How To Train Your Universe

Imagine you set off a Big Bang. Imagine you set off many Big Bangs. Newborn universes are all small, grubby and just a little disorganised. But you love them, and want each of them to mature into a well-balanced cosmos with an evenly distributed population of stars and galaxies, and an almost (but not quite!) featureless microwave background. What should you do?

Most cosmologists would suggest a dose of inflation, delivered at an early age. During inflation, your baby universe grows almost exponentially, ensuring that the mature universe is smooth and flat. This dramatic expansion not only smooths away any irregular remnants of the Big Bang, but generates the primordial ripples that eventually grow into galaxies; points that were once almost touching now find themselves on opposite sides of the sky, as part of a microwave background that is almost identical in every direction. 


During inflation, the universe expands dramatically, diluting any remnants of the Big Bang itself. 

Cosmologists don’t give tips to newbie universe-builders, but we do ask how our universe evolved. It was quickly discovered that a simple Big Bang needed special and apparently arbitrary initial conditions in order to grow into the universe we now inhabit. But In 1980, physicist Alan Guth, then a post-doc at SLAC, realised that a mechanism he dubbed inflation made these “initial conditions problems” manageable, even if it didn’t solve them completely. Inflation rapidly became part of the theorists’ tool kit – and like all successes, it has many parents.

What does inflation look like? In a smooth, expanding universe the distance between any two points gets larger, but inflation ensures that the speed at which they recede from one another is increasing. (NB it is the empty space between galaxies that expands; “bound” objects, such as atoms, solar systems, galaxies, people, kittens, etc, don’t expand).

The key ingredient of inflation turns out to be negative pressure, which might sound weird, but it’s not. You know what positive pressure looks like: stuff that expands, releasing energy that can be turned into motion. Like a shaken up bottle of ginger beer, or this bike pump powered water rocket:


Just as stuff with positive pressure wants to expand, stuff with negative pressure wants to contract — like a stretched rubber band that goes “ping”. See, you knew what negative pressure was all along.

So negative pressure may not be mysterious, but is it something that makes a universe expand. Surely you’d want positive pressure for that?

Um. Yes. Well, sort of. If the universe is pushed apart by the stuff inside it, the pressure drops as space expands. And if pressure helps universe expand, expansion slows as the universe gets bigger and the pressure gets smaller. But inflation is accelerated expansion, and paradoxically needs negative pressure. Bear with me here. 

The next apparent paradox is the density of the universe during inflation. The density of a box of rocks goes down if the box gets bigger (and nothing enters or leaves the box). Double the size of the box and the density drops by a factor of two. But wait! Stretching something with negative pressure stores energy within it, and thanks to E=em-cee-squared this extra energy has mass. In an inflating universe energy gets diluted as the universe expands, but the negative pressure adds extra mass. For realistic models of inflation the density is almost constant — the stretching of space adds energy almost as fast as the expansion dilutes it: the universe can grow a trillion, trillion, trillion times bigger during inflation but the density only falls by a factor of 1000 or so..  

[You might worry that inflation somehow undercuts the conservation of energy, but when you get down to brass tacks, it’s actually conservation of energy that makes it work this way.]

So what has negative pressure? To a particle physicist at least, a “scalar field” is the easiest answer. All fundamental subatomic particles are associated with a field (and vice versa) — a scalar field corresponds to a particle with no spin. A scalar field can store a fixed energy at each point in space (thanks to its “potential”), and the total energy in the field grows as the universe expands and the volume of space increases. Which means the pressure is negative, and inflation is underway.

Indeed, Guth discovered inflation by thinking about scalar fields, not the other way round. 

So which scalar field made inflation happen? We don’t know. Haven’t a clue. The Higgs-boson (discovered in 2012) is a scalar, and so far, it’s the only one we know about. But not all scalar fields lead to inflation, and the Higgs is the kind that doesn’t. This is where cosmology doubles down — we’re not just looking at the beginning of the universe, we are exploring undiscovered vistas in particle physics. 

In fact, inflation could have happened a trillion, trillion, trillionth of a second after the Big Bang, at energies a trillion times beyond the reach of the Large Hadron Collider (and difficult to replicate experimentally!). At these energies you must take “theories of everything” seriously (string theory, among other contenders) — and if we know inflation happened, we also know that any successful “theory of everything” must let inflation happen. And since inflation underlies the hot and cold spots in the microwave background and leads to the formation of galaxies, any theory of inflation must account for exactly what we see in the sky. Thus theory meets the real world.  

The word is that the BICEP team has discovered B-modes, specific patterns in the polarisation of the microwave background. B-modes are the “smoking gun” of inflation, and their strength is tied to the energy density of the universe during inflation – if the rumours hold up, we could soon know that inflation DID happen a trillion, trillion, trillionth of a second after the Big Bang, and at energies a trillion times beyond the reach of the Large Hadron Collider.

There would still be several steps to nailing it down — not all B-mode signals must come from inflation, and not all inflationary models must produce detectable B-modes. And there’s one more thing: the B-mode is sourced by gravitational waves — ripples in the fabric of space — generated during inflation. Gravitational waves are a key prediction of Einstein’s General Relativity and their existence is still not conclusively confirmed. Even so, gravitational waves are getting second billing to what we will learn about the early universe: we will know that inflation is a compelling answer to cosmology’s initial-conditions problems, and humankind will have looked directly into the cosmic dawn. 

No pressure. No pressure at all. Negative pressure, in fact.

For a more detailed discussion, check out Sean Carroll’s blog which describes the connection between B-modes and inflation, and much else. Tomorrow is going to be an interesting day.