COSMOLOGY:
Cosmology is the study of the structure and evolution of the Universe. We discuss the Universe in terms of the Big Bang Theory , a theory which describes nicely how the Universe has evolved from its birth to its present ago and the possible ways in which it will evolve in the distant future.

Cosmology is an interesting science in many respects. A particularly fascinating thing is that Cosmology is a science for which, in the past, progress was driven by belief as well as by empirical data. Today, the situation is completely different. Modern cosmological thinking is driven primarily by empirical data, the quality and quantity of which improves day-by-day. In Astronomy 123: Cosmology, we review the current state of our understanding of the Universe and our place in the Universe through consideration of theoretical models and the observational facts that underlie and drive the modeling.
Although, on occasion, theoretical progress was driven by philosophical concepts, we do not imply that the theories which were invented were unfalsifiable (not testable) by data and therefore unscientific. We mean that sometimes workers chose to explore and develop ideas about the Universe driven by philosophy but that these notions had to lead to predictions amenable to experiment and thus testable (falsifiable) within some reasonable time frame.
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 of our Universe, and there is only one of our Universe. Consequetly, we use the capital U. If we spoke of universes in general we would then use lower case "u" in our descriptions. It is unfortunate that we know of only one universe because we are never sure if our Universe is exceptional or if it is just one of the gang. This, however, cannot be helped and we must play the hand we have been dealt.

Scientific Models of the Universe

We do not discuss the true nature of things in a sense, rather, we construct models which can reproduce the actions and behavior of the Universe and, more importantly, models which lead to predictions of phenomena not yet observed. This last point is crucial for this is what makes a model scientific (as it makes it falsifiable).

Observational Facts
We do not know a ton of things about the Universe (such as we do about the Solar System, for example), but there are several important things which we do know. We will expand greatly on each bit of information in the next few weeks.

the Universe is expanding
the Universe is bathed in a pervasive and uniform sea of radiation, the Cosmic Micro-wave Background Radiation (CMBR)
the chemical composition of the Universe is ~ 90 % hydrogen and ~ 10 % helium and tiny amounts of everything else
spatial and temporal structure is observed in the Universe

BIG BANG
Today, we interpret these observations (and some further ones) within the context of what is known as the Big Bang Theory. We will say much more on the Big Bang later. However, note that the theory asserts only that the Universe in past was much smaller and denser and has been expanding since that time. These notions are amply supported by the current observations. Most of the current work improves on the details of the model, not its general validity. We will next investigate some ideas (principles) which have guide (or guided) how we view the Universe.

Cosmological Principles

Copernican Revolution: Principle of Mediocrity
Cosmological Principle
Perfect Cosmological Principle
Anthropic Principle

Scales of the Universe

Size scales of the Universe
Comment: We use two different length scales in astronomy when dealing with cosmology. The scales are the light year, the distance a beam of light travels in 1 year or
1 light year = 9.5 trillion kilometers ~ 5.9 trillion miles.
The other length scale used is the parallax second or more commonly the parsec which is
1 parsec = 3.08x10¹³ kilometers ~ 3.24 light years

Time line of the Universe

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