Radiation Transforms Into Matter (HSC SSCE Physics): Revision Notes

Radiation transforms into matter

Introduction to the Big Bang Theory

The Big Bang Theory (BBT) is our best scientific explanation for how the Universe began and developed over approximately 13.8 billion years. This theory helps us understand how all the elements in the periodic table were formed, from the lightest hydrogen to the heaviest elements.

The BBT developed through careful collection and analysis of evidence from across the Universe. Since we cannot physically collect samples from distant regions of space, scientists study the characteristic electromagnetic radiation that different elements emit. Each element produces a unique "fingerprint" of radiation across the electromagnetic spectrum, including microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Energy and matter in the early Universe

According to the BBT, energy is fundamental to understanding the Universe's origin. At the very beginning, energy was all that existed. This energy emerged from an incredibly dense and hot point called a singularity. The temperature at this moment was unimaginably high.

As this energy spread outward, it became distributed across increasingly vast distances. This expansion caused the temperature in any given region to drop significantly compared to the conditions at the singularity.

The transformation of energy into particles

A crucial principle of the BBT is that particles can form from pure energy. At extremely high temperatures, any particles that formed were immediately destroyed again. However, as temperatures decreased, particles became more stable and longer-lasting. This stability allowed them to begin combining with each other.

Over time, conditions became suitable for atoms and molecules to form. All the energy from the original singularity remains in the Universe today - it has simply been redistributed into the matter and radiation we observe. The total amount of energy remains constant, just as it was at the beginning.

Elementary particles

What are elementary particles?

An elementary particle is a particle that cannot be broken down into smaller components. The search for these fundamental building blocks has revealed much about the nature of matter.

Antiparticles and matter-antimatter annihilation

Every particle has a corresponding antiparticle with opposite properties. During the early Universe, slightly more matter than antimatter formed overall. This small imbalance explains why we observe mainly matter today, rather than antimatter. Scientists still don't fully understand why this imbalance occurred.

When matter and antimatter meet, they undergo annihilation, converting back into pure radiant energy.

Types of elementary particles

Elementary particles fall into two main categories: quarks and leptons.

Quarks

Quarks are tiny elementary particles that combine to form larger particles like protons and neutrons. There are six types of quarks, each with its own antiquark:

Quark	Symbol	Antiquark	Symbol
up	$u$	anti-up	$\bar{u}$
down	$d$	anti-down	$\bar{d}$
strange	$s$	anti-strange	$\bar{s}$
charm	$c$	anti-charm	$\bar{c}$
bottom	$b$	anti-bottom	$\bar{b}$
top	$t$	anti-top	$\bar{t}$

Leptons

Leptons are elementary particles that include the familiar electron along with five other particles. Leptons interact through gravitational, electromagnetic, and weak forces, but not through the strong force. The six leptons and their antiparticles are:

Lepton	Antiparticle
electron	positron
electron neutrino	electron anti-neutrino
muon	anti-muon
muon neutrino	muon anti-neutrino
tau ($\tau$)	anti-tau ($\bar{\tau}$)
tau neutrino	tau anti-neutrino

Hadrons and field particles

Hadrons

Hadrons are relatively large particles built from elementary particles (quarks). They are divided into two categories based on their size and composition:

Mesons are middle-sized particles composed of one quark and one antiquark. These particles eventually decay into electrons, positrons, neutrinos, and photons.

Baryons are heavier particles made from three quarks. The most familiar baryons are protons and neutrons. All baryons eventually decay into protons, and sometimes into other particles like neutrons.

Field particles

Field particles are special particles that carry forces between other particles through exchange mechanisms. For example, when charged particles interact electromagnetically, they exchange photons.

The four fundamental forces and their mediating particles are:

• Electromagnetic force - mediated by photons
• Weak force - mediated by W and Z bosons
• Gravitational force - mediated by gravitons (not yet detected experimentally)
• Strong force - mediated by gluons and pions

Timeline of the Big Bang

The first microsecond

According to the BBT, the most dramatic changes occurred incredibly quickly after the Big Bang:

Formation of hadrons

As temperatures continued to fall, hadrons such as protons and neutrons were able to form from quarks. These particles became stable enough to persist rather than being immediately destroyed.

Nuclear era

During this extended period, atomic nuclei began to form, primarily hydrogen, helium, and some lithium. However, these were charged ions rather than neutral atoms, as conditions were still too energetic for electrons to remain bound to nuclei.

The Universe becomes transparent

This marked a crucial transition when conditions finally allowed neutral hydrogen atoms to form. Because neutral atoms don't scatter radiation like charged ions do, radiation could now travel freely through space. The Universe became transparent to radiation for the first time.

Evidence for the Big Bang Theory

Cosmic microwave background radiation

The strongest evidence supporting the BBT comes from detecting residual radiation that hasn't formed into matter. This cosmic microwave background radiation (CMBR) has a characteristic wavelength that matches BBT predictions precisely.

Discovery and measurement

In 1965, Robert Wilson and Arno Penzias made the first measurement of this predicted radiation, providing crucial evidence for the BBT.

Since then, sophisticated satellite missions have measured the CMBR with extraordinary precision:

• COBE (NASA, 1989-1993)
• WMAP (NASA, 2001-2011)
• Planck (European Space Agency, 2009-2013)

Universe expansion and evolution

The diagram illustrates how the Universe has expanded from the Big Bang through various eras to the present day. Notice the accelerating expansion in recent cosmic history, discovered through work by researchers including Australian Nobel Laureate Brian Schmidt and colleagues at the Australian National University. This acceleration appears to be driven by dark energy overcoming the gravitational attraction between matter and dark matter.

Summary timeline of cosmic history

Time since Big Bang	Era	Temperature (K)
$10^{-43}$ s	Planck	$>10^{32}$
$10^{-35}$ - $10^{-32}$ s	Inflation	$10^{14}$
$10^{-10}$ s	Force separation	$10^{13}$
$10^{-3}$ s (1 ms)	Particles	$10^{12}$
3-20 min	Nuclear fusion / Nucleosynthesis	$10^9$
Up to 380,000 years	Atoms	Falls to 3000
380,000 to 200 million years	Dark ages	Falls to 100
200 million to 380 million years	First stars	Falls below 100
400 million years to present	Galaxies	Falls to 2.7

Major events in each era

Planck era: Space and time were established. Temperatures were incredibly high.

Inflation era: Fundamental particles formed and unformed repeatedly. The strong force became distinct, and cosmic inflation occurred.

Force separation: Fundamental particles lasted long enough to interact. An explosion of photons resulted as matter and antimatter collided. The strong force, weak force, and electromagnetism separated.

Particle era: Fundamental particles stabilised. Protons, neutrons, electrons, and neutrinos became stable enough to persist.

Nuclear fusion era: Fusion processes began and then ceased. The primordial ratios of elements were established. A plasma of hydrogen, helium, and electrons filled the Universe.

Atom era: Photons that had been trapped were freed when atoms formed. The Universe became transparent, though no stars yet existed to produce light - only a faint hydrogen glow.

Dark ages: The Universe was transparent but dark, with cosmic background radiation travelling unimpeded.

First stars: Light from the first stars (50-500 solar masses) began to re-ionise surrounding gas clouds.

Galaxy era: Galaxies appeared and evolved. The Universe developed into its current state.