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Astronomers usually do not measure the distance to the universe’s farthest galaxies by surveying their positions directly. Instead, they measure how much the galaxies’ light has been stretched by cosmic expansion, determine a redshift from recognizable features in that light, and use a cosmological model to infer a distance or a lookback time. The redshift is an observation; the distance calculated from it depends on the model and on which kind of distance is being reported.
What astronomers measure—and what they infer
For a remote galaxy, astronomers receive light that began its journey long ago. As the universe expands, the light’s wavelengths stretch while it travels. A feature emitted at one wavelength can therefore arrive at a longer wavelength. Astronomers call this shift toward longer wavelengths redshift.
The observed wavelengths and the pattern of features in a galaxy’s light are measurements. Redshift is calculated by comparing an observed feature with the wavelength at which that feature is known to be emitted. The galaxy’s distance is then inferred from its redshift using a model of the universe’s expansion.
This distinction matters because redshift is not itself a distance in light-years. Converting it into a distance requires assumptions about how the universe expanded, including its expansion rate, geometry and energy content. A reported result should therefore identify the distance convention being used rather than presenting a redshift as though it were a directly measured distance. NASA’s explanation of cosmological redshift describes this model-dependent connection.
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How astronomers find and measure a distant galaxy
1. Select candidates from images
Observations often begin with images taken through several filters. Each filter records the average brightness of an area of the sky over a particular wavelength band. For infrared observations, instruments such as the James Webb Space Telescope’s NIRCam can capture the light from very distant galaxy candidates in multiple bands.
Astronomers compare the measured brightnesses and colors with models of galaxies at different redshifts. The result is a photometric redshift: an estimate, usually expressed as a range or probability distribution, of how far the galaxy’s light may have shifted. This method is useful for finding candidates because a camera can observe many objects at once, including faint ones that are difficult to study in detail.
Photometry does not usually settle the redshift question on its own. Different combinations of galaxy properties and redshift can produce similar colors in broad filters. A promising image-based estimate is therefore a candidate identification, not the same kind of evidence as a redshift measured from a spectrum. NASA’s December 2023 overview of galaxy-distance measurements explains this role for multi-filter imaging.
2. Look for a shifted spectral feature
To estimate redshift from a recognizable feature, astronomers need to know both where that feature appears in the observed light and its known emitted, or rest-frame, wavelength. In simplified form, redshift is the difference between the observed and rest-frame wavelengths divided by the rest-frame wavelength. The farther the feature has shifted toward longer wavelengths, the greater the measured redshift.
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One useful feature is the Lyman break, a sharp drop in ultraviolet light associated with hydrogen absorption. In a distant galaxy, cosmic expansion shifts this ultraviolet feature to longer observed wavelengths. It may then be detected in infrared observations. Astronomers can compare where the break appears with where it is expected to fall at different redshifts.
Identifying a feature is crucial: a shift is meaningful only if astronomers can determine what produced it. A single apparent change in brightness or color is not enough to establish a galaxy’s distance. The pattern across wavelengths—and the confidence that a particular feature has been identified correctly—helps distinguish a plausible redshift from a misleading estimate.
3. Use spectroscopy to refine or confirm the estimate
A spectrograph separates incoming light into many narrower wavelength samples, producing a spectrum. This gives astronomers a more detailed view of the galaxy’s light than broad imaging filters provide. They can search for emission lines or spectral breaks, identify a feature, and compare its observed wavelength with its known rest-frame wavelength to calculate redshift.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Spectroscopy can make the redshift estimate more precise and can provide stronger evidence for a candidate’s interpretation. It is not always easy: very faint galaxies may not provide enough light for a useful spectrum. That is one reason astronomers first use imaging to identify promising targets. NASA’s 2016 explanation of redshift and remote-galaxy distances describes the spectral approach, while its 2023 overview notes the practical value of imaging when spectroscopy is difficult.
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Photometric and spectroscopic results answer related but different questions. Imaging can help identify likely high-redshift objects efficiently; spectroscopy can test the estimate with finer wavelength information. When comparing two claims, check whether each redshift came from colors across filters or from identified spectral evidence, and whether the reported result includes a redshift range or uncertainty.
How redshift becomes a distance
Once astronomers have a redshift, they use a cosmological model to relate it to the universe’s expansion history. The calculation depends on the model’s account of the expansion rate, geometry and energy content. Changing those assumptions can change the inferred distance, even if the observed spectral feature and its redshift are unchanged.
There is also more than one useful way to describe the separation between us and a remote galaxy. Lookback time answers how long the light traveled to reach us: because the journey took time, we see the galaxy as it appeared in the past, not as it is now. A distance measure describes separation according to a specified convention in an expanding universe. These quantities are related, but they are not interchangeable. A statement such as “the light has been traveling for” gives a time; a distance in light-years or another unit needs its distance convention stated.
This is why a redshift alone should not be translated casually into a single distance figure. A clear report gives the measured redshift, describes the evidence behind it, names the cosmological assumptions or model where available, and specifies whether the reported quantity is a lookback time or a particular distance measure. NASA’s cosmological-redshift explanation provides the central caution: the conversion from redshift to distance relies on a model.
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Where the cosmic distance ladder fits
The cosmic distance ladder is a chain of methods used to calibrate distances, especially for nearby galaxies and measurements of the universe’s expansion rate. It is related to the distant-galaxy question, but it is not usually the direct method used to infer distances to the earliest galaxy candidates described here.
Calibrating standard candles
Some stars change brightness in ways that allow astronomers to estimate their intrinsic brightness. Cepheid variables and RR Lyrae stars are examples of standard candles. If astronomers can estimate how bright an object is intrinsically and compare that with how bright it appears, they can infer its distance.
Geometric measurements in the Milky Way neighborhood help calibrate these indicators. Calibrated Cepheids and Type Ia supernovae can then extend distance measurements to more distant galaxies. NASA’s material on the distance ladder and on steps toward measuring the Hubble constant describes how such calibration links contribute to broader distance and expansion measurements.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →The important boundary is that a standard candle is not directly observed in every farthest galaxy. For the most distant candidates, the route described here is to estimate redshift from imaging or spectral features and then use cosmology to infer a distance. The distance ladder supplies essential calibration context for astronomy’s broader distance scale, rather than replacing that redshift-based inference for the earliest objects.
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A dated example: JADES-GS-z14-0
On May 30, 2024, NASA reported that spectroscopy with JWST’s NIRSpec measured a redshift of about 14 for JADES-GS-z14-0. NASA described it as the most distant known galaxy at the time of that announcement. The figure is a reported redshift, not a distance in light-years.
“Most distant known” is a time-bounded description: new observations can change which object holds that status. NASA’s May 2024 report establishes what was announced then; it does not, by itself, establish the current record. When quoting a record claim, include its date and do not silently turn the dated announcement into a present-day ranking.
How to judge a claim about a galaxy’s distance
When an article or study reports a very distant galaxy, use these checks to understand what the number means and how strong the evidence is:
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- Method: Was the redshift estimated from broad-band colors, or measured spectroscopically?
- Feature: Which emission line or spectral break was identified, and how confident is that identification?
- Result: Is the redshift reported with a range or uncertainty, or only as an approximate value?
- Conversion: Does the report give a redshift, a lookback time, or a distance? If it gives a distance, does it identify the convention and cosmological model?
- Date: Is a “most distant known” claim tied to a publication date? A record from one announcement is not automatically a current record.
These checks keep three different things separate: the light observed by a telescope, the redshift inferred from that light, and the model-dependent distance calculated from the redshift. That separation is the key to reading distant-galaxy claims accurately.
What the measurement does—and does not—require
The measurements discussed here rely on professional observatories capable of sensitive infrared imaging and spectroscopy. The JWST/NIRSpec example illustrates the level of instrumentation involved. An ordinary consumer telescope is not a way to reproduce the spectral measurements used to establish redshifts for the faintest, most distant galaxy candidates.
The result is not a direct surveyor’s measurement of a galaxy’s present-day position. Astronomers collect its ancient, expansion-stretched light, identify spectral evidence, and use a cosmological model to interpret what that evidence says about distance and cosmic history.
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