The Birth of a Universe
Fundamental Forces
Where did all the stuff that makes up the universe come from? To answer this question, cosmologists—astronomers who study the large-scale structure of the universe—have turned to the microscopic world of particle physics. Particle physics (sub-atromic and nuclear physics) is the study of matter and energy at its most fundamental level and particle physicists seek to understand the basic structure of matter and the fundamental forces, sometimes called interactions, that exist between matter in the universe. According to current theory, there are four basic interactions that govern the interaction of matter:
Gravitation
Gravity
attracts all matter but is too weak to have any significant affect at subatomic
levels (at least as we understand it today; a subatomic, or quantum, theory
of gravitation has yet to be formulated.) Gravity appears to becomes
important when the quantity of matter grows to approximately 500 miles
in size. In other words, as big as moons and the largest asteroids or dwarf planets.
Electromagnetism
This plays an important role in the attractive/repulsive force between charged subatomic
particles such as electrons and protons. It is responsible for the oscillating
electric and magnetic fields we know as electromagnetism and is mediated, or transferred, through photons.
The Weak Nuclear Force
The weak interaction is a force that is involved in the nuclear beta
(electron) decay and other radioactive processes.
The Strong Nuclear Force
It is this force that binds atomic nuclei together and stops them
from flying apart due to the mutual electrostatic repulsion of their protons.
All these four forces are mediated by carrier particles which are elementary particles that 'carry' the force from one particle to another. Aaccording to the laws of quantum physics, the particle and wave behaviors exist simultaneously although experiments are able to observe only one 'state' at a time.
The W± and Z0 particles
that mediate the weak nuclear force were discovered in 1984 using
the Super Proton Synchrotron particle accelerator at CERN in Geneva,
the gluon that mediates the strong
nuclear force has been detected by indirect methods, while the
photon is the particle that mediates the electromagnetic
interaction. Only the hypothesized graviton awaits a quantum
theory of gravitation and has so far not been observed. Perhaps string theory or quantum loop gravitation will provide the necessary clues so that physicists can not only predict but also test and confirm evidence for a quantum theory of gravitation.
In the Beginning...
Astronomers believe that the infant universe was dominated by radiation. To understand why, we must understand a key concept in particle physics called pair production. Symbolically this concept can be expressed as
photon + photon = particle + antiparticle
The reverse is also possible. Thus, a particle and its antiparticle can collide and annihilate each other producing two high energy gamma ray photons:
particle + antiparticle = photon + photon
For pair production to occur the following conditions must be met:
- The energy of the photons must be greater than the mass-energy of the created particles.
- Pair production must obey the law of conservation of energy.
Cosmologists model the early universe as a hot, high energy photon gas and the kinetic energy of the photons depends on their temperature. Creation and annihilation of particles occurs in equal numbers so that as many particles are made per second as are being destroyed and the universe is in a state of thermal equilibrium. Particle physicists call this state of balance between matter and antimatter symmetry . The average energy of the photons in a gas can be approximated by the equation E = kT. It is important to understand that the creation of different types of particles by pair production depends on the temperature of the photons.
Eras of the Universe
Cosmologists divide the history of the universe into four periods or eras of time each one corresponding to a particular range of temperatures. These are
- A heavy particle era
- A light particle era
- A radiation era
- A matter era
Heavy Particle Era: Temp < 1033 K;
Time after Big Bang > 10–43 sec.
During this period the universe is hot enough for all massive
elementary particles to be created by pair production. The universe
is in thermal equilibrium and expands rapidly with the first stable
protons being formed at about 10–6 sec after the
Big Bang.
Light Particle Era: Temp < 1010 K;
Time after the Big Bang > 10–4 sec
The temperature of the universe is no longer hot enough for massive
particles to be made. Lighter particles such as electrons and positrons
can still be produced and protons and electrons combine to make
neutrons. We live in a matter-dominated universe and for this to
be the case, there must have been an excess of particles over antiparticles
during the transition from the heavy to light particle eras. Particle
physicists refer to this condition as a symmetry
breaking.
Radiation Era: Temp <
1010 K; Time after the Big Bang > 10 sec
In this era protons and neutrons left over from the heavy and light
eras interact to form the first stable nuclei. The most important
nucleus to form during this era is deuterium (2H) and
from this, nuclei of helium (4He and 3He)
as well as beryllium 7Be and lithium 7Li,
which are manufactured by simple fusion reactions. The production
of light elements formed in the heavy and light eras is called primordial
nucleosynthesis (don't confuse this with stellar nucleosynthesis
which is the formation of heavier elements inside stars) and the
initial abundance of helium in the early universe (about 25%) was
set at this time. The remaining 75% being mainly hydrogen with
trace amounts of beryllium and lithium. The Big Bang model predicts
that the universe should contain a helium abundance of at least
these percentages with more being created by subsequent nuclear
reactions in stars, and observations of the chemical composition
of various celestial objects tends to support this.
Matter Era: Temp =
3,000 K; Time after Big Bang = 375,000 yrs
In previous eras matter and radiation interacted with each other
and were locked together. The radiation could not escape and the
universe was opaque to radiation. However, when the temperature
of the universe dropped to 3,000 K, the first stable hydrogen atoms
formed and matter and radiation were no longer coupled together.
The universe became transparent to radiation and cosmologists call
this event photon decoupling. Prior to this time, any hydrogen
atoms formed were immediately destroyed as electrons absorbed radiation
and ionization occurred. Due to local variations in density, matter
started to clump together and material condensation occurred. Cosmologists
believe that large scale structures such as galaxies could first
start to form at this time. The radiation that spread out after
photon decoupling now comes to us in the form of the 3 K cosmic
microwave background (CMB) which over the last 14 billion
years (give or take a few) has been redshifted from a temperature
of 3,000 K to 3 K due to the exansion of the universe (recall Wien's
law). The smoothness—the radiation is both homogeneous
and isotropic—suggests that matter and radiation must have
been uniformly distributed about 300,000 years after the Big Bang.
The most recent observations from the WMAP (Wilkinson Microwave
Anisotropy Probe) indicate that the time of photon decoupling occurred
375,000 years after the Big Bang.
Dark Ages: Temp < 3,000
K; Time after Big Bang: 375,000 – 500,000,000 yrs.
During this time, the universe continued to expand and cool. The
small variations in density and temperature became the seeding
grounds for the first stars and primordial galaxies as nuclei cooked
up in the heat following the Big Bang accumulated to form vast
clouds of gas. Recent observations indicate that the earliest stars
(called Population III in the convoluted scheme created by astronomers
in the first half of the twentieth century) may have formed about
half-a-billion years after the Big Bang. The tantalizing evidence
of their discovery is still being teased from long-exposure images
taken by the Hubble Space Telescope and the Spitzer Space Telescope.
As the Population III stars were forming, their host quasars (primordial
galaxies) were most likely forming too.