Lecture 41

Evolution of the Universe (continued)

Extrasolar Planets

• The Beginning and Fate of the Universe

• Observations of Extrasolar Planets

Chapter 18.9 18.14

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 Main Forces

There are 4 major forces in today’s universe:Gravity, electromagnetism, the strong force, and the weak force.

At very high temperatures, the electromagnetic, weak, and strong force merge one force.

The theories that predict these mergers are called grand unified theories (GUT).Two forces – gravity and GUT force – operated in the universe between 1043 and 1038 sec (GUT era).

Inflation

At a temperature of 1029 K the strong force separated from the GUT force.

In just 1036 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 separation probably resulted in a huge energy release that caused a sudden and dramatic expansion called inflation.

The Electroweak Era

This era lasted until 1010 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 1010 to 103 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.

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 temperatureand 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.

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 1029 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 matter.

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