Physical Cosmology, Lead: Eberhard Möbius

2/4/2003

Summary by Conor Horigan:

Introduction to Physical Cosmology

• How far can we see?
We have found that there are certain limits to how far we can see into the Universe. There exists a “curtain” that we cannot see past and as we get closer and closer to this curtain it seems that things move faster sand faster towards the speed of light. Before approaching this point we simply reach our observable limits. Currently that distance or limit is approximately 20 billion light years away. When we attempt to grasp this distance it is often times quite difficult for many of us. Just thinking about how large our own galaxy is a rather humbling experience. How many tens of thousands of factors things are from one another really helps to put where we are in perspective and to really illustrate how much of a pinpoint we really are.

• The Expanding Universe
As you stand along the side of the road and hear the sound of a car horn as it blasts past you what you are experiencing is the Doppler effect. The frequency of the horn at first is rather high and as the passing car recedes away from you its frequency diminishes. The wavelengths become longer as the car becomes further. Similarly this is the principle that is used to identify and exhibit how the universe we live in is expanding.

During the 1920’s Edwin Hubble developed a method proving that our universe was expanding. By measuring the frequency of light received here on Earth from distant galaxies we have been able, to some extent, to figure out that objects in space are moving away from us. This procedure is accomplished by measuring the spectra or “fingerprint” of certain elements. Light emitted from a moving object will appear to have different wavelengths whether moving towards or away from a relative position in space. Objects, such as galaxies, receding from our relative position in the universe emit a red set of longer waves whereas we would detect a blue set of shorter wavelengths if an object were approaching us. Again similar to the sound frequency of a passing horn, when dealing with light properties we can recognize a change in color and by recognizing this change, or red shift as it is often called, we can calculate the speed at which these objects are moving away from us.

By using this method we also come to the conclusion that the further galaxies are from us the faster they are moving away. This is calculated linearly by relating distance with speed. It has been found that the furthest objects (quasars and pulsars) are moving at 95 to 97 percent the speed of light. Now that we can somewhat grasp at what speed things are receding from us at we can furthermore figure out the approximate location of other galaxies in the universe. We can slowly begin to map it out.

So far technology and time have allowed us to map out small slices of what is our observable universe. For example if you picture the universe as a globe, or the Earth, we have only so far mapped out a portion of it roughly the size of Rhode Island.

After Edwin Hubble provided ways to prove the idea of an expanding universe, by showing that all galaxies seem to move away from us, it was and is easy to assume that Earth was at its center. But once again is this a totally naive idea to accept, an idea similar to that of the astronomers of ancient times who believed that the Sun rotated around the Earth. What one needs to do is visualize the universe in an altogether different manner. Think of the universe similar in composition to that of unbaked raisin bread, one where the raisins are galaxies and the dough is the space between. As the bread is cooked and expands the raisins (galaxies) move farther and further from each other. It is a direct effect of the amount of dough in between them. Lets say that if there was double the amount of dough between two specific raisins than two others they would wove away from each other at a speed double that of the set of raisins with half as much dough separating them. Our universe seems to operate much like this; it can be viewed as an expanding homogeneous mixture of properly aligned elements.

A common question that seems to arise, and one that did in class is the question of what is outside of the universe? What is it that the universe is expanding in to? We know that it isn’t space because space is only defined with matter. How can this be that we are expanding if there is nothing for us to expand into? It seemed that the consensus of the class was that the concept, that the universe creates space it doesn’t move into it, is simply something we must accept.

• Let There Be Light
The universe expands, but into what? The universe creates space, it doesn’t move into it. Space is something that can only be defined by matter. And if this theory of expansion is true what does that mean? If something is expanding is it not expanding outwards from something else and at some point in time a beginning? Perhaps a “Big Bang”, a moment where everything fell into place and rapid expansion ensued, eventually slowing down with time. But what do we have as proof of this occurrence and at this point what would time be anyway? Who would be there to hold a clock during those split seconds, or ages, and if there were a beginning would it not have been so massive and gravitational that the force needed, or escape velocity, to expand, would be tremendous? Dwarfing that velocity needed to escape the Earths gravitational pull for instance. Would this same specific velocity similarly still apply today? Without reaching it wouldn’t everything simply fall back on itself in an incredible “Crunch”?

Possibly, we really cannot say with any great certainty what events took place. Did everything originate out of pure radiation, maybe out of the palm of “God’s hands”? Did the Big Bang actually occur? If so there should still be remnants of its radiation around today. But what was the cooking recipe for the big bang? In the universe today we can find about 25% helium, which is what we would expect to find, but there must be more matter than we can see. What is this matter and furthermore where is it. What is it that is preventing the universe from collapsing on itself.

• Problems with the Bang
So far we have encountered three significant problems facing the theory behind the Big Bang. The first is called the flatness or fine tuning problem. How did the universe beat out the odds? To exist as it does today it has to exist within one order of magnitude. Wow, that’s amazing that it exists upon such a fine line between expansion and total collapse. I mean we are talking about a lot of decimal places here.

The second problem we have come across is what is known as the horizon problem. For the most part either direction we glance into the universe it appears to be relatively similar. But at the time of the big bang there is no possible way that there could have been any communication between the opposite ends of the universe as it rapidly expanded. How can we explain this homogeneity of the universe?

The third problem is that of matter in the universe. If it all began as radiation and then through the process of “pair production”, the direct conversion of radiation into matter, we somehow got to the point where we are today. But lets say for example two pounds of light is equivalent to one pound of matter plus one pound of anti-matter, then what do we have left? Where is all of this anti matter that must be somewhere in the Universe and if the radiation converts to one part matter and one part anti-matter would these two pieces not have simply destroyed each other?
There are many questions still to be answered in the universe. We are only just now beginning to put some tiny pieces together. The problems that face the big bang are essentially just questions that are out there to prove or disprove its theory, although they are by no means a contradiction, science simply does not quite yet have the answers required.

In relation to the big bang and the events that took place the class slightly diverged to speak about stars and their different life forms. This discussion was relevant to the events of the big bang because possibly stars and supernova events, their processes, and others are similar to the ones that occurred at the time of the big bang. For example they could help us to predict with more precision the age of the universe and maybe help us to further understand what the moments right before and after the bang were like.

• The Anthropic Principle
Why is it that we have life within our universe? What are the physical constants and properties that have allowed life to evolve in such a manner? Are they solely unique to us here within this universe or do things possibly fall so seemingly perfectly into position elsewhere? So far science has not been able to answer these questions for us. Currently our best answers seem to lye within the theories of The Anthropic Principle. Within this principle we have developed two possible answers, the “weak” Anthropic Principle and The “Strong” Anthropic Principle. The “weak” basically stating that we live in a universe that was simply designed correctly to support life. The “Strong” stating that we live in a universe that is one of many and this one just so happens to be the right one. Is it that we are living in an ideal location and time or possibly an ideal universe? But, whatever the principle chosen one is only faced with another set of questions. Why is this the right universe? Who/what/how put or placed things in the right order? How many universes are there? And so and so on.

Through the class discussion we talked a little not only about the Anthropic principle but also about life. Not only about how it is that the conditions are right for life but how is it that life came to be. Did it evolve out of the primordial soup that once was or are we a result of Panspermia and was life possibly deposited here from somewhere else? Is there anyway of knowing the answers to these questions? How convenient it is that the curtain is so easily dropped in front of us.