Though the Sun is gone, I have a light. -Kurt Cobain
Last time we visited dark energy, we discussed its initial discovery. This came about from the fact that supernovae observed with a certain redshift (i.e., moving away from us) appear to be systematically fainter than we were able to explain.
But we weren’t satisfied with simply saying that there must be dark energy. We asked a lot of critical questions about why these supernovae might appear so faint.
First off, we asked the question, “Could these supernovae from far away be different than the type Ia supernovae we have today?”
Unfortunately, the answer is a resounding no. So long as atoms work exactly the same way, they require the same pressure to collapse at all points, times, and places in the Universe. The process of forming a Type Ia Supernova — having a white dwarf accrete mass until the core collapses and it explodes — should be independent of location and time.
Well, if the supernovae are constant, could the environments that they form in be different than the environments today? Of course they could. So, is there any way to make them appear fainter without them actually being fainter, and without having to resort to dark energy? Sure, you might say, block some of that light! All you need is some dust, like so.
What a simple idea, right? Problem solved?
Not so fast. Dust, in real life, is made up of real particles (atoms, molecules, grains, etc.), with real sizes. This means they affect light differently at different wavelengths. Not just red, green, and blue, but X-rays, ultraviolet, infrared, and more. We don’t see this light dimmed more in one spectral band than any other; it’s dimmed equally at all wavelengths!
So, real dust is out. But what if we invented some new type of dust that absorbed light the same at all wavelengths? We can give it a name: grey dust. We have no idea what would cause it, but it’s a lot more believable that there’s some new kind of dust out there than there is a whole new type of energy pervading the Universe.
Well, if this grey dust were there, then the light from distant supernovae would simply continue to appear dimmer and dimmer the farther away they were. Whereas, if the Universe had dark energy, the supernovae should start to appear relatively brighter beyond a certain distance. Take a look at the graph below to compare some different theories with the data.
As you can see, grey dust (the top line) is as inconsistent as a Universe with only normal matter (bottom line) when compared to the data.
So you can’t simply blame it on a trick of the light. In fact, if we look at the most modern supernova data, it clearly favors dark energy significantly over even a flat, low-density Universe.
Other “light-blocking” schemes, such as photon-axion oscillations, suffer from the same problem; they don’t give the right turn-over as shown above. If we’ve got the right laws of gravity, there’s pretty much no way around dark energy.
But we don’t like relying on only one source of data. Supernovae are nice, but what happens when we look at all the other evidence? Does that tell us there must be dark energy too, or could it be that the supernova data just cannot be trusted? Seems like a job for part 4, and so I’ll see you then!