Cosmological Models

In 1929, astronomer Edwin Hubble measured the light from distant galaxies and found something startling: nearly all of them were moving away from us, and the farther away a galaxy was, the faster it moved. That single observation upended thousands of years of belief that the universe was fixed and unchanging.

How the geocentric and heliocentric models differ historically. What evidence supports the Big Bang model of the universe. How redshift reveals the universe's expansion. What cosmic microwave background radiation is and why it matters.

For over 1,400 years, Ptolemy's geocentric model (Earth at the center) dominated Western astronomy. In 1543, Nicolaus Copernicus published a heliocentric model placing the Sun at the center, which Galileo's telescope observations in 1610 -- including the moons of Jupiter -- helped confirm.

When a light source moves away from an observer, its wavelengths stretch and shift toward the red end of the spectrum -- this is called redshift, similar to how a siren's pitch drops as an ambulance drives away (the Doppler effect). Hubble's 1929 data showed nearly all distant galaxies were redshifted, meaning the space between galaxies is expanding.

If galaxies are moving apart today, running the clock backward implies the universe was once compressed into an extremely hot, dense state approximately 13.8 billion years ago. This is the Big Bang model, and it is supported by two major pieces of evidence beyond redshift.

In 1965, Arno Penzias and Robert Wilson accidentally detected faint microwave radiation coming from every direction in the sky. This cosmic microwave background (CMB) is the leftover afterglow from about 380,000 years after the Big Bang, when the universe first became transparent to light, and it earned them the 1978 Nobel Prize in Physics.