“Cosmologists are often in error, but never in doubt.” –Lev Landau
I’ve been telling you about the Big Bang, the greatest story ever told, and the entire natural history of the Universe. Let’s remind you — historically — of how our conception of the Universe changed as we learned more about our surroundings.
Maybe the first astronomical observation ever made was that the Sun rises in the East, passes overhead, and sets in the West. And it does this day after day, every day. It’s no wonder that our first “cosmological model” of the Universe was that the Earth is stationary, and the Sun orbits around it once every day. Why not, after all? That’s what we see on Earth!
But as we started making careful observations of the night sky, we discovered that the motions of planets were inconsistent with that picture. While the Sun looks like it orbits the Earth, another explanation could be that the Earth is rotating, and so the Sun would simply appear to revolve around the Earth. The better picture that superseded the old one was to put the Sun at the center, and let the Earth be just like any other planet: orbiting the Sun.
But it wasn’t long before we realized our Sun wasn’t unique, and that nearly all of those points of light in the night sky were also Suns, just like our own. So rather than the Sun being the center of the Universe, it became just another star — one of billions — in our galaxy: the Milky Way.
And amazingly, we thought that our single galaxy made up the entire Universe until the 1920s! (You can read about the discovery that changed everyone’s mind here.) But now we know that our galaxy is just one of many billions in the entire Universe, which is billions of light years across, expanding, and which started with the big bang just under 14 billion years ago.
Our telescopes see farther than ever before, the amount of information we have about space is vastly greater than at any point in history, and you can find some of the most amazing images ever taken simply by using google.
But surely the current cosmological model — the Big Bang — isn’t the final, complete story. So I thought I’d point out the three greatest problems (in my opinion, anyway) with the current cosmic model of the Universe.
1.) What happened before the Big Bang? The big bang wasn’t the beginning; something had to have happened to create the Universe we have today with the properties it has. We have a word describing what must’ve happened before — inflation — but we don’t know how inflation happened, or even which model of inflation is the right one! So if we want to understand where our Universe came from, we’ll need to answer this question, and inflation — while brilliant and compelling — isn’t a complete answer.
2.) What the hell is most of the matter in the Universe made of? When you look out at the Universe, you can measure the amount of mass out there (thank you, laws of gravity), and you can also measure how much normal matter is out there (thanks to light). It turns out that when we add up all the normal stuff out there — stars, planets, gas, dust, plasma, etc. — it’s only about 15% of the total amount of matter in the Universe that gravity tells us needs to be there!
So what is this matter, and where is it? Or, quite possibly, are our laws of gravity fundamentally flawed, and is our understanding of how gravitation works on the largest scales in the Universe simply incomplete? That’s the second great mystery, known as the dark matter problem.
3.) Why is the Universe’s expansion speeding up? You can imagine that if things start off hot, dense, and expanding, that there are three possibilities. Gravity is going to fight that expansion, and try to pull everything back together. Maybe gravity wins, and will eventually pull things back together, causing the Universe to recollapse? Or maybe the expansion wins and gravity loses, and everything flies off into the infinite abyss? Or maybe we’re in this wonderful balance between gravity and the expansion, where the expansion rate will asymptote to zero, but will never quite run away and will never reverse into a recollapsing state.
What we actually observe is none of these. It looks — for the first few billion years of the Universe — like it’s in this perfectly balanced state. But then all of a sudden, the expansion rate picks up again, and the farthest galaxies from us accelerate away! This is known as the dark energy problem, and (while there are many speculative theories at this point) there is no compelling explanation for this phenomenon.
And those are the three biggest problems, from my point of view, facing cosmology today. What will the resolution be? Right now, nobody knows. But these are the questions, and now you can go think about them, and tell me what you think the solutions might be!