Once you can accept the Universe as being something expanding into an infinite nothing which is something, wearing stripes with plaid is easy. -A. Einstein
But accepting the expansion of the Universe is easy compared to accepting the existence of dark energy. Why — and how — is there some mysterious property inherent to space that prevents the expansion rate from dropping to zero? Why is the expansion rate as large as it is today? Why, of the four options we can reasonably conceive of, is the Universe obeying this “accelerating” picture below?
The why and how are questions that we do not yet have an answer to. Nonetheless, dark energy is practically as universally accepted among cosmologists as evolution is among biologists. In this new series, I’d like to take you through our current understanding of dark energy, and why we can’t just wave our hands and explain it away.
How did this all get started? Well, you look out at things in the Universe, at things far away, and you’re actually looking back in time. You’d think that if you looked at something one million light years away, you’d be looking backwards one million years in time, since light travels at the speed of light.
And you’d actually be wrong. Why? Because the Universe has been expanding during that one million years! Well, if you look at things in the Universe that happen at a whole bunch of different distances, you can figure out how the Universe has expanded over its history.
And if you know how it’s expanded, you can learn what is it made up of. Well, if the Universe were all matter, it would do one of the first three cases in the top image, which I’ll repeat for you here.
But if the Universe has dark energy in it, it should do the “accelerating” case at the far right. How can we tell them apart? Well, Hubble’s law links two special things: distance and velocity. But the way these things are linked depends on what’s in your Universe, like so:
The big thing is that, in a Universe with dark energy, distant objects will appear to be fainter than in a Universe without it. And in 1998, that was exactly what was discovered: Type Ia supernovae, formed the same way at all times in the Universe, were fainter than they should have been at large distances!
This was measured by two independent teams and subsequently confirmed numerous times. In fact, this was my discovery of the decade for the 1990s! And yet, it’s one of the most unsettling parts of our picture of the Universe, that over 70% of the total energy in the Universe is this mysterious dark energy. We’ve been trying to explain it away, do without it, or come up with a reasonable alternative for our observations ever since, and we haven’t been able to do it.
And I think it’s worth telling you about all the different ways we’ve tried, and all about why those ways don’t work. And at the end, you can decide whether it’s hard to kill or not. So come back soon for parts 2, 3, and everything after!