Do your course evaluations. I will add 5 points on your final exam if you complete the evaluation.

Quasars are supermassive black holes at the centers of galaxies, which are accreting material. The accretion disk is what gives off the luminosity. When galaxies merge, material can be deposited in the vicinity of the super massive black hole, causing an accretion disk to form At very large distances (very long ago) galaxies were forming through mergers. This is why we only see quasars at large distances.

The straight line represents a mass-less universe with no gravity. In other words, the expansion began and now the universe is coasting. Growing in size but at a constant rate. When the data falls below the sloping straight line it means that the universe was moving more slowly in the past and the expansion is speeding up. How can this be?

Gravity is only attractive. It can only work to help slow the expansion of the universe. If the expansion is speeding up, then there has to be a repulsive force that is acting to force the expansion to grow more rapidly Gravity can’t do this. This repulsive force has been named “Dark Energy” It fits into Einstein’s equations just like his old cosmological constant. But this isn’t keeping the universe static. It is forcing it to grow more rapidly.

Using relativity we can model what will happen in the future as well.

Past Future

Our location in the Universe A distant galaxy or quasar Today

Our location in the Universe A distant galaxy or quasar Today

The model shows that the galaxy is, today, much farther away than it was when the light left the galaxy billions of years ago. Also, notice that what ever direction you look, when you look out into space, you are looking back toward the earlier universe. All directions point back toward the center of expansion. Also, there is no observable edge of the universe. You are located at the edge of the universe. Any direction you look, is backward in time, toward the center of expansion.

What is this expansion? Remember back to the gravitational red shift that is caused when light near the event horizon of a black hole moves away from the black hole. The red shift is caused by the clock near the event horizon running slow compared to the observer far from the black hole. The effect is caused by the slowly running clock. The expansion of the universe is similar. The space- time metric is growing as a function of time. What that means is that today, in our portion of the universe, a “meter” is larger than it was in the past.

Suppose we used the wavelength of light as our “meter” stick. Distant past Intermediate Past Today

When we look into the distant past and up to today, we see that the “meter” stick grew in size. I use quotes around “meter” stick, because a real meter stick (say made of wood and held together by chemical bonds) would not have changed in size. The space-time metric for the universe is growing as a function of time. Light is a wave and is not held together by forces. When space-time grows, so does the distance between peaks in the wave. Since the time when the Earth formed, it has not grown in size with the expansion of the universe. Earth’s matter is held together by its gravity. But the distance between galaxies that are not held together by gravity, has grown as time has gone by.

In our Local Group of galaxies, we do not see the expansion of the universe between our galaxies because we are bound by gravity. In relativity, we would say that we all agree on the length of a “meter”. Just like different observers far from a black hole can all agree on the wavelength of light. These observers have clocks that run at the same rate. The Local Group of galaxies all have the same “meter” stick. So we all agree on the distance and we do not see the universe expanding between us.

Final word on expansion The galaxies are not moving apart from each other in the universe. It is the universe that is growing. Remember, space-time is what we mean by the universe. When space-time grows, when the meter stick changes length, then the distance between objects is seen to grow. Long ago, the distance between objects was much less than it is now. The universe was far more dense.

The growth of the space-time metric does not have to be grow at a linear rate with time. In a closed, finite universe the growth is rapid at first, then slows to a stop and then the metric begins to shrink. In an open universe, the growth of the metric slows over time but never stops growing.

Think back… What causes space-time to warp? 1.High velocities 2.light 3.mass

In all cases, the universe was exceedingly dense long ago.

A B D E C F

A. The Plank Era Time is about seconds after Big Bang It is hypothesized that all four forces of nature were combined into one at this time. (gravity, electromagnetism, weak nuclear and strong nuclear) Currently we can not describe this era. Physics does not have a unified model for combing gravity with the other three forces.

During this time the universe is nearly infinitely dense. Notice, I did not say infinitely small. Small would suggest that the universe could be viewed from the outside. There is no outside. The space-time metric is so compressed that there is virtually no distance between locations. As an analog, think about the twin paradox and how the traveling twin thought the distance to Alpha Centauri was only 0.6 light years. Not the 4.5 light years that we measure.

A B D E C F

B. Inflation At the time between seconds and seconds, gravity breaks free from the other forces of nature. This break releases an enormous about of energy into the universe and causes the universe to expand at a rate far faster than the speed of light. The temperature drops from to degrees.

A B D E C F

C. Particles begin to form At around seconds, particles begin to form. Here I have a series of questions for you. After the inflation era (B) how was light changed?

1.It had a longer wavelength 2.It had a shorter wavelength 3.It disappeared

What does a longer wavelength mean in terms of Energy? 1.The light had more energy 2.The light had less energy 3.The energy stayed the same

Particles, made of matter begin to form. Where do these particles come from? 1.The first supernova explosions 2.The expanding universe 3.The light (E=mc 2 )

C. Particles begin to form At around seconds, particles begin to form. During this time quarks and anti-quarks where forming from radiant energy. ϒ + ϒ quark + anti-quark It is important that the gamma-ray light must have energy sufficient to make these particles. E = mc 2

The process is also reversible. quark + anti-quark But if this were the only thing happening then then the quarks and the light would be in balance. But the light is rapidly losing energy due to the expansion of the universe. The quarks annihilate each other, but a slight asymmetry allows produces more quarks than anti-quarks. ϒ + ϒ

For every 1 billion quark/anti-quark pairs that are produced, there are 3 extra quarks. At the end of this era, for every 999,999,997 quark/anti-quark pairs, there are 3 lone quarks. With the exception of these extra quarks, all the rest is turned back into light. But the light looses energy do to the expansion of the universe, and no more quarks can be produced. Since most of the quarks/anti-quarks produced light, the matter is only 3 parts per billion, compared to the photons of light.

A B D E C F

D. Baryons begin to form At a time of about 1 second, the remaining quarks begin to form neutrons and protons out of quarks. This could not happen earlier, because the light had so much energy that it would break apart any quarks that combined. Electrons also begin to form out of a similar process as the quarks.

A B D E C F

E. Nucleosnythesis After the protons and neutrons form, they begin to collide and make Deuterium. (That’s an isotope of hydrogen that has a nucleus with one proton and one neutron. At first, these nuclei can not survive, because the light has so much energy it splits them back apart. At around 3 minutes after the Big Bang, the expansion causes the light to loose energy to the point that it can no longer break the nuclei apart.

During the next few minutes, the reactions that occur are similar to that in the Sun, except the Deuterium is made by combining free protons and free neutron.

Even in this Era, where there are many free particles with high energy, it takes a bit of time to fuse Helium out of protons and neutrons. But in the early universe, time is not something that we have a lot of. By the time that the universe is 10 minutes old, the expansion of the universe has stopped nucleosynthesis. This only provided enough time to make helium and very trace amounts of Lithium.

The new universe will end up with a composition of about 75% hydrogen and 25% helium and a miniscule amount of lithium. Now think back to what reaction rates in the Sun depend on.

What do nuclear reaction rates depend on? 1.temperature 2.volume 3.Density 4.Red shift 5.1 &

Reaction rates depend on the temperature and the density. Temperature because nuclei must have sufficient kinetic energy to over come the repulsion force between protons. Density, because these tiny particles have to have head-on collisions in order to “stick”. So knowing the exact ratio of Hydrogen to helium, or measuring the exact amount of lithium in the universe, can tell us about the density of the universe. And density decides the shape of the universe.

A B D E C F

F. Electrons combine with nuclei After 10 minutes, the universe is filled with protons, helium nuclei, electrons and light. The photons of light still have huge amounts of energy. If an electron binds to a nucleus, it is immediately ionized by light. Let’s think back to the stellar spectroscopy.

Why do O-type stars have weak hydrogen absorption lines? 1.They have very little hydrogen 2.They are too hot for hydrogen to hold on to its electrons 3.They are very young stars

In order for electron to really stay bound to the nucleus we need temperatures that are less than an A-star would have. This is around 10,000 degrees. So the light in the universe has to be stretched to a wavelength (by the expansion) that gives the photons less than the ionization energy of hydrogen. This happens after about 300,000 years of expansion.