formula.expert

Constants library

6 values, each with its units, its uncertainty, and where it came from.

Astronomical 6

Light-Year exact

ly=9.4607304725808×1015 m\mathrm{ly} = 9.4607304725808 \times 10^{15}\ \text{m}

mDistance light travels in one Julian year of 365.25 days — exactly 9 460 730 472 580 800 m, since both c and the year are defined.

Parsec exact

pc=3.085677581491367×1016 m\mathrm{pc} = 3.085677581491367 \times 10^{16}\ \text{m}

mDistance at which one au subtends one arcsecond — 648000/π au exactly, about 3.26 light-years, the working unit of stellar astronomy.

Hubble Constant measured

H0=67.4 km/(s⋅Mpc)H_0 = 67.4\ \text{km/(s}{\cdot}\text{Mpc)}

km/(s·Mpc)Present expansion rate of the universe, 67.4 km/(s·Mpc) from the cosmic microwave background — a galaxy 1 Mpc away recedes at 67 km/s.

Age of the Universe measured

t0=4.354×1017 st_0 = 4.354 \times 10^{17}\ \text{s}

sTime since the Big Bang, 13.797 billion years or 4.35 × 10¹⁷ seconds, from fitting the ΛCDM model to the microwave background.

Critical Density of the Universe measured

ρc=8.53×10−27 kg/m3\rho_c = 8.53 \times 10^{-27}\ \text{kg/m}^{3}

kg/m³Density 3H₀²/8πG that makes the universe spatially flat, about 8.5 × 10⁻²⁷ kg/m³ — some five hydrogen atoms per cubic metre.

Cosmic Microwave Background Temperature measured

TCMB=2.72548 KT_{\mathrm{CMB}} = 2.72548\ \text{K}

KTemperature of the relic radiation from the Big Bang, 2.725 48 K — the most perfect blackbody spectrum ever measured, anywhere.