Research On Semi - Holographic Universe

In the 1990 s, researchers found the universe is accelerating. Since then,researchers proposed dark energy to account for this observation. Dark energy is an unknown form of energy filling all the space, and it accelerates the universe. However,dark energy theory merely can’t explain some observations. For this reason researchers proposed another dark component and called this dark component dark matter. We can’t see dark matter by telescope, but it dominates all the matter of the universe. Compared to dark matter and dark energy, ordinary matter is so little that we can neglect it in some situations. The coincidence problem is a very important problem of the universe that need to be solved. It asks why the density of dark energy is equal to the dark matters’ in the universe. We are going to find a way to solve this problem.We will introduce the study of the thermodynamics of spacetime by Jacobson and the study of the entropy of the universe by D. H. Hsu and Reeb. The work of Jacobson pioneers the way how to find the proper holography besides the case of black hole. It seems the apparent horizon is the right holography of the universe. And the work of D.H. Hsu and Reeb indicates that total entropy of all the known matters is still much lower than the holographic bound. So we will come to some useful results if a dark component of the universe strictly obeys the holographic principle.Next, we present a new cosmological model i.e., Semi-Holographic Universe,according to the holographic principle. If dark energy strictly obeys the holographic principle, its entropy is one fourth of the apparent horizon.There are decisive evidences that our observable universe evolves adiabatically after inflation in a comoving volume, that is, there is no energy-momentum flow between different patches of the observable universe. It indicates that the existence of the other dark component(dark matter) is compulsory, as a compensation of dark energy, based on the first law of thermodynamics. By using the method of dynamical system analysis,we find that there exists a stable dark energy-dark matter scaling solution at late time,which is helpful to solve the coincidence problem. In a numerical example, we find that the deceleration parameter can be well consistent with observations. The baron baryon matter is so little that could be neglected.Eventually, we find that no matter the temperature of the universe changes rapidly or slowly,the equations of dark energy and dark matter always evolve in thesame way. This result is helpful to solve the coincidence problem. Hence, we expect that the holographic component may have remarkable effects in the early universe.