For a long time, humans had no idea what was going on in the Universe. To help, we made up stories to either help us explain what we saw, or to make us feel better about what we didn’t understand. But then science came along, and we started to understand more. We could test our ideas, and as we got more confident in the process, our ideas grew. The field of cosmology was born, this is the study of the cosmos itself. And now, after centuries of speculation and just-so-stories, we’re starting to get a grasp of the biggest ideas there are. The nature of the Universe.
By the turn of the 20th century, scientists knew that the Earth was old. Darwin’s theory of Evolution (or Darwinism) strongly implied that the Earth was at least millions of years old, and Lord Kelvin (a hugely respected physicist confirmed that the Earth was ancient, given that it must have cooled from an initially molten state. Which must have taken a while, at least a million years. How old exactly, no one knew. As for the Universe itself, it logically must be as old or older than Earth.
A popular model for the Universe was that it is static, the way it has and always will be. Stars may be born and they may die. However, overall things pretty much stay in balance. For example, the Universe always existed, always will, always had galaxies in it and so on. As with pretty much everything in science, there are variations on this idea, but that’s it in a nutshell, and it’s what many astronomers believed. This is important, because when we try to understand observations in astronomy, we fit them into a framework of understanding or things that we think we already know. When something doesn’t fit, it’s a problem. Perhaps the observation is wrong, or maybe we’re misinterpreting it, but sometimes, rarely, but sometimes, the framework is wrong! If you think of Science as a Tapestry, when you yank at one thread, the whole thing may need reviewing. And exactly that happened.
The thread that got pulled in this picture was first uncovered in 1912. Astronomer, Vesto Slipher started taking spectra of the “Spiral nebulae”, hoping to get a better insight on their characteristics . It took him several years, but by 1917 he had observed 25 of them, and he found something astonishing. When he examined their spectra, he saw that almost all of them were highly redshifted. In other words, it looked like most of these objects were rushing away from us at high speeds of up to millions of kilometres per hour.
At this point, two different lines of work began to converge. One was by a Belgian theoretical physicist named George Lemaitre. In the 1920’s he had been studying Albert Einstein’s work, or more specifically, the equations dealing with the behavior of the Universe as a whole. Einstein had concluded that the Universe was static and unchanging, but Lemaitres disagreed. He found that an expanding or contracting Universe fit the equations better, and given the redshifts observed by Slipher, he proposed that the Universe was getting bigger, which is why the galaxies appeared to be moving away from us.
At the same time, astronomers Edwin Hubble and his assistant were trying to determine the distances to neighbouring galaxies. They observed the great spiral nebula M31 (or the Andromeda Galaxy as we now know it), using what was at the time the largest telescope in the world. They found dozens of pulsating stars in it, literally stars that changed their brightness in a regular, periodic fashion, called Cepheid variables. They were critically important, because it was known that the time it took them to pulse was directly related to its luminosity. That means that if you measure their period, you can determine how far away they are, simply by measuring their apparent brightness. The distant they found M31 to be from Earth was 900,000 light years, clearly outside even the largest estimates of the size of the Milky Way.
They then observed some of the same galaxies Slipher did, and measured their distances. When they compared distances to the redshifts Slipher observed, they found that the farther away the galaxy was, the faster it was moving away from us. This tied into what Lemaitres had concluded – the Universe is expanding!
There are lots of different ways of looking at this. Lemaitre himself suggested imagining the cosmic clock running backwards. The raisin pudding analogy helps us to mentally picture how Hubble's Law relates to an expanding universe. Consider a loaf of raisin pudding (or bread dough) baking in an oven, with raisins sprinkled evenly throughout. As the pudding expands during cooking, all of the raisins are moved farther and farther apart from each other. Seen from the viewpoint of any individual raisin, all the other raisins in the pudding appear to be receding away with some velocity. The nearby raisins recede more slowly and distant raisins recede more quickly. It is important to remember that the raisins are NOT flying apart on their own due to some sort of explosion, they are simply being carried along by the expansion of the dough. In the same way, the galaxies are carried apart by the expansion of space itself.
Right now, as time inexorably marches on, all galaxies in the sky are getting farther and farther away from us. But that means that in the past, they were closer together. Run the clock back far enough and they get closer and closer together until at some point in the past, everything in the entire Universe was crammed into an infinitely dense atom. Lemaitre called this a “Primeval Atom” or more colourfully, the “Cosmic Egg”. But this itself has implications.
If everything in the Universe were to be squeezed into one place, then this place is going to be unimaginably hot. Then, for some reason, (scientists are still working on the reasoning for this sudden event), it suddenly expanded violently and started cooling. This sounds a lot like an explosion or bang, involving the entire Universe, which is big. What else would you call this other than, “The Big Bang”.
This is all mind boggling to comprehend, and astronomers had hard time accepting it. After all, it went against everything they thought was true at the time. In science, a hypothesis needs to make testable predictions before it can be taken seriously. What predictions could The Big Bang model of the Universe make that we can observe today?
The speed of light is fast, about 300 000 kilometres per second, or close on a 1-billion kilometres per hour. Fast, but not infinitely fast. The sun is 150 million kilometres away. It takes light about 8 minutes to reach the Earth, so in a sense you are seeing the Sun as it was 8 minutes ago. The nearest star system to us is Alpha Centauri, 4.3 light years away, so we see it as it was 4.3 years ago. The Andromeda galaxy is about 2.5 million light years away, meaning that the light that is reaching us now, left that galaxy when Homo habilis (a species of early humans), famous for being the first species to use carved stone tools, walked the Earth. The farther away something is, the farther in the past we see it. This is called the “lookback” time, and is a crucial tool in the field of Cosmology. By observing very distant objects, we can see the Universe as it was when it was ‘young’. You might think that we could see all the way back to the moment of the Big Bang, but there is a problem. At some point back in time, the Universe as so hot and dense that is was the same temperature as the surface of a star. It would’ve been very luminous, but also opaque. As it expanded, it cooled, and became transparent. If we look back far enough, that moment in time when it cleared up is as far back as we can see.
By looking at the physics of the Big Bang (the math that describes how matter, energy, space and time behave), astronomers could predict when this moment happened in the lifetime of the Universe- a few hundred thousand years after the bang itself. Using the idea of the lookback time, they could predict how far away it would be from us, and therefor calculate the redshift. Remember redshift stretches the wavelength of light. The light of the Universe emitted at the time would’ve been like a star, in the visible part of the electromagnetic spectrum, but the light that reaches us billions of years later – now – should be redshifted into microwave wavelengths.
Hubble’s law states that the red shifts in the spectra of distant galaxies (and hence their speeds of recession) are proportional to their distance. This means that the Universe had a beginning. By measuring how quickly the Universe is expanding, we can use math to run the clock backwards, and determine the age of the Universe (the inverse of Hubble’s constant should equal to the age of the Universe). Currently the best measurement for the age of the Universe is 13.82 Billion years!