Study On The Ultracompact Dark Matter Minihalos

Since Einstein created general relativity, We have had a theory to describe the evolution of our Universe for the first time. For the simplicity of research,Einstein proposed the cosmological principle:the distribution of matter in the universe is ho-mogeneous and isotropic when viewed on a large enough scale. Meanwhile,to get a static solution of cosmological,Einstein introduce the cosmological constant. But this solution is soon found not stable.Afterwards Hubble observed an overall red shift from 24 nearby galaxies,and red shift of the galaxy is proportional to its distance from earth. This is the direct evidence that the universe is expanding,also suggesting the uniform of the universe. Twenty years later,Gamow put forward the hot big bang theory which our universe originate from,and calculate the background temperature of relic phonon. Then Arno Penzias and Robert Woodrow Wilson,two engineer of Bell Laboratories, found the cosmic microwave background radiation that theoretical physicist is looking for,therefore Confirmed the prediction of Gamow. Moreover the big bang cosmology well interpreted the abundance of light elements. In the 1970s,a large number of ob-servations from Galaxy rotation curves,Velocity dispersions of galaxies gravitational lensing and so on,show that most of matter is dark matter which don’t involving in electromagnetic interaction at any significant level. Observed data from la supernovae in 1998 show that the expansion of the universe is accelerating, hint the existence of dark energy with negative pressure if general relativity is correct in the cosmological scale.It is well known from various observation that the total energy of the universe contains 4.9% ordinary matter,26.8% dark matter and 68.3% dark energy. In this paper, we mainly study a kind of new dark matter structure recently discovered.Chapter 1 summarizes the knowledge of cosmology. We first briefly reviews the history of cosmology,then introduce the primary concept in general relativity,finally get the cosmological eqution of evolution. For radiation,matter and dark energy,we obtained the relation of energy density p and cosmological scale factor a,as well as the equation of scale factor with the increase of time.Chapter 2 Systematically describes the observational evidence for dark matter,then classify the dark matter candidates and discussed the detection method of dark matter. Weakly interaction massive particles(WTMPs) are the ideal candidates of cold dark matter,and the lightest supersymmetric particle is most commonly researched WIMP. We give their fraction Ωχ in the universe as a function of parameter,and with present observation we obtain the constraints of dark matter parameter,such as WIMP mass and cross section. A typical example of hot dark matter is three types of neutrinos,we constaint the mass of neutrinos, we also discuss the nonthermal production mecha-nism and give the expression of Ωχ dependent of the parameter.Especially,we discuss the simplest example of nonthermal relics-the condensate of a weakly interacting mas-sive scalar field. Axion,introduced to solve the strong CP problem,is a attractive non-thermal relic candidate. There are methods of detecting dark matter:direct detection experiments measure Nuclear recoil of atomic nucleus,indirect detections measure the particle of standard model produced by dark matter annihilation or decay.In Chapter 3,we study a new dark matter structure-ultracompcat minihalos (UCMH-s) which proposed by Ricotti and Gould in 2009.Primordial black holes can be formed if there are region of primordial density perturbation dp> 0.3.If primodial densi-ty perturbation is less than this critical value but larger than 10-3,ultracompact mini-halos instead of primordial black holes would be produced.Compared with normal dark matter halos,the density of UCMHs is steeper and its formation time is earli-er.If the dark matter is composed of weakly interactive massive particles,particles like γ-ray from UCMHs due to dark matter annihilation or decay would be detected Fer-mi satellite or Air Cherenkov Telescopes,and neutrino signals would be detected by IceCube/DeepCore or other neutrino telescopes.For the given model we calculate the lower bound of UCMH fraction which would result from the detections of γ-ray anni-hilate by WIMPs from an UCMH by Fermi,and the upper limit of UCMH fraction if γ-ray from UCMHs are not detected,and translate UCMH fraction constraints into the limit on the primordial curvature perturbation on small scales.But if WIMPs are not an-nihilated,decay could be important, we compute the γ-ray signal by WIMP decay from UCMH,and obtain the corresponding constraints on UCMH fraction and primordial curvature perturbation.In addition to γ-ray signals.we also research on neutrino signal from UCMHs.Although no excess of neutrino signals from nearby UCMHs has been observed, we give the UCMHs fraction and translated these constraints into the limit on the primordial curvature perturbation on samll scale.Finally,we summary our research works in this thesis,and Prospects future studies.