Why is the night sky dark despite the infinity of stars in the universe?
- Analysis suggests that the sky should be as bright as the surface of the sun – this is not the case, which puzzles astronomers.
- It’s a paradox that took nearly 300 years to resolve – this arose from the steady state theory, which was the most widely accepted viewpoint on the universe at the time.
- The steady state theory said that the universe has been and always will be like it is today, and it was infinite in size, had no beginning and was uniform in all directions, and did not expand.
- Initial attempts to resolve this paradox involved speculating that most stars were too far away to be seen; however, this was dismissed as the steady state theory itself stated that the universe had existed infinitely long, thus allowing light from even distant stars to reach us and contribute to a bright sky.
- Johannus Kepler came up with a different solution to the paradox; he suggested the stars in the universe extended to a finite distance beyond which the viewer encountered only empty space.
- This solution prompted the questions how far way is the boundary and what lies beyond it. Additionally, the notion of a finite universe posed challenges – it would eventually crash on itself under the gravitational pull of its stars and planets.
- This lead most astronomers to continue to assume the universe was infinite, and thus the Paradox and the steady state Theory remain as is, unresolved and forgotten.
- Untill after nearly 2 centuries, Heinrich Wilhelm Olbers, a renowned astronomery breath a new life into the paradox and once again asked the question why is the night sky dark.
- The common belief during his time was that the universe was static, infinitely old, and infinite in size.
- Wilhelm Olbers had a simple solution – if we assume the universe is infinite, it is logical to deduce the opaque bodies in it would also be infinite in number – dust and rock that obstruct light from distant stars, making it appear fainter – the cosmic dust gradually absorbs and blocks the light of stars as it journeys through space.
- Most people today subscribe to this theory, drawing parallels between it and the way clouds obstruct the sun’s light and view. Cosmic rocks and dust likewise could block and eclipse light from stars.
- However, Olbers’ solution does not entirely solve the problem, as any rock and dust would heat up and radiate the absorbed energy, and the universe would still contain about the same amount of radiation, and the energy we detect on earth would be the same.
- So what’s the solution to the paradox?
- After waiting for another 100 years, the solution to the paradox would be unveiled.
- In 1929, when Edwin Hubble peered through his powerful telescope, he hoped for a universe that would be collapsing – that would prove a finite universe just like Kepler said, but to his shock, the universe was not collapsing in on itself, but rather the galaxies were moving away from us as if the space between was expanding.
- In fact, the further away the Galaxy, the faster it was moving away. The Universe seems not to be falling in on itself but rather expanding.
- This raised a new question: if the universe is expanding, then what does that mean for the early universe? Cosmologists now believe that the universe was once incredibly small and dense, and it expanded rapidly in a phenomenon known as cosmic inflation.
- As the universe expanded, it cooled down and eventually stars and galaxies formed – this discovery ultimately led to the development of the Big Bang Theory.
- Scientists believe this happened about 13.8 billion years ago – it is the most accurate model of the birth of our universe.
- Today, most scientists think the universe has a finite age – about 13.8 billion years.
- When we look at 13.8 billion year old light it’s not that we don’t see stars just because light from them hasn’t gotten to us yet, we don’t see any stars because we’re getting a glimpse before any stars had formed – a starless universe – the further we look the farther back in time we observe until we reach the edge of the universe – the after glow of the big bang known the Cosmic Microwave background – furthest point in universe we can observe from earth.
- When we point our telescopes past the earliest stars, we see light left over from the Big Bang – forming a background beyond the stars.
- The Big Bang theory introduces a new problem – it states that the universe was much brighter in the past, especially at the end of the recombination era – all points of the local sky at that era were comparable in brightness to the surface of the sun due to the high temperatures of the universe in that era.
- So the Cosmic microwave background should actually be dominating our night skies back on earth, resulting in a constant need to wear sunglasses all the time to avoid being blinded by the intense brightness of the universe. This is also offset by the fact that the universe is expanding and the expansion has not stopped since the Big Bang.
- The expansion of the universe has only gotten faster, meaning new space is constantly being created even faster than the speed of light itself.
- This seems to violate the principle of relativity, but infact it is not – the expansion of space is not caused by objects moving through space but rather by the stretching of space itself due to effects of dark energy, a mysterious force that is causing the expansion of the universe to accelerate.
- The expansion of space is measured by the Hubble constant, 73.3 km/sec, which means that the distance between 2 points in space will double in size in about 9.8 billion years.
- Light being a wave, as stars move away from us, their light wave get stretched out – the human eye only see certain wavelengths of light, and when the wavelengths get too long, we can nologer see them. This is called Red-Shift.
- For very distance objects, the light waves get stretched so much that they become infrared, and we can nologer see them – so in a sense the night sky is lit up, we just can’t see it because of our limited human eyballs but telescopes can.
- Because of the finite speed of light and the fact universe has a finite age of 13.8 billion years – we can only see light from galaxies that have had enough time to travel to us, the expansion of space complicates things even more, in the far out regions of space where space is expanding faster than the speed of light, the light from those galaxies will never outrun the expansion no matter what we do or how long we wait we’ll never be able to see light from those galaxies
- What we see in the night sky is just a tiny fraction of the visible universe, and we cannot see beyond this horizon – it is not a horizon in space, it is a horizon in time. Beyond this horizon, the light will never outrun this expansion.
