What we see is only the “Icing on the Cake” II – The Dark Force
Lead: Alex Miklos and Lee Petersen

3/4/2008

Summary by Jeff Tessein

Things we can’t see: What is the universe made up of?

Slide 1 of the presentation began with a pie chart showing what the universe is made up of and the proportions of each: 4% normal matter, 22% dark matter and 74% dark energy: the existence of dark matter and energy was first theorized when it was realized that “normal matter” was not all the matter in the universe. Fritz Zwicky discovered that that there was insufficient matter for several reasons. Motion in galaxy clusters and galaxy rotation curves mandate that the “normal” matter in the universe was not enough to account for what was being observed in the universe. To date dark matter has not been physically observed. However, there are certain types of normal matter that may allow us to know that dark matter is present. These are known as MACHOS (MAssive Compact Halo ObjectS) and they came about as possible evidence of dark matter in the form of thus far unseen compact objects. This led to a discussion of how dark matter is known to exist. WIMPS (Weakly Interacting Massive ParticleS) are a possible candidate for dark matter, but we’re not sure because WIMPS have not been observed. We decided that even though we cannot directly observe WIMPS because they will not interact with normal matter, indirect observations (like gravitational lensing) are what allow astronomers to observe MACHOS. Another observation that is evidence for the existence of dark matter is that neutrinos outnumber nucleons in the universe. A WIMP is much like a neutrino in that it is equally undetectable, but with a much higher mass.

A neutrino is an elementary particle with a very small mass. Some important characteristics of the neutrino were relevant in the solar neutrino problem. The solar neutrino problem occurred when theorists had realized that nuclear fusion has to release a neutrino each time a reaction happens. The neutrino was first discovered by beta decay in the 1950s. If solar masses were stacked between here and Alpha Centauri, and neutrinos sent through these masses, more than half of them would make it from here to Alpha Centauri without reacting with the material. Hence neutrinos would emerge freely from the Sun’s center. However, astronomers studying thermonuclear fusion in the sun by looking at the neutrinos emitted from the Sun’s core found that the observed number was way too low for the Sun’s energy production (only 1/3 of the amount of neutrinos predicted by theory). This turned out to be the solution of the problem because there are three types of neutrinos, and each type makes up 1/3 of the total number of neutrinos. Electrons, muons and tau particles make up one family. The partners to these are electron neutrinos, mu neutrinos and tau neutrinos, but only the electron neutrino was visible in most of the observations, and neutrinos switch between all three varieties.

The presentation moved on to talk about the different theoretical particles. The –ino suffix indicates supersymmetric partnership. The s- prefix stands for supersymmetric partners to leptons. Up to this point, supersymmetric pairs have not been detected from particle accelerator experiments, but it is possible that sometime in the future they will be detected when higher energy accelerators are built.

At this point the question was asked, “Is there a difference between normal matter and normal energy?” The answer is no. Normal energy is made up of electromagnetic radiation converted to mass using E = mc^2. However, this is a very insignificant portion of the total amount of normal matter, so in reality, normal matter is essentially just mass since the energy (which comes from all the photons in the universe) is so small in comparison.

A question brought up by the presenters was, what would happen if there was a significant increase in dark energy over time? Davies suggests that this could cause the Big Rip.

The discussion then moved on to the concept of the expanding universe. We talked about how the universe is expanding at an accelerating rate, but theoretically should be slowing down because of gravitational attraction. This unexpected effect may be caused by an anti-gravitational force. Lee Petersen reported on a possible theory to modify gravity that adds another term. An equation that may look something like the following:

-GMm/x^2 + kf(x) k<<G

In this form it could reconcile the expanding universe problem because over extremely large distances gravity would begin to point in the opposite direction. However, this modification is not taken seriously in most circles because of the observed variation of rotation rate of galaxies with distance from the center. This model is in conflict with these galaxy rotation curves. We concluded this subtopic by discussing that the constants used in astronomical calculations seem to be just correction factors, and a more accurate theory to come along sometime in the future would exclude them.

The next subtopic in our discussion was how the universe was able to form from nothing; it would seem that mass is not conserved. To explain this Davies says that energy and mass sum in the universe to zero. According to General Relativity, mass and kinetic energy of expansion curve spacetime in opposite directions. That means that half the mass (or energy) has a negative sign and half is positive so it will all cancel out to zero mass. This even applies to dark matter and dark energy. Thus we are not violating fundamental conservation laws.

A new question was brought up at this point: How would you view a universe where the “event horizon” (the point beyond which we cannot see because of limitations on speed of light) has shrunk so small that we can only see our own galaxy? This will happen eventually with the way the universe is expanding. Would we still know as much about the larger universe if our view were reduced to only our local galaxy cluster? The class decided that our limit to see the universe’s history from observation would be reduced and we wouldn’t know nearly as much about the universe as we know now. Someone asked if this would still happen before the universe falls prey to “entropy death”. The answer is yes. The two events are on entirely different time scales. Within 100 billions years the event horizon will shrink to our local galaxy cluster. At this time small stars will still be producing energy. In 100 trillion years the last star will burn out and only after that entropy effects would take over.

Questions asked at the end of class:

Q: Is the Big Crunch still a relevant theory?

A.: No. That has gone out the window with the discovery of accelerated expansion of space.

Q: Is dark energy Hawking Radiation?

A: No. Hawking Radiation is specific to black holes. There is a probability that over time particles can escape black holes through quantum effects. Over a long period of time, a black hole will eventually evaporate. This would still happen even with the cosmic microwave background feeding the black holes because the local area around the black
hole would be empty. The time it might take for a black hole to evaporate is around 10^60 years.

End of class.