Redshift <-> recessional velocity API

Converts between cosmological redshift z and recessional velocity using the relativistic (default) or classical Doppler relation, and optionally returns the Hubble distance from H0. Answers 'Velocity from redshift z?', 'Redshift for this recession speed?'.

Price$0.04per request

MethodPOST

Route/v1/space/redshift-velocity

StatusLive

MIME typeapplication/json

Rate limit120/minute

Cache0s public

spaceastronomyredshiftdopplerrecessional-velocityhubblecosmologyrelativity

API URLhttps://x402.hexl.dev/v1/space/redshift-velocity

Integration docs

Example request

{
  "redshift": 0.5,
  "hubbleConstant": 70
}

Example response

{
  "redshift": 0.5,
  "velocityKms": 115304.791538,
  "velocityFractionC": 0.384615385,
  "model": "relativistic-doppler",
  "hubbleDistanceMpc": 1647.211308,
  "hubbleDistanceMly": 5372.484735,
  "interpretation": "z=0.5 ⇔ recession 115305 km/s (38.46% c)."
}

Input schema

{
  "type": "object",
  "properties": {
    "redshift": {
      "type": "number",
      "examples": [
        0.5
      ]
    },
    "velocityKms": {
      "type": "number"
    },
    "hubbleConstant": {
      "type": "number",
      "description": "H0 in km/s/Mpc",
      "examples": [
        70
      ]
    },
    "relativistic": {
      "type": "boolean",
      "description": "Use relativistic Doppler (default true)"
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}