Convert redshift (z) into lookback time and the age of the universe
When we look at distant galaxies, the light they emit arrives at our telescopes with a longer wavelength than it started with. In the visible spectrum, longer wavelengths are redder—hence the name "Redshift." Unlike the Doppler effect (which is caused by objects moving through space), cosmological redshift is caused by space itself expanding while the light is in transit.
The redshift value ($z$) is a direct measure of how much the universe has grown since that light was emitted. For example, at a redshift of $z=1$, the universe was half its current size when the light began its journey. By measuring redshift, astronomers can map the history of the universe's expansion over billions of years.
Because the speed of light is finite, looking at high-redshift objects is literally looking back in time. This calculator uses the ΛCDM Model (the standard model of cosmology) to turn that color shift into a time value. When you see a high redshift, you are seeing the universe as it was when it was much younger, hotter, and denser.
The rate at which the universe expands is governed by the Hubble Constant ($H_0$). Even though we call it a constant, its exact value is one of the biggest mysteries in modern physics. Different measurement methods give slightly different results, a problem scientists call the "Hubble Tension." This calculator uses the latest data from the Planck satellite to provide its estimates.