1. Jack Oughton / 07023367
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 origination
Explaining 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.

2. Jack Oughton / 07023367
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)

3. Jack Oughton / 07023367
It is within the context of this big bang hypothesis that most modern
galactic formation theories exist.
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

4. Jack Oughton / 07023367
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
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 &

5. Jack Oughton / 07023367
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)
Fig 3 Velocity vectors from LG and Virgo. The Virgo infall velocity (
Vi
nfall ) 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.
Conclusion
In 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

6. Jack Oughton / 07023367
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, and often conflict. It is probable that there is more
than one mechanism at work in galaxy formation.
Alladin, S.M. & Narasimhan, K.S.V.S., 1982. Gravitational interactions
between galaxies. Physics Reports, 92(6), 339-397.
Atwood, W., 2006. Prospects for observing dark-matter remnants with
GLAST. Advances in Space Research, 37(10), 1862-1867.
Courteau, S., 2000. Cosmic flows 1999 : towards an understanding of
large-scale structure : proceedings of a conference held on the
Campus of the University of Victoria, on the Island of Vancouver,
British Columbia,, San Francisco Calif.: Astronomical Society of
the Pacific.
Dekel, A., 1995. Dark Matter from Cosmic Flows: How Much? Where?
What is it? Nuclear Physics B - Proceedings Supplements, 38(1-3),
425-434.
Del Popolo, A., 2002. On the evolution of aspherical perturbations in the
universe:
An analytical model. Astronomy and Astrophysics, 387(3), 759-777.
Eggen, O.J., Lynden-Bell, D. & Sandage, A.R., 1962. Evidence from the
motions of old stars that the Galaxy collapsed. The Astrophysical
Journal, 136, 748.
Fixsen, D.J. & Mather, J.C., 2002. The Spectral Results of the Far‐
Infrared Absolute Spectrophotometer Instrument on COBE. The
Astrophysical Journal, 581(2), 817-822.
Kitching, T., Massey, R. & Richard, J., 2010. Title: The dark matter of
gravitational lensing. arxiv.org. Available at:
http://arxiv.org/abs/1001.1739.
Longair, M.S., 2008. Galaxy Formation (Astronomy and Astrophysics
Library) Second Edition., Springer Berlin / Heidelberg.

7. Jack Oughton / 07023367
Milgrom, M., 1983. A modification of the Newtonian dynamics as a
possible alternative to the hidden mass hypothesis. The
Astrophysical Journal, 270, 365.
Moffat, J.W., 2006. Scalar– tensor– vector gravity theory. Journal of
Cosmology and Astroparticle Physics, 2006(03), 004-004.
Sandage, A., 1999. Bias Properties of Extragalactic Distance Indicators.
VII. Correlation of Absolute Luminosity and Rotational Velocity for
S[CSC]c[/CSC] Galaxies over the Range of Luminosity Class from I
to III– IV. The Astronomical Journal, 117(1), 157-166.
Sean M. Carroll, 2001. The Cosmological Constant. Living Reviews in
Relativity, 4.
Searle, L. & Zinn, R., 1978. Compositions of halo clusters and the
formation of the galactic halo. The Astrophysical Journal, 225, 357.
Steidel, C.C., 1999. Observing the epoch of galaxy formation. Proceedings
of the National Academy of Sciences of the United States of
America, 96(8), 4232-4235.
Szalay, A., 1985. Formation of galaxies. Nuclear Physics B, 252, 113-126.