CIE IGCSE Physics (0625) The Universe Study Notes - New Syllabus
CIE IGCSE Physics (0625) The Universe Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of The Universe
Key Concepts:
- The Milky Way, Galaxies, and the Universe
- Redshift
- Cosmic Microwave Background Radiation (CMBR)
- Speed of a Galaxy Using Redshift
- Hubble’s Constant and the Expanding Universe
The Milky Way, Galaxies, and the Universe
The Milky Way, Galaxies, and the Universe
The Milky Way is the galaxy that contains our Solar System. It is just one of the many billions of galaxies in the Universe. Each galaxy is a massive system of stars, dust, gas, and dark matter held together by gravity.
Key Facts
- The Milky Way is a spiral galaxy.
- It contains over 100 billion stars, including our Sun.
- Its diameter is approximately 100,000 light-years.
- The Solar System is located on one of the spiral arms, about 27,000 light-years from the center.
- The Universe contains billions of galaxies, each with billions of stars.
Galaxy vs Universe
| Term | Description |
|---|---|
| Milky Way | The galaxy we live in, containing the Sun and solar system. About 100,000 light-years across. |
| Galaxy | A huge collection of stars, gas, dust, and dark matter held together by gravity. |
| Universe | All of space and everything in it, including all galaxies, stars, planets, and cosmic matter. |
Example:
Light from a star located on the far side of the Milky Way takes about 100,000 years to reach Earth. What does this imply?
▶️ Answer/Explanation
Since the diameter of the Milky Way is 100,000 light-years, it means that light from the far edge of our galaxy takes 100,000 years to reach us. So, when we observe that star, we’re actually seeing it as it was 100,000 years ago.
Redshift
Redshift
Redshift is the increase in the observed wavelength of light (or other electromagnetic radiation) emitted by a source that is moving away from the observer.
This means that the light is shifted toward the red end of the visible spectrum (which has longer wavelengths).
Why Does Redshift Occur?
- When a star or galaxy moves away from Earth, the light waves are stretched, increasing their wavelength.
- This shift in wavelength causes the light to become redder hence, the name “redshift”.
Key Concepts
| Term | Definition |
|---|---|
| Redshift | Increase in the observed wavelength of light from objects moving away from us. |
| Receding Galaxy | A galaxy that is moving away from the Earth and shows redshifted light. |
| Expanding Universe | The idea that galaxies are moving away from each other and space itself is stretching. |
Observational Evidence
- Light from distant galaxies is redshifted when compared to light from sources on Earth.
- The further away a galaxy is, the more its light is redshifted – meaning it’s moving away faster.
- This suggests that the Universe is expanding in all directions.
Redshift and the Big Bang Theory
The fact that galaxies are moving away from us (and from each other) implies that the Universe was once much smaller and denser. This supports the Big Bang Theory, which states that the Universe began from a very small, hot, and dense point and has been expanding ever since.
Example:
Light from a distant galaxy is observed to have a longer wavelength than the same light emitted on Earth. What does this tell us about the galaxy’s motion?
▶️ Answer/Explanation
This is evidence of redshift. The increased wavelength means the galaxy is moving away from us. The redshift indicates that space between us and the galaxy is expanding. Therefore, this supports the idea that the Universe itself is expanding, as described by the Big Bang Theory.
Cosmic Microwave Background Radiation (CMBR)
Cosmic Microwave Background Radiation (CMBR)
Cosmic Microwave Background Radiation (CMBR) is a type of low-energy microwave radiation that is observed in all directions in space. It has a very uniform distribution and is considered a “snapshot” of the early Universe.
Key Facts About CMBR
| Fact | Description |
|---|---|
| Type of radiation | Microwave (electromagnetic wave with long wavelength and low frequency) |
| Frequency range | ~160 GHz (very specific frequency observed everywhere in space) |
| Uniformity | Detected from all directions in the Universe |
Origin of the CMBR
Shortly after the Big Bang (around 380,000 years later), the Universe cooled enough for atoms to form, allowing radiation to travel freely. This initial radiation was very high-energy gamma rays.
As the Universe expanded, space itself stretched — this caused the high-energy radiation to be redshifted into the microwave region. This is now observed as CMBR.
How Expansion Affects the Radiation
- Original radiation from the early Universe was high-frequency (gamma/infrared).
- As the Universe expanded, the wavelengths were stretched (redshifted).
- This lowered the frequency, converting the radiation into microwave radiation.
Importance of CMBR
- CMBR provides strong evidence for the Big Bang Theory.
- It shows that the Universe had a hot, dense origin.
- Its uniform presence supports the idea that the early Universe was very uniform and homogeneous.
Example:
A student claims that microwave radiation from space is evidence of the Big Bang. Explain why this is correct.
▶️ Answer/Explanation
After the Big Bang, the Universe was hot and filled with high-energy radiation. As the Universe expanded, this radiation cooled and was redshifted into the microwave region. The fact that we still detect this microwave radiation in all directions is evidence that the Universe started from a hot, dense state – supporting the Big Bang Theory.
Speed of a Galaxy Using Redshift
Speed of a Galaxy Using Redshift
The light from galaxies moving away from Earth is redshifted – its wavelength becomes longer.
The faster a galaxy is receding, the greater the redshift.
Redshift Formula for Speed
\(\frac{\Delta \lambda}{\lambda} = \frac{v}{c} \)
- \( \Delta \lambda \) = observed increase in wavelength (i.e., redshift amount)
- \( \lambda \) = original emitted wavelength
- \( v \) = speed at which the galaxy is receding from Earth
- \( c \) = speed of light (\(3 \times 10^8\) m/s)
Explanation:
- When a galaxy moves away, the light waves stretch out → wavelength increases → redshift.
- The faster the galaxy moves, the greater the increase in wavelength (\( \Delta \lambda \)).
- Using the redshift formula, we can calculate the galaxy’s velocity \(v\).
Distance to a Galaxy Using Supernova Brightness
Certain types of supernovae (e.g., Type Ia) have a known peak brightness (they are “standard candles”). Comparing their apparent brightness from Earth with their known actual brightness allows us to determine their distance.
Inverse Square Law of Light
\( \text{Apparent Brightness} \propto \frac{1}{\text{distance}^2} \)
- If a supernova appears faint, it must be far away.
- If a supernova appears very bright, it must be closer.
- By comparing observed brightness to expected brightness, astronomers calculate the distance to the galaxy hosting the supernova.
Why These Two Methods Matter Together
- Redshift → gives the speed at which a galaxy is moving away.
- Supernova brightness → gives the distance to the galaxy.
- Together, they provide evidence for the expanding Universe and allow us to determine the Hubble constant \(H_0\) using Hubble’s Law:
\( v = H_0 \cdot d \)
Example:
A distant galaxy shows a redshift such that \( \frac{\Delta \lambda}{\lambda} = 0.02 \). Calculate how fast the galaxy is moving away from the Earth.
▶️ Answer/Explanation
Use the redshift formula: \( \frac{\Delta \lambda}{\lambda} = \frac{v}{c} \Rightarrow v = \frac{\Delta \lambda}{\lambda} \cdot c = 0.02 \cdot 3 \times 10^8 = 6 \times 10^6 \text{ m/s} \)
Final Answer: \( \boxed{6 \times 10^6 \text{ m/s}} \)
Hubble’s Constant and the Expanding Universe
Hubble’s Constant and the Expanding Universe
The Hubble constant \( H_0 \) is the ratio of the speed at which a galaxy is receding from the Earth to its distance from the Earth.
Hubble’s Law Equation
\( H_0 = \frac{v}{d} \)
- \( H_0 \) = Hubble constant (in s\(^{-1}\))
- \( v \) = velocity of galaxy moving away (in m/s)
- \( d \) = distance to the galaxy (in m)
Current Estimate of $H_0$
\( H_0 \approx 2.2 \times 10^{-18} \ \text{s}^{-1} \)
- This value means that for every meter of distance, a galaxy moves away at a rate of about \( 2.2 \times 10^{-18} \) m/s.
- It is derived from astronomical observations of redshift and supernova distances.
Estimating the Age of the Universe
If galaxies have been moving apart since the Big Bang, the inverse of the Hubble constant gives an estimate of how long they’ve been separating – the age of the Universe.
\( \frac{d}{v} = \frac{1}{H_0} \)
- \( \frac{d}{v} \) gives a time (in seconds) — an estimate of the time since galaxies started separating from a common origin.
- This suggests the Universe began from a single point: strong evidence for the Big Bang Theory.
Interpretation:
- The equation \( \frac{1}{H_0} \) gives a rough age of the Universe.
- Using \( H_0 = 2.2 \times 10^{-18} \ \text{s}^{-1} \):
\( \text{Age} \approx \frac{1}{2.2 \times 10^{-18}} = 4.545 \times 10^{17} \ \text{s} \)
Convert to years: \( \frac{4.545 \times 10^{17}}{60 \times 60 \times 24 \times 365} \approx 14.4 \times 10^9 \ \text{years} \)
→ Estimated age of the Universe ≈ \(\boxed{14.4 \ \text{billion years}}\)
Example:
A galaxy is moving away from Earth at a speed of \( 3.3 \times 10^5 \ \text{m/s} \). Estimate its distance from Earth using \( H_0 = 2.2 \times 10^{-18} \ \text{s}^{-1} \).
▶️ Answer/Explanation
Using \( H_0 = \frac{v}{d} \Rightarrow d = \frac{v}{H_0} \)
\( d = \frac{3.3 \times 10^5}{2.2 \times 10^{-18}} = 1.5 \times 10^{23} \ \text{m} \)