Overview on Physical Cosmology
Lead: Eberhard Möbius

1/29/2014

Summary by Daniel Martin

Everything out of Nothing? - Sense of Place and Paradigm Shifts

The first lecture was given by Professor Eberhard Mӧbius on physical cosmology.  It mainly focused on the first of the three “unicates”: the universe, life, and consciousness.  Various aspects of the universe were discussed including how it is currently believed to have formed – the Big Bang Theory, the history of cosmological knowledge, and limits of what we as humans can know given our place in the universe.  We also discussed the physical constraints placed upon us by the way physics works.  Cosmology may be the only unicate that stays with us forever – that is, while biological life and consciousness may be found elsewhere in our universe, we will never be able to directly observe another universe.

The limitation of being confined to one universe was the first topic discussed.  Professor Mӧbius told the class he wondered about the limits of the universe, if there are any, when he was a child.  He brought attention to mankind’s inability to escape our universe and look at it from the outside.  Everything we know and do is confined to our current universe, and whether or not there are other universes is a question that is likely never to be answered.  While we can observe and compare events on different planets, galaxies, etc. we only have one universe.

Then Olbers’ Paradox was presented.  Olbers’ Paradox states that if the universe is infinite and eternal, the entire night sky should be full of light, not only filled with a few stars.  Why is the night sky black?  This concept was compared to a forest of trees.  As one looks out through a forest, trees close up take up a portion of one’s field of view.  Then, each tree a medium distance away takes up less space, but there are a larger number of trees, so they take up the same percentage of one’s field of vision as the closer trees.  This proportion stays the same as trees become farther away but more numerous, ultimately filling a person’s entire field of view.  If the universe is infinite, theoretically the entire night sky should be filled with light just as a view of the forest is entirely trees.  Furthermore, as far as we know, all galaxies are evenly distributed/in the same density in the universe, just as trees are evenly distributed in a forest.  One assumption this theory makes, however, is that there has been adequate time for light from every star to reach us.

The lecture then moved to distances in the universe.  The distance to other celestial bodies is measured using parallax.  This uses the difference in how an object is seen from two different points to measure the object’s distance.  Using this we have been able to discover that the Earth is not in the center of the universe by the change in distance between Earth and celestial bodies.  However, the Earth is in the center of the observable universe. The time it takes for light to reach earth from other galaxies can be incredibly large.  For example, light from the star α-Centauri takes 4.3 years to reach Earth.  Anything we observe in α-Centauri actually happened 4.3 years ago.

In the 1920’s, Edwin Hubble found the light emitted from galaxies that are smaller and farther away was shifted slightly to the right (they were slightly redder than normal).  This is called the red shift, which occurs because the wavelength of light is slightly elongated as the source moves away from the observer, making the source appear redder (red has the longest wavelength of visible light).  As space is stretched, the waves of light are also stretched.  A comparison was made to drawing a wave on a rubber band and then stretching it, elongating the wave.  The red shift shows that the universe is expanding in all directions, and that everything is getting farther away from everything else.  More recently, in the past 15 years, it has also been shown that the expansion is increasing in speed.  An analogy was made between the universe’s expansion and bread with raisins in it.  As bread with raisins in it rises in the oven, all the raisins get farther from each other.  Also, things that are farther move faster in relation to the point of observation.  This applies no matter which raisin one observes from.  This gave rise to a key cosmological principle: the universe is the same everywhere.  There is nothing special about where the Milky Way and the Earth are in the universe.  The most distant galaxies are moving away at a speed approaching that of light, which indicates the existence of more galaxies beyond the observable universe.  This leads to two possible solutions to Olbers’ Paradox: we cannot see galaxies that are beyond our observable universe and/or we cannot see galaxies whose light needs longer to reach us than the age of the universe.  If one takes the distance between galaxies and divides it by their speed, we find the time it took for the galaxies to reach their current distance.  By calculating this we find the age of the universe, meaning it must have a beginning.

From this, it was calculated that at 300,000 years after the big bang, the universe was 3000K and had radiation like a red star.  The background radiation from this has been observed at the value predicted by calculations.  Furthermore, the ratio of helium to hydrogen in stars and interstellar material is also at the predicted value.

However, there are still problems with the Big Bang Model.  While these problems do not contradict the Big Bang, they remain unexplained.

• The Flatness or Fine-tuning problem.  The universe is finely tuned, on the verge of collapsing on itself or expanding forever.  According to the current model, certain parameters of the universe had to be accurate within 10-15 after 3 minutes of existence, in order for our universe to exist now as it does. 
• The Horizon Problem.  The universe is the same in opposite directions, but no communication has been possible at the speed of light from one end to the other up to the time the observable background radiation was released.  Different regions of the universe have the same temperature and other properties, while being too far away to have influence on each other. 
• The Matter Problem.  Two pounds of photons would transform into one pound matter and one pound anti-matter and vice versa.  However, we now have an abundance of matter and a deficiency of anti-matter.

An inflation model offers an explanation for these observations.  The inflation model describes a very rapid exponential expansion of the universe in less than the first second after the big bang. 

• The Flatness or Fine-tuning problem.  After an enormous amount of inflation, the surface of a round object appears flat.  For example, the earth’s surface may appear flat in certain places, despite being a globe.  Similarly, this may apply to the universe.
• The Horizon Problem.  Rapid inflation during the beginning of the universe may lead to the uniform nature of the observable universe.
• The Matter Problem.  Here a question still remains: why is any matter left at all?  The presence of matter without anti-matter shows a break in symmetry.  This is thought to be a result of rapid cooling, leading to a transition to a state with less symmetry, similar to transitioning from smooth water to chunky ice.

In order for our universe to exist as it does require fine-tuning, as seen in the inflation model.  If there were enough gravity to cause the universe to collapse every few 100 million years, there would be no heavy elements, no planets, and no human beings.  If the gravitational force were 2% stronger or weaker than the nuclear force, in the first 100 seconds either all hydrogen would have turned into helium, and we would not exist, or hydrogen burning into helium observed in normal stars would not be possible and hence there would be no stars like the sun and no human race.

The lecture concluded with a discussion of the Anthropic Principle.  Our universe is perfect for the development of intelligent life, and there have been many theories over time of how it came to be.  Presently our best answer is the inflation model.  However, this still leaves many things unexplained, including how inflation itself came.  There are two possible answers for that question. 
1. The strong anthropic principle: the universe was set up to provide for life. 
2. The weak anthropic principle: there are many universes and we live in one of the few capable of supporting life.

Finally, what happened before the big bang?  Although the big bang and inflation models provide an explanation of how our universe formed, it does not provide an explanation for existence itself.