Andromeda Galaxy

How Far Can Humans See In Space?

The universe is mind-boggling vast. The diameter of the observable universe is estimated to be about 93-billion light years across. With just our eyes, we can generally only see a few thousand light years worth of distance, although there are some objects we can see that are much further away. The farthest object you can see with the naked eye is the Andromeda Galaxy located 2.5-million light years away, yet it is only visible if there is little to no light pollution. In order to see farther into space, we must rely on telescopes. How far can we see using the most powerful telescopes?

Size Of The Universe

Cosmic web
This image is a computer generated model of the observable universe, usually called the Cosmic Web

Before explaining the farthest distance we can see, it is important to know how big the universe is. As was already stated, the observable universe is estimated to be around 93-billion light years across. However, that number may seem contradictory as the universe itself is only 13.8-billion years old. How can we see beyond 13.8-billion light years if light hasn’t had enough time to reach our eyes? Since the universe is expanding, and the speed of light is finite, we see objects as they were rather than as they are. A galaxy that is, say, one-billion light years away, is actually now located at a much further distance due to the expansion of space. We simply see the galaxy as it was one-billion years ago, yet it has traversed a tremendous amount of distance since then. When astronomers account for the expansion of space, they end up with the estimated size of the universe being 93-billion light years. To simplify things, a better way of thinking of distance is to view it as how far back in time we can see. That is to say, how close to the Big Bang can we see?

Hubble And Webb

Hubble deep field
The Hubble Deep-Field image shows some of the farthest, oldest known galaxies. Image credit: NASA/ESA

Until the launch of the James Webb Space Telescope (JWST) in 2021, the Hubble Space Telescope was the record holder for farthest visible distance in space. Although Hubble is smaller than some Earth-based observatories, it is able to see the universe in much higher detail due to the fact that it is in space, where it does not have to deal with any sort of atmospheric distortion. Although no longer the most powerful telescope, Hubble is still able to see the universe as it was a mere 500-million years after the Big Bang. The JWST is now the most powerful telescope ever built, and it is able to see the universe as it was only 200-million years after the Big Bang. That means that the JWST is able to piece together an additional 300-million years of cosmic history compared to Hubble. The JWST will be able to study some of the first galaxies to form after the Big Bang. 

The Cosmic Microwave Background Radiation 

The CMBR is the oldest form of light in the universe at nearly 13.8-billion years old. Image credit: NASA/WMAP

The farthest physical distance we can see is the Cosmic Microwave Background Radiation (CMBR). The CMBR can be thought of as the echo of the Big Bang, as it is the leftover radiation from the birth of the universe. The CMBR itself is the farthest possible distance humans can see as it represents the moment that the universe became transparent to light. Although light did exist before the CMBR, gas and dust were simply too dense for light to escape and traverse space. The CMBR formed only 380,000 years after the Big Bang, and so we are seeing the universe prior to the formation of even the first stars. 

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