Objectives

• Describe the characteristics of the universe immediately after its birth.

• Explain how matter emerged from the primeval fireball.

Early Origins

• Astronomers are unable to observe the universe when it was very young, because truly far-away and long-ago events were engulfed in a sea of intense radiation

• Only subatomic particles existed—not only the protons, neutrons and electrons we know today, but also, we think, various strange and exotic elementary particles predicted by current theory.

• Surprisingly, part of this group of particles that characterized the early universe can now be studied here on Earth, in huge particle accelerators

Early Origins

• Only subatomic particles existed

Density

• On the very largest scales we can regard the universe as a mixture of matter and radiation.

• The overall density of matter is not known with certainty, but it is thought to be at least a few tenths of the critical density, about 10-26 kg/m3

Density

• Most of the radiation in the universe is in the form of the cosmic microwave background, the low-temperature (3 K) radiation field that fills all space.

• For our current purposes, then, we can regard the cosmic microwave background as the only significant form of radiation in the universe

• The reason for this is that stars and galaxies, though very intense sources of radiation, occupy only a tiny fraction of space.

Density

• We can express the energy in the microwave background as an equivalent density by first calculating the number of photons in any cubic centimeter of space, then converting the total energy of these photons into a mass using the relation E = mc2.

• we arrive at an equivalent density for the microwave background of about 5 x 10-31 kg/m3

Density

• Density for the microwave background of about 5 x 10-31 kg/m3

• at the present moment :– the density of matter (around 10-26 kg/m3) in

the universe far exceeds the density of radiation.

– Matter dominated

Density

matter-dominated universe

A universe in which the density of matter exceeds the density of radiation. The present-day universe is matter-dominated.

Density

Even though today the radiation density is much less than the matter density, there must have been a time in the past when they were equal.

Before that time, radiation was the main constituent of the cosmos. The universe is said to have been radiation-dominated then.

Density

radiation-dominated universe

Early epoch in the universe, when the density of radiation in the cosmos exceeded the density of matter.

Density

As a result, the density of radiation fell faster than the density of matter as the universe grew. Tracing the curves back from the densities we observe today, we see that radiation must have dominated matter at early times—that is, at times before the crossover point.

As the universe expanded, the number of both matter particles and photons per unit volume decreased.

However, the photons were also reduced in energy by the cosmological redshift, reducing their equivalent mass, and hence their density, still further.

Particle Production In The Early Universe

pair production

The process in which two photons of electromagnetic radiation give rise to a particle—anti-particle pair.

Particle Production In The Early Universe

(a) Two photons can produce a particle—antiparticle pair—in this case an electron and a positron—if their total energy exceeds the mass energy of the particles produced.

(b) The reverse process is particle—antiparticle annihilation, in which an electron and positron destroy each other, vanishing in a flash of gamma rays.

Particle Production In The Early Universe

At the same time it provides the energy to produce an electron—positron pair (the spiral paths, which curve in opposite directions in the detector's magnetic field because of their opposite electric charges).

(c) Tracks in a particle detector allow us to visualize pair creation. Here a gamma ray, whose path is invisible because it is electrically neutral, arrives from the left; it dislodges an atomic electron and sends it flying (the longest path).

Particle Production In The Early Universe

As an example of how pair production affected the composition of the early universe, consider the production of electrons and positrons as the universe expanded and cooled.

At high temperatures—above about 1010 K—most photons had enough energy to form an electron or a positron, and pair production was commonplace.

Particle Production In The Early Universe

• As a result, space seethed with electrons and positrons, constantly created from the radiation field and annihilating one another to form photons again.

• Particles and radiation are said to have been in thermal equilibrium

• new particle—antiparticle pairs were created by pair production at the same rate as they annihilated one another.

Particle Production In The Early Universe

As the universe expanded and the temperature decreased, so did the average photon energy.

By the time the temperature had fallen below a billion or so kelvins, photons no longer had enough energy for pair production to occur, and only radiation remained.

Particle Production In The Early Universe

(b) Below about 1 billion K, photons have too little energy for pair production to occur.

(a) At 10 billion K most photons have enough energy to create particle—antiparticle (electron—positron) pairs, so these particles exist in great numbers, in equilibrium with the radiation.

Particle Production In The Early Universe

Pair production in the very early universe was directly responsible for all the matter that exists in the universe today. Everything we see around us was created out of radiation as the cosmos expanded and cooled

Particle Production In The Early Universe

The first few hundred seconds of the universe's existence saw the creation of all of the basic "building blocks" of matter we know today

protons and neutrons froze out when the temperature dropped below 1013 K, when the universe was only 0.0001 s old.

The lighter electrons froze out somewhat later, about a minute or so after the Big Bang, when the temperature fell below 109 K. This "matter-creation" phase of the universe's evolution ended when the electrons—the lightest known elementary particles—appeared out of the cooling primordial fireball