Explore 13.8 billion years of cosmic history, an early exponential stretch, and a puzzle scientists are still trying to solve.
Our expanding universe
Rewind the cosmic clock
Space between far-apart, freely moving regions grows as the universe expands. Rewind to see how a fixed region of space changes size. The Earth, Solar System and gravitationally bound galaxies do not stretch along with this flow.
Enable JavaScript to explore. The example shows the present day in an approximate cosmological model.
Relative size
1.000
Scale factor · today = 1
Expansion rate
67.4
km/s per megaparsec in this model
Redshift
0.00
z = 1 ÷ scale factor − 1
Comoving tracers, not real objects. The display uses a logarithmic size scale so early times remain visible. Read the scale factor for physical size.Redshift describes how light emitted at the selected scale factor would be stretched by today. The curve models the period after early cosmic inflation. It is an educational approximation, not a reconstruction of every stage of the universe.
Read the graph as a table
Selected scale factors in the fixed model
Scale factor
Time since hot Big Bang
0.000909
366 thousand years
0.0100
16.6 million years
0.100
542.5 million years
0.250
2.14 billion years
0.500
5.84 billion years
1.00
13.79 billion years
An unsolved question
How fast is space expanding today?
Scientists infer today’s expansion rate in different ways. A model fitted to the early universe and measurements of the nearby universe give different answers. This disagreement is called the Hubble tension.
One megaparsec is about 3.26 million light-years. This illustration uses speed = H₀ × distance for an idealised present-day Hubble flow.
Planck · flat ΛCDM
Planck 2018 analysis · published 2020
67.40 ± 0.50
km/s per megaparsec · reported uncertainty
H₀ Distance Network
Distance-network study · 2026
73.50 ± 0.81
km/s per megaparsec · reported uncertainty
At 100 Mpc, the two values give recession speeds differing by 610 km/s.
The Planck value is inferred from cosmic microwave background observations under flat ΛCDM. The 2026 H₀ Distance Network combines nearby-distance measurements. These are two examples of current approaches, not the entire field. Their reported errors do not capture every possible model or systematic uncertainty. This comparison does not resolve the tension or prove new physics.
Galaxies also have local motions. This simple relation does not predict a particular galaxy’s observed redshift. The expansion rate changes with cosmic time, so 1 ÷ H₀ is not automatically the age of the universe.
A very early stretch
Cosmic inflation is a different idea
Early cosmic inflation proposes an extremely rapid expansion in the universe’s first tiny fraction of a second. It is distinct from the later expansion shown above—and unrelated to rising shop prices.
Try a mathematical stretch. Each e-fold multiplies a length by e (about 2.718). This control illustrates exponential growth; it does not measure how long inflation lasted or how many e-folds actually occurred.
After 10 e-folds, a length is multiplied by about 22,026.
Equal steps on this ruler are powers of ten. The drawing is not a picture of the universe’s physical size.
The Big Bang describes an expanding universe evolving from a hot, dense state, not a blast from one location into empty space. Expansion of space can separate distant regions faster than light without an object locally overtaking a light beam.
What this model assumes
The clock and curve use a flat ΛCDM approximation with H₀ = 67.4 km/s/Mpc, matter fraction 0.315, effective radiation fraction 0.000092, and dark-energy fraction 0.684908. We integrate dt = da ÷ [a H(a)] numerically. The resulting present age is about 13.79 billion years, close to the familiar 13.8-billion-year estimate. Radiation is simplified; this is not a precision cosmology solver.
“First light” refers to photons travelling freely after recombination, not the first stars. The model’s approximate value may differ from the rounded 380,000-year landmark. Earth’s formation is shown roughly 4.54 billion years before today. These landmarks are approximate, not exact anniversaries.