Astronomical Glossary.
H.
Chemical symbol for hydrogen.
The most abundant chemical in the universe.
H2 is the symbol for the molecular hydrogen molecule which is abundant in giant clouds in our galaxy and can be detected by its infrared spectrum..
It is described by the equation v = H0D, with H0 the constant of proportionality—the Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmological time .
The Hubble constant, H0, or its dimensionless equivalent, "little h", is a fundamental cosmological property that is now known to an accuracy better than a few per cent.
Despite its cosmological nature, little h commonly appears in the measured properties of individual galaxies.May 14, 2020.
The Hubble constant, H0, or its dimensionless equivalent, "little h", is a fundamental cosmological property that is now known to an accuracy better than a few per cent.
Despite its cosmological nature, little h commonly appears in the measured properties of individual galaxies.May 14, 2020.
where h is the dimensionless Hubble parameter, pronounced 'little h'.
H(z), often called the Hubble parameter, will change with time in different ways depending on the mass (ΩM), curvature (Ωk), and dark energy (ΩΛ) densities of the Universe..
Modern estimates, using measurements of the cosmic microwave background radiation left over from the big bang, place the value of H0 at about 67 km (42 miles) per second per megaparsec..
To get a sense of the uncertainty in a property or result between two little h cosmologies, we show the fractional change in the property, relative to h = 0
7, when little h is changed continuously from 0
60 to 090 (this brackets the currently favoured range)
The Hubble constant is a global cosmological property of the Universe (at least in the vanilla ΛCDM paradigm) and not a local property of the individual objects within it (galaxies, gas,
)H 0 (commonly refereed to by its alter ego, “little h ”, defined below) often appears in the measurement of galactic properties at cosmological distances, and more generally throughout computational cosmology
Its presence is required whenever an assumption about the underlying cosmology must be made, no matter how subtle
