arXiv subjects
Publications and source records attributed to Kazuhide Ichikawa.
We show that the cosmic microwave background (CMB) data of WMAP can give subelectronvolt limit on the neutrino mass: m_nu < 0.63 eV (95% CL). We also investigate its degeneracy with other cosmological parameters. In particular, we show the Hubble constant derived from the WMAP data decreases considerably when the neutrino mass is a few times 0.1 eV.
We investigate the modifications to predictions for the abundances of light elements from standard Big-Bang nucleosynthesis when exotic late-decaying particles with lifetimes exceeding ~1 sec are prominent in the early Universe. Utilising a model-independent analysis of the properties of these long-lived particles, we identify the parameter space associated with models that are consistent with all observational data and hence resolve the much discussed discrepancies between observations and theoretical predictions for the abundances of Li^7 and Li^6.
We discuss constraints on the effective number of neutrino species Nnu from recent cosmological observations such as CMB, LSS, BBN, including our own analysis which uses the WMAP and the Luminous Red Galaxy power spectrum data. We also discuss their implications on some non-standard cosmological scenarios such as the low (MeV-scale) reheating temperature scenario and the scenario with decaying particles between BBN and structure formation.
We present models of decaying particles for increasing the effective number of neutrinos N_nu after big bang nucleosynthesis but before the structure formation begins. We point out that our scenario not only solves the discrepancy between the constraints on N_nu from these two epochs, but also provides a possible answer to deeper inconsistency in the estimation of the matter power spectrum amplitude at small scales, represented by sigma_8, between the WMAP and some small scale matter power measurements such as the Lyman-alpha forest and weak lensing. We consider (a) saxion decay into two axions; (b) gravitino decay into axino and axion; (c) Dirac right-handed sneutrino decay into gravitino and right-handed neutrino.
We derive constraint on the effective number of neutrino species N_nu from the cosmic microwave background power spectrum of the WMAP and galaxy clustering power spectrum of the SDSS luminous red galaxies (LRGs). Using these two latest data sets of CMB and galaxy clustering alone, we obtain the limit 0.9 < N_nu < 8.2 (95% C.L.) for the power-law LambdaCDM flat universe, with no external prior. The lower limit corresponds to the lower bound on the reheating temperature of the universe T_R > 2 MeV.
We investigate observational constraints on the curvature of the universe not restricting ourselves to a cosmological constant as dark energy, in particular allowing a dark energy equation of state to evolve with time in several ways. We use type Ia supernovae (SNeIa) data from the latest gold data set which includes 182 SNeIa, along with the CMB shift parameter and the baryon acoustic oscillation peak. We show quantitatively that the constraint on the curvature of the universe depends on dark energy model: some popular parametrizations give constraints closely around the flat universe at 5% level (2 sigma C.L.) whereas some parametrizations allow the universe to be as open as Omega_k ~ 0.2.
We investigate cosmological evolution and implications of cosmic strings with time-dependent tension. We derive basic equations of time development of the correlation length and the velocity of such strings, based on the one scale model. Then, we find that, in the case where the tension depends on some power of the cosmic time, cosmic strings with time-dependent tension goes into the scaling solution if the power is lower than a critical value. We also discuss cosmic microwave background anisotropy and matter power spectra produced by these strings. The constraints on their tensions from the Wilkinson microwave anisotropy probe (WMAP) three year data and Sloan digital sky survey (SDSS) data are also given.
We investigate how the nature of dark energy affects the determination of the curvature of the universe from recent observations. For this purpose, we consider the constraints on the matter and dark energy density using observations of type Ia supernovae, baryon acoustic oscillation peak and cosmic microwave background with several types of dark energy equation of state. Although it is usually said that the combination of current observations favors a flat universe, we found that a relatively large parameter space allows the universe to be open for a particular model of dark energy. We also discuss what kind of dark energy model or prior allow a non-flat universe.
We study constraints on time variation of the fine structure constant alpha from cosmic microwave background (CMB) taking into account simultaneous change in alpha and the electron mass m_e which might be implied in unification theories. We obtain the constraints -0.097 < Delta alpha/alpha < 0.034 at 95% C.L. using WMAP data only, and -0.042 < Delta alpha/alpha < 0.026 combining with the constraint on the Hubble parameter by the HST Hubble Key Project. These are improved by 15% compared with constraints assuming only alpha varies. We discuss other relations between variations in alpha and m_e but we do not find evidence for varying alpha.
We derive an upper limit on the neutrino mass from the WMAP three-year data alone by employing a deterministic minimisation method based on a grid search in multi-parameter space. Assuming the flat LCDM model with power-law adiabatic perturbations, we find the sum of m_nu < 2.0 eV in agreement with the result of the WMAP team. This result, the limit being nearly the same as that from the WMAP first-year data, means that the fundamental limit on the neutrino mass obtainable from the cosmic microwave background alone is already nearly met, as anticipated from the previous analysis. We also clarify the role of the polarisation data in deriving the limit on the neutrino mass.
We revisit the constraint on the primordial helium mass fraction Yp from observations of cosmic microwave background (CMB) alone. By minimizing chi square of recent CMB experiments over 6 other cosmological parameters, we obtained rather weak constraints as 0.17 < Yp < 0.52 at 1 sigma C.L. for a particular data set. We also study the future constraint on cosmological parameters when we take account of the prediction of the standard big bang nucleosynthesis (BBN) theory as a prior on the helium mass fraction where Yp can be fixed for a given energy density of baryon. We discuss the implications of the prediction of the standard BBN on the analysis of CMB.
We discuss the constraints on the time-varying equation of state for dark energy and the curvature of the universe using observations of type Ia supernovae from Riess et al. and the most recent Supernova Legacy Survey (SNLS), the baryon acoustic oscillation peak detected in the SDSS luminous red galaxy survey and cosmic microwave background. Due to the degeneracy among the parameters which describe the time dependence of the equation of state and the curvature of the universe, the constraints on them can be weakened when we try to constrain them simultaneously, in particular when we use a single observational data. However, we show that we can obtain relatively severe constraints when we use all data sets from observations above even if we consider the time-varying equation of state and do not assume a flat universe. We also found that the combined data set favors a flat universe even if we consider the time variation of dark energy equation of state.
We study how the oscillations of the neutrinos affect their thermalization process during the reheating period with temperature O(1) MeV in the early universe. We follow the evolution of the neutrino density matrices and investigate how the predictions of big bang nucleosynthesis vary with the reheating temperature. For the reheating temperature of several MeV, we find that including the oscillations makes different predictions, especially for $^4$He abundance. Also, the effects on the lower bound of the reheating temperature from cosmological observations are discussed.
It is shown that a subelectronvolt upper limit can be derived on the neutrino mass from the CMB data alone in the Lambda CDM model with the power-law adiabatic perturbations, without the aid of any other cosmological data. Assuming the flatness of the universe, the constraint we can derive from the current WMAP observations is \sum m_nu < 2.0 eV at the 95% confidence level for the sum over three species of neutrinos (m_nu < 0.66 eV for the degenerate neutrinos). This constraint modifies little even if we abandon the flatness assumption for the spatial curvature. We argue that it would be difficult to improve the limit much beyond \sum m_nu \lesssim 1.5 eV using only the CMB data, even if their statistics are substantially improved. However, a significant improvement of the limit is possible if an external input is introduced that constrains the Hubble constant from below. The parameter correlation and the mechanism of CMB perturbations that give rise to the limit on the neutrino mass are also elucidated.
Within the standard big bang nucleosynthesis (BBN) and cosmic microwave background (CMB) framework, the baryon density measured by the Wilkinson Microwave Anisotropy Probe (WMAP) or the primordial D abundance is much higher than the one measured by the $^4$He or $^7$Li abundances. To solve the discrepancy, we propose a scenario in which additional baryons appear after BBN. We show that simply adding the baryons can not be a solution but the existence of a large lepton asymmetry before BBN makes the scenario successful. These extra baryons and leptons, in addition to the initial baryons which exist before the BBN, can be all produced from $Q$-balls.
We calculate primordial abundances of light elements produced during big bang nucleosynthesis when the fine structure constant and/or the cosmic expansion rate take non-standard values. We compare them with the recent values of observed D, He4 and Li7 abundances, which show slight inconsistency among themselves in the standard big bang nucleosynthesis scenario. This inconsistency is not solved by considering either a varying fine structure constant or a non-standard expansion rate separately but solutions are found by their simultaneous existence.
In this paper, we investigate the dynamics and the evolution of the scale factor of a probe Dp-brane which move in the background of source Dp-branes. Action of the probe brane is described by the Born-Infeld action and the interaction with the background R-R field. When the probe brane moves away from the source branes, it expands by power law, whose index depends on the dimension of the brane. If the energy density of the gauge field on the brane is subdominant, the expansion is decelerating irrespective of the dimension of the brane. On the other hand, when the probe brane is a Nambu-Goto brane, the energy density of the gauge field can be dominant, in which case accelerating expansion occurs for $p \leq 4$. The accelerating expansion stops when the brane has expanded sufficiently so that the energy density of the gauge field become subdominant.
We reconsider the evolution of strongly degenerate neutrinos in the early universe. Our chief concern is the validity of the entropy conservation after the neutrino annihilation process has frozen out (so that the establishment of chemical equilibrium is not trivial). We argue that the entropy indeed conserves because elastic scattering keeps the neutrino and antineutrino distribution functions in the equilibrium form and the sum of their chemical potential keeps zero even after the neutrino annihilation freeze-out. We also simulate the evolution of the degenerate neutrino spectrum to support the argument. We conclude that the change in the neutrino degeneracy parameter when the relativistic degrees of freedom in the universe decreases is calculated using the entropy conservation and the lepton number conservation without worrying about at what temperature the neutrino annihilation process freezes out.