This is an enhanced version (with some upgraded images and text) of an article I first wrote over two years ago. It is just as valid today as it was back then, only today, I have a special offer to go with it. Next week, a bunch of cosmologists and myself are getting together and all writing about dark energy. And I want you to have your say.
So at the end of this post, ask your dark energy questions. Ask anything and everything you ever wanted to know about dark energy. I’ll choose the best one (or, space & time permitting, more than one) and write a special post on it for you then. Enjoy the read, and then let’s see what you’ve got!
“This is what science is all about; getting thrown a curveball by Nature and plunging in to find out what’s going on”. -Andy Albrecht
Imagine waking up in the morning and heading out into the sand dunes. They never look exactly the same from day to day. But each day that you go out, they’ll look somewhat like this.
You consider yourself smart and well-informed, and you have a sense of adventure. So each morning, you venture out a little farther into the dunes. You find a variety of different features, but everything pretty much just looks like, well, sand dunes. Plain, ordinary sand dunes. Yes, they shift in detail over time, but nothing ever appears out of the ordinary.
One day, however, all of that changes. You venture out farther than you’ve ever gone before, and you see something that simply bewilders you.
Something like this, that appears to be dinosaur footprints! Well, suddenly you have this huge problem in front of you! On the one hand, you know that dinosaurs have been extinct for 65 million years. On the other hand, you examine these footprints, and you find that they’re fresh. You walk around more, and you discover that this area is covered in dinosaur footprints. The tops of the dunes, the deepest valleys, the rolling straightaways, all of it.
So what do you do? How do you make sense of this? I mean, it seems that there are two major possibilities. Either these footprints were left by a creature that is going to cause you to drastically change your worldview,
or something — perhaps a roguish person, a coincidence of unlikely natural events, or a poorly understood phenomenon — is conspiring to make the land appear like it’s inhabited by a creature that, according to your present understanding, doesn’t and shouldn’t exist.
But which one is it? Has your conception of nature just been turned on its head, and are you going to have your foundations rattled, or is something deceiving you? In other words, you might find yourself asking the following question:
Is what you’re seeing indicative of a fantastic, paradigm-shifting theory, or is there another, more mundane cause for the effect you observe?
In the case of dinosaur footprints, you may be much more likely to think that you’re being deceived. And I’m the same way. If you’re anything like me, your intuition in this situation will tell you that you need lots of hard, convincing evidence before you’re ready to change your worldview so drastically.
Coming into that situation, it’s probably fair to say that you didn’t expect you’d be believing in the highly improbable, but now you’ve got a mystery to solve. While this dinosaur situation never actually happened, this is essentially what happened to cosmologists in 1998, when the first evidence for dark energy was discovered. We had measured the expansion rate of the Universe relatively nearby, and had figured out what the Hubble constant was. But at this point, we didn’t know what the fate of our Universe would be. But we could figure that out, if only we knew what the contents of the entire Universe were. We looked at extremely distant supernovae, trying to discover what the expansion rate of the Universe was in the past, and hence, what the fate of our Universe would be.
Would the Universe recollapse, like the yellow line shows? Would it expand off into infinity, watching its size increase ever-so-slowly as the expansion rate drops to zero, begging for just one more proton, as that would be enough to cause a recollapse, like the green line? Or would it expand off into the abyss of emptiness, like the blue line? In any case, it would never follow that ridiculous curve shown by the red line; nobody even considered that. Yet in 1998, the data came in, and of those four cases, what did the data indicate?
Well, it was hard to tell, but it was more consistent with the red line than any others. You couldn’t rule out the blue line, and the green line was pretty inconsistent, but it was hard to tell. When I started graduate school in 2001, more evidence had rolled in, but I wasn’t ready to believe it yet.
Yet of the four major possibilities, the last one — that we lived in an accelerating, dark-energy-filled Universe — was suddenly one that needed to be taken seriously.
But, like I said, I wasn’t ready to buy it just yet. I needed to find out more information first, and I approached it the same way I’d approach the dinosaur footprint situation: assuming that there was an alternative explanation for why we observe this oddity.
The possibilities exploded both in my mind and in the scientific literature of the time.
- Could something be blocking the light from back then? In other words, was there some new form of “grey dust” that could block the light from these extremely distant supernovae, equally at all wavelengths?
- Could supernovae have been intrinsically different when the Universe was so young? Could some fundamental physics have been different billions of years ago, causing an intrinsic difference in the Type Ia supernovae at incredibly high redshift?
- Could perhaps the supernovae have been the same, but the environments in which they detonated were different, giving an intrinsically less bright explosion?
- Could something be happening to systematically remove a fraction of the light (like photon-axion oscillations) that mimic the effect of dark energy?
I looked at these and other alternative explanations — in incredible depth — and it turned out that they just couldn’t account for what we saw. (See my recent dark energy series.) More distant supernovae ruled out possibilities 1 and 4, while evolutionary effects were inconsistent with other high-redshift observations.
In the meantime, data from other sources was rolling in. The cosmic microwave background, galaxy clusters, and many other distance indicators all pointed towards a Universe that was inconsistent with physics unless it had dark energy in it. By 2004, it was clear that even if the supernovae observations didn’t exist at all, we would still have enough evidence to strongly favor dark energy over all other cosmological models.
It took a little over three years to convince me to start to change my beliefs, and to accept dark energy, but it was coming along, and I certainly had moved from the “No way is this thing right” camp into the “Wow, there’s a lot of evidence for it and no real wiggle room to get out of it” camp. The nail in the coffin (for me) came when a very smart man proposed an interesting alternative explanation for dark energy. While researching whether his explanation was feasible or not (and — while it’s an interesting idea — I concluded that it’s not, and even got a good paper out of it), I really became very well-informed about dark energy, its alternatives, and its implications, among other things.
It suddenly occurred to me that if you wanted to eliminate this one thing, dark energy, it would take a minimum of six separate observations to be overturned. And that was it; it was suddenly unreasonable to me that I would reject dark energy. But it took three to four years of studying it intensely, becoming some type of expert in it (as much as one can be), and writing papers right on the cutting edge of the science to convince me. It’s the only scientific conclusion I’ve had in my life where I’ve had to reject it and take up the antithesis of my original position, based on new observations.
And now, today, I defend dark energy, because I understand why we need it. So don’t believe that dinosaurs exist just because you’ve found footprints. But when you’ve exhausted all other alternatives, and you’ve found lots more evidence for them, don’t cling too dearly to your old beliefs in spite of the new evidence. (And no, there aren’t dinosaurs in the dunes; I made that part up.)
So choose your experts wisely, don’t be afraid to challenge even your most cherished beliefs about how things work in the face of new evidence, and now that you’ve come this far, let’s hear it: what else would you like to know about dark energy?