Why is the Sky Dark at Night?

 

For hundreds if not thousands of years mankind has always looked into the night skies amazed at the twinkling of stars above against a very dark sky.

But why is the sky dark at night? This simplistic answer of an idiot would say, this is because there is no sun at night. But that’s not the answer. We have an estimated 250 billion stars in the Milky Way Galaxy each contributing a tiny quantum of light no matter how far away or how dim, but collectively would have set the night skies as bright as the day. But the night skies are still dark?

Let me give a simple explanation on my own.

Let us divide the Universe into layers of shells, each shell containing untold billions of stars like in our Milky Way. We see the Milky Way as a river of faint lights stretching from one end of the horizon to the other end, along with other brighter stars scattered across the night sky. Let us say what we see with the naked eyes are the nearest stars,

However, all of them, whether near or far, are enclosed inside the closest shell of ours – the Milky Way Galaxy.  We can see the night skies, if without clouds and light pollution from cities are ablaze with stars and star lights.  

Let us now take the nearest galaxy to our Milky Way, which is the Sagittarius Dwarf Spheroidal Galaxy, 0.0.7 million light years away as our next nearest shell of stars. The stars there are dimmer of course and we need a telescope to see them. Then the third major shell of stars contains the Andromeda Galaxy, 2.54 million light years away. The stars there too collectively give us some light albeit dimmer.  

We then go further to the next outer 4^th shell, that contains the nearest giant galaxy called Maffei 1, which is 11 million light years away. We then look further and further into the outer shells beyond. As we see further and further away, each shell of stars adds on their lights to the inner ones towards Earth. The furthest ones can hardly be seen from Earth, but clearly visible as a river of lights to its closest neighbouring shell of galaxies, the same as we can see the Milky Way as our closest shell of stars and their lights.

Hence if we assume the Universe is infinite, each shall add on their lights to its closest inner shell towards Earth. Logically, no matter how faint the star lights were, whether from a near distance or to infinite distances, they all add up and contribute their bits of quantum energy to each shell all the way from infinity till here until our night skies would be aglow far brighter than our day skies. But it is not.

Our night skies are still dark except for star lights and our Milky Way. In simple thinking, the Universe must be limited with stars and their lights.  Our Observable Universe is believed to be 46.508 billion light years in radius, beyond which, if it is expanding, we cannot see their lights as they are still travelling towards us if they are not pulled back again in some distant future by the total sum of their gravitational forces into a single point (singularity) as it was before the Big Bang.

Let us now look at this problem in another way, in my personal way. Shall we?  Let's go! 

The distance to the Galactic Center of our Milky Way is approximately 8 kiloparsecs (26,000 light years away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula. Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars. These stars form a large disk whose diameter is about 100,000 light years. Some put their estimate at 400 billion stars. Let us say they form the first inner shell in volume of the Observable Universe.  Since the universe has been expanding for 13.8 billion years, the comoving distance (radius) is now about 46.6 billion light-years. Thus, volume (4/3 πr³) equals 3.58×10⁸⁰ m³ and the mass of ordinary matter equals density (4.08×10⁻²⁸ kg/m³) times volume (3.58×10⁸⁰ m³) or 1.46×10⁵³ kg. Since the radius of the Observable Universe is 46.5 billion light-years or 4.40×10²⁶ m, its surface area (A) = 4 pi r2 would be 2.7 x 10 ²²  (27 followed by 21 zeros) square light years.

If we take the Andromeda galaxy 2.54 million light years away as the next shell, then its shell surface area would be 8 x 10¹³ square light years and a shell layer which is its volume minus the volume of our Milky Way. This works out to be 6.8 x 10¹⁹ – 5.2 x 10¹⁴ = 6.8 x 10 ¹⁹ cubic light years. But as we go further and further away, both the spherical areas, as well as their volume becomes larger and larger.  In such a scenario we expect the Universe to contain more and more galaxies emitting more and more light. But this may not be the case. The galaxies may be uniformly distributed throughout the Universe no matter how far away. We shall discuss this shortly.

Nevertheless, given such horrendously massive volume and surface areas, the Observable Universe would contain as many as an estimated 2 trillion (2 followed by 24 zeros) galaxies and, overall, as many as an estimated 10²⁴ stars, far more stars (and earth-like planets) than all the grains of beach sand on planet Earth. Imagine all the lights from those untold myriads of stars shining collectively towards Earth had Earth been the centre of the Universe which of course it is not. Earth then theoretically at night would be aflame far brighter than the Sun. But it is not.

So, our theory may not be right here because we assume that the expanding Universe is like a balloon filled with water with dust particles in them, each particle representing the galaxies and stars inside with more and more particles, each as a “galaxy” towards the skin of the balloon as it expands with water. The water-filled balloon will be aglow like a powerful electric bulb on its surface, less at its centre. But it is not.  Hence, we need to think out of the box in another direction.

In a technical forum paper, I presented at Oxford in November 2019 just before the Covid pandemic, I suggested the expanding Universe is like a balloon filled with air, not water, with the galaxies marked outside on the surface of the balloon, and not inside. In such a scenario, as the Universe expands, the markings of particles representing each galaxy will fly away (separate away) from each other on the surface, and not from the centre of the balloon unlike the case of a water -filled balloon. In other words, the Universe has no centre. The galaxies just fly away from each other with no fixed centre.  As the Universe expands it gives no chance of any light to accumulate at any point. That would, I suggested in my paper, the reason why there was no accumulation of light at any point, and hence this would be the reason in my personal analysis, why it is dark at night.   

The question we need to answer now is, is the Universe finite with a finite age and finite size?  This is 200-year-old question astronomers like Olbers have asked. Olbers had come to a strange conclusion based on all that was known about the Universe at that time, the night sky should not have been dark. In fact, the entire heavens should have been glowing as brightly as the Sun because it was already thought that if the universe was infinite, it should contain an infinite number of stars. If this was so, then no matter how faint or distant the stars were, each of the infinite number of stars would then contribute a tiny bit of light from the nearest to the most distant ones, making the entire sky look as bright as day. But it is not. The sky at night is still dark. So, there must be another reason.

This is the problem Olbers raised in his paper of May 7, 1823, the cosmological model of the time suggested every point in the sky should be as bright as the surface of the Sun. There should be no night. Olbers proposed a solution: the light from more distant stars was absorbed by dust or other material floating in space. The English astronomer John Herschel later pointed out this couldn’t be right because anything absorbing that much light would eventually heat up enough to glow. When Olbers died on March 2, 1840, at the age of 81, the riddle we know today as Olbers’ paradox was unsolved.

For my lay gentle readers here, as well as for those who are trained and qualified in astrophysics and cosmology further away, let us admit that the night sky basically dark no matter what our explanation.  

But a bit of thinking will tell us if there is a uniform distribution of stars in space there should not be dark anywhere in the sky.

If we look in any direction, we should be able to see a star somewhere. But this argument does not work, and the sky is still dark at night. This phenomenon is called Olber’s paradox, named after Dr. Heinrich M, W. Olber who was actually an ophthalmologist, a specialist eye doctor who turned to become an astronomer to ask this question why the sky is dark at night although this question has been discussed more than a hundred years earlier. This is similar to my interest from medicine and medical research during my adult working life into my interest in astronomy since a child.

One might think that the easiest way out of this paradox, as was realized in Olber’s time, is simply to say that there is dust or other interstellar matter absorbing the light from the distant stars and galaxies. But this energy would heat up the interstellar dusts slowly bit by bit and given enough astronomical length of time will cause all the matter in the Universe to glow up intensively, brightening the night skies uniformly. So, we are back to this paradox Olber asked, why then is the sky dark at night?

One answer to Olbers paradox lies in part the presence of the red shift in the expansion of the Universe. This does not mean that the answer to this paradox is just that the visible light from the distant galaxies is redshifted out of the visible, for at the same time ultraviolet light is continually being redshifted out of the ultraviolet into the visible

The point is rather that each quantum of light undergoes a real diminution of energy as it is redshifted. Thus, we do not see the level of brightness emitted at the surface of a faraway star. Consequently, we do not see the level of brightness emitted at the surface of a faraway star or galaxy because the energy that was emitted has been diminished by this redshift effect before it reaches us.

Since the energy, E, of a quantum at a given wavelength, A, corresponding to a certain frequency, v, is E = hv = hc / λ, the quantum has at first an energy corresponding to its original wavelength. As its wavelength gets longer, its frequency decreases because of the Doppler effect and Hubble’s law, the formula E = hc / λ shows that its energy diminishes. This energy is actually lost from the quantum.  

Of course, in Olber’s time, the large telescopes that later made the discovery of Hubble’s law possibly had not even been twinkles in the eyes of their builders who were not yet born. The discovery that one resolution of the paradox depended on the expansion of the universe had to await later generations.

There are some complications to this major point that we could mention. As we look out in space, we are looking back in time since light has taken a finite amount of time to travel. If we could see far enough, we could see the beginning of time before the formation of the Universe. This may be an equally valid way out from this dilemma. This may be a more valid contribution to the existence of Olber’s paradox than explaining it through redshifts of distant stars as old as 10²⁴ (1 trillion, trillion) years ago. However, the Universe is not that old based on Hubble’s law that puts it at 10¹⁰ (10 billion) years old that would be more appropriate.

We have to know about the expansion and the age of the Universe to answer Olber’s question, namely why is the sky dark at night? We may conclude that either it is expanding till the lights from distant stars near the edge of an expanding Universe could not reach us, or it is too young, assuming that if it had old enough to generate lots of stars that still we cannot see to ablaze the night skies with lights as bright as in the day.   

Having said this, British cosmologist Edward Harrison resolved the conflict in 1964. He showed that the main factor determining the brightness of the night sky is the finite age of the stars. The number of stars in the observable Universe is extremely large, but it is finite. This limited number, each burning for a limited time, spread over a gigantic volume, lets darkness manifest itself between the stars. Harrison later realised this solution had already been proposed not only by Edgar Allan Poe, but by British physicist Lord Kelvin in 1901. Observations in the 1980s confirmed the resolution proposed by Poe, Kelvin and Harrison. Olbers’ paradox had finally been put to rest.

In summary, a combination of the finite age and size of the observable universe, the absorption and scattering of light, the redshift of distant galaxies, and the selective direction of our line of sight contribute to the darkness of the night sky.

Other alternative explanations offered in summary form for this apparent paradox are:

1.      Finite Age of the Universe: The universe has a finite age, and light takes time to travel from distant stars. Therefore, we only see light from stars within a certain observable distance. Stars beyond this distance haven't had enough time for their light to reach us.

2.      Finite Size of the Observable Universe: The observable universe is not infinite. It has a finite size, and beyond a certain distance, galaxies are moving away from us faster than the speed of light due to the expansion of the universe. As a result, their light will never reach us.

3.      Absorption and Scattering of Light: Interstellar dust and gas can absorb and scatter light, reducing its intensity before it reaches us. This can contribute to the darkness of the night sky.

4.      Redshift: The expansion of the universe causes the light from distant galaxies to be redshifted. This means that the wavelength of the light is stretched, moving it towards the red end of the spectrum. In some cases, this can shift the light into the infrared spectrum, making it invisible to our eyes.

5.      Incomplete Sky Coverage: Even though there are many stars in the universe, our line of sight is not always directed towards them. The night sky appears dark because we are not constantly looking at the light-emitting objects in the universe.

It's fascinating to see how our scientific understanding has evolved over time and how various astronomers and scientists have contributed to solving this intriguing puzzle.

The finite age and finite size of the observable universe, along with the limited lifespan of stars, indeed provide a satisfactory explanation for why the night sky is dark despite the vast number of stars in the cosmos. It's a great example of how scientific inquiry and collaboration contribute to our understanding of the universe.

(A 2,759 worded essay in 9 pages)

Lim ju boo 

Postdoctoral CE Astronomy (University of Oxford)