The Biggest Mystery in Physics
In 1998, two teams of astronomers made a shocking discovery: the expansion of the universe is not slowing down — it is speeding up. Distant supernovae were dimmer than expected, meaning they were farther away than a decelerating universe would allow. Something was pushing the cosmos apart with increasing force. We call it dark energy, and it constitutes 68% of everything that exists. We have almost no idea what it is.
The Cosmological Constant
The simplest explanation for dark energy is Einstein's cosmological constant Λ — a fixed energy density inherent to space itself. As the universe expands, matter dilutes but Λ stays constant, so dark energy eventually dominates. The simulation above lets you adjust the matter and dark energy densities to see how the balance between gravity and repulsion determines the universe's fate.
Cosmic Fate
The dark energy equation of state parameter w determines the universe's destiny. If w = -1 (cosmological constant), the universe ends in a Big Freeze — eternal, accelerating expansion into cold darkness. If w < -1 (phantom energy), the expansion rate itself accelerates, leading to a Big Rip that tears apart galaxies, stars, atoms, and eventually spacetime itself in finite time. The simulation computes the scale factor evolution for any combination of parameters.
The Hubble Tension
One of the most pressing puzzles in modern cosmology is the Hubble tension: different methods of measuring the universe's expansion rate give inconsistent answers. The cosmic microwave background gives H₀ ≈ 67 km/s/Mpc while local distance ladder measurements give H₀ ≈ 73 km/s/Mpc. This discrepancy may point to new physics beyond our standard model — perhaps a clue to the true nature of dark energy.