What is the expansion history of the Universe?

Overview

Besides the existence of life elsewhere, the origin and history of the Universe is arguably the biggest question of our time. How did the expansion start, and what is the dark energy driving today’s acceleration? Is the expansion homogeneous in time and isotropic in space? The lowest and most uncertain rungs of the cosmic distance ladder rely on time-variable objects, pulsating stars and Type Ia supernovae, and the physics of both, including how metallicity and dust change their properties, is still open. Gravitational-wave sirens offer a new route to distances once next-generation detectors see mergers throughout cosmic time, while wide binary stars may help to constrain the nature of dark matter.

Why the TDT?

The TDT will monitor thousands of Type Ia supernovae along many lines of sight, not just across the sky as a whole, over a large fraction of cosmic time, testing whether the expansion is isotropic as well as homogeneous. Precision measurements over look-back times of up to 10 Gyr reach back to when dark energy did not yet dominate the expansion, so the TDT will trace spectroscopically the onset and evolution of dark energy beyond the cosmic inflection point at z ≈ 0.6.

Two-panel plot against redshift from 0 to 1.3. Top: distance modulus rising from about 33 to 45 magnitudes, with 5000 simulated supernovae lying tightly along the standard Lambda-CDM curve. Bottom: the difference from a coasting universe with no acceleration; the Lambda-CDM curve rises to about 0.1 magnitudes near redshift 0.5 to 0.6 and returns to zero by redshift 1.3. — Simulated Hubble diagram of 5000 Type Ia supernovae, spread evenly in redshift out to z ≈ 1.3. The lower panel shows the inflection point in the cosmic expansion around z ≈ 0.6, caused by the onset of dark energy (H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_m = 0.3, Ω_Λ = 0.7).

Science themes

Type Ia supernovae as tracers

Gravitational-wave sirens

The cosmic distance ladder, near and far

Observing requirements

Requirement	Value
Target classes and magnitude range	Thousands of Type Ia supernovae out to z ≈ 1.3 (look-back times up to 10 Gyr), along many lines of sight; pulsating stars; gravitational-wave sirens; wide binaries

White papers

See all TDT publications.