“If you wish to make an apple pie from scratch, you must first invent the Universe.” –Carl Sagan
The Universe is a lot like an extremely intricate cake. We view it today as a snapshot in time, now that it’s complete.
We can view the whole thing from one point of view, determined by where we happened to be located. But we can also look intricately inside certain sections of it, so long as we have the right tools.
But if we truly want to understand it, rather than to just describe it, we need to know how the cake was made. And this doesn’t merely mean that we need to know that it’s got cake, filling, icing and frosting.
It means we need to know all of the ingredients, and how they came to assemble, step-by-step, the cake you wind up with in the end.
If we understand this properly, we should be able to come up with a perfect recipe to reproduce this cake, and both experiments and simulations should reliably reproduce a cake that looks, smells and tastes exactly like the model cake we were studying.
And this is exactly what the field of cosmology — my field, for what it’s worth — sets out to do.
Our Universe has been incredibly well-studied over the past century, and we now know — down to a very intricate level of detail — what it looks like. We know how galaxies are distributed throughout the Universe: how they clump and cluster together in groups, clusters, superclusters and along filaments.
The thing is, we also have a whole slew of other observations about the Universe, including (but not limited to) the Cosmic Microwave Background, observations of colliding galaxy clusters, the brightnesses of distant Type Ia supernovae, measurements of the abundances of the light elements, the Hubble expansion of the Universe, the age restrictions on stars, galaxies and globular clusters, the isotropy and homogeneity of the Universe on large scales, and the observed flatness and uniform temperature and density properties across billions and billions of light years.
If we really understood what was going on with the Universe, we’d be able to take our best understanding of the laws of physics, start with some initial conditions, and run a simulation that resulted in a Universe that — aside from some normal statistical variations — is indistinguishable, cosmologically, from the Universe we observe.
Now, there’s only one way we’ve ever found that gives us a “cake” that looks like our Universe:
- A Big Bang cosmology that’s governed by General Relativity,
- where the energy density of the Universe is equal to the critical density,
- with roughly 4.5% of that energy density composed of normal matter (protons, neutrons and electrons),
- a small (~0.01%) amount of energy density in the form of radiation (photons),
- another small (less than 1% but maybe as much as 0.3%) amount in the form of neutrinos,
- with a large chunk (around 22%) in the form of dark matter
- and the rest (maybe 73%) in the form of dark energy.
Additionally, a number of remaining puzzles (flatness, horizon, spectral index) are solved by adding inflation as a way to set up the Big Bang.
The thing is, that’s it.
The cake looks amazing, but there are still a few things that you’d prefer to be tweaked. There are a few details that don’t quite match about structure formation on small scales (like, individual galaxies and smaller). There are some possible issues with the ages of some of the oldest globular clusters (that may appear to be as old as, if not a few percent older than, the age of the Universe). And… that’s really it.
In other words, we’ve got an amazing cake that matches up, but perhaps it’s a little too bitter and not quite sweet enough for the most discriminating of tastes. We’d like the cake to be a little better.
There are alternative models that come out all the time that fix this problem. They are sweet enough. They aren’t too bitter, either.
The big problem is this: they aren’t cakes.
It’s tantalizing and controversial to make a contention like Do We Really Need Dark Matter? But it’s a lie, and a poor lie at that. You can’t solve one minor problem at the expense of six or seven major ones, and that’s what every alternative that’s ever been proposed to dark matter does, including the latest one.
Yes, we don’t have all the answers, and other than knowing a whole host of properties of dark matter (energy density, interacts gravitationally, no strong or electromagnetic interactions, highly restricted self-interactions and cross-sections, etc.), we aren’t entirely sure what it is. But the simple fact of the matter is this: as far as we’ve been able to determine, we can’t have the Universe we live in without it.
The next time someone proposes an alternative to dark matter, remember to hold it up to this level of scrutiny: can you reproduce the Universe — including the CMB, the observed expansion rate, the large-scale structure, and the abundances of the light elements, just to name a few — without it? If the answer is yes, then you do, in fact, have an amazing breakthrough! But without it, you’re just trading a minor problem for multiple catastrophic ones.
As I said earlier today: You can complain that the Dark Matter cake isn’t sweet enough, but a bowl of sugar is no substitute.