Space observation started out with people observing the night sky and looking through visible light telescopes that couldn’t magnify the sky enough to get detailed observations about objects outside the solar system. In recent years, however, space observation technology has evolved to the point that astronomers are able to observe deep space objects billions of light years away using infrared light instead of just visible light. These advancements have provided an exciting opportunity to expand the observable universe and see objects extremely far from Earth. In the early 20th century, astrophysicists noted that there appears to be a very strong trend with the appearances of observed star systems. Galaxies closer to Earth usually emit cool-hued visible light while appearing older and more structured. On the other hand, farther ones tend to emit redder visible light or are only visible in infrared light. They virtually always look less structured than closer ones, implying they’re younger. The consistency of these trends led physicists to wonder why these observations are the way they are, leading to the rise of a theory that has shaped our knowledge of the universe.
To understand the way space observations are, one must also know how the majority of space telescopes actually create images. This process is analogous to how digital cameras work, except on a much larger scale. In cameras, electromagnetic waves are focused through a lens that uses glass to redirect them to a single point on a sensor. The sensor is divided into millions of red, green, and blue pixels. When light hits each pixel, it gets converted to a certain amount of energy proportional to the brightness of the light that hit it along with registering the wavelength of the light that hit it and the others around it. A computer inside the camera measures the energy from each pixel to put together approximate shapes, colors, and brightness of things in an image. The technology in space telescopes is extremely similar, except that it uses sets of mirrors instead of lenses and has more sophisticated, larger detectors rather than sensors.
For light to hit a mirror, it must first travel to it. Light in a vacuum, such as space, travels at a speed of 299,792,358 meters (186,282 miles) per second. This speed seems extremely fast on paper, however the observable universe is so massive that even light can take several years to travel between places depending on their distance (hence the distance measurement, the light year). The light emitted from or reflected off of an object determines how an image of it will look, and this appearance will not change with the distance its photons have travelled.
When light from an object that traveled from a far distance hits a telescope’s mirror, the object will appear as it was at the time the light was emitted. For example, if light from an object one light year away (the distance light travels in a year) hits a telescope, it will appear as it was a year ago. As stated before, when looking deep into space, galaxies such as JADES-GS-z14-0 located billions of light years away from the Earth tend to look younger than those near the Earth. This appearance can be explained by inferring the galaxy was at an extremely far distance from Earth when it was younger. In other words, light emitted while it was younger traveled an extremely long distance to Earth, explaining why it looks newer in images of it from telescopes. Farther objects like it probably look more developed as of now (or might not exist), however it would be impossible to observe them in their present state due to their distance. However, the speed of light alone still does not explain why far-away objects tend to look redder than those near the Earth.
This is where the Doppler effect comes into play. Objects that emit light emit it in electromagnetic waves that travel out from all directions. When an object is at rest, the wavelengths, and therefore the frequencies, will be the same in all directions. However, when an object moves, the distance between it and the waves in the direction it’s moving will decrease while the gap between it and the waves it’s moving away from will increase. This can cause the wavelengths of electromagnetic waves to be shortened or lengthened depending on which way the object is moving because of a change in distance between them. When an object is moving away from the waves, this distance will increase, causing the waves to have a longer wavelength, and because wavelength is inversely proportional to frequency, a lower frequency. When electromagnetic waves’ frequencies decrease, they tend to move left on the electromagnetic spectrum, or in other words, get “redder” (a process astronomers call “redshift”).
After determining the Doppler effect could be observed in galaxies relative to Earth, astrophysicists had another question; why do distant galaxies tend to be more redshifted than those near us? For redshift to occur, a galaxy would have to be moving away from the Milky Way. Although distant galaxies tend to be more redshifted, implying that they’re traveling at a faster rate, almost all known galaxies, regardless of distance, are redshifted. Noticing this trend, Edwin Hubble introduced Hubble’s Law in 1929.
Hubble’s Law states that a galaxy’s velocity and redshift is directly proportional to its distance from the Earth (not velocity), implying that space itself is expanding. Gradually, more space is added between objects because of the expansion of the universe. Objects that were initially far from the Earth appear to be moving away faster than those near it. This observation occurs because there is a greater quantity of constantly-expanding space in between them that adds more distance between them than closer objects (less room for space to be added). In other words, galaxies aren’t actually moving away from the Earth while the Milky Way stays still; the space between all objects in the universe is constantly growing, and they appear to have a velocity to astronomers because they are being carried away from each other due to that expansion rather than movement. In this case, the Doppler effect isn’t happening because of moving objects creating distance between emitted waves and itself, but rather that space’s expansion was doing this. The reason that celestial bodies and systems don’t get torn apart by this expansion is simply because gravitational force is much stronger than this growth and can resist it.
Hubble’s Law and the modern understanding of the speed of light has changed the way astrophysicists observe space today. Astronomers have determined how to calculate redshift (denoted by “z”) using the shifted absorption spectrums of elements in the compositions of galaxies and comparing them to those elements’ normal spectrums. This value has allowed them to calculate objects’ exact distances from Earth, even though we see them as they were in the past, and compare those distances to each other. For example, the extremely high redshift of JADES-GS-z14-0, z = 14.32 has allowed astronomers to determine the galaxy is located 13.4 billion light years away from Earth and is the farthest galaxy from it we know of (It’s so far that its only visible on the infrared spectrum). Astronomers can also determine if an object is moving towards Earth because its redshift would be negative if this is the case, such as in the Andromeda galaxy. They have also been able to estimate Hubble’s constant, the rate at which the universe is expanding, to be between 69.8 km/s to 74 km/s per megaparsec. In other words, for every 3.26 million light-years of distance, the universe is expanding an extra 70 kilometers per second. These estimates and understandings of distances have made mapping the observable universe to scale more accurately possible, along with being able to predict how the universe will look in the very far future. Overall, light speed and Hubble’s law are crucial parts of modern astronomy and have given people a much better understanding of the universe around them, and will likely be the basis for even more major breakthroughs in the future. But of course, it can be used to answer the commonly-asked question of, “Why do distant galaxies look like that?”
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