Explore cosmic expansion using Hubble's Law. Calculate recession velocities, redshifts, and the age of the universe from distant galaxies.
Last Updated: 5/6/2026
1 Mpc ≈ 3.26 million light-years; Andromeda ≈ 0.77 Mpc
Current estimates: ~67–74 km/s/Mpc (tension between local & CMB measurements)
In 1929, Edwin Hubble discovered that nearly all galaxies are moving away from us, and crucially, the farther they are, the faster they recede. This observation revolutionized our understanding of the cosmos, providing the first direct evidence that the universe is expanding—a cornerstone of the Big Bang theory. The universe is not a fixed stage where galaxies move through space; rather, space itself is stretching, carrying galaxies apart (except for local gravitational clusters). This expansion continues today, and discoveries in 1998 revealed that it is accelerating, driven by a mysterious phenomenon called dark energy that comprises ~68% of the universe's total density.
The Hubble Constant (H₀) quantifies the current expansion rate in units of km/s per megaparsec of distance. Its reciprocal (1/H₀) provides a rough estimate of the age of the universe—approximately 13.8 billion years—assuming constant expansion (a simplification, since acceleration has varied). Today, a significant "Hubble tension" exists: local measurements yield ~73 km/s/Mpc while measurements from the cosmic microwave background suggest ~67 km/s/Mpc, hinting at unknown physics or systematic errors that future data may resolve. The concept of redshift (z), shown by the calculator, measures how much light from distant galaxies has been stretched toward the red end of the spectrum, a direct signature of cosmic expansion.
Input the distance to a galaxy in megaparsecs (Mpc). For reference: Andromeda ≈ 0.77 Mpc, Virgo cluster ≈ 20 Mpc, Coma cluster ≈ 100 Mpc.
Use the current best estimate (typically 70 km/s/Mpc) or explore the tension by trying 67 or 73 km/s/Mpc to see how different measurements affect age estimates.
The result shows the galaxy's radial velocity—how fast it is receding from Earth due to cosmic expansion. Nearby galaxies have small velocities; distant ones recede at millions of km/s.
Redshift (z) quantifies spectral shift; the age derived from 1/H₀ is a simple estimate that assumes constant expansion and does not account for acceleration or spatial curvature.
Scenario: Calculate the recession velocity, redshift, and lookback time for a hypothetical receding galaxy at a distance of 0.77 Mpc (the distance to Andromeda, our nearest large galaxy, though Andromeda itself is locally approaching Earth due to local gravity, not participating in Hubble expansion). This example illustrates how Hubble's Law applies to distant galaxies in the expansion field.
Interpretation: At distances of ~0.77 Mpc, Hubble expansion is weak compared to local gravitational dynamics. Only beyond the Local Group (~3–10 Mpc) does Hubble flow clearly dominate. Andromeda at exactly this distance is actually approaching; it has not yet entered the Hubble expansion regime due to local gravitational attraction. Distant galaxies (100+ Mpc) show clean Hubble recession with easily measurable redshifts. The age estimate (~13.8−14.3 billion years) is consistent with independent measurements from the cosmic microwave background.
Space itself is expanding, carrying all galaxies apart. Galaxies are not flying through a static universe; they remain relatively stationary within their local space, which stretches. This is why the expansion law works consistently across all velocities.
Different measurement methods yield H₀ ≈ 67–74 km/s/Mpc. Local ladder measurements give ~73, while cosmic microwave background (CMB) data imply ~67. If real, this tension may indicate unknown physics, dark energy variations, or systematic errors—an active area of cosmology research.
Yes, due to expansion. Distant galaxies recede at speeds exceeding c without violating relativity, which limits local motion through space. This is allowed by general relativity; space expands while the speed of light remains constant within any local reference frame.
Redshift (z) measures the fractional shift of light wavelength toward the red (longer-wavelength) end due to expansion. Higher z means greater distance and recession velocity. Spectroscopy of redshift is crucial for mapping the universe and measuring cosmic acceleration.
It's a simplification. The true age accounts for how expansion rate has changed due to matter density and dark energy. The universe is approximately 13.8 billion years old; the Hubble time (1/H₀ ≈ 14.3 Gy at H₀ = 70) is close but not exactly equal.
The mechanism is unknown, but observations (1998 Nobel Prize) show cosmic expansion is accelerating. Dark energy (~68% of the universe) acts like a cosmological constant with negative pressure. Its nature—true constant, dynamic field, or modified gravity—remains one of physics' biggest mysteries.
The local ladder uses type Ia supernovae (standard candles), Cepheid variables, and parallax distances to calibrate distance. CMB-based methods use the geometric properties of the early universe. The tension between methods drives new observations and refinements in calibration.
Yes, we observe their light despite superluminal recession. Distant galaxies emitted light in the past when they were closer; we see that light today. Thanks to expansion, light from regions receding faster than c still reaches us if it was emitted in the early universe.
Related Tools
Calculate probability of love.
Calculate exoplanet properties.
Calculate lunar phase.
Explore Olbers' paradox.
Calculate sun angle.
Calculate sunrise and sunset times.