Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.aaa.org/s932/images/09fallclass5.pdf Дата изменения: Wed Sep 7 23:30:07 2011 Дата индексирования: Tue Oct 2 00:54:04 2012 Кодировка: Поисковые слова: magnitude

Class 1 Introduction, Background History of Modern Astronomy The Night Sky, Eclipses and the Seasons Kepler's Laws Newtonian Gravity General Relativity Matter and Light Telescopes Class 2 Solar System Characteristics Formation Exosolar Planets Class 3 Stars The Sun Stellar Evolution of Low and High Mass Stars Deaths of Stars

Class 4

Galaxies

Galaxy Classification Formation of Galaxies Galactic Evolution

Class 5 Cosmology
Large-Scale Structure of the Universe
What do we see?

Big Bang Cosmology
What model explains what we see?
Class 6 Special Topics Why is Pluto no longer a planet? IAU planet classification Observing with a Telescope


Large Scale Structure
The observables: Local Group (size 1 Mpc) Virgo cluster (size 2+ Mpc) Virgo supercluster (size ~33 Mpc) Voids (size ~ 10-150 Mpc), Walls and sheets, aka filaments (size ~ 50-80+ Mpc)


1 parsec
The nearest star is 1.2 parsecs away (Proxima Centuri). The solar system is about 8000 parsecs away from the center of our Milky Way. 8000 parsecs = 8 kiloparsecs = 8 kpc The diameter of the Milky Way is about 30,000 parsecs or 30 kpc. The nearest galaxy like our own is Andromeda at close to 1,000,000 parsecs away. 1,000,000 = 1 Megaparsec = 1 Mpc


Large Scale Structure
The observables:

Local Group
The Local Group is found on the outskirts of a larger structure dubbed the Local Supercluster which itself is flattened and is roughly centered on the Virgo Cluster.
Virgo cluster Virgo supercluster Voids, Sheets Filaments


The group comprises over 30 galaxies, mostly dwarfs. Between Andromeda and the Milky Way is about 780 kpc. In total it contains about 1.3 x 1012 M, whereas the Milky Way is about 6 x 1011 M. M= 2 x 1030kg kg=kilogram


Large Scale Structure
The observables: Local Group

Virgo cluster
The Virgo cluster is roughly the center of the Local Supercluster.
Virgo supercluster Voids, Sheets Filaments


The Virgo Cluster is a cluster of galaxies at a distance of about 18.0 Mpc away in the constellation Virgo. It's mass is about 1.2 в 1015 M out to 8 degrees from the cluster's center ­ i.e. a radius of about 2.2 Mpc. The cluster has about 1300 ­ 2000 member galaxies.

As we look on larger and larger scales, we find that a larger and larger fraction of the matter in the universe is dark.




Large Scale Structure

The observables: Local Group Virgo cluster

Virgo supercluster (aka Local Supercluster)
Voids, Sheets Filaments

The mass and velocity of the Virgo cluster indicate that the Local Group is probably not off far enough away to escape, so that its recession from Virgo will probably be halted at one time, and then it will fall and merge into the cluster.




Large Scale Structure

The observables: Local Group Virgo cluster

Virgo supercluster
Voids, Sheets Filaments

Upward of 90 percent of the matter in the universe is dark


Once thought to be the largest structures in nature, superclusters are now understood to be subordinate to enormous walls or sheets, usually called "filaments", sometimes called "super cluster complexes".
Walls or sheets can span hundreds of megaparsecs in length.


Sloan Great Wall
the largest known structure in the universe ~ 400+ Mpc in length and ~300 Mpc from Earth.


Large Scale Structure: Galaxy surveys demonstrate the homogeneity and isotropy of the universe for scales ~200Mpc and larger
http://cosmicweb.uchicago.edu/images/mov/s02_full2.mpg The formation of clusters and large-scale filaments in the Cold Dark Matter model with dark energy shows the evolution of structures in a 43 Mpc box from redshift of 30 to the present. Between the last frames there is little change because the expansion of the universe is in the stage of acceleration as the "dark energy" becomes dominant; gravity cannot compete with the dark energy-driven acceleration and the growth of structures ceases. As the contraction of largescale structures is halted they expand with the universe.

http://cosmicweb.uchicago.edu/images/mov/r1c1_full2.mpg

Formation of a group of galaxies quite similar to our Local Group in which our galaxy, the Milky Way, is approaching our biggest neighbor the Andromeda Galaxy. The region shown here is 1/10 of the box shown in the filament formation page and is equal to 4.3 megaparsec or 14 million light years.


Big Bang Cosmology


Before the developments of the last century, including the pivotal discovery of the cosmic background radiation (CBR), there was a long history of theorizing about the nature of our universe on the largest scales. Some of our biases, our assumptions, have changed over time as discoveries developed, even if we are still asking the same kinds of questions like: is the universe infinite or finite? is the universe static or dynamic? An old assumption was that the universe was infinite and static. Even Einstein believed the universe was static. An infinite static universe, however, should always have a bright sky regardless of day or night.


Hubble's Law resolves Olber's paradox which is from cosmological principle (that the universe is homogenous and isotropic) plus assumptions of an infinite, unchanging universe.

Galaxies are measured to be moving away from each other via the cosmological redshift of their light.

Galaxies are observed to be moving away from each other due to the expansion of space.


Here is the spectrum of an emission-line galaxy with z=0.0886. The prominent features of such a galaxy in emission include the hydrogen, nitrogen II, sodium II, and oxygen III lines. The wavelengths have been stretched by cosmic expansion. Simply measure wavelength displacement to get a galaxy's distance.


Hubble's Law is the observation that recession velocity scales with distance. The farther away, the faster it moves away. This is interpreted as the cosmic expansion of space.
H0 = about 70 (km/s) / Mpc


The developments of the last century, in particular, bring us to the following most accepted picture of our understanding of the evolution of our universe, which contains 4.6% ordinary matter, about 23% dark matter and 72% dark energy:


In the beginning of space and time, about 13.7 billion years ago, an initial hot and dense state of energy began expanding according to:
Expansion rate Matter density Dark energy

Radiation density

Curvature


Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.This singularity signals the breakdown of general relativity.
Between, t=0 seconds, i.e., the Big Bang and the first
5 100000000000000000000000000000000000000000000

second

is the Planck era.


Planck time
According to quantum theory, 1 Planck time should be the smallest unit of time physics can reason about in a meaningful way and it is derived all from constants of nature. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length.

During the Planck time all four forces of nature are unified. At the end of the Planck epoch, gravity breaks from the electronuclear force (electromagnetism, weak force and strong force combined) and Grand Unification models describe the evolution of the universe.


If the grand unification energy is taken to be 1015 GeV, this corresponds to temperatures higher than 1027 K. During this period, three of the four fundamental interactions-- electromagnetism, the strong interaction, and the weak interaction--were unified as the electronuclear force.





Big Bang Nucleosynthesis

Conditions in the cooling, thinning universe became less and less conducive to further fusion reactions as time went by. For all practical purposes, primordial nucleosynthesis stopped at helium-4. Detailed calculations show that, by about 15 minutes after the Big Bang, the cosmic elemental abundance was set, and helium accounted for approximately one-quarter of the total mass of normal matter in the universe. The remaining 75 percent was hydrogen. It would be almost a billion years before nuclear reactions in stars would change these figures.


..till recombination and decoupling


Hierarchical structure formation