Modern Theoretical and Observational Cosmology: Proceedings of the 2nd Hellenic Cosmology Meeting, h

MODERN THEORETICAL. AND OBSERVATIONAL. COSMOLOGY. Proceedings of the 2nd Hellenic Cosmology. Meeting, held in the National Observatory of.
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Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available, spanning centuries or millennia. On the other hand, radio observations may look at events on a millisecond timescale millisecond pulsars or combine years of data pulsar deceleration studies.

The information obtained from these different timescales is very different. The study of our very own Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Our understanding of our own Sun serves as a guide to our understanding of other stars. The topic of how stars change, or stellar evolution, is often modeled by placing the varieties of star types in their respective positions on the Hertzsprung—Russell diagram , which can be viewed as representing the state of a stellar object, from birth to destruction.

Theoretical astrophysicists use a wide variety of tools which include analytical models for example, polytropes to approximate the behaviors of a star and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on.

Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen. Theorists in astrophysics endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.

Theorists also try to generate or modify models to take into account new data.

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In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model. Topics studied by theoretical astrophysicists include: Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole astro physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astrophysics, now included in the Lambda-CDM model , are the Big Bang , cosmic inflation , dark matter, dark energy and fundamental theories of physics. Wormholes are examples of hypotheses which are yet to be proven or disproven. The roots of astrophysics can be found in the seventeenth century emergence of a unified physics, in which the same laws applied to the celestial and terrestrial realms.

In modern times, students continue to be drawn to astrophysics due to its popularization by the Royal Astronomical Society and notable educators such as prominent professors Lawrence Krauss , Subrahmanyan Chandrasekhar , Stephen Hawking , Hubert Reeves , Carl Sagan and Neil deGrasse Tyson.


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The efforts of the early, late, and present scientists continue to attract young people to study the history and science of astrophysics. From Wikipedia, the free encyclopedia. This article is about the use of physics and chemistry to determine the nature of astronomical objects. For the use of physics to determine their positions and motions, see Celestial mechanics. For the physical study of the largest-scale structures of the universe, see Physical cosmology.

For the journal, see Astrophysics journal. Cosmology portal Physics portal. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each photon that travels through intergalactic space loses energy due to the redshift effect.

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This energy is not obviously transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of conservation of energy. Thermodynamics of the universe is a field of study that explores which form of energy dominates the cosmos — relativistic particles which are referred to as radiation , or non-relativistic particles referred to as matter.

Relativistic particles are particles whose rest mass is zero or negligible compared to their kinetic energy , and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light. As the universe expands, both matter and radiation in it become diluted. However, the energy densities of radiation and matter dilute at different rates.

As a particular volume expands, mass energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the wavelength of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands.


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    • The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well analytically.

    As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion. The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period.

    The standard cosmological model is known as the Lambda-CDM model. Within the standard cosmological model , the equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant. At first, the expansion is slowed down by gravitation attracting the radiation and matter in the universe.

    However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago. During the earliest moments of the universe the average energy density was very high, making knowledge of particle physics critical to understanding this environment. Hence, scattering processes and decay of unstable elementary particles are important for cosmological models of this period.

    As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe. Observations suggest that the universe began around The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth.

    Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses. Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted.

    Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars , and ultimately galaxies, clusters of galaxies and superclusters formed. Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order.

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    One is that there is no compelling reason, using current particle physics, for the universe to be flat , homogeneous, and isotropic see the cosmological principle. Moreover, grand unified theories of particle physics suggest that there should be magnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period of cosmic inflation , which drives the universe to flatness , smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles.

    Another major problem in cosmology is what caused the universe to contain far more matter than antimatter. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be X-rays and gamma rays produced as a result of annihilation , but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this currently not understood process is called baryogenesis.

    Three required conditions for baryogenesis were derived by Andrei Sakharov in , and requires a violation of the particle physics symmetry , called CP-symmetry , between matter and antimatter. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.

    Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment , rather than through observations of the universe. Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its temperature dropped below that at which nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced.

    Starting from hydrogen ions protons , it principally produced deuterium , helium-4 , and lithium. Other elements were produced in only trace abundances. It is frequently referred to as the standard model of Big Bang cosmology. The cosmic microwave background is radiation left over from decoupling after the epoch of recombination when neutral atoms first formed.

    At this point, radiation produced in the Big Bang stopped Thomson scattering from charged ions. The radiation, first observed in by Arno Penzias and Robert Woodrow Wilson , has a perfect thermal black-body spectrum. It has a temperature of 2.

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    Cosmological perturbation theory , which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments COBE and WMAP [42] and many ground and balloon-based experiments such as Degree Angular Scale Interferometer , Cosmic Background Imager , and Boomerang. The results of measurements made by WMAP, for example, have placed limits on the neutrino masses. On 17 March , astronomers of the BICEP2 Collaboration announced the apparent detection of B -mode polarization of the CMB, considered to be evidence of primordial gravitational waves that are predicted by the theory of inflation to occur during the earliest phase of the Big Bang.

    Understanding the formation and evolution of the largest and earliest structures i. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling.

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    Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments , superclusters and voids. Most simulations contain only non-baryonic cold dark matter , which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter.

    More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy. Other, complementary observations to measure the distribution of matter in the distant universe and to probe reionization include:.

    These will help cosmologists settle the question of when and how structure formed in the universe. The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies.

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    Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. High Colour Fermions, D. Natural Chaotic Inflation, E. Farakos, Koutsoubas and E. Georgalas, Lahanas and E, Papantonopoulos, Phys. Filter Development in the Auger Experiment, M. Brane Inflation from Mirage Cosmology , E. Brane Cosmology , E. Inflation in String Theory and how you can get out of it , E. Brane Worlds with Induced Gravity, G. B raneworld Cosmological Models, E.

    Brane-bulk matter relations for a purely conical codimension-2 brane world, E. Cosmology in Six Dimensions, E.


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