Even though space is filled with stars, it itself is dark. But why? This seemingly simple question is in fact so complex that it even received a special name – Olbers' paradox. Judge for yourself – according to astronomers, there are about 200 billion trillion stars in the observable Universe. Many are as bright as our Sun, but many more stars shine much brighter. So why, then, is space not filled with dazzling light? Astronomers rightly believe that the answer may lie in distance – many of the stars we observe are very far from Earth. And the further away a star is, the less bright it appears. But this is only part of a puzzle for which various solutions have been proposed over the centuries. Now, with the help of the James Webb Space Telescope, researchers are proposing a new solution to the famous paradox.
It’s dark in space. However, astronomers have been unable to understand why for several centuries. Image: miro.medium.com
Contents
- 1 Olbers' Paradox
- 2 In Search of an Answer
- 3 “New Horizons”
- 4 How Much Light Is There in the Universe and How Can We Find It Out?
Olbers' Paradox
Olbers' Paradox is named after Heinrich Wilhelm Olbers, the German astronomer who popularized it in the 19th century. In fact, this paradox was first discussed by earlier thinkers, including Thomas Digges, Johannes Kepler, Edmond Halley, and Jean-Philippe de Chaize. They all wondered why the night sky is not as bright as the daytime sky, given the assumption that the universe is infinite and full of stars.
To understand Olbers' paradox, let's imagine a simple analogy. Suppose you are in a closed room without windows or doors, and you have an electric light bulb in your hand. If you turn it on, the room will become bright. Now suppose that in your other hand you have another light bulb, which you also turn on, which will make the room even brighter, since there are more light sources.
If space is full of stars, why is it dark? Image: www.syfy.com
It turns out that if you add more and more light bulbs to a room, it will eventually become so light that it will be difficult to see anything. This happens because the light from all the bulbs fills every corner of the room, which means there is simply no more room for darkness.
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Now let's apply this analogy to the night sky. If the Universe is infinite and full of stars, then there should be a star in every direction, no matter how far away they are. The light from these stars must eventually reach us, even if it takes a very long time. The night sky should be as bright as the daytime sky, or even brighter. But why is it not like that in reality?
Finding the answer
The answer may seem obvious: the stars are too far away, and they are also too dim to be observed with the naked eye. But this is an unsatisfactory explanation – even if the stars are very far away, and the light from them is very dim, their number should compensate for both distance and brightness.
The answer to why space is dark is still unknown. Image: mediaproxy.salon.com
For example, if you look at a distant mountain range, you won’t be able to see every single tree or rock on it, but we will see their shape and color. Likewise, when looking at a distant galaxy, it will be impossible to see every star and planet, unlike the shape of the galaxy and its color.
Read also: Why is outer space not as dark as we think?
Read also: Why is outer space not as dark as we think?
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Therefore, if there are enough stars in the sky in all directions, their combined light should make the night sky bright. But in reality, this brightness is again not enough. Even if we assume that the Universe is homogeneous and static, we would need about 10 Suns in each direction to get the «desired» result.
Moreover, the Universe is heterogeneous – it has regions with different densities and temperatures. Some of the light coming from stars can be blocked or scattered by dust, gas and other objects in space. Moreover, the Universe is not static. On the contrary, it is dynamic and evolving.
This means that stars are not eternal, and some of them have long disappeared, reducing their contribution to the brightness of the night sky.
The universe is expanding at an accelerating rate. Image: content.api.news
As you can see, there may be other factors or phenomena that affect the appearance of the night sky, and Olbers' paradox is not just a curiosity or a mystery. In fact, it is a window into the nature of the Universe and its history, which challenges everything we know about the cosmos.
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«New Horizons»
Four years ago, astronomers got a stunning insight into a new type of scientific research they could do: they would finally be able to determine the presence (or absence) ofcosmic optical background—the sum of radiation in optical wavelengths emanating from objects located outside the Milky Way over the entire history of the Universe.
In 2022, astrophysicists at the Rochester Institute of Technology analyzed hundreds of images of background light taken by the Long Range Reconnaissance Instrument (LORRI) of the New Horizons space probe. (New Horizons). The results, published in The Astrophysical Journal, confirmed that the spacecraft is observing much more light than it should.
NASA spacecraft «New Horizons» studies the brightness of the Universe. Image: media-ecn.s3.amazonaws.com
It is also interesting that another group of scientists from various research institutes and observatories had previously arrived at similar results. They also used data from the LORRI device, but focused on single images. Taken together, these results mean that there must be sources of radiation unknown to usin the Universe, and space beyond the Milky Way is much brighter than thought.
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How much light is there in the Universe and how to find out?
So, if the sum of all this light, that is, the cosmic optical background, matches the light that is predicted to come from galaxies and their black holes, then we can confirm the current picture of the Universe. However, if this is not the case and deep space is not completely dark, then there is something in the Universe that we have no idea about.
The situation, however, became even more confusing after the publication of the results of a new study, which can be found on the ArXiv preprint server (this means that the work has not been peer reviewed). The new discovery refutes previous studies that suggested there was a “cosmic optical background” on top of the light from known galaxies.
The deepest image of galaxies in the night sky taken by NASA using the James Webb Space Telescope (JWST). Image: bigthink.com
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Theoretically, the only “cosmic optical background” that should be present in the Universe is the light emitted by stars, which should be limited to galaxies and larger clumps of matter, plus a little extra reflected light from within the same structures. But from Earth and even from space within our solar system, we cannot make these measurements – there is too much «scattered light» from our star, bouncing off tiny particles in interplanetary space to reveal true darkness.
In the space between these galaxies there are many indistinguishable faint galaxies that contribute to the cosmic optical background. Quantifying this background at all wavelengths is vital to ensuring that our cosmological model accurately reflects reality, astronomers say.
Scientists have already conducted an extraordinary census of faint galaxies at a wide range of distances, using powerful high-resolution deep-survey techniques from observatories such as the James Webb Telescope and Hubble, and using lower-resolution wide-view techniques from telescopes such as the Sloan Digital Sky Survey, Gaia and Euclid. But all these telescopes have a significant drawback: they are not located far enough from the Sun.
Where does light come from in the Universe? Image: i.gzn.jp/img
As a result, there is a risk that stray light, from both the Milky Way and zodiacal dust, pollutes these observatories. To explore the depths of interstellar space, we need to eliminate these sources of, for lack of a better term, «light pollution».
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Thus, the only real option, according to astronomers, is to fly very, very far, over vast distances, where the density of interplanetary dust particles is extremely low. It turns out that although stars, galaxies and the Milky Way are a familiar sight in the night sky, what happens beyond it is a mystery, and Olbers' paradox still has no solution.