In the previous article, we discussed the Big Bang theory and saw that this theory makes two major predictions:
1. The universe is expanding.
2. A large number of photons were released in the early universe when energy levels were low enough for electrons to bond with nuclei.
A theory is only as good as its predictive power so let's put each of these predictions to the test.
1. Expanding Universe
If you've ever wondered how we can possibly know how far away the distant stars and galaxies are, you're not alone. In fact up until the early 1900s, most scientists did not believe there were any galaxies outside of our own Milky Way. Although countless galaxies are visible in even the smallest telescopes, they were thought to be clouds of gas residing in our own galaxy simply because we had no way of measuring their distance.
Imagine you want to measure the distance between you and a friend while both of you are standing on the top of two mountains. The distance is much too far to use a tape measure so you have to get creative. One way you can do it is using a flashlight. If someone shines a flashlight at you from a few feet away, the light is blinding. However if they are standing a few hundred feet away it isn't nearly as bright. The change is brightness is related to how far away the light is so if you measure the brightness of a flashlight at close range, then send your friend up the mountain and tell him/her to shine it, you can use the change in brightness to figure out how far away your friend is.
The same is true of stars, if we know how bright a certain star should be, then we can measure how bright it looks to us to calculate how far away it is. The only problem is finding a star that has a known brightness. Just looking at the night sky, it is impossible to tell which stars are dim but close and which ones are bright but far. Two stars can appear to be almost the same brightness in the sky but in reality be at very different distances from the Earth.
Stars can look the same in the night sky even when they are vastly different sizes and distances
Fortunately astronomer Henrietta Swan Leavitt solved that problem when she discovered that the true brightness of a certain type of star called a Cepheid Variable can be predicted. Variable stars go through cycles where they change (vary) in brightness, hence the name 'variable' star. In the case of Cepheid Variables, the time it takes for their brightness to change is directly related to the true brightness of the star. If we know how bright the star truly is, and how bright it looks from Earth, we can use the difference to figure out how far away it is.
Cepheid variables have a known brightness so their apparent brightness from Earth can be used to find their distance
Using this knowledge, another astronomer Edwin Hubble (after whom the Hubble Space Telescope was named) was able to show that some objects in the night sky were much further away then prevoiusly thought. So far in fact that they couldn't possibly be within our galaxy. The discovery that there are other galaxies in the universe was surprising enough but there was another surprise waiting for astronomers The light coming from these distance galaxies appeared shifted towards the red part of the spectrum which indicated that they were moving away from us.
The shift towards red is a result of the Doppler effect, the same effect that causes the sound of an ambulance to change as it passes you. As the ambulance approaches, the siren sounds higher pitched than when the ambulance is moving away. Similarly, the light from objects approaching us is shifted into the blue part of the spectrum (blueshift) and the light from objects moving away is shifted to the red part of the spectrum (redshift). It was expected that some galaxies would be moving towards us and some away from us but astronomers were puzzled to find that almost all galaxies were moving away from us. Moreover, the further the galaxies were the faster they appeared to be receding from us.
Almost all other galaxies appear to be moving away from us
Astronomers quickly realized though that it wasn't just us, all the galaxies are moving away from each other as well. So how is this possible for all the galaxies to be simultaneously moving away from each other? The space between them is getting bigger, much like the space between raisins becomes larger in a loaf of raisin bread when it's baked. Just like the bread batter rises and expands, the space between galaxies itself is expanding.
As the bread expands, the distance between all raisins increases
The discovery that the universe is expanding flew in the face of thousands of years of belief that the universe was eternal and unchanging. If this comes as a surprise to you don't feel bad, even one of the greatest minds of all time had trouble wrapping his head around it. By the time the universe was discovered to be expanding, Albert Einstein had already published most of the work that would make him famous, however he was troubled by one of the predictions in his equations for General Relativity. His equations seemed to indicate that the universe should be expanding or contracting depending on how they were interpreted. Einstein strongly believed that the universe was eternal and unchanging so he added a correction called the Cosmological Constant into the equation to balance things out (preventing expansion or contraction). Once he saw the work of Hubble and others that proved the universe was expanding, he regretted changing his equation; referring to it as the biggest blunder of his life
A universe that is still expanding today is exactly what we would expect to see if the universe did indeed have a sudden beginning 13.5 billion years ago. There are many good mathematical reasons for this but perhaps the most intuitive way to think about it is to consider the fact that the universe is expanding and rewind the process. If everything is moving apart and has been doing that for billions of years, at some point in the past everything must have been very close together.
2. Photons in the Early universe
At the end of the previous article, we saw that once electrons were able to bond with nuclei, the universe became transparent allowing photons produced during the big bang to pass through it easily. However this is a pretty bold claim given that no one was around to see that light 13.5 billion years ago so how can we know for sure?
Photons were able to travel with minimal interference once atoms formed
Saying that the universe is a big place would be an understatement. Even light, the fastest thing we know, can take millions or even billions of years to cross sections of it. Space is so big that we measure distances in terms of how long it takes light to cross them. The distance light travels in a year is called a light-year. The closest stars are a few light-years away, our galaxy is hundreds of thousands of light-years across and one of the closest galaxies to us, the Andromeda galaxy, is 2 million light years away.
The Andromeda galaxy sits about 2 million light-years away
An interesting consequence of such distances is that we see objects in the night sky not as they are now, but as they were in the past. Think about it, if light coming from the Andromeda galaxy takes 2 million years to travel to Earth, then the light we see today must have started its journey 2 million years ago. In other words, the image you see above is what the Andromeda galaxy looked like 2 million years in the past even though the picture was taken recently. Likewise, if there is anyone in the Andromeda galaxy looking back at us, they would just be receiving our light from 2 million years ago when our earliest ancestors were discovering how to make and use stone tools.
Recall that the universe is 13.5 billion years old, meaning that the furthest any light could have possibly traveled since then is 13.5 billion light-years. Anything further than that would be impossible for us to see because there hasn't been enough time in the history of the universe for it to reach us. Even more interesting, if we could see something 13.5 billion light-years away it would be the light from the big bang itself since this is the only thing that happened 13.5 billion years ago!
Now its time to bring back the photons from the beginning of this article. We theorized that shortly after the Big Bang, when atoms were formed, that a whole bunch of photons suddenly found themselves able to travel freely across the universe. Some of those photons, specifically the ones that started out in a place 13.5 billion light years away, should just be reaching us now. In every direction in the sky, there exists some point that is 13.5 billion light-years away so we should be seeing light from the Big Bang coming at us from all directions.
We should see light from the Big Bang coming from all directions
If you've ever been outside at night you've probably noticed that the night sky is pretty dark except for the spots where there are stars. There isn't light coming from all directions as the theory suggests. Or at least we don't see light coming from all directions when we look at the night sky. But let's not forget that our human eyes can only see a fraction of the light spectrum. Specifically we see from red on the lower end to violet on the higher end. But anything below infrared and above ultraviolet is invisible to our eyes.
The spectrum of light extends far above and below what our eyes can see
Now its time to recall the concept of redshift that we touched on in the previous section. Remember that the universe is expanding which means all the distant galaxies are moving away from us and the further they are the faster they recede. In turn, the faster they recede, the further into the red part of the spectrum their light is shifted. The edge of the observable universe is 13.5 billion light years away and objects at that distance are moving away from us faster than anything else. Therefore we would expect the light from the big bang to be shifted very far down into the red part of the spectrum. In fact it should be shifted so far that it would appear mostly in the microwave part of the spectrum where our eyes can't see it.
Light from the Big Bang has been shifted into the microwave part of the spectrum on its way to Earth
In order to 'see' this light, you need something that can detect low frequency light waves. Astronomers use expensive radio telescopes but really any radio receiver or old TV with an antenna will do. All you need to do is flip your TV set or radio dial to a channel that is nothing but static. Most of the static comes from random electrical signals that are picked up but a small portion of it is coming from the beginning of the universe. Your TV or radio is able to detect the low frequency light waves that left the edge of the observable universe 13.5 billion years ago during the big bang and are just arriving now.
Most of us consider static nothing more than an annoyance and the first people to discover the light signal coming from the big bang felt the same way at first. Radio astronomers Arno Penzias and Robert Wilson had set up their radio antenna for a completely different experiment but could not seem to eliminate some unexpected noise. They tried to pinpoint the source of the noise but it consistently came from all directions, day and night so it couldn't be the sun or anything within our own galaxy. Eventually the two became aware that astronomers were searching for the very signal that they had discovered by accident: the cosmic microwave background.
The cosmic microwave background is light from the Big Bang that shines at us from all directions in the universe
The cosmic microwave background is the result of atoms forming in the early universe, allowing the photons generated during the big bang to travel through space with little interference. The photons that were freed from the edge of our observable universe have been travelling freely since they were released 13.5 billion years ago and are just now reaching the Earth from all directions. This causes light to come at us from all directions but this light has been redshifted down into the microwave part of the spectrum due to the expansion of the universe. As a result, we don't see it shining down on us when we look at the night sky, however it does make itself known to some extent as annoying static on your car radio.