Big Bang Theory Lecture Notes

The Beginning of Everything

Learning Intentions

• Understand the timeline of key events of the Big Bang Theory.
• Explain how evidence has led to the development of the Big Bang Theory.
• Understand the changing nature of scientific theories.

The Big Bang Theory

Scientists have gathered extensive evidence about the Universe and developed the Big Bang Theory.

The Big Bang Theory states:

• About 13.7 billion years ago, all matter in the Universe was concentrated into a singularity (an incredibly tiny point).
• A singularity is a point where current mathematical models fail to calculate size and density due to its infinitesimally small size.
• This singularity rapidly expanded in a hot expansion, which continues today.

Rapid Expansion

• The theorized singularity was extremely small, containing all the matter in the universe.
• All matter possessed energy, making the singularity incredibly hot and dense.
• The singularity could not contain the mass and energy, leading to rapid expansion.
• This rapid expansion initiated the universe, and objects have been moving away from each other since.

Gravity: Bringing Everyone Together

• The universe is lumpy, with regions of empty space.
• Matter attracts more matter due to gravity, causing it to clump together and grow.
• Anything with mass has an attractive force (gravity) that draws more mass toward it.
• Gravity caused atoms to form large enough masses to create stars.
• Gravity can slow down the momentum of expanding objects.
• Sir Isaac Newton proposed the theory of gravity in 1687.

Big Bang Theory: A Rough Timeline of Key Events

• 13.7 billion years ago: The entire Universe was inside a singularity, hotter and denser than anything imaginable.
• Time, space, and matter all began with the Big Bang.
• In a fraction of a second, the Universe grew from smaller than a single atom to bigger than a galaxy.
• No atoms formed initially due to extreme heat; only protons and neutrons existed.
• 380,000 years after the Big Bang:
  • The universe cooled to 3000 Kelvin.
  • Atoms formed with nuclei and electrons.
  • Photons could scatter and stream through space, making visible light possible.
• 380,000 – 1 million years:
  • Dark Ages: a period with limited information.
  • Atoms collided and gathered mass.
  • The first stars and galaxies formed approximately 100 million years after the Big Bang.
  • The oldest observable stars formed around 600 million years after the Big Bang.
  • 9 billion years Our solar system forms.

Future of the Expanding Universe

• Everything observable in the universe is due to light detected from distant stars.
• The oldest stars are 13.7 billion light-years away.
• Movement is tracked by observing the movement of distant galaxies.
• Our night sky shows stars of the Milky Way galaxy only.
• If the universe keeps expanding indefinitely, galaxies might become unobservable, impacting the future of astronomy.

Light in the Universe

• Each night sky is different due to the expansion of the universe.
• Light from stars has been traveling for billions of years to reach us.
• Analysis of starlight provides insights into distant stars and galaxies.

The Doppler Effect

• The Doppler effect allows us to measure the relative movement of stars in relation to the Earth.
• Movement can be towards or away from the Earth.
• We can estimate the direction a star is traveling using the Doppler effect.

The Doppler Effect and Sound

• The Doppler effect can be demonstrated using sound, as it follows similar principles.

Doppler Effect Explained

• Sound moves in waves; higher frequency waves (more bunched up) correspond to higher pitch.
• As an object moves closer, we encounter higher frequency waves, resulting in a higher-pitched sound.
• As an object moves away, we encounter lower frequency waves, resulting in a lower-pitched sound.

Visual Representation of Doppler Effect

• Cars sound lower pitched as they speed away.
• Galaxies look redder as they speed away.
• Stretched waves correspond to lower pitch and redder light.
• Bunched waves correspond to higher pitch and bluer light.

Light Waves and the Electromagnetic Spectrum

• Lower frequency light waves appear red.
• Higher frequency light waves appear blue.
• Light energy is a small part of the electromagnetic spectrum.

Spectroscopy

• Spectroscopy involves passing emitted light from a luminous object through a prism.
• The light is split into its component colors on the spectrum.
• Observing the spectrum reveals the chemicals absorbed or emitted by the light source.

Using Light Spectrums

• In a light spectrum, black lines indicate specific elements that have absorbed light.
• Different elements display unique patterns of light absorption, resulting in different black bands.
• The wavelengths that absorb light indicate the elements present in a star.

Shifting Lines on the Spectrum – Doppler Effect in Action

• Tracking the movement of stars relative to Earth is possible by observing the shift in their emission or absorption spectrum lines over time.
• The patterns in the lines remain the same but shift along the visible light spectrum depending on the star's movement.
• Light sources moving away from the observer shift towards the lower frequency red end of the spectrum, known as redshift.
• Light sources moving towards the observer shift towards the higher frequency blue/violet end of the spectrum, known as blueshift.

Evidence for an Expanding Universe

• The spectrum of hydrogen gas is the unique fingerprint of that element.
  When we see a repeat of the pattern we saw in the lab, we know hydrogen is present.

• Observing the displacement of repeating patterns of lines in a galaxy towards the red indicates its movement away from us. For example, Galaxy UGC 12915 is moving at a velocity of 4350 km/s.

• The further the galaxy, the more the shift to the red with Galaxy UGC 12508 moving at a velocity of 9100 km/s.

• Greater redshift indicates faster recession. Galaxy KUG 1750 moving at a velocity of 15,400 km/s.

• The redshift is caused by the expansion of space. Galaxy KUG 1217 moving at 31,400 km/s and has redshifted into the infrared.

Findings from Distant Galaxies

• Scientists found all the same lines shifted to the red side of the spectrum, indicating a redshift.
• The Doppler Effect explains this redshift.

Hubble’s Diagram

• Using redshifts of distant galaxies, astronomers found a linear relationship between the distance to galaxies and their recession velocity.
• The degree of redshift indicates the recession velocity.

Hubble’s Diagram - Axes Units

• Velocity axis is measured in kilometers per second (km/s).
• Distance axis is measured in Megaparsecs (Mpc).

Hubble’s Diagram - Megaparsec

• One megaparsec (Mpc) equals 3.26 million light-years, or 3.08 \times 10^{19} kilometers

Age of the Universe from Hubble’s Law

• The age of the universe can be estimated from Hubble's Law by taking the reciprocal of the Hubble constant.
• Assuming a constant rate of expansion:
  distance = velocity \times time
  time = \frac{distance}{velocity}
• Rearranging Hubble's Law for \frac{d}{v} provides an expression for the time it would take for distant galaxies to move away from Earth.

Summary of Evidence for the Big Bang

• The Doppler effect:
  • All galaxies are moving away from us, indicated by redshift.
  • The further away a galaxy is, the faster it is moving.
  • These features are consistent with explosions.
• Cosmic Microwave Background Radiation (CMBR):
  • Detected from all parts of the Universe.
  • Thought to be the heat left over from the original explosion.