The Big Bang, the story of how our Universe came to be the way it is, may be one of the most successful theories in science.
The basic idea behind the Big Bang model is simple: the Universe is expanding and cooling. If this is true, then running the clock backwards means that in the distant past, the Universe would have been much hotter and denser than it is today. But while many lines of evidence provide great support for the Big Bang and have led to its nearly universal approval across the astronomical community, it is not without its problems.
The Universe is big. However, the Universe that we see is only a tiny sliver of what is out there. If a galaxy is 14 billion light years away, that means it takes the light 14 billion years to reach us. But what if there hasn’t been enough time for that to happen? What if the time required for light to get from there to here is actually greater than the age of the Universe?
The Universe is 13.7 billion years old. If there is a galaxy sitting 13.8 billion light years away1, then there hasn’t been enough time since the Universe was formed for the light to reach us yet! This “edge” of our vision is like a curtain drawn all around us and is known as the cosmic horizon. Like the horizon here on Earth, it’s a line beyond which we can not see no matter how hard we try. The space within that horizon is the “visible Universe” – a tiny bubble within which enough time has elapsed for light to have reached the Earth.
The cosmic horizon for a galaxy many billions of light years away is different than our own. The denizens of that galaxy can see parts of the Univere that we can not and vice versa. But imagine two galaxies separated by such a great distance that their horizons do not overlap. They see two completely different Universes and have absolutely no knowledge of one another. There hasn’t been enough time for any information to get from one bubble to another. And this is a bit of a problem.
When we look at two opposite points on the sky, the Universe looks exactly the same. The densities and temperatures in one direction are the same as they are in the opposite direction. It doesn’t matter where you point your telescope, you’re going to see basically the same Universe. But a point at one end of our visible Universe is outside the horizon of a point on the other side. The parts of the visible Universe that sit opposite each other on our cosmic horizon have no knowledge of one another. How is it then remotely possible that all these disconnected regions of the cosmos ended up with exactly the same properties?
There is also a problem with the Universe being so flat. Yes, space has a “shape”, usually referred to as its “curvature“. This is easier to picture if you imagine a two dimensional Universe: one that exists entirely on a sheet of paper, for example. You could lie the paper flat, you could fold it around a soccer ball – you could contort in a number of different ways. Folded onto a sphere, the Universe is “closed”. If you started moving in one direction and kept going in a straight line, eventually you’d come back to where you started. A flat Universe is just what it sounds like. There are no curves to it all; it’s much like lying your paper Universe down on the floor. There is also the possibility of an open Universe, where the paper takes on a shape similar to a saddle. All our observations point to a flat Universe. So, why is this a problem? It’s because a flat Universe is also extraordinarily improbable.
Maintaining a flat Universe is like trying to balance a pencil on it’s point. The tiniest deviation from perfectly balanced sends the pencil quickly toppling one way or another. The curvature of the Universe behaves similarly. An infinitesimal deviation away from being perfectly flat gets magnified as the Universe ages. If the Universe was even slightly open or closed in it’s formative years, then it would have long ago blown apart or collapsed. And yet, here it is. To still be here, the Universe would have to have been within one part in 10 to the 60th power of being perfectly flat. But there’s no real reason for it to be so. Much like the horizon problem, leaving things the way they are relies a bit too much on coincidence for anyone’s comfort.
But even if coincidence is the reason, the Big Bang has one other problem. While it is remarkably successful at predicting the primordial abundance of elements, it completely misses the mark when predicting the abundance of what are known as “magnetic monopoles”. One defining attribute of anything magnetic is the presence of two “poles”: a north pole and a south pole. No matter how small you slice it, any magnet will always have two poles, never just one. At least it’s never been observed. And there’s the problem: the Big Bang model predicts that isolated magnetic poles – monopoles – should exist in relative abundance in the Universe. So…where are they?
In the early 1980s, a number of astronomers wrestled with these and other quirks of the Big Bang model and came up with a rather nifty solution. It came to be known as the theory of “inflation”.
The main idea behind the inflation model is that very early in its history – only one billionth of a billionth of a billionth of a second after the Big Bang – the Universe went through an extraordinarily brief period of very rapid expansion. This period of inflation drove the Universe to become 10 to the 78 times larger in the tiniest fraction of a second. A region of the Universe the size of a proton almost immediately became the size of a grapefruit.
How does this little modification solve any of these problems? Before inflation began, all of what we know as the Universe would have been in close contact, close enough to share information like temperature and density. Inflation drove these regions apart at a speed much greater than that of light, permanently cutting off any further contact. But with the initial conditions the same all over, these isolated bubbles of space would evolve in similar ways giving us the uniformity we see when we look all over the sky.
Rapid expansion also smooths out the Universe. If you have a deflated balloon, its surface is highly curved and lumpy. Blow into it and its surface becomes smoothed out and any tiny region on the balloon starts to look flatter and flatter. Inflation means the Universe could have any shape right after the Big Bang but that curved space would get smoothed out in less than an eyeblink leaving a Universe that is uniformly flat2.
Inflation also takes the monopoles that formed in the very earliest epochs and scatters them throughout a cosmos that is much bigger than the visible Universe. The density of monopoles plummets so far that you might find at most one monopole within our cosmic horizon. This doesn’t affect the density of matter that would go on to form stars and galaxies because that material would not condense out of the cosmic stew until long after inflation was done.
In the past decade, space missions have mapped the cosmic background radiation – the sea of microwave energy left over from the blast of radiation sent out into the cosmos when the Universe released all its pent up energy – to remarkable precision. By measuring tiny fluctuations in the energy across the sky, astronomers see imprinted on this glow from the early Universe the tell tale signs of quantum fluctuations that would have been imprinted on the early Universe as the inflationary period came to a close. These fluctuations in energy which exist on only the smallest of scales would have been immensely magnified as the Universe expanded at an inconceivable rate. Careful measurements of the sizes of these fluctuations and how they are distributed on the sky match what the theory of inflation predicts.
These fluctuations are the seeds from which all that we see came into being. The hydrogen and helium of the infant Universe would flow into these denser regions like puddles after a rain storm. From these primordial pools of the most basic of elements came the first stars and galaxies. Those early stars would seed the Universe with heavier elements which would get recycled through succeeding stellar generations and eventually come to be the seeds from which planets – and life – would grow. Our very existence is owed to energy fluctuations of only one part in 10,000 on the smallest scales imaginable frozen into the Universe after an unimaginable epoch of exponential expansion 13.7 billion years ago.
1 As pointed out in the comments (and in an upcoming article!) a galaxy whose light has taken 13.8 billion years to reach us is actually much, much farther away than 13.8 billion light-years because the Universe has expanded in that time. For 13.8 billion years of light travel time, that galaxy is currently at a physical distance of closer to 46 billion light years from Earth.
2 Also pointed out in the comments, this is only approximately true. Without dark energy, the energy density of the Universe decreases as it expands and this in turn actually has an effect on the curvature. Curvature and energy density are intricately linked. So you need a dark energy component as well to compensate — but that’s a topic for a whole other post. Thanks to Steve Nerlich for the clarification!