The Universe & Big Bang

Year 10 answers

Fluency

Moon $<$ planet $<$ star $<$ solar system $<$ galaxy $<$ galaxy cluster.
About $200$ billion.
Andromeda (M31).
The distance that light travels in one year, about $9.5$ trillion km.
A region of space where gravity is so strong that nothing, not even light, can escape.

Fluency

Redshift (Hubble’s law); cosmic microwave background; abundance of light elements (H and He).
The recession speed of a galaxy is proportional to its distance: $v = H_0 d$ .
The faint thermal radiation from when the universe first became transparent ( $\sim 380\,000$ years after the Big Bang). It is direct evidence of the hot, dense early universe.
About $75\%$ hydrogen, $25\%$ helium by mass (of ordinary matter).
About $13.8$ billion years.

Fluency

Nuclear fusion of hydrogen into helium (and later heavier elements).
A star that is steadily fusing hydrogen to helium in its core — the longest, most stable life stage.
Red giant, planetary nebula, white dwarf.
Supernova followed by a neutron star or black hole.
In supernova explosions and neutron-star mergers.

Reasoning

A passing ambulance’s siren is higher pitched as it approaches and lower as it recedes. Similarly, light from receding galaxies is stretched to longer (redder) wavelengths. Galaxies all showing redshift indicates the universe is expanding.
If all galaxies are moving apart, running the film backwards shows they were all together at one point in the past. Extrapolating gives a starting time about $13.8$ Gyr ago.
Before recombination, the universe was so dense it behaved like a uniform plasma; all regions had the same temperature. Inflation ironed out any variations, so the CMB is almost perfectly uniform except for tiny fluctuations.
Light takes time to travel. The further away we look, the longer the light has been in transit, so we see objects as they were in the past.
Massive stars burn their fuel much faster to counteract their stronger gravity; higher temperatures and pressures consume hydrogen in $\sim 10$ Myr vs the Sun’s $\sim 10$ Gyr.

Problem solving

$d = \dfrac{21\,000}{70} = 300$ Mpc $\approx 978$ million ly.
$\sim 2.5$ million years. You are seeing Andromeda as it was $2.5$ Myr ago.
$5 \times 10^9 \text{ yr} \times 3.156 \times 10^7 \text{ s/yr} \approx 1.58 \times 10^{17}$ s.
A supernova releases in seconds roughly the same energy the Sun emits over its entire $10^{10}$ -year lifetime. Supernovae are extraordinarily energetic, visible across the universe.

Reasoning

As the universe expands, photon wavelengths are stretched by the same factor. Going from $\sim 3\,000$ K (emission) to $2.73$ K (today) corresponds to a $\sim 1\,100$ -fold stretch — the universe is now $1\,100$ times larger than at the epoch of recombination.
$z = 1.0$ means observed wavelength is twice emitted. Wavelength has been stretched by a factor $1 + z = 2$ , meaning space itself has doubled in scale since the light was emitted. So the universe was half its current size.
The two methods probe different epochs (early universe vs nearby universe). If both are right, something in between (new physics, dark energy behaviour) may be missing from the standard model of cosmology.
Life requires carbon, oxygen, nitrogen, calcium, iron — none of which existed in the pristine Big Bang hydrogen/helium mix. Stars had to form, die, and seed the gas clouds with heavy elements before planets and life were possible. This pushes first-life possibilities back only once enough massive stars had exploded — likely $\sim 1$ - $2$ Gyr after the Big Bang.