Principle of limitation of physical quantities and cyclic universe

A close study of Heisenberg uncertainty principles reveals many significant facts, and all four major physical quantities, energy, time, momentum and length, have both lower and upper limits. Now, many questions come up. What are these limits? Some answers may lead to the understanding of the development of our universe. What is the shortest limit of time? At the beginning of big bang, there exists a tremendously short time, the Planck time. This may be just the shortest time limit in our universe. The longest time limit might be the lifetime of our universe. The longest length might be the final diameter of our expanding universe. All these lead to a finite universe. Two more coupling formulae are formed for the other two pairs of physical quantities, mass and speed, thermal energy and temperature. These four physical quantities must also have limits. We already knew that speed has upper limit and temperature has lower limit. By these two formulae, Planck and Einstein equations are derived directly. Since most other physical quantities are somewhat related to these major physical quantities, it seems that there exists a principle of limitation of physical quantities. A quantitative sketch of big bang is described. It also shows that our universe will contract back to another big bang. The principle of limitation opens up some fields of investigation. It may bring nature back to the harmony and determined world described by classical physics.     

第一讲微波背景辐射的各向异性、偏振及宇宙电离的历史

文章对微波背景辐射的各向异性、偏振及宇宙电离的历史给出了评述性介绍．从大爆炸理论的预言，到观测的发现，到其各向异性及偏振的探测，微波背景辐射(CMB)向人们揭示了丰富的宇宙学信息．文章在对基本理论作了简单介绍后，着重讲述了最新的CMB的观测结果及其物理意义．特别对微波背景各向异性探测器(Wilkinson Microwave Anisotropy Probe，WMAP)的偏振观测及其对宇宙重新电离的限制给出了较详细的叙述．     

Quantum gravity,the origin of time and time's arrow

The local Lorentz and diffeomorphism symmetries of Einstein's gravitational theory are spontaneously broken by a Higgs mechanism by invoking a phase transition in the early universe, at a critical temperature T_c below which the symmetry is restored. The spontaneous breakdown of the vacuum state generates an external time, and the wave function of the universe satisfies a time-dependent Schrödinger equation, which reduces to the Wheeler-deWitt equation in the classical regime for T<T_c, allowing a semiclassical WKB approximation to the wave function. The conservation of energy is spontaneously violated for T>T_c, and matter is created fractions of seconds after the big bang, generating the matter in the Universe. The time direction of the vacuum expectation value of the scalar Higgs field generates a time asymmetry, which defines the cosmological arrow of time and the direction of increasing entropy as the Lorentz symmetry is restored at low temperatures.     

Brane Formation and Cosmological Constraint on the Number of Extra Dimensions

Special relativity is generalized to extra dimensions and quantized energy levels of particles are obtained. By calculating the probability of particles' motion in extra dimensions at high temperature of the early universe, it is proposed that the branes may have not existed since the very beginning of the universe, but formed later. Meanwhile, before the formation, particles of the universe may have filled in the whole bulk, not just on the branes. This scenario differs from that in the standard big bang cosmology in which all particles are assumed to be in the 4D spacetime. So, in brane models, whether our universe began from a 4D big bang singularity is questionable. A cosmological constraint on the number of extra dimensions is also given which favors N ≥ 7.     

Cosmological baryon-number domain structure and the first order phase transition of a vacuum

If CP-nonconservation arises from spontaneous symmetry breaking in the very early universe, the universe will have a domain structure of baryon number. We propose a model of the early universe in which domains are stretched exponentially and the radius of the domains is much greater than that of the horizon of the standard big bang model, provided that the grand unified theory undergoes a first order phase transition. If the size of the stretched domains is sufficiently big to avoid pair annihilations of baryon and antibaryon domains, the difficulties of the baryon symmetric universe may be removed.     

Dynamical Horizon Entropy Bound Conjecture in Loop Quantum Cosmology

The covariant entropy bound conjecture is an important hint for the quantum gravity, with several versions available in the literature. For cosmology, Ashtekar and Wilson-Ewing ever show the consistence between the loop gravity theory and one version of this conjecture. Recently, He and Zhang [J. High Energy Phys. 10 (2007) 077] proposed a version for the dynamical horizon of the universe, which validates the entropy bound conjecture for the cosmology filled with perfect fluid in the classical scenario when the universe is far away from the big bang singularity. However, their conjecture breaks down near big bang region. We examine this conjecture in the context of the loop quantum cosmology. With the example of photon gas, this conjecture is protected by the quantum geometry effects as expected.     

Dynamical Horizon Entropy Bound Conjecture in Loop Quantum Cosmology

The covariant entropy bound conjecture is an important hint for the quantum gravity, with several versions available in the literature. For cosmology, Ashtekar and Wilson-Ewing ever show the consistence between the loop gravity theory and one version of this conjecture. Recently, He and Zhang [J. High Energy Phys. 10 (2007) 077] proposed a version for the dynamical horizon of the universe, which validates the entropy bound conjecture for the cosmology filled with perfect fluid in the classical scenario when the universe is far away from the big bang singularity. However, their conjecture breaks down near big bang region. We examine this conjecture in the context of the loop quantum cosmology. With the example of photon gas, this conjecture is protected by the quantum geometry effects as expected.     

Entropy bounds and Cardy–Verlinde formula in Yang–Mills theory

Using gauge formulation of gravity the three-dimensional SU(2) YM theory equations of motion are presented in equivalent form as FRW cosmological equations. With the radiation, the particular (periodic, big bang – big crunch) three-dimensional universe is constructed. Cosmological entropy bounds (so-called Cardy–Verlinde formula) have the standard form in such universe. Mapping such universe back to YM formulation we got the thermal solution of YM theory. The corresponding holographic entropy bounds (Cardy–Verlinde formula) in YM theory are constructed. This indicates to universal character of holographic relations.     

Why Large Hadron Collider?

I discuss LHC physics in the historical perspective of the progress in particle physics. After a recap of the Standard Model (SM) of particle physics, I discuss the high energy colliders leading up to LHC and their role in the discovery of these SM particles. Then I discuss the two main physics issues of LHC, i.e. Higgs mechanism and supersymmetry. I briefly touch upon Higgs and SUSY searches at LHC along with their cosmological implications.     

Implications of the Higgs boson discovery

With the discovery of the Higgs boson by the LHC in 2012, a new era started in which we have direct experimental information on the physics behind the breaking of the electroweak (EW) symmetry. This breaking plays a fundamental role in our understanding of particle physics and sits at the high-energy frontier beyond which we expect new physics that supersedes the Standard Model (SM). In this review we summarize what we have learned so far from LHC data in this respect. In the absence of new particles having been discovered, we discuss how the scrutiny of the properties of the Higgs boson (in search for deviations from SM expectations) is crucial as it can point the way for physics beyond the SM. We also emphasize how the value of the Higgs mass could have far-reaching implications for the stability of the EW vacuum if there is no new physics up to extremely large energies.     

Big bang as the collapse of an ordered spin system

The entropy of a spin system interacting with a free particle representing the inertia of the universe in the early stages is calculated. The conversion from a state of minimum entropy and minimum inertia with maximum spin order to a state of maximum entropy andmaximum inertia is analogized to the big bang.     

Quantum Oscillations Can Prevent the Big Bang Singularity in an Einstein-Dirac Cosmology

We consider a spatially homogeneous and isotropic system of Dirac particles coupled to classical gravity. The dust and radiation dominated closed Friedmann-Robertson-Walker space-times are recovered as limiting cases. We find a mechanism where quantum oscillations of the Dirac wave functions can prevent the formation of the big bang or big crunch singularity. Thus before the big crunch, the collapse of the universe is stopped by quantum effects and reversed to an expansion, so that the universe opens up entering a new era of classical behavior. Numerical examples of such space-times are given, and the dependence on various parameters is discussed. Generically, one has a collapse after a finite number of cycles. By fine-tuning the parameters we construct an example of a space-time which satisfies the dominant energy condition and is time-periodic, thus running through an infinite number of contraction and expansion cycles.     

Quantum bounce and cosmic recall

Loop quantum cosmology predicts that, in simple models, the big bang is replaced by a quantum bounce. A natural question is whether the universe retains, after the bounce, its memory about the previous epoch. More precisely, does the Universe retain various properties of the state after evolving unitarily through the bounce, or does it suffer from recently suggested cosmic amnesia? We show that this issue can be answered unambiguously at least within an exactly solvable model. A semiclassical state at late times on one side of the bounce, peaked on a pair of canonically conjugate variables, strongly bounds the fluctuations on the other side, implying semiclassicality. For a model universe growing to 1 megaparsec, the change in relative fluctuation across the bounce is less than 10(-56) (becoming smaller for larger universes). The universe maintains (an almost) total recall.     

Looking for physics beyond the Standard Model

Motivations for new physics beyond the Standard Model are presented. The most successful and best motivated option, supersymmetry, is described in some detail, and the associated searches performed at LEP are reviewed. These include searches for additional Higgs bosons and for supersymmetric partners of the standard particles. These searches constrain the mass of the lightest supersymmetric particle which could be responsible for the dark matter of the universe. To cite this article: P. Binétruy, J.-F. Grivaz, C. R. Physique 3 (2002) 1235–1243.     

The Shelter Island Conferences Revisited: “Fundamental” Physics in the Decade 1975–1985

The focus of this broad historical overview of “the steady evolution of theoretical ideas” from Shelter Island I in 1947 to Shelter Island II in 1983 is some of the developments in “fundamental” physics after the establishment of the standard model, in particular, the adoption of the view that all present day field theories are “effective field theories” based on the gauge concept; taking seriously big bang cosmology, grand unified field theories (GUTs), and inflation; and the emergence of a new symbiosis of physics and mathematics.     

第四讲暴涨宇宙学的研究与进展

文章简单介绍了标准(大爆炸)宇宙模型的成功和困难，着重介绍了暴涨宇宙学的研究历史和最近的进展，并展望了今后人们可能的关注方向     

The Higgs sector of the MSSM in the decoupling limit

We study the heavy Higgs sector of the MSSM composed of the and particles in the so-called decoupling limit where . By integrating out these heavy Higgs particles to one-loop, we compute the effective action for the electroweak gauge bosons and find out that, in the decoupling limit, all the heavy Higgs effects can be absorbed into redefinitions of the Standard Model electroweak parameters. This demonstrates explicitely that the decoupling theorem works for the heavy MSSM Higgs particles. This is also compared with the paradigmatic and different case of the Standard Model heavy Higgs particle. Finally, this work together with our two previous works, complete the demonstration that all the non-standard particles in the MSSM, namely, squarks, sleptons, charginos, neutralinos and the heavy Higgs particles, decouple to one-loop from the low energy electroweak gauge boson physics. Received: 2 March 2000 / Revised version: 13 July 2000 / Published online: 8 September 2000     

Georges Lemaître,Pioneer of Modern Theoretical Cosmology

No other scientist may have had a greater impact on modern cosmology than the Belgian physicist, astronomer and priest Georges Lemaître. In 1927 he predicted the expansion of the universe on the basis of the cosmological field equations; and four years later he proposed what he called the primeval-atom hypothesis, the first version of the later big bang universe. In all his work on cosmology the cosmological constant Λ played a significant role. A recognized expert in the theory of general relativity, Lemaître also contributed significantly to the theoretical clarification of local and global singularity problems. Still, when he died in 1968, at a time when the standard big bang model celebrated its first victories, he was largely forgotten or recalled only as a somewhat shadowy figure of the past. This essay reviews in a historical context the scientific work of Lemaître with particular attention to his seminal contributions in the decade between 1925 and 1934.