Red Light, Blue Light?
If you read the James Webb Space Telescope article and wondered what redshift was, this is for you. Just an update on JWST. It is now in space with power, its mirror fully deployed, and is now making its way to the second Lagrange point. But if you remember, I talked about redshift and gave a very simple explanation. Well, this is the deep-dive into it. Just a heads up, I wrote this article at 4 am while watching the JWST launch, so some of the wording may be completely off. But that is fine.
 |
| Planck Satellite Cosmic Background Radiation |
Redshift is defined as the change in the wavelength of the light divided by the wavelength that the light would have if the source was not moving — called the rest wavelength: At least three types of redshift occur in the universe — from the universe's expansion, from the movement of galaxies relative to each other and from "gravitational redshift," which happens when light is shifted due to the massive amount of matter inside of a galaxy. The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. In astronomy and cosmology, the three main causes of electromagnetic redshift are The radiation travels between objects which are moving apart ("relativistic" redshift, an example of the relativistic Doppler effect) The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift) The radiation travels through expanding space (cosmological redshift). The terms derive from the colors red and blue which form the extremes of the visible light spectrum. The observation that all sufficiently distant light sources show redshift corresponding to their distance from Earth is known as Hubble's law. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.
The redshift of an object is measured by examining the absorption or emission lines in its spectrum. These lines are unique for each element and always have the same spacing. When an object in space moves toward or away from us, the lines can be found at different wavelengths than where they would be if the object were not moving (relative to us). Redshift occurs when an object emitting electromagnetic radiation recedes from an observer. The light detected appears "redder" than it should be because it is shifted toward the "red" end of the spectrum. This phenomenon was first observed in a 1938 experiment performed by Herbert E. Ives and G.R. Stilwell, called the Ives–Stilwell experiment. Since the Lorentz factor is dependent only on the magnitude of the velocity, this causes the redshift associated with the relativistic correction to be independent of the orientation of the source movement. In contrast, the classical part of the formula is dependent on the projection of the movement of the source into the line of sight which yields different results for different orientations.
To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. If the same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. In order to calculate the redshift, one has to know the wavelength of the emitted light in the rest frame of the source: in other words, the wavelength that would be measured by an observer located adjacent to and comoving with the source. When the redshift of various absorption and emission lines from a single astronomical object is measured, z is found to be remarkably constant. Although distant objects may be slightly blurred and lines broadened, it is by no more than can be explained by the thermal or mechanical motion of the source. For these reasons and others, the consensus among astronomers is that the redshifts they observe are due to some combination of the three established forms of Doppler-like redshifts. When photometric data is all that is available (for example, the Hubble Deep Field and the Hubble Ultra Deep Field), astronomers rely on a technique for measuring photometric redshifts. For example, if a Sun-like spectrum had a redshift of z = 1, it would be brightest in the infrared rather than at the yellow-green color associated with the peak of its blackbody spectrum, and the light intensity will be reduced in the filter by a factor of four, (1 + z)2.
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. This correlation was first observed by Edwin Hubble and has come to be known as Hubble's law. Vesto Slipher was the first to discover galactic redshifts, in about the year 1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his 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. 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) exhibiting blueshifts as we fall towards a common barycenter, and redshift maps of clusters showing fingers of god effect due to the scatter 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 (the ratio of a galaxy's mass in solar masses to its brightness in solar luminosities), an important tool for measuring dark matter. The redshifts of galaxies include both a component related to recessional velocity from the expansion of the universe and a component related to peculiar motion (Doppler shift). Popular literature often uses the expression "Doppler redshift" instead of "cosmological redshift" to describe the redshift of galaxies dominated by the expansion of spacetime, but the cosmological redshift is not found using the relativistic Doppler equation which is instead characterized by special relativity; thus v > c is impossible while, in contrast, v > c is possible for cosmological redshifts because the space which separates the objects (for example, a quasar from the Earth) can expand faster than the speed of light.
Currently, the objects with the highest known redshifts are galaxies and the objects producing gamma-ray bursts. The most reliable redshifts are from spectroscopic data, and the highest-confirmed spectroscopic redshift of a galaxy is that of GN-z11, with a redshift of z = 11.1, corresponding to 400 million years after the Big Bang. Slightly less reliable are Lyman-break redshifts, the highest of which is the lensed galaxy A1689-zD1 at a redshift z = 7.5 and the next highest being z = 7.0. The highest-known redshift radio galaxy (TGSS1530) is at a redshift z = 5.72 and the highest-known redshift molecular material is the detection of emission from the CO molecule from the quasar SDSS J1148+5251 at z = 6.42. Scientists measured the redshift of GN-z11 to see how much its light had been affected by the expansion of the universe. The Sloan Digital Sky Survey is an ongoing redshift project that is trying to measure the redshifts of several million objects.
Doppler effect is an important phenomenon in various scientific disciplines, including planetary science. Doppler effect in physics is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. The opposite of a redshift is a blueshift. In visible light, this shifts a color towards the blue end of the spectrum.
Comments
Post a Comment