This article is about the light phenomenon. For other uses of the phrase "Red Shift", see Red Shift

Redshift describes a change in the wavelength of light, in which the wavelength is longer than when it was emitted at the source. This can happen when the source moves away from the observer, known as the Doppler effect. The term redshift is also used for the observation that light emitted by distant galaxies is shifted to longer wavelengths (towards the red end of the spectrum, hence the name) when compared to 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

where f is frequency and λ is wavelength.

Causes

It can be due to three reasons:

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. 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

z ≈ v/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 receeding, but in general relativity stretching the space is different to 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.

The overall cosmological red shift of distant galaxies is controversially claimed to be discretely quantized when viewed from the perspective of a certain velocity frame of reference with respect to our locality. Because this would imply a preferred frame of reference contrary to special relativity, the existance of the quantized red shift of galaxies has been denied as impossible. Yet, the experimental observations are strong. If observationally true, no explanation for a quantized redshift has been accepted or even seems possible. In the opinion of some, the resolution of this conundrum points to a final theory of physics.

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. Gravitational redshift was offered as an explanation of the redshift of quasars in the 1960s, although this is not widely accepted now.

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 Big Bang.

For galaxies more distant than the Local Group, but within a thousand megaparsecs or so, the redshift is proportional to the galaxy's distance, a fact discovered by Edwin Hubble and known as Hubble's law. This redshift is thought to be 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. It turns out that 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.

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.

See also

• phase shift
• blue shift
• Fingers of God - cosmological effect causing elongation of galaxies in redshift space