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Abstract: In this article I argue that current research indicates that an isothermal model of galactic expansion appears to tentatively confirm empirical evidence. I discuss fundamental cosmological principles which mass accreting theories of galaxy formation are derived from. I discuss earlier research which has established the framework. I also highlight ongoing problems that face us in developing a working model of galaxy formation and accretion in the early universe.

Introduction: Big bang – origin of the universe and the precursor to galactic originationExplaining the process of galaxy formation is a tenuous hypothetical endeavor. We do not know the initial conditions in the primordial Universe with enough accuracy to reconstruct the process with scientific accuracy.

The big bang, which is current scientific consensus for the origin of the universe, confines all matter and energy at the beginning of the universe within an infinitely small space, homogenously distributed. After staggeringly short and energetic periods of time in which subatomic and elementary particles formed, and energetic forces became distinct from each other, we reach a period where the universe is beginning to resemble its present self.

This point has been described as the opaque era of the universe, when light and matter were intertwined. Photons of light collided with free protons (hydrogen ions), neutrons, electrons and helium nuclei, trapping the light in a thick particle plasma. After about 300,000 years of expansion, following the big bang, the universe had cooled enough to allow atoms of hydrogen and helium and trace elements to form, in an event called recombination. As these primordial atoms started to combine, photons that were trapped in the plasma were liberated, and the universe became transparent to light. The process that produced this blast of free energy is known as photon decoupling, and this period of time is known as the decoupling epoch. From this decoupling of matter and energy the cosmic microwave background as we see it today was created. This period of time is important as it marks the boundary to the cosmic dark age. Unfortunately, periods before the decoupling are invisible to us, and are the reason why scientific experiments which replicate the conditions of this era are important in our understanding of these processes.

We are now in the era of galaxies, where the trend in mass accumulation has increased to the scale of galaxy formation.

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A ripple on a flat surface is an area which has mass that breaks the uniformity of the background pattern. It is believed that there were ripples in this primordial superdense plasma, of a few parts per million. Although tiny, these ripples would have implications as the Universe expanded, and their scale was magnified exponentially in tandem with the growth of the matter cloud. These fluctuations in gas may not have been the only ripples, some propose that at dark matter may have also began to form clumps at the same time, or before ordinary matter. Because of it’s gravitational influence, it may have had effects on the normal matter around it, possibly disrupting the ripples in conventional matter.(Atwood 2006)

Fig. 1. Spectrum of the Cosmic Microwave Background Radiation as measured by the COBE satellite. Within the quoted errors, the spectrum is precisely that of a perfect black-body at radiation temperature T = 2.728 ± 0.002 K(Fixsen & Mather 2002) Although we are unable to ‘see’ it, this is the footprint for our primordial fireball and strong evidence for the hot big bang hypothesis.

Observational evidence from a variety of sources currently points to a universe which is (at least approximately) spatially flat. We happen to live in that brief era, cosmologically speaking, when both matter and vacuum are of comparable magnitude. At early times, the cosmological constant would have been negligible, while at later times the density of matter will be essentially zero and the universe will be empty.(Sean M. Carroll 2001)

Today the visible universe is highly inhomogeneous, with mass unevenly distributed between denser areas of isolated galaxies, denser still areas of galactic superclusters and giant voids of intergalactic space. As we progress to larger and larger scales, the distribution of galaxies becomes smoother, but still contains significant non-random features(Longair 2008)

It is within the context of this big bang hypothesis that most modern galactic formation theories exist.

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Dark Matter – Cosmic Scaffolding for Building Galaxies

Fig 2.This is a 3-D map of dark matter. Clumping of the dark matter is more pronounced in the more recent times (left) than in the early universe (right). NASA / ESA / R. Massey

Dark matter (DM) takes its name from the idea that 5/6ths of matter in the universe is invisible to us; ‘dark’, though it’s gravitational effects can be measured on luminous matter, such as that contained in galaxies(Dekel 1995). DM appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light. However, interacts via gravity, and has been observed through the gravitational lensing it creates (Kitching et al. 2010)

Dark matter is essential to fill missing values in astrophysical models, such as the cosmological constant, in which a figure with a positive energy density would drive an accelerating expansion of empty space. DM was originally hypothesized to explain the abnormally high rotation speeds of galaxies, which would otherwise be torn apart if they did not contain hidden mass. We do not yet have a definitive model of how galaxies form. Many observational barriers have been overcome in the last few years; and it is now possible to observe galaxies over >90% of the age of the universe. This is due to technological advancements such as new telescopes, instruments, and techniques. (Steidel 1999) Most theories about the early universe make two assumptions; it was filled with hydrogen and helium, and that some areas were slightly denser than others. How this larger, more homogenous field of gas resulted in the formation of galaxies is an area where astrophysicists differ.

In our region of mass overdensity (an area where localised mass is accumulating to densities greater than the cosmic mean), eventual

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decoupling from the Hubble flow [the normal speed of universal expansion] results in a turnaround point. At this point the mass in our localized region is sufficient to cause gas to fall inwards and join the growing overdensity(Del Popolo 2002).

Broadly speaking there are two scenarios, which address the mechanism of mass overdensity.

Top Down [Adiabatic]In this theory, the first objects to have separated from the homogonous background, and gravitationally bound to themselves would have had masses at the level of 1015 M☉. They would be irregular or flattened, resembling cosmic pancakes.This scenario supports the classical formation theory of Eggen, Lyden Bell and Sandage, who hypothesized that the Milky Way resulted from the dynamic collapse of a large glass cloud. As the cloud collapsed into a superdense centre of mass, the gas surrounding it would begin to spin up into a rapidly rotating disk. (Eggen et al. 1962)

Top down sequences would be expected if long wavelength ripples in the pregalactic gas cloud carried more power than shorter ones. Theoretically matter would clump on the largest scales first and this effect would be compounded, with a gravitational bias towards the larger wavelengths as they accumulated more and more mass.

Bottom Up [Isothermal]Contrastingly, if smaller scale fluctuations in the cloud where more important, then the first systems to become gravitationally independent would be smaller. This theory was developed by Searle and Zinn. They hypothesized that the galactic formation occurs through a process of accretion (Searle & Zinn 1978).These smaller, denser areas would over time combine together in a process known as hierarchical galaxy formation. It would not be unlike gravitational galactic interactions we observe today, in which galaxies appear to be in the process of merging or distorting their mutual structure. It has been suggested that tidal interactions modify galactic structures, and can contribute to either a deformation of a galaxy structure, or if interaction is prolonged, a full scale merger between them (Alladin & Narasimhan 1982). Galactic cannibalism is the process in which a more massive galaxy assimilates smaller galaxies on it’s periphery. The redshifts of many galaxies within the local group show evidence of a virgocentric flow. This speed of this flow (the actual difference between the Virgo cluster peculiar velocity and the peculiar velocity of the Local Group (LG) in the direction of Virgo) is estimated to be 220 km/s (Courteau 2000)

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Fig 3 Velocity vectors from LG and Virgo. The Virgo infall velocity ( Vinfall ) is vectorially subtracted from the LG’s MBR (microwave background radiation) motion.

Signs of similar flow have also been detected in the redshift distance relation for galaxies proximate to similar massive objects in clusters outside the local group (Allan Sandage 1999).

This presents strong evidence that gravitational attraction between galaxies is responsible for changes in universal mass distribution, and supports an isothermal system of mass accretion.

ConclusionIn my personal opinion, some derivative of the bottom up hypothesis of galaxy formation is most credible at this time. I say this as there is ample observational evidence of hierarchical clustering. However, our knowledge in this area is built on shaky foundations.

There is ongoing argument about the fundamental mechanics of gas cloud behavior, regarding the mass problem, with conflicting explanations coming from the Lambda Cold Dark Matter hypothesis and modified gravitational theories such as Modified Newtonian Cosmology, proposed by Milgrom(Milgrom 1983), and Moffatt’s MOG(Moffat 2006). The relation between peculiar velocities and the correlation function of galaxies points to the possibility that galaxies do not form uniformly everywhere(Szalay 1985). None of the present theories of galaxy formation can account for all facts in a natural way,

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and often conflict. It is probable that there is more than one mechanism at work in galaxy formation.

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