The expansion of the universe is demonstrated primarily through several key measurements:
1. Redshift-Distance Relationship (Hubble's Law)
- Edwin Hubble discovered that galaxies are moving away from us, and their recessional velocity increases with distance. This relationship is quantified by the Hubble constant (H0), which measures how fast the universe expands per unit distance. For example, recent measurements give values around 70 to 74 kilometers per second per megaparsec (km/s/Mpc), meaning that for every megaparsec (about 3.26 million light-years) farther away a galaxy is, it appears to be moving away about 70 km/s faster
2. Standard Candles: Cepheid Variables and Type Ia Supernovae
- Astronomers use objects with known intrinsic brightness to measure distances accurately. Cepheid variable stars pulsate at rates that correlate with their true brightness, allowing distance measurement. Type Ia supernovae have a consistent peak brightness, serving as reliable distance markers even in faraway galaxies. By comparing their known brightness to observed brightness, distances can be calculated, and combined with redshift data, the expansion rate is derived
3. Cosmic Microwave Background (CMB) Radiation
- The CMB is the relic radiation from the early universe, providing a snapshot of the universe about 380,000 years after the Big Bang. Measurements of the CMB’s temperature fluctuations and anisotropies by satellites like Planck help determine cosmological parameters, including the expansion rate. The CMB data predicts a slightly different expansion rate than local measurements, which has led to ongoing research and debate
Summary
- The main evidence for the universe’s expansion comes from measuring how fast galaxies recede as a function of their distance (Hubble’s Law), using standard candles like Cepheid variables and Type Ia supernovae to establish distances, and analyzing the cosmic microwave background radiation to understand early universe conditions and expansion history. These methods collectively show that the universe is expanding and that this expansion rate can be precisely measured, though some tension remains between different measurement techniques
This multi-faceted approach confirms that space itself is stretching, causing galaxies to move away from each other over time.
