Universe Expansion
Compute recession velocity and distance from redshift using the Hubble law and cosmological expansion formulas.
Hubble's Law and Cosmic Expansion
Distant galaxies recede from us because space itself is expanding. Edwin Hubble found that recession velocity grows with distance. The Hubble constant \(H_0\) links how fast a galaxy moves away to how far it is.
Key Formulas
$$v = H_0 \cdot d$$ $$d = \frac{c \cdot z}{H_0} \quad \text{(small } z \text{)}$$For nearby galaxies with small redshift \(z\), recession velocity is approximately \(v \approx c \cdot z\). Distance \(d\) is in megaparsecs (Mpc) when \(H_0\) is in km/s/Mpc and \(c\) is in km/s. This tool uses \(H_0 = 70\) km/s/Mpc.
Redshift and Distance
Redshift measures how much a galaxy's light has stretched toward longer wavelengths. A redshift of \(z = 0.1\) means wavelengths increased by 10%. For small \(z\), this corresponds to a recession velocity of about 30,000 km/s and a distance near 428 Mpc.
Related tools: Antipode Calculator and Latitude Longitude Distance.
Frequently Asked Questions
What is the Hubble constant?
The Hubble constant \(H_0\) describes the current expansion rate of the universe. A common value is 70 km/s/Mpc, meaning a galaxy 1 Mpc away recedes at about 70 km/s.
When is the small-z approximation valid?
The relation \(d = cz/H_0\) works well for \(z\) much less than 1. For larger redshifts, more detailed cosmological models account for changes in expansion history.
What is a megaparsec?
One megaparsec (Mpc) equals about 3.26 million light-years. Astronomers use parsecs and megaparsecs for intergalactic distances.
How is recession velocity measured?
Astronomers measure the shift of spectral lines in a galaxy's light. The fractional wavelength change gives redshift \(z\), which relates to recession velocity for distant objects.
Does expansion move galaxies through space?
Cosmic expansion stretches space itself. Galaxies are carried apart as the fabric of space grows, rather than flying through a fixed background like ordinary motion.
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