Particle horizon

About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.

Determination of the Hubble con

Abstract: Establishing accurate extragalactic dis- tances has provided an immense challenge to astronomers since the 1920s. The situation has improved dramatically as better detectors have become available, and as several new, promising techniques have been developed. For the first time in the history of this difficult field, relative distances to galaxies are being compared on a case-by-case basis, and their quantitative agreement is being established. New instrumen- tation, the development of new techniques for measuring distances, and recent measurements with the Hubble Space telescope all have resulted in new distances to galaxies with precision at the ?5-20% level. The current statistical uncer- tainty in some methods for measuring Ho is now only a few percent; with systematic errors, the total uncertainty is ap- proaching ?+10%. Hence, the historical factor-of-two uncer- tainty in the value of the Ho is now behind us. Though there has been remarkable progress in measuring the cosmological parameters, the accuracy of these quantities is determined by the available technology and measurement techniques and is still not sufficiently high to discriminate among the various existing world models (1). Because of the fundamental dependence on the cosmological parameters in all of the models, accurate determinations are critical to make reliable predictions based on the current models. For instance, a reliable value of the Hubble constant is required to constrain the density of baryons from nucleosynthesis at an early epoch of the universe. The Hubble constant sets the time and length scale at the epoch of equality of the energy density of matter and radiation. In the structure formation paradigm based on gravitational instability, the horizon scale at matter-radiation equality specifies the critical range of the density perturbation spectrum turnover, and an accurate knowledge of the Hubble constant allows a quantitative comparison of the anisotropies in the cosmic background radiation and theories of the large- scale structure of the universe. In addition, in the issue addressed in this session, that of the age of the universe, there is a direct confrontation between the expansion age inferred from the Hubble constant in the standard model and age dating of the oldest objects in the universe. The reason for testing the cosmological model by using the age of the universe is obvious: there should be no astronomical object in the universe older than the universe itself. Consequently, the oldest objects known provide the minimum age of the universe. What is required to measure an accurate value of Ho? According to the Hubble law, what is needed are measure- ments of both redshifts of galaxies (via spectral lines), and distances to galaxies (at sufficiently large distances where

Cosmological Parameters and Quintessence From Radio Galaxies

Abstract: FRIIb radio galaxies provide a tool to determine the coordinate distance to sources out to redshifts of two. The coordinate distance depends on the present values of global cosmological parameters, quintessence, and the equation of state of quintessence, and provides one of the cleanest determinations of global cosmological parameters because it does not depend on the clustering properties of any mass-energy components. The modified standard candle method (using type Ia supernove) of determining the coordinate distance is compared in detail with the modified standard yardstick method (using FRIIb radio galaxies); the methods are found to be complementary. The most significant difference between the methods is that the radio galaxy method is completely independent of the local distance scale and the properties of local sources, while the supernovae method is very closely tied to the local distance scale and the properties of local sources. FRIIb radio galaxies provide one of the very few reliable probes of the coordinate distance to sources with redshifts out to two. The method indicates that the current value of the density parameter in non-relativistic matter must be low irrespective of whether the universe is spatially flat, and of whether a significant cosmological constant or quintessence pervades the universe at the present epoch. FRIIb radio galaxies indicate that the universe is currently accelerating in its expansion if the primary components of the universe at the present epoch are non-relativistic matter and quintessence, and the universe is spatially flat.

Observed Cosmological Redshifts Support Contracting Accelerating Universe

Abstract: The main argument that Universe is currently expanding is observed redshift increase by distance. However, this conclusion may not be correct, because cosmological redshift depends only on the scaling factors, the change in the size of the universe during the time of light propagation and is not related to the speed of observer or speed of the object emitting the light. An observer in expanding universe will measure the same redshift as observer in contracting universe with the same scaling. This was not taken into account in analysing the SN Ia data related to the universe acceleration. Possibility that universe may contract, but that the observed light is cosmologically redshifted allows for completely different set of cosmological parameters $\Omega_M, \Omega_{\Lambda}$, including the solution $\Omega_M=1, \Omega_{\Lambda}=0$. The contracting and in the same time accelerating universe explains observed deceleration and acceleration in SN Ia data, but also gives significantly larger value for the age of the universe, $t_0 = 24$ Gyr. This allows to reconsider classical cosmological models with $\Lambda =0$. The contracting stage also may explain the observed association of high redshifted quasars to low redshifted galaxies.

The Arrow of Time In a Universe with a Positive Cosmological Constant Λ

Abstract: There is a mounting evidence that our universe is propelled into an accelerated expansion driven by Dark Energy. The simplest form of Dark Energy is a cosmological constant Λ, which is woven into the fabric of spacetime. For this reason it is often referred to as vacuum energy. It has the “strange” property of maintaining a constant energy density despite the expanding volume of the universe. Universes whose energy ismade of Λ posses an event horizon with and eternally finite constant temperature and entropy, and are known as DeSitter geometries. Since the entropy of DeSitter spaces remains a finite constant, then the meaning of a thermodynamic arrow of time becomes unclear. Here we explore the consequences of a fundamental cosmological constant Λ for our universe. We show that when the gravitational entropy of a pure DeSitter state ultimately dominates over the matter entropy, then the thermodynamic arrow of time in our universe may reverse in scales of order a Hubble time. We find that due to the dynamics of gravity and entanglement with other domain, a finite size system such as a DeSitter patch with horizon size H 0 -1 has a finite lifetime ∆t. This phenomenon arises from the dynamic gravitational instabilities that develop during a DeSitter epoch and turn catastrophic. A reversed arrow of time is in disagreementwith observations. Thus we explore the possibilities that: Nature may not favor a fundamental Λ, or else general relativity may be modified in the infrared regime when Λ dominates the expansion of the Universe.