The UniverseCambridge IGCSE Physics: Revision notes
Section 1
What is the structure of the Universe and the Milky Way?
The Universe contains billions of galaxies, each containing billions of stars. Our own galaxy, the Milky Way, is one of these. The Milky Way has a diameter of approximately 100,000 light-years, which is the distance light travels in one year through the vacuum of space.
- A light-year is a unit of distance, not time, used to measure vast cosmic distances
- Each galaxy consists of billions of stars held together by gravity
- The Milky Way is a relatively large spiral galaxy, but is dwarfed by the scale of the Universe containing billions of other galaxies
Think of the Universe as a vast ocean, galaxies as continents, and our Solar System as a single town on that continent — we are incredibly small on a cosmic scale.
Section 2
What is redshift and how does it indicate an expanding Universe?
Redshift is the increase in observed wavelength of electromagnetic radiation emitted from receding (moving away) stars and galaxies. When a source moves away from an observer, the waves are stretched, increasing their wavelength and shifting them towards the red end of the electromagnetic spectrum.
- Light from distant galaxies is redshifted compared to light emitted on Earth
- This redshift occurs because the galaxies are moving away from us due to the expansion of the Universe
- The greater the recession velocity of a galaxy, the greater the redshift observed
- Redshift provides direct observational evidence that the Universe is expanding and supports the Big Bang Theory
The Big Bang Theory states that all matter in the Universe originated from a single point and has been expanding ever since.
Examiners expect you to clearly link redshift to the expanding Universe and the Big Bang Theory. Always state that redshift is evidence that galaxies are moving away from us, which proves the Universe is expanding.
Students often confuse redshift with the colour of the object itself. Redshift is not about the colour of the galaxy — it's about how the wavelength of its light has been stretched due to motion away from us.
Section 3
What is cosmic microwave background radiation (CMBR) and what does it tell us?
Cosmic microwave background radiation (CMBR) is microwave radiation of a specific frequency observed at all points in space. This radiation is uniform throughout the Universe, coming from all directions.
- The CMBR was produced shortly after the Universe was formed, within the first few hundred thousand years of the Big Bang
- As the Universe expanded rapidly, this radiation was stretched into the microwave region of the electromagnetic spectrum
- The CMBR is a direct remnant of the hot, dense early Universe and provides strong evidence for the Big Bang Theory
- The fact that CMBR is detected everywhere in space suggests the Universe originated from a single point
- Modern measurements show CMBR has a very consistent temperature across all directions, supporting the Big Bang model
The CMBR is sometimes called the "afterglow" of the Big Bang.
Examiners expect you to explain that CMBR is evidence for the Big Bang by stating: it was created in the early hot Universe, has been stretched to microwave wavelengths by cosmic expansion, and is observed uniformly in all directions.
Section 4
How is the recession velocity of a galaxy determined from redshift?
The recession velocity of a galaxy — the speed at which it is moving away from Earth — can be calculated from the change in wavelength of its emitted light due to redshift.
When analysing light from distant galaxies:
- Measure the observed wavelength (λ_observed) of specific spectral lines
- Compare this to the reference wavelength (λ_rest) measured from the same element on Earth
- Calculate the change in wavelength (Δλ) = λ_observed − λ_rest
- Use the relationship: the greater the change in wavelength, the greater the recession velocity
- Spectral lines from hydrogen and other elements provide reference points for comparison
- The redshift magnitude directly correlates with how fast the galaxy is receding
- This method relies on the Doppler effect, which explains how motion affects observed wavelength
- A larger redshift indicates a galaxy moving away faster
If hydrogen's spectral line is normally at 656 nm but appears at 700 nm in a distant galaxy, the change in wavelength is 44 nm. This redshift indicates the galaxy is receding from Earth at a calculable velocity using the Doppler effect relationship.
Section 5
How is the distance to a far galaxy determined and what is the Hubble constant?
Distance to a far galaxy can be determined by measuring the brightness of a supernova in that galaxy. A supernova (an exploding star) has a known intrinsic brightness (absolute magnitude). By comparing the observed brightness to the intrinsic brightness, astronomers can calculate distance using the inverse square law.
The Hubble constant (H₀) relates the recession velocity of a galaxy to its distance:
H₀ = v / d
Where:
- v = recession velocity of the galaxy (in metres per second)
- d = distance to the galaxy (in metres)
- H₀ = Hubble constant = 2.2 × 10⁻¹⁸ per second (current estimate)
Rearranging the equation: d = v / H₀
- Hubble's Law states that more distant galaxies recede faster from us
- The Hubble constant is the proportionality constant linking velocity and distance
- By measuring both the redshift (which gives v) and observing supernovae (which gives d), we can verify Hubble's Law
When using the Hubble equation, always ensure you substitute the correct value for H₀ (2.2 × 10⁻¹⁸ s⁻¹). Always check your units and show your working clearly in exam questions.
If a galaxy has recession velocity v = 6.6 × 10⁷ m/s and H₀ = 2.2 × 10⁻¹⁸ s⁻¹, then distance d = v/H₀ = (6.6 × 10⁷) / (2.2 × 10⁻¹⁸) = 3 × 10²⁵ m.
Section 6
What does the age of the Universe tell us about the Big Bang?
An estimate for the age of the Universe can be found using the rearranged Hubble equation:
d/v = 1/H₀
Since distance divided by velocity gives time, this equation estimates how long the Universe has been expanding:
Age of Universe ≈ 1/H₀
Calculating: 1 / (2.2 × 10⁻¹⁸) ≈ 4.5 × 10¹⁷ seconds ≈ 14 billion years
Evidence for the Big Bang:
- This calculation assumes the Universe has been expanding at a constant rate since a single point
- The fact that we can trace all matter back to one location in the past supports the Big Bang Theory
- This represents the time since all matter was present at a single point of infinite density and temperature
- The agreement between the age derived from Hubble measurements and other independent methods (like the age of the oldest stars) strengthens confidence in the Big Bang model
Examiners want to see that you understand the equation d/v = 1/H₀ estimates the age of the Universe and represents evidence that all matter was once at a single point. Always state this as an estimate, not a definitive fact, as the expansion rate may have changed over time.
Must Know
- The Milky Way is one of billions of galaxies, each containing billions of stars, with a diameter of approximately 100,000 light-years
- Redshift (increase in observed wavelength from receding objects) is evidence that the Universe is expanding and supports the Big Bang Theory
- Cosmic microwave background radiation (CMBR) is microwave radiation observed uniformly throughout space, created shortly after the Big Bang and stretched by the expanding Universe
- Hubble's Law: H₀ = v/d, where v is recession velocity, d is distance, and H₀ = 2.2 × 10⁻¹⁸ s⁻¹
- Distance to far galaxies is determined using the brightness of supernovae; recession velocity is found from redshift measurements
- The age of the Universe (≈14 billion years) is estimated from 1/H₀, providing evidence that all matter originated from a single point
That's the notes covered.
Carry on to the next subtopic.