A History of the Universe

Flashcards covering key cosmological concepts, discovery of galactic expansion, Big Bang evidence, radiometric dating, and cosmic timescales.

Olbers' Paradox

The paradox asking why the night sky is dark if the universe is infinite and filled with an infinite number of stars, resolved by the fact that the universe is not infinitely old.

Edwin Hubble

The astronomer ($1889$–$1953$) who established that galaxies are vast, distant systems and discovered that nearly all galaxies are moving away from Earth.

Hubble Plot

A plot of galactic velocity in $\text{km/sec}$ against distance in $\text{MLY}$, illustrating that a galaxy's recession velocity increases linearly with distance.

Uniform Expansion

The phenomenon where every part of the universe expands equally, causing every galaxy to recede from every other galaxy without Earth or any single point being the center.

Balloon Analogy

An analogy demonstrating cosmic expansion, where galaxies on the expanding surface of a balloon move away from one another without any point on the surface being the center.

Singularity

The initial, extremely dense point in space-time from which all matter and energy in the universe originated at the Big Bang.

Georges Lemaitre

The Belgian Catholic priest and mathematician ($1894$–$1966$) who proposed that the universe had a definitive origin in the past and is expanding from a Big Bang.

Observable Universe

The region of space within approximately $14\text{ billion light years}$ of Earth, bounded by the distance light has been able to travel since the universe began.

Cosmic Microwave Background Spectrum

The blackbody radiation spectrum measured by COBE that provides direct evidence for the hot Big Bang fireball, shifted into microwave radio frequencies by expansion.

Cosmic Microwave Background Map

A full-sky map showing tiny temperature fluctuations (brighter and darker regions) in the early universe that reveal how matter began clumping together.

Radioactive Parent Atom

An unstable radioactive isotope that decays into a stable daughter atom over time at a fixed rate.

Radioactive Decay Curve

A graph plotting the exponential decrease in the number of radioactive parent atoms and the corresponding increase in stable daughter atoms over successive half-lives.

Half-Life

The amount of time required for $50\%$ of a sample's radioactive parent atoms to decay into stable daughter atoms.

Radioactive Elements Table

A list of parent isotopes, stable daughter products, and half-lives, such as Potassium-$40$ decaying to Argon-$40$ with a half-life of $1.25\text{ billion years}$.

Age of the Solar System

The age determined to be $4.6\text{ billion years}$ by measuring isotope ratios in the oldest solid meteorites and mineral grains.

Age of the Earth

Calculated from radiometric dating to be approximately $4.54\text{ billion years}$, slightly younger than the earliest solar system grains.

Carl Sagan's Cosmic Calendar

A scale model compressing the entire $14\text{-billion-year}$ timeline of the universe into a single calendar year, placing the Big Bang on January 1 and human history in the final minutes of December 31.