WTF, Universe? Some weird shit went down right after the Big Bang
The Universe rapidly accelerated in a cosmic inflation. Maybe.
Have a seat. Relax. Set your mind at ease. This is going to get weird.
If you’ve read previous posts, you may be thinking that we crossed that bridge long ago, and you’d be right. But this next topic is about the inflationary picture of the early Universe, and when I say “early” I mean “so fucking early you cannot even imagine how small an increment of time it is, nor can I put it into words because we don’t have words for such tiny numbers.” Cosmic inflation and its potential implications have caused professional scientists, who think about weird shit for a living, to write public articles about how inflation is so weird it’s not even science! (OH SNAP!)
The stuff we’ve talked about so far in the first 5 posts, while weird as fuck, are all true. Scientists are as sure about them as they are about anything, even though it is hard to get us scientists to say things like “absolute certainty” *shudder*. But just as we know that gravity is the reason apples fall to the ground and that human activity is causing the global warming of the planet, so do we know that the Universe is expanding, that gravitational waves are ripples in the fabric of space-time, and that the global geometry of the Universe is flat.
Inflation, however, is not necessarily true. In fact, it’s not even a theory, but a “paradigm,” which is a fancy way of saying it describes a class of models. These models could all turn out to be bullshit. Or, maybe we haven’t come up with the right one yet. Science is hard.
Yes, inflation IS science, and any haters can cash me outside.
The Universe is flat. Disturbingly flat.
In the last post, I explained how we can measure the global geometry of the Universe by drawing huge-ass triangles on space-time. We have done so, and the result is that the Universe is flat.
The problem with a flat Universe, besides the fact that it is SUPER BORING not to have a Pringle-shaped Universe with hyperbolic geometry, is that curvature grows over time. Flat today means very small (close to zero) curvature today, and if we extrapolate to the past, this means the curvature was incredibly fucking tiny right after the Big Bang. Unnaturally tiny, even.
Incredibly tiny things are often a problem in physics. If the value of a physical parameter, in this example the curvature of the Universe, could be anything, then why the fuck would it be zero? We’re cool with 0.1, or maybe even 0.001, but once you start adding lots of zeros in there — let’s say, 60 zeros — then that shit gets negligibly small. For all intents and purposes, such numbers are the same as zero. This begs for a reason, a mechanism to make it zero.
Inflation is that mechanism.
The basic idea is that, unlike the steady expansion that took place for most of the Universe’s history, the very early Universe experienced rapid, accelerated expansion. And by early, I mean around 1/10³⁴ seconds after the Big Bang. During this period, instead of growing over time, the curvature gets smaller. In fact, it gets so small as to be practically zero.
To see why, imagine you are standing on a basketball-sized sphere in space. This sphere is very massive, so you don’t float away from it. And you’re wearing a space suit so you can breathe and — look, just suspend disbelief for a bit, okay? Now, imagine this sphere rapidly expands under your feet until it is the size of the Earth. Locally, it would be very close to flat. This is what inflation does, but for the whole Universe.
Inflation also explains some other problems with the simple expansion model, like why things in the Universe are the same, and why they are different.
The first is known as the “horizon problem,” but I’m not even going to attempt to explain how inflation solves the problem of why (apparently) causally disconnected patches of the Universe appear the same. The short answer is: inflation brings the entire observable Universe into causal contact.
Okay, so things are the same because they were once in thermal equilibrium in the early Universe. So why are they different? Or, more scientifically, what is the origin of the primordial density perturbations?
Welp, due to inflation, small differences between nearby patches of the early Universe that eventually grew to be stars and galaxies — otherwise known as “density perturbations” — grew from the quantum fluctuations of virtual particles popping into and out of existence and wait wait we’re getting into quantum mechanics here and that is a whole ‘nother level of weird. NEVER MIND.
So what’s with the inflation haters?
One reason some scientists are not so keen on inflation is that the idea has been around for a while, but we have yet to find any direct evidence for it. Sure, it is an idea that solves some problems, but there are other ideas that also claim to solve those problems. We like our theories to be not only clever, but also true.
The inflation paradigm does make some predictions about the polarization pattern of the earliest light in the Universe, the Cosmic Microwave Background radiation. But most of the polarization is caused by boring things in our Milky Way galaxy.
Many experiments are in various stages of trying to measure the cosmological polarization signal very precisely, including some in Antarctica and some sent up into the upper atmosphere on balloons. There was a brief period where one experiment announced positive evidence of inflation, but it turned out to be polarization caused by dust in our own galaxy. Seriously, all it does is get in the way! Stupid fucking galaxy.
Anyway, another reason inflation makes some scientists feel icky is that one of its possible consequences is the dreaded multiverse. Some models predict that inflation stops in our part of the Universe (which is obviously true), but not in others. Since the volume of the inflating parts is a giga-mega-bajillion times larger than the parts where inflation ended, this “eternal inflation” effectively creates an infinity of island universes separated from each other — a multiverse.
And multiple universes make people uncomfortable. They just do. Even though the effective multiverse — or multiple universe domains — of eternal inflation is not really the same as a true multiverse, in my opinion, it still smacks of being untestable, which is one of the worst insults in science*. Thus we have an inflationary controversy.
*Other scientific insults include insignificant, trivial, handwaving, dickwaving, and unfalsifiable.
The next posts will step into more controversial ground with dark matter and dark energy. It turns out that when your best model of cosmology supposes that the overwhelming majority of the mass and energy in the Universe consists of weird stuff we have no direct evidence for, some people remain unconvinced.