6 May 2011

** FINAL EXAM 8AM Wednesday 11 May 2011- MUST have an OU ID **

Answer key for 3rd exam, reading for last part of course and "most missed questions" from midterms are under link "Stuff for Final".

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Today I will talk about Cosmology- the study of the origin and evolution of the entire universe- all the matter and energy there is.

Two "simple to ask" but "hard to answer" questions are: How big is the Universe? and How old is the Universe?

Olbers paradox IF the Universe were infinitely big and also infinitely old, then the sky would not be dark at night, as we would see a star in every possible direction, like you see a tree trunk in every possible direction if you are in a large forest. This "paradox" is often called "Olbers paradox". It is not really a paradox, as it is "solved" by the simple fact that the Universe in not infinitely old- it had a definite beginning about 14 billion years ago.

There are various ways of measuring the age. The easiest to understand is the Hubble Law. If a distant galaxy is a certain distance away, and it is moving away at a certain speed (from the Doppler shift), then we can "run the Universe movie backward" and claculate when all the galaxies were in one place.

Large Hadron Collider Physicists use particle accelerators to speed subatomic particles to speeds very close to the speed of light. This is a map showing the large hadron collider (LHC) on the border between France and Switzerland. The main ring is a tunnel 17 miles in circumference buried betneath the ground. In this tunnel, particles are accelerated and collided. By smashing these particles together, they can learn about the behavior of matter and energy at VERY high temperatures. These studies have allowed us to model how the very early universe- which was in a hot dense state called the hot big bang- would behave. (If you want to see something beyond nerdy, and you have 5 minutes to kill, check out the "Large Hadron Rap" on YouTube.)

Only helium made in big bang During the first few seconds after the big bang, the Universe was hot and dense enough for nuclear fusion. However, due to the lack of a stable nuclei with 5 nucleons (protons or nuetrons), elements heavier than helium were NOT made. As the Universe was expanding and cooling rapidly, fusion soon stopped, with about 1/4 of the mass of the Universe turned into helium, while the rest remained as protons or hydrogen nuclei. (In stars, heavier elements can be "cooked" as there is much more time for fusion to occur, and Nature has various "tricks" to jump the 5 nucleon gap, tricks which require lots of time.

Wherever we look in the universe, we see stars with roughly 1/4 helium and 3/4 hydrogen, which some heavier elements. Some helium is "cooked" in stars, but no where near enough to account for the helium seen. The helium must come from a source other than stars, and the hot big bang provides a natural explanation for the helium abundance we see.

The Cosmic Microwave Background Scientists realized in the early 1960s that if the Universe had gone through a hot dense phase, we might be able to see the "leftover glow" of that era. The photons that had been around then would now be very low energy photons, as the expansion of the Universe would stretch them. At the same time, two scientists from ATT (The phone company!) were trying to find the source of "static" or "hiss" that was heard on transoceanic phone calls sent through newly invented communications satellites. These two blokes, Penzias and Wilson, discovered the "hiss" came from everywhere in the sky. This "hiss" turned out to be the leftover glow of the hot big bang!! Penzias and Wilson were awarded the 1978 Nobel prize in physics for their discovery.

More recent observations using satellite telescope (to get above the "glow" of the Earths atmosphere) have revealed the leftover glow in great detail. The leftover glow is usually called the CMB - cosmic microwave background. The peak in energy of the photons of the CMB is in the microwave (short radio) part of the EMR spectrum.

Lookback time As we look to more and more distant parts of the Universe, we are also LOOKING BACK IN TIME, as the light we see originated IN THE PAST, as it has taken time to get to us. This is called the "lookback time". Thinking about the Universe and how we see different parts of it at different age is a sure-fire way to get a headache!

Although this "time machine" nature of light is very useful in studying how the Universe has changed with time, it is a very frustrating , selective time machine. For example, we can *NOT* see what the Milky Way looked like in the past, as the light that left the Milky Way billions of years ago is now billions of light years away!!

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Final image- Earthrise If I had to pick one single image to show in this course, this would be it. It was taken by a human with a camera who was in a spaceship orbiting the Moon. The picture contrasts the dry, airless, lifeless Moon with the beautiful water world of our birth and death. Earth- love it and take care of it cause we and generations to follow ain't leaving it!

For those of you who care about more than whats on the next exam, I hope you have learned something from this introduction to the Universe- in the words of T. S. Eliot:

We shall not cease from exploration

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time

There is another famous picture of Earth and some words worth reading in the "Afterwords" in the book (p. 473-474). Some background information on these photographs can be found by looking at "Earthrise" and "Pale blue dot" on wikipedia and references therin.