Physical cosmology

Physical cosmology, as a branch of astronomy, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. Cosmology involves itself with studying the motions of the celestial bodies and the first cause. For most of human history, it has been a branch of metaphysics. Cosmology as a science originates with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on earth, and Newtonian mechanics, which first allowed us to understand those motions. This is now called celestial mechanics. Physical cosmology, as it is now understood, began with the twentieth century development of Albert Einstein's theory of general relativity and better astronomical observations of extremely distant objects.

The twentieth century advances made it possible to speculate about the origins of the universe and allowed scientists to establish the Big Bang as the leading cosmological theory, which most cosmologists now accept as the basis for their theory and observations. Vanishingly few researchers still advocate any of a handful of alternative cosmologies, but professional cosmologists generally agree that the big bang best explains observations. Physical cosmology, roughly speaking, deals with the very largest objects in the universe (galaxies, clusters and superclusters), the very earliest distinct objects to form (quasars) and the very early universe, when it was nearly homogeneous (hot big bang, cosmic inflation and the cosmic microwave background radiation).

Cosmology is unusual in physics for drawing heavily on the work of particle physicists' experiments, and research into phenomenology and even string theory; from astrophysicists; from general relativity research; and from plasma physics. Thus, cosmology unites the physics of the largest structures in the universe to the physics of the smallest structures in the universe.

Fundamental questions in astrophysisc

In astrophysics, the questions of galaxy formation and evolution are:

* How, from a homogeneous universe, did we obtain the very heterogeneous one we live in?
* How did galaxies form?
* How do galaxies change over time?

After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand.

The most accepted view today is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, hierarchically forming superclusters, clusters, galaxies, star clusters and stars. One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe. Hence the distribution of galaxies is closely related to the physics of the early universe.

The observed components of galaxies (including our own Milky Way) that must be explained in, or at least not be at odds with, a theory of galactic evolution, include:

* the stellar disk is quite thin, dense, and rotates
* the stellar halo is very large, sparse, and does not rotate (or has perhaps even a slight retrograde rotation), with no apparent substructure
* halo stars are typically much older and have much lower metallicities than disk stars (there is a correlation, but there is no absolute connection between these data)
* some astronomers have identified an intermediate population of stars, variously called the "metal weak thick disk", the "intermediate population II", et al. If these are indeed a distinct population, they would be described as metal-poor (but not as poor as the halo stars), old (but not as old as the halo stars), and orbiting very near the disk, in a sort of "puffed-up", thicker disk shape.
* globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods)!
* in each globular cluster, all the stars were born at virtually the same time (except for a few globulars that show multiple epochs of star formation)
* globular clusters with smaller orbits (closer to the galactic center) have orbits which are somewhat flatter (less inclined to the disk), and less eccentric (more circular), while those further out have orbits in all inclinations, and tend to be more eccentric.
* High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).

On the 11th July 2007, using the 10 metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasedena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old [1].

Recent research as a part of the Galactic Zoo project suggests that there is an unexplained parity violation, with a greater proportion of the galaxies rotating in an anticlockwise direction when seen from the Earth[2].

Fundamental questions

In astrophysics, the questions of galaxy formation and evolution are:

* How, from a homogeneous universe, did we obtain the very heterogeneous one we live in?
* How did galaxies form?
* How do galaxies change over time?

After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand.

The most accepted view today is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, hierarchically forming superclusters, clusters, galaxies, star clusters and stars. One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe. Hence the distribution of galaxies is closely related to the physics of the early universe.

The observed components of galaxies (including our own Milky Way) that must be explained in, or at least not be at odds with, a theory of galactic evolution, include:

* the stellar disk is quite thin, dense, and rotates
* the stellar halo is very large, sparse, and does not rotate (or has perhaps even a slight retrograde rotation), with no apparent substructure
* halo stars are typically much older and have much lower metallicities than disk stars (there is a correlation, but there is no absolute connection between these data)
* some astronomers have identified an intermediate population of stars, variously called the "metal weak thick disk", the "intermediate population II", et al. If these are indeed a distinct population, they would be described as metal-poor (but not as poor as the halo stars), old (but not as old as the halo stars), and orbiting very near the disk, in a sort of "puffed-up", thicker disk shape.
* globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods)!
* in each globular cluster, all the stars were born at virtually the same time (except for a few globulars that show multiple epochs of star formation)
* globular clusters with smaller orbits (closer to the galactic center) have orbits which are somewhat flatter (less inclined to the disk), and less eccentric (more circular), while those further out have orbits in all inclinations, and tend to be more eccentric.
* High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).

On the 11th July 2007, using the 10 metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasedena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old [1].

Recent research as a part of the Galactic Zoo project suggests that there is an unexplained parity violation, with a greater proportion of the galaxies rotating in an anticlockwise direction when seen from the Earth[2].