big-bang

Hydrogen makes up approximately 74% of all ordinary matter in the universe by mass — about 92% of all atoms. Helium accounts for roughly 24% of ordinary mass. Everything else — every carbon atom in every living cell, every silicon atom in every rock, every iron atom in every star's core — adds up to approximately 2% of the universe's total atomic mass. The dominance of hydrogen and helium is not a coincidence or a geological quirk. It is a direct record of the conditions that existed in the first three minutes after the Big Bang. Understanding why the cosmos is almost entirely these two lightest elements tells you how the universe was born and how it will eventually end.

In 1964, Penzias and Wilson found persistent noise in a Bell Labs antenna they could not explain. It turned out to be the afterglow of the Big Bang. Here is how they found it.

Hubble's 1929 redshift observation tied galaxy speed to distance. See what the 24-galaxy data showed, why his constant was off 7x, and the tension today.

George Gamow predicted the cosmic microwave background, co-authored the αβγ paper on Big Bang nucleosynthesis, and bridged physics and DNA science.

The observable universe is 93 billion light-years across, but the real universe is far bigger — possibly infinite. Here's how we know.

The Big Bang explained: evidence from Hubble's law, the CMB, and primordial element abundances, plus the timeline of cosmic history and the universe's fate.

The Big Bang wasn't an explosion in space — it was an expansion of space itself. And the evidence for it is overwhelming.

In 1929, Edwin Hubble showed that galaxies recede from us at speeds proportional to their distance, the first direct evidence that the universe is expanding. The slope of that relation, the Hubble constant, is still debated: two precise methods disagree by about 5 sigma.

The cosmic microwave background (CMB) is thermal radiation from the universe's first 380,000 years, now cooled to about 2.7255 K. Its tiny ripples seeded every galaxy.

Cosmic inflation is a proposed burst of exponential expansion in the first fraction of a second after the Big Bang, stretching space by a factor of at least about 10²⁶. It explains why the universe is flat and uniform and where galaxies came from. Its best-known direct test, a 2014 claim by BICEP2, turned out to be dust.

Nucleosynthesis is the process by which atomic nuclei are built from simpler components. Hydrogen and helium were made in the first three minutes after the Big Bang. Every heavier element was forged inside stars or in their violent deaths. Much of the gold and platinum is made in neutron star mergers — cosmic events detected as gravitational waves.

The universe is 13.8 billion years old. But how do we know? Three independent methods — the cosmic microwave background, the oldest stars, and Hubble's constant — converge on the same answer. Here's the evidence.