={c}^{2}\cdot \rho ,$ Where$\rho$  mass energy density of the medium,$c$  the speed of light.
Pressure is expressed through the equation of state$p(\epsilon ),$ or use the dimensionless parameter  the ratio of pressure to energy density$w=\frac{p}{\epsilon},$ then the equation of state:
 $p=w\cdot \epsilon .$
For different environments$w$ has a different meaning. Below we assume that the density of the medium is above zero. The following 9 options are possible:
1. Phantom energy (phantom energy) (see phantom cosmology ) is a medium with negative gravity greater (in magnitude ) than that of a vacuum.
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With such an equation of state, the density of the medium increases with time, the negative gravity increases and after a finite time becomes infinite, and a Big Gap will occur in the Universe. Another feature of such an environment is that the speed of sound in it is higher than the speed of light.$c.$
2. Vacuum  a medium with negative gravity.
 $w=\mathrm{one.}$
Respectively:
 $p}_{V}={\epsilon}_{V$ (only such an equation of state is compatible with the definition of vacuum as a form of energy with a constant density everywhere and always, regardless of the reference system).
 ${\epsilon}_{G}=2\cdot {\epsilon}_{V}.$
In the Einstein equations, the vacuum energy is described by the cosmological constant$\mathrm{\Lambda}=\frac{\mathrm{eight}\cdot \pi \cdot G}{{c}^{\mathrm{four}}}\cdot {\epsilon}_{V}.$
According to the latest data ^{[1],} the vacuum energy density in the Universe is$\mathrm{\Omega}}_{\mathrm{\Lambda}}={0.728}_{0.016}^{+0.015$ from critical density .
3. Quintessence  a medium with negative gravity is lower than that of a vacuum.
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Only when$$ there is negative gravity, therefore, only under this condition the expansion of the universe accelerates, that is, the nature of dark energy is either a vacuum, or phantom energy, or quintessence.
4. The environment in which there is no positive and negative gravity.
 $w=\frac{\mathrm{one}}{3}.$
5. The environment in which gravity is lower than that of dust.
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6. Dust cloud, ordinary baryonic matter and cold dark matter (no environmental pressure,$p=0$ ).
 $w=0$
Respectively:
 ${p}_{M}=0;$
 ${\epsilon}_{G}={\epsilon}_{M}.$
According to the latest data ^{[1],} the energy density of ordinary cold baryonic matter in the Universe is${\mathrm{\Omega}}_{b}=0.0456\pm 0.0016$ from the critical density , and the density of cold dark matter is${\mathrm{\Omega}}_{c}=\mathrm{0,227}\pm 0.014$ from the critical density, which in total gives$\mathrm{\Omega}}_{m}={0.272}_{0.015}^{+0.016$ from critical density.
7. The environment in which gravity is higher than that of dust, but lower than that of radiation.
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8. Ultrarelativistic environment (radiation, photons, and other ultrarelativistic particles), including relic radiation ; also massive particles in the early Universe, when the temperature (expressed in energy units) far exceeds the masses of the particles:
 $w=\frac{\mathrm{one}}{3}.$
The behavior of the Universe was determined by a close to this equation of state in the time interval from the Planck era to the recombination era.
Respectively:
 ${p}_{R}=\frac{\mathrm{one}}{3}\cdot {\epsilon}_{R};$
 ${\epsilon}_{G}=2\cdot {\epsilon}_{R}.$
9. The environment in which gravity is higher than that of radiation.
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Similarly, when$w>\mathrm{one}$ the speed of sound in such an environment is higher than the speed of light$c$ .