E1 Scales of the Universe

Topic

The observable universe spans distance scales covering more than twenty-six orders of magnitude, ranging from the size of an atomic nucleus to the radius of the observable universe itself.

The parsec is the natural unit of distance in parallax astronomy: it is the distance at which one astronomical unit subtends an angle of one arcsecond; thus, the smaller the measured parallax angle of an object, the greater the distance separating that object from the observerspecifically, the distance increases in inverse proportion to the measured parallax angle.

Hubble's law establishes that the universe is expanding and that the recession velocity of galaxies is directly proportional to their distance from us, with a constant of proportionality of approximately seventy kilometers per second per megaparsec: the farther away a galaxy is, the faster it is receding from us.

Cosmological redshift, in turn, measures the extent to which the wavelength of light has stretched during its journey through expanding space; a redshift value of one indicates that the wavelength has doubled compared to its value at the time of emission.

Luminosity distanceused to correct the measured apparent brightness of distant sourcesincreases more rapidly than comoving distance as redshift rises, because an additional amplification factor, which also depends on the redshift, is applied to the comoving distance; ignoring this correction leads to a systematic underestimation of the actual distance of an object with a high redshift. Thus, from the orders of magnitude with which this document begins to the systematic underestimation with which it concludesencompassing the parsec, Hubble's law, and cosmological redshiftthe nature of the observable universe dictates that measuring a cosmic distance relies on entirely different tools depending on the scale involved.

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