The James Webb Space Telescope has spotted some of the earliest and most distant galaxies, but how can we be sure these early galaxies aren't closer and more recent?
The more distant the galaxy, and thus the further back in time it is, the more shifted its originally visible light is down to near-infrared or even infrared. This is why the JWST views the universe in infrared and near infrared; it is the best way to view very early and distant galaxies. with greater values indicating a greater redshift, thus early galaxies have a high z. For instance, the earliest galaxy spotted by Hubblehas z-11.1.
Scientists look at the wavelength of light an element such an oxygen emits and absorbs in a lab here on Earth and compare that with what they see in the universe.called OIII at a wavelength of 88 micrometers here on Earth. When ALMA looked at the spectra from a distant galaxy, astronomers saw OIII at a longer wavelength, 1,160 micrometers meaning it had definitely been redshifted.
From the difference in wavelength between the lab and the astronomical observation it's easy to calculate the redshift. So for this ALMA galaxy z=/88=12.2, this means the galaxy has a redshift of 12.2. With some clever mathematics, this value of z reveals that the galaxy is seen as it was when the universe was just 360 million years old.
The chemical fingerprint of distant galaxies can be used to confirm the time period at which galaxies are seen in another way. The early universe was a sea of hydrogen and helium with just a smattering of heavier elements, which astronomers call"metals" like oxygen and nitrogen.
That means astronomers can look at the spectra of light from a galaxy to see how abundant in heavy elements it is to get a picture of the age of the universe at which it is seen. If it's lacking in metals, that confirms it's probably a galaxy in the early universe. have a light output that is so uniform that they are referred to as"standard candles.
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