WHAT IS COSMOLOGY?

Cosmology is the study of the origin and evolution of the Universe as a whole. The study of the observable features of the Universe, really the constituents of the Universe, is known as Cosmogony.

Cosmology is an interesting science in many respects. A particularly fascinating thing (to me) is that Cosmology is a science where short-term progress is sometimes driven by belief and/or philosphical notions rather than by empirical data. This, however, is not to say that one can invents theories which are not falsifiable (i.e., not testable-- therefore unscientific) but, rather, sometimes one chooses to explore and develop ideas about the Universe that are driven by aesthetics. This becomes less true as time passes, however, as our technology develops and we learn more and more things (facts) about the Universe.

Before we go on, let's make an observation about Universe versus universe. Why do I use a capital "U" rather than a lower case "u"? Well, because we are speaking about our Universe, and there is only one of our Universe, we use the capital "U". When we speak of universes in general we use the lower case "u." It is a little unfortunate that we know of only one universe because we are then never sure if we are exceptional or simply one of some large gang of universes. This, however, cannot be helped and we must play the hand we are dealt.

People view the Universe from different perspectives:

• religious viewpoint --> realm of spirit, a theistic creation
• artist, poets, ... --> a series of forms as felt by sensitive practioners
• philosophers --> a thing of analytic and synthetic structures disciplined by logic
• scientific viewpoint --> a body of intricate structures, elucidated by theory disciplined by observation and experiment

So What is the Universe? Which view is the correct view of the Universe? In a sense, all have something to offer; some contribute to our understanding of the Universe and others contribute to our appreciation of the Universe. Consequently, we must define what we hope to accomplish in this course to set the ground rules. In this class, we discuss,

Scientific Models of the Universe

We do not necessarily discuss the true nature of things, rather, we construct models that reproduce the observable actions and behavior of the Universe and, more importantly, models that can predict phenomena that have not yet been observed, but phenomena amenable to experiment which may support or disprove the prediction, i.e., they are testable. A tweak on this idea of testability concerns whether it is currently testable or whether it will be testable in the reasonably near future. This last point is crucial because this is what makes models falsifiable. It is the possibility that a model can be proven false that plays a large role in whether we call a model scientific or not.

OBSERVATIONAL FACTS

We do not know many things about the Universe, but with the development of Observational Cosmology, there are important things which we do know:

• the Universe is expanding (Hubble Law)
  • the Universe is flat (shape [topology]) which raises a host of problems (e.g., the flatness problem).
• the Universe is bathed in a pervasive and uniform sea of radiation, The CMBR is nearly the same in whatever direction we look; this gives rise to something known as the Horizon Problem.
• the chemical composition of the Universe is ~ 90 % hydrogen and ~ 10 % helium and tiny amounts of everything else ( which is a strong prediction of the Big Bang Theory).
• Observation of Structure (galaxy clustering, mass distribution, galaxy formation, stars, ...)
• ....

Today, we interpret the above observations within the context of the Big Bang Theory.

Let us now start to investigate some ideas (principles) that have guided how we view the Universe.

Cosmological Principles
• Copernican revolution and its Implication
• Cosmological Principle
• Perfect Cosmological Principle
• Anthropic Principle
Scales of the Universe
• Size scales of the Universe
• Timeline of the Universe; detailed timeline, simpler timeline