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					Density of the Universe
How will the universe end? Which force will win: gravity or expansion? Hubble constant is a measure of the current kinetic energy of the universe. The strength of the gravitational pull depends on the density of matter in the universe.

Critical Density
The greater the density, the greater the overall strength of gravity  the higher the likelihood that gravity will eventually halt the expansion. Gravity can win if the current density of the universe exceeds 1029 g/cm3. This density is called the critical density.

The visible matter contributes only <1% of the matter density needed to stop the expansion. The fate of the universe depends on the dark

Critical Density
To contribute enough mass into the critical density, the average mass-to-light ratio should be ~1000. Clusters of galaxies have this ratio of ~ a few hundred. If the proportion of dark matter in the universe is similar to that in clusters, the universe will expand forever.

Fate of the Universe
There are 4 categories for the change of the future expansion rate of the universe: recollapsing, critical, coasting, accelerating. Recent observations of white dwarf supernovae show that the expansion of the universe appears to be speeding up.

This behavior suggests the presence of a repulsive force, which is called dark energy.

Fate of the Universe

Summary
Dark matter seems to be real, but we do not yet know what it is. Galaxies contain a lot more mass in dark matter than in stars. Dark matter holds the key to the fate of the universe. Current knowledge is consistent with the idea of eternally expanding universe.

Chapter 17
The Big Band Theory

• The Big Bang Theory • Evidence for Big Bang

Early Days of the Universe
The universe is filled with a faint glow of radiation, a remnant from the Big Band heat. It prevents us from seeing directly to earlier times. We need to use models to study what happened with the universe in its first ~380,000 years. In the beginning the universe was dense and

The Big Bang Theory
This is the theory of the universe’s earliest moments. It presumes that the universe began from a tiny, hot, and dense collection of matter and radiation. It describes how expansion and cooling of particles could have led to the present universe of stars and galaxies.

It explains several aspects of today’s universe with a very good accuracy.

Conditions in the Early Universe
The temperature was so high that photons could be transferred into matter and back (E=mc2). Example: When 2 photons collide with a total energy greater than 2mec2, they create a pair of particles – electron and antielectron (positron). When an electron and positron meet, they annihilate and transform their mass-energy into photon energy.

The First Instant
We cannot describe the conditions in the universe during the first 1043 second. There are 2 separate current theories which describe very small and very big. Quantum mechanics is a theory of subatomic particles. General relativity is a theory of the large-scale structure of space-time.

The Main Forces
There are 4 major forces in today’s universe: Gravity, electromagnetism, the strong force, and the weak force. Gravity dominates large-scale action. Electromagnetic force dominates chemical and biological reactions. The strong force binds nuclei together. The weak force mediates nuclear reactions.

The GUT Era
At very high temperatures, the electromagnetic and weak force merge into one electroweak force. At even higher temperatures, the electroweak force may merge with the strong force. The theories that predict these mergers are called grand unified theories (GUT). Two forces – gravity and GUT force – operated in the universe between 1043 and 1038 sec.

Inflation
At a temperature of 1029 K the strong force separated from the GUT force. The separation probably resulted in a huge energy release that caused a sudden and dramatic expansion called inflation. In just 1036 sec, the universe grew from the size of an atom to the size of the solar system. Inflation might explain some today’s universe features.

The Electroweak Era
This era lasted until 1010 sec from the Big Bang. It came to an end at a temperature of 1015 K, when all the four forces have finally separated. Its end marked the appearance of weak bozons in the universe. In 1983, these particles were directly observed in a particle accelerator experiment.

The Particle Era
The particle era lasted from 1010 to 103 sec. The particles (electrons, neutrinos, quarks) were spontaneously created and annihilated.

After its end, quarks combined in groups of three to form protons and neutrons. The temperature dropped down to 1012 K. At this point the universe was no longer hot enough to produce particles from pure energy.

Particles in the Universe
During the particle era, protons and antiprotons (and other matter and antimatter particles) were created from energy. If the numbers of particles and antiparticles were equal, they would annihilate at the end of the particle era.

We observe now a ratio of photons-to-protons of ~1 billion  the number of protons was slightly larger than that of antiprotons.

The Era of Nucleosynthesis
After the first 0.001 sec, the protons and neutrons attempted to fuse into heavier nuclei. This era lasted until 3 minutes after the Big Bang and the temperature dropped down to 109 K. Fusion ceases at such “low” temperatures. The result of the primordial fusion was 75% protons and 25% alpha-particles (helium atom nuclei).

The Era of Nuclei
At the end of the nucleosynthesis, the universe consisted of a very hot plasma of hydrogen nuclei, helium nuclei, and free electrons. The fully ionized nuclei moved independently of free electrons for the next 380,000 years. Finally, hydrogen and helium captured electrons and formed stable atoms at ~3000 K. The universe became transparent. Photons were released from electrons and are seen today as the cosmic microwave

The Era of Atoms and Galaxies
The era of atoms began after the era of nuclei. The universe was a mixture of plasma and neutral atoms. The matter was distributed unevenly throughout the universe, giving rise to protogalactic clouds. First galaxies formed ~1 billion years after the Big Bang, marking the beginning of the era of galaxies. This era continues to this day.

Evidence for the Big Bang
The Big Bang theory is a model, which explains some facts (observations). It should be able to make predictions that can be verified through observations or experiments. Two important predictions:

1. Cosmic microwave background radiation. 2. Fusion of original hydrogen into helium.

The Cosmic Microwave Background
An unexpected noise was found during testing a microwave antenna at Bell Labs in 1965. The noise was coming from every direction. At the same time, physicists from Princeton calculated the expected radiation from the initially hot universe. They suggested that this radiation could be detected with a microwave antenna.

The result was a Nobel Prize in physics for 1978.

The Cosmic Microwave Background
The background consists of photons arriving at Earth directly from the end of the era of nuclei (when the Universe was about 380,000 years old). Neutral atoms captured most of the electrons. Photons were released and have flown freely through the universe ever since. This background radiation can be detected with a small TV antenna as part (1%) of static “snow”. The redshifted spectrum of the background radiation has now a temperature of 2.73 K.

The COsmic Background Explorer
COBE was launched in 1991 to test the predictions of the Big Bang theory. It has measured the mean CMB temperature and detected its fluctuations (~1/100,000). The temperature fluctuations reflected the density variations in the early universe. This discovery also supported the idea of Weakly Interacting Massive Particles.

Synthesis of Helium
The current CMB temperature tells us precisely how hot the universe was when it appeared. It tells us how much helium was initially A helium nucleus contains 2 protons and 2 produced. neutrons. At T > 1011 K, nuclear reactions converted protons into neutrons and back, keeping their numbers nearly equal. Between 1010 and 1011 K, neutron – proton reactions favor protons, because neutrons are heavier than protons.

Synthesis of Helium

Energy is required to convert protons to neutrons. At T < 1010 K, only neutrons can be changed into protons. However, fusion continued to operate and protons and neutrons combined into deuteriu Then deuterium fused into helium. During the early era of nucleosynthesis, helium nuclei were being destroyed by gamma-rays. At ~1 minute, gamma-rays were gone and the

Summary
It is easy to predict conditions in the early universe, it is harder to predict the behavior of matter and energy. Our current understanding of physics allows us to reconstruct the conditions in the universe back to 1010 sec. Observations support some predictions of the Big Bang theory.


				
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