Redshift

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

$z={\frac {f_{emitted}-f_{observed}}{f_{observed}}}={\frac {\lambda _{observed}-\lambda _{emitted}}{\lambda _{emitted}}}$

where f is frequency and λ is wavelength.

Causes

It can be due to four 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.

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.

4. The Wolf Effect. Physicist Emil Wolf has found that two non-Lambertian sources that emit beamed energy, can interact in a way that causes a shift in the spectral lines. It is analagous to a pair of tuning forks with similar frequencies (pitches), connected together mechanically with a sounding board; there is a strong coupling that results in the resonant frequencies getting "dragged down" in pitch. The Wolf Effect can produce either redshifts or blueshifts, depending on the observer's point of view, but is redshifted when the observer is head-on.

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.

Non-standard cosmological models

Some studies suggest that cosmological redshift is quantized rather than continuously variable (e.g. http://public.lanl.gov/alp/plasma/downloads/Tifft.pdf). However, it is fair to add that these studies remain controversial, and that the overwhelming majority of the cosmological community regard cosmological redshift as not quantised. The same can be said of Halton Arp's famous but controversial interpretation of cosmological redshift. While both models have their proponents, neither model is part of the scientific mainstream.

Redshift can also be caused by two sources that emit beamed energy (eg. lasers, quasars) interacting with each other, an effect discoverd by physicist Emil Wolf which he called Noncosmological Redshift [Ref.], and others have called the Wolf Effect [Ref.]. This phenomenon has been verified experimentally, and although it is not at all widely accepted as an explanation for cosmological redshift, there are some proponents of this position (including Wolf himself).

See also

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