# Redshift

Redshift describes a change in the wavelength of light, in which the wavelength is longer, or redder, than when it was emitted at the source. This can happen when the source moves away from the observer, known as the Doppler effect. It is also observed when light emitted by distant galaxies is shifted to longer wavelengths than the spectrum of closer galaxies. This is taken as evidence that the universe is expanding and that it started in a Big Bang.

In general, redshift (and blueshift, the observation of shorter wavelength light than emitted) is quantified by

$\displaystyle z = \frac{f_{\mathrm{emitted}} - f_{\mathrm{observed}}}{f_{\mathrm{observed}}} = \frac{\lambda_{\mathrm{observed}} - \lambda_{\mathrm{emitted}}}{\lambda_{\mathrm{emitted}}}$

where f is frequency and λ is wavelength. This quantity is unitless.

## Causes

The most commonly-cited causes of redshift are:

1. Movement of the source. If the source of the light is moving away from the observer, then redshift (z > 0) occurs; if the source moves towards the observer, then blueshift (z < 0) occurs. This is true for all waves and is explained by the Doppler effect. Consequently, this type of redshift is also called the Doppler redshift. If the source moves away from the observer with velocity v and this velocity is much smaller than the speed of light c, then the redshift is approximately given by :zv/c
However, it is important to note that this expression is only approximate, and needs modification for speeds close to the speed of light. (For an exact equation for the frequency shift, see the article on the relativistic Doppler effect).
2. Expansion of space. The current models of cosmology assume an expanding space. Light will experience a redshift if it travels through expanding space, because the expanding space stretches the light ray, which makes the wavelength longer, which is another way of saying the light gets redder. If the Universe were contracting instead of expanding, we would see distant galaxies blueshifted instead of redshifted. This redshift of distant galaxies looks like a Doppler effect from receding, but in general relativity stretching the space is different from moving the source. These galaxies are not believed to be receding; instead, the intervening space is believed to be stretching, which is subtly different. Nevertheless, astronomers (especially professional ones) sometimes refer to 'recession velocity' in the context of the redshifting of distant galaxies from the expansion of the Universe, because they all know it's only an apparent recession. This can sometimes be confusing to the intelligent lay person who does not realise the astronomers are just talking in a shorthand, and aren't in fact ascribing this redshift to a real recession movement of the source. This type of redshift is also called the cosmological redshift or Hubble redshift.
3. Gravitational effects. The theory of general relativity holds that light moving through strong gravitational fields experiences a red- or blueshift. This is known as the Einstein shift. The effect is very small but measurable on Earth using the Mossbauer effect. However it is significant near a black hole and as an object approaches the event horizon, the red shift becomes infinite. It is also the dominant cause of large angular scale temperature fluctuations in the cosmic microwave background radiation. This type of redshift is also called Gravitational redshift.

## Observation in astronomy

The redshift observed in astronomy can be measured because the emission and absorption spectra for atoms are distinctive and well known. When analyzing light from distant galaxies, one observes absorption and/or emission features which appear shifted to lower frequencies. More distant objects generally exhibit larger redshifts; these more distant objects are also seen as they were further back in time, because the light has taken longer to reach us. The largest observed redshift so far, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background radiation; the numerical value of its redshift is about z = 1089 (z = 0 corresponds to present time), and it shows the state of the Universe about 13.7 billion years ago, and 379,000 years after the initial moments of the Big Bang.

For galaxies more distant than the Local Group and the nearby Virgo Cluster, but within a thousand megaparsecs or so, the redshift is approximately proportional to the galaxy's distance (in the context of conventional cosmological models). This was first proposed by Edwin Hubble and is known as Hubble's law. In the widely-accepted cosmological model based on general relativity, redshift is mainly a result of the expansion of space: this means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us. Hubble's law follows in part from the Copernican principle. Measuring the redshift is often easier than more direct distance measurements, so redshift is sometimes in practice converted to a crude distance measurement using Hubble's law.

Gravitational interactions of galaxies with each other and clusters cause a significant scatter in the normal plot of the Hubble diagram. The peculiar velocities associated with galaxies superimpose a rough trace of the mass of virialized objects in the universe. This effect leads to such phenomena as nearby galaxies (such as the Andromeda Galaxy) exhibitting blueshifts as we fall towards a common barycenter and redshift maps of clusters showing a Finger of God effect due to the spread of peculiar velocities in a roughly spherical distribution. This added component gives cosmologists a chance to measure the masses of objects independent of the mass to light ratio, an important tool for measuring dark matter.

Vesto Slipher was the first to discover galactic redshifts from ~1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his Law.

For more distant galaxies, the relationship between current distance and observed redshift becomes more complex. When one sees a distant galaxy, one is seeing the galaxy as it was sometime in the past, when the expansion rate of the Universe was different from what it is now. At these early times, we expect differences in the expansion rate for at least two reasons: (1) the gravitational attraction between galaxies has been acting to slow down the expansion of the Universe since then, and (2) the possible existence of a cosmological constant may be changing the expansion rate of the Universe. Recent observations have suggested the expansion of the Universe is not slowing down, as expected from (1), but accelerating (see accelerating universe). It is widely, though not quite universally, believed that this is because there is a cosmological constant. Such a cosmological constant also implies that the ultimate fate of the Universe is not a Big Crunch, but instead will continue to exist foreseeably (though most physical processes within the Universe will still come to an eventual end).

The expanding Universe is a central prediction of the Big Bang theory. If extrapolated back in time, the theory predicts a "singularity", a point in time when the Universe had zero size. The theory of general relativity, on which the Big Bang theory is based, breaks down at this point. It is believed that a yet unknown theory of quantum gravity would take over before the size becomes zero.

## Redshift interpretations in non-standard cosmologies

Proponents of non-standard cosmologies try to cast doubt on the standard explanation of the cosmological redshift and the resulting Hubble Law since most of the ideas do not involve an expanding universe. Since the Big Bang has been accepted by essentially the entire professional scientific community in this field as the scientific theory that is the best supported by the evidence, many advocates spend a lot of their time critiquing the Big Bang rather than developing their own ideas. Some of these alternative models are discussed below, but it is important to recognise that they have very little or no support in the professional scientific community. In other words, the current paradigm disfavours these models. Caveat lector!

One example of this was the proposed idea of Halton Arp's quantized redshifts which was an idea based on analyses done before the sky surveys increased the number of measured redshift by several orders of magnitude. The idea was that the cosmological redshift might be showing evidence of periodicity which would be difficult to explain in a Hubble's Law universe that had the feature of continuous expansion. However, most astronomers agree that the analysis suffers from poor methodology and small number statistics.

Another critique of cosmological redshift also came from Arp, who continues to find anecdotal support in the existence of apparently connected objects with very different redshifts. Conventional cosmological models regard these as chance alignments.

Proponents have refered to other observed effects, such as Brillouin scattering, Compton scattering, Raman scattering and the Wolf effect which they claim can, under certain circumstances, mimic frequency shifts in certain bands of the electromagnetic spectrum. These are frequency-dependent effects and do not, for example, apply across an entire emitted spectrum unlike the most commonly cited causes.

Advocates of plasma cosmology have from time to time supported various redshift interpretations involving scattering, but these all suffer from being unable to adequately reproduce the Hubble Law on large scales.