Calculating Galactic Velocity: Astronomers determine how fast a galaxy is moving relative to Earth by comparing the observed wavelength of light () to a known laboratory reference wavelength (). The velocity () is found using the ratio of the change in wavelength to the reference wavelength, multiplied by the speed of light ().
Measuring Red-shift (z): The degree of red-shift is quantified by . By plotting this value against the distance of galaxies, astronomers have observed a linear relationship: the further a galaxy is, the greater its red-shift and the faster its recession speed.
Mapping the CMB: Space-based telescopes are used to detect microwave radiation from above the Earth's atmosphere to avoid absorption. These maps show a nearly uniform temperature of 2.73 K, with tiny fluctuations that correspond to regions of slightly higher or lower matter density.
| Feature | Big Bang Theory | Steady State Theory |
|---|---|---|
| Origin | Expanded from a single hot point | No beginning or end |
| Density | Decreases as space expands | Constant (new matter created) |
| Evidence | Red-shift and CMB | Red-shift only |
| CMB Explanation | Remnant of the initial hot stage | Cannot explain it |
The Doppler Formula: In exams, you must be careful not to confuse the observed wavelength () with the reference wavelength (). Always subtract the smaller unshifted wavelength from the larger shifted wavelength to find for receding objects.
Evidence Pairing: When asked for evidence for the Big Bang, always provide both: 1) Galactic red-shift, which proves expansion, and 2) CMB radiation, which proves the Universe had a hot, dense beginning. Mentioning only one is often insufficient for full marks.
Balloon Analogy Logic: If asked why the density of galaxies falls, use the balloon analogy. As the 'skin' of the balloon (space) stretches, the dots (galaxies) get further apart while their number stays the same, meaning there are fewer galaxies per unit of volume.
Expansion vs. Movement: A common mistake is thinking galaxies are 'flying through space' like shrapnel from a bomb. In reality, galaxies are relatively stationary within their local space, but the 'fabric' of space between them is growing, which causes the observed separation.
The Center of the Universe: Students often look for a 'starting point' on a map. However, because space itself is expanding, every observer in every galaxy sees all other galaxies moving away from them, making it appear as if they are at the center when no true center exists.
Microwaves vs. Visible Light: Do not assume the Big Bang produced microwaves directly. It produced high-energy gamma radiation; it is only because the Universe has stretched so much over 14 billion years that these waves have been lengthened into the microwave region.
Supernovae as Standard Candles: The evidence for accelerated expansion comes from observing Type Ia supernovae in distant galaxies. Their known brightness allows us to measure exact distances, confirming that distant galaxies are receding even faster than predicted by early models.
The Fate of the Universe: The expansion rate suggests different possible futures for the Universe, depending on the total mass and dark energy content. If expansion continues to accelerate, the Universe will continue to cool and galaxies will eventually become invisible to one another.
