Why darkness between stars reveals more about the universe than light

When looking up at the night sky, light from stars draws attention. But the darkness between the light can reveal even more about the universe, says Nobel prize-winning astrophysicist Adam Riess

The darkness between cosmic light is worth looking at too
ESA/Hubble & NASA, E. Noyola, R. Cohen

The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.

I remember one of the first times I really looked at the night sky. I was astonished to learn that the bright lights were mostly stars like our sun and astounded that we can only ever see them as they were many millions of years ago – thanks to their great distances and the “slow” speed of light. If this didn’t provide enough awe for an 8-year-old, I was told that many of these stars had already passed the end of their lives and were no longer there! Years later I can now reflect that where my eyes were drawn was not even the most amazing part of the night sky. It is the darkness between the stars that I now find even more staggering. This is not a reflection on the Zen of Nothingness. Rather, the darkness tells us more about the universe than the light.

One of the first people to really appreciate the strangeness of the darkness of the night sky was German astronomer Heinrich Wilhelm Olbers in the early 19th century. He framed the “dark night sky paradox”, also known as Olbers’s paradox, which questions why the sky should appear mostly dark at night. Answering this question allows us to understand something fundamental about the very nature of our universe.

The paradox begins by considering what the night sky should look like if the universe were infinitely filled with stars (or galaxies) randomly distributed across the sky. That would mean that no matter where one looks in the sky, one should always be able to see a bright object that would banish the darkness.

We can imagine this scenario mathematically. Dividing space into concentric shells around us, the amount of light we receive from the bright objects in each shell declines the further it is from us. Simultaneously, the further a shell is from us, the larger it is and so it contains more such objects. These effects exactly cancel so that every shell provides us an equal amount of light. Therefore, if the universe was infinite, eternal and static, always appearing as we see it now, the sum of the light we received from the sum of all shells would be infinite and the night sky would be blindingly bright.

It is not unreasonable to expect that something as unfathomable as the universe would have no limits in time or space. After all, what could be bigger or older than the universe? So, assuming the universe is infinite, eternal and static is not a bad guess and not something we could reject out of hand without contrary evidence.

However, we do observe the sky to be dark at night. Why? There are many ways out of this paradox. If the universe as we see it is not eternal but rather started a finite time ago, then there is not enough time to receive light (which travels at finite speed) from the distant shells. If the universe is not static but rather is expanding, then light from distant shells is stretched to wavelengths beyond what we can see. If the universe is filled with non-luminous objects (dust or black holes) then the light of distant shells may be absorbed, converted into unseen sources of energy or used up. Lastly, if the universe is a finite size, the parade of ever more distant luminous shells would come to an end.

Remarkably, one of the first suggestions at a solution to the paradox was presented by the famous 19th-century writer Edgar Allan Poe who had spent an unusual amount of time contemplating the dark and macabre. In 1848, he published Eureka, an essay on the nature of the universe. Posited from his intuition rather than deduced from scientific data, it still managed to anticipate discoveries that would come over the next century about the finiteness of the universe and how it began.

The first crack in the belief that the universe was everything, everywhere and all the time came in the early 20th century with the discovery that the universe is, in fact, expanding. This suggested that not only is the universe finite, but that there was a point when the expansion first started – an origin story now known as the big bang. The discovery in 1964 that there is radiation left over from the big bang confirmed that the universe was not eternal. My colleagues and I showed in 1998 that the universe’s expansion is getting faster, further increasing the stretching and dimming of the most distant light that we see. These studies explained the reasons for what our eyes had shown us long ago – that the universe is not fully illuminated.

However, as our telescopes become more powerful, we are finding even the apparent voids of darkness often contain distant, luminous objects. One look at a “deep-field” image from the Hubble Space Telescope or the new James Webb Space Telescope makes that abundantly clear. Yet even these deep exposures contain a lot of dark regions whose exploration will require ever more powerful telescopes.

The field of cosmology today is still absorbed with understanding dark properties of the universe but those properties are due to the presence of something rather than nothing. About 96 per cent of the universe is composed of dark matter and dark energy – neither of which emit light. And neither of which we really understand. We can’t directly see dark matter and dark energy, but we can infer that they are there through the gravitational effects they have on the luminous objects.

This darkness will tell us even more about the visible parts of the universe ­­– I’m sure of that – even if we don’t know what yet. Today, when I look up into the night sky, I am still filled with awe. Awe for everything that we have discovered from the gaps between the stars and awe for all the discoveries that are yet to come.

Adam Riess is a Nobel prize-winning astrophysicist based at John Hopkins University and the Space Telescope Science Institute- in Maryland. He’s spent his career measuring luminous objects in space, such as supernovae, Cepheid variables (a type of pulsating star) and even the universe itself.