Modern Cosmology -
Part VI: Cosmic Web and Formation of Galaxies
Jacobs University Bremen
Caption: The cold dark matter model has become the leading theoretical paradigm
for the formation of structure in the Universe. Together with the theory of cosmic inflation,
this model makes a clear prediction for the initial conditions for structure formation and
predicts that structures grow hierarchically through gravitational instability.
Testing this model requires that the precise measurements delivered by galaxy surveys
can be compared to robust and equally precise theoretical calculations.
The image combines the largest simulation of the growth of dark matter structure ever carried out
with new techniques for following the formation and evolution of the visible components.
Baryon-induced features in the initial conditions of the Universe are reflected
in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain
the nature of Dark Energy with next generation surveys.
[Image courtesy: Volker Springel Millennium simulation]
|1. Structure Formation and Universe on the Computer
Caption: The frames show the evolution of structures in a 43 million parsecs (or 140 million light years) box from redshift of 30 to the present epoch. At the initial epoch (z=30), when the age of the Universe was less than 1% of its current age, distribution of matter appears to be uniform. This is because the seed fluctuations are still fairly small. As time goes on, the fluctuations grow resulting in a wealth of structures from the smallest bright clumps which have sizes and masses similar to those of galaxies to the large filaments. Notice the filament spanning the entire box from left to right and how it becomes more and more pronounced with time.In a universe dominated by matter and radiation (as opposed to dark energy), the mutual gravitational pull of all the particles tends to slow down the expansion rate as the universe expands. When the universe was smaller and more dense, it therefore follows that the expansion rate was much larger than it is today. Indeed, as we extrapolate the universe further back in time, we reach a point where the density, temperature, and expansion rate were all infinitely large. This point is a singularity, which we refer to as the Big Bang (although that term is also used for the entire cosmological model that includes the later universe as well). At the Big Bang, our knowledge of what happens gives out; the fact that physical quantities become infinite is a sign that we don't know what is going on. Presumably, in the real world there is no singularity; instead, something happens that cannot be described by physics as we currently understand it.
|2. Large-Scale Structure and Clusters of Galaxies
The visible large-scale structure of the universe is sponge-like, with galaxy superclusters arranged into enormous filaments and sheets that are separated by giant voids where very few if any galaxies reside.
Caption: A supercomputer simulation of the distribution of matter in the Universe produced by cosmologists at the University of Durham,.
Among the nearest components of the cosmic large-scale structure are the Local Supercluster, the Great Wall, the Great Attractor, and the Shapley Concentration. In addition, observations by the Chandra X-ray Observatory have revealed part of an intergalactic web of hot gas and dark matter that is crucial in defining the cosmic landscape. The hot gas alone, which appears to lie like a fog in channels carved by rivers of gravity, is more massive than all the stars in the universe. Its detection may eventually enable astronomers to map the distribution of dark matter.
|3. Final Examination
||Final Exam|| Final Examination Key Knowledge
||Key Knowledge|| Final Examination Solutions
|4. Gravitational Lensing and Dark Matter
A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general theory of relativity. Although Orest Chwolson is credited as being the first to discuss the effect in print in 1924, the effect is more commonly associated with Einstein, who published a more famous article on the subject in 1936. Fritz Zwicky posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this effect was confirmed by observation of the so-called "Twin QSO" SBS 0957+561.
There are three classes of gravitational lensing:
1. Strong lensing: where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images.
2. Weak lensing: where the distortions of background sources are much smaller and can only be detected by analyzing large numbers of sources to find coherent distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the center of the lens. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys. These weak lensing surveys must carefully avoid a number of important sources of systematic error: the intrinsic shape of galaxies, the tendency of a camera's point spread function to distort the shape of a galaxy and the tendency of atmospheric seeing to distort images must be understood and carefully accounted for. The results of these surveys are important for cosmological parameter estimation, to better understand and improve upon the Lambda-CDM model, and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on Dark Energy.
3. Microlensing: where no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the Milky Way in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant quasar. The effect is small, such that (in the case of strong lensing) even a galaxy with a mass more than 100 billion times that of the sun will produce multiple images separated by only a few arcseconds. Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of megaparsecs away from our Galaxy.
|not treated||Lecture Notes: Part VI
|LN: Part VI|
|End of Lectures on Cosmology|