Is the universe expanding and, if so, what will become of it?
The expansion of the universe was first theorized by Alexander Friedmann in 1922, who derived the Friedmann equations to show that the cosmos could be getting bigger. His theory was confirmed in 1929 by Edwin Hubble, who was the first to observe that distant galaxies appeared to be moving away from us. Since then, it has generally been accepted that the universe is expanding. Some think that it will expand faster and faster, some think that it will reach a stable flat state and others believe it will eventually start to contract, culminating in a ‘Big Crunch’. But what’s for certain is that our observations clearly show that for now it is growing, and objects further away from us are moving faster than those that are closer.
A good way to imagine cosmic expansion is in a smaller, more everyday scenario. Imagine that you have a deflated balloon, and on it you draw lots of dots. As you blow up the balloon all of the dots start to move away from one another, but there is no central point from which all the dots move. Indeed, dots further away from one another appear to move at a greater speed and vice versa. This is essentially what is happening in the universe, but on an infinitely larger scale, with entire galaxies appearing to move away from one another. There is, however, no centre of the cosmos. In other words, if we stood on a planet in another galaxy, we would observe the same phenomenon as on Earth – namely that everything is moving away from us, with further objects moving more rapidly.
Every galaxy has something known as red shift, which is the primary method through which the expansion of the universe was confirmed. This can be best explained by comparison to the Doppler effect. When a police car drives past you with its siren on, the sound waves are compressed and subsequently expanded as it zooms by. As it comes towards you the sound waves are squashed, while as it moves away the waves are stretched.
This decreases and increases the frequency of the sound waves, respectively. The same thing happens with light coming from a distant galaxy. As the galaxy is moving away from us, the light from it appears to stretch towards the red end of the electromagnetic spectrum. The further away – and thus the faster a galaxy is moving from us – the greater the red shift will be. As an aside, if a galaxy is spinning, the side moving towards us, will be squashed towards the blue end of the spectrum, known as blue shift, although this is unrelated to the expansion of the universe.
One of the most compelling pieces of evidence for cosmic expansion was something known as Olbers’ Paradox. In 1823 German astronomer Heinrich Wilhelm Olbers posited if the universe was infinite and stationary, then the night sky should be as bright as a star because all stellar light should be constantly entering our atmosphere. Olbers suggested, therefore, that the night sky was black as the universe was expanding. The light from distant stars was being stretched and bent and, as a result, Earth was not being bombarded by constant light. Coupled with direct observations of distant galaxies, it’s clear the universe is growing, however what its eventual fate will be is still very much up for debate.
Fate of the cosmos
The rate of expansion of the universe is determined by the Hubble Constant, Ho, the equation for which is also a measure of how old the cosmos is. By calculating the gravitational strength of galaxies in the universe, and determining the rate of acceleration of each, the shape of the cosmos can be estimated.
The fate of the universe is ultimately governed by its density, but this is currently unknown. A benchmark known as the “critical density’ – a point at which the universe can be said to be in its final stable state – provides us with three main theories as to its eventual destiny.
Contract – A density greater than the critical density will lead to a ‘Big Crunch’. The universe’s expansion will slow and the gravity in the cosmos will result in a contraction. All matter will move towards everything else at a faster and faster rate until the reverse of the Big Bang occurs, when everything is compressed into an infinite point.
Expand – If the density of the universe is less than the critical density, then the universe will expand forever. However, under this theory, everything in the cosmos will continue to move away from everything else until the universe is so sparse that nothing can interact with anything else, ultimately leading to its demise.
Flat – If the density of the universe is equal to the critical density, then the universe will eventually become stable. The expansion of the cosmos will slow and come to a halt, but it will not reverse, so everything will remain stable and the universe could potentially go on forever.