It is well known that when we observe the universe , when we look at distant stars and galaxies with our telescopes, we are seeing the past. This is simply because the speed of light is very small compared to the distances that separate us from the rest of the universe. Therefore, the light that reaches us from distant stars has needed to travel for years or thousands of years before reaching the earth and our detector, be it a telescope or your eyes. One way to understand this fact in a more intuitive way would be to consider the following: when someone tells us news, they always tell us about the past. If a friend tells you about a problem they have had in class or at work or the news tells you about the decisions made yesterday by your government, this information has previously needed to travel to you at a limited speed , or else you would have received that information. information instantly.
An even clearer example is to consider how information was transmitted before the internet, the telephone, or even electricity . Think of a general of an army of some expanding ancient empire reporting his victory in conquering new territories to the capital of the empire. Think of Marcus Astronomer, the famous and fictional Roman general telling Rome of his victory at the edge of the empire. This information would hopefully take a few days and probably a couple of months to reach Caesar , so he could only know what was happening in Britain in the past, never what was happening at that very moment. Something analogous occurs in our study of the universe. We see the Moon as it was just over a second ago, the Sun as it was eight minutes ago, Pluto as it was about four and a half hours ago, and Alpha Centauri as it was more than four years ago . And in the same way, we see the Andromeda galaxy as it was two and a half million years ago or the Virgo cluster galaxies as they were sixty-five million years ago .
However, we can use this limitation to our advantage. Since seeing far is seeing into the past, we can try to see as far as possible to study the early universe . To study what the cosmos was like billions of years ago without the need to be present at that time. No need to travel back in time. I think there is no better example of all this than this image taken by the James Webb Space Telescope just a few weeks ago, known as the First James Webb Deep Field . The vast majority of the objects in this picture are not planets, stars, or even nearby nebulae. They are galaxies. This image contains so many different galaxies, several thousand in all, that it is in itself a small catalog of the types of galaxies present in the universe everywhere and at all times. In it we can see some galaxies as they were just a few million years ago and others as they were billions of years ago when the universe was not even 10% of its current age. By studying similarities and differences between the galaxies of each epoch or if certain types or certain properties predominate in some epochs or others, we can know in detail the processes of galactic evolution throughout those billions of years of existence.
But what’s so special about taking a picture of a few galaxies? This photo shows a galaxy cluster known as SMACS 0723 and located more than four billion light-years away. This cluster is found in our night sky near the constellation Volans , visible from the southern hemisphere. These galaxies pack so much mass that light from even more distant galaxies is distorted by their gravity , creating the stretched-out images seen in the center of the picture. But that is not the only thing we see, because together with the cluster of galaxies we also see hundreds and thousands of other galaxies, which have nothing to do with it.
This image captures a tiny region of the sky. If you were to divide the entire sky into 26 million equal parts , this photograph would correspond to only one of those parts. This photograph is also the result of combining a large number of different photographs, using 6 different monochrome filters and with a total exposure time of about twelve and a half hours . By comparison, Hubble’s first deep-field image required almost 142 hours of exposure , or almost 6 days of uninterrupted light collection. If the James Webb Space Telescope were dedicated solely and exclusively to taking pictures like this of the entire area of the sky, it would take more than 37,000 years to do so .
All deep field photographs obtained show a similar picture. They all show the same density of galaxies and the same percentages of the different types and ages . This is further evidence in favor of the cosmological principle which says that the universe observed on large enough scales is homogeneous and isotropic. In closer terms, this cosmological principle tells us that human beings do not occupy a privileged position .
Such an image can only be taken from space , not only because James Webb observes at a wavelength for which the Earth’s atmosphere is opaque, but because telescopes on the Earth’s surface are affected in their observations by turbulence that distorts the image and the amount of background light noise , coming from the reflection in the atmosphere of the light emitted from our cities. This means that despite the limitations that arise when sending telescopes into space, such as the obvious limitation in the size and mass of the telescopes that we can load on a rocket, the photos obtained from outside the Earth’s atmosphere are more sensitive and have greater definition than those obtained from surface observatories. This is why we had to wait for Hubble to get the first deep field, and this is also why we look forward to even deeper pictures from the James Webb Telescope .
Rob Garner, 2022, NASA’s Webb Delivers Deepest Infrared Image of Universe Yet, NASA
2022, Webb’s First Deep Field (NIRCam Image), NASA, webbspacetelescope.org