Wednesday, 8 July 2009

Galaxy Evolution

Galaxy Evolution:
The phenomenon of lookback time allows us to actually observe the evolution of
galaxies. We are not seeing the same galaxies as today, but it is possible to trace
the behavior of galaxies types with distance/time.
It is known that galaxies form from large clouds of gas in the early Universe. The
gas collects under self-gravity and, at some point, the gas fragments into star
cluster sized elements where star formation begins. Thus, we have the expectation
that distant galaxies (i.e. younger galaxies) will be undergoing large amounts of
star formation and producing hot stars = blue stars. The study of this phenomenon
is called color evolution.

Computer simulations also indicate that the epoch right after galaxy formation is a
time filled with many encounters/collisions between young galaxies. Galaxies that
pass near each other can be captured in their mutual self-gravity and merge into a
new galaxy. Note that this is unlike cars, which after collisions are not new types of
cars, because galaxies are composed of many individual stars, not solid pieces of
matter. The evolution of galaxies by mergers and collisions is called number

Thus, our picture of galaxy evolution, incorporating both these principles, looks
like the following:

Some types of galaxies are still forming stars at the present epoch (e.g. spiral and
irregular galaxies). However, the past was marked by a much higher rate of star
formation than the present-day average rate because there was more gas clouds in
the past. Galaxies, themselves, were built in the past from high, initial rates of star
The time of quasars is also during the time of first star formation in galaxies, so the
two phenomenon are related, the past was a time of rapid change and violent
activity in galaxies.
Space observations called the Hubble Deep Field produced images of faint galaxies
and distant galaxies at high redshift which confirmed, quantitatively, our estimates
of the style and amount of star formation. Nature lends a hand by providing images
of distant galaxies by gravitational lensing, as seen in this HST image of CL0024.
Interestingly enough, it is often easier to simulate the evolution of galaxies in a
computer, then use the simulations to solve for various cosmological constants,
such as Hubble's constant or the geometry of the Universe. The field of
extragalactic studies is just such a process of iteration on the fundamental constants
of the Universe and the behavior of galaxies with time (i.e. galaxy evolution).
Creation Event:
The debate about the origin of the Universe presupposes that there was an origin.
Instead of a beginning, the Universe may be experiencing an endless number of
cycles. Ancient Chinese believed that all events formed a periodic pattern driven
by two basic forces, Yin and Yang.

The Hindu cosmological system consisted of cycles within cycles of immense
duration (one lifecycle of Brahma is 311 trillion years). Cyclicity cosmologies, and
their associated fatalism, is also found in Babylonian, Egyptian and Mayan
Judeo-Christian tradition was unique in its belief that God created the Universe at
some specfic moment in the past, and that events form an unfolding unidirectional
sequence. Key to this philosophy is that the Creator is entirely separate from and
independent of His creation. God brings order to a primordal chaos.

Belief that a divine being starts the Universe then `sits back' and watchs events
unfold, taking no direct part in affairs, is known as deism. Here God is considered a
cosmic engineer. In contrast, theism is the belief in a God who is creator of the
Universe and who also remains directly involved in the day-to-day running of the
world, especially the affairs of human beings. God maintains a personal and
guiding role. In both deism and theism, God is regarded as wholly other than, and
beyond, the physical Universe. In pantheism, no such separation is made between
God and the physical Universe. God is identified with Nature itself: everything is a
part of God and God is in everything.
A Creation event implies that everything came from nothing (creation ex nihilo)
since if there were something before Creation, than an earlier Creation is needed to
explain that something. God existed before Creation, and the definition is not
limited to work with pre-existing matter or pre-existing physical laws either. In
fact, the most obvious distinction between the Creator and the created Universe is
that the Creator is eternal and the created Universe had a beginning.
Hot Big Bang:
The discovery of an expanding Universe implies the obvious, that the Universe
must have had an initial starting point, an alpha point or Creation. In other words,
there existed a point in the past when the radius of the Universe was zero. Since all
the matter in the Universe must have been condensed in a small region, along with
all its energy, this moment of Creation is referred to as the Big Bang.
A common question that is asked when considering a Creation point in time is
``What is before the Big Bang?''. This type is question is meaningless or without
context since time was created with the Big Bang. It is similar to asking ``What is
north of the North Pole?''. The question itself can not be phrased in a meaningful
The Big Bang theory has been supported by numerous observations and, regardless
of the details in our final theories of the Universe, remains the core element to our
understanding of the past. Note that an alpha point automatically implies two
things: 1) the Universe has a finite age (about 15 billion years) and 2) the Universe
has a finite size (its expanding at a finite speed in a finite time).

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