Inflation Theory.


In which I tackle one of the things that for a long time seemed like a colossal fudge to me, but which has an ever-increasing weight of evidence supporting it: inflation theory.

How big is the universe, really? Nobody knows, and it’s entirely probable that nobody will ever know. We’re limited in our measurements to observations of the bits of it we can actually see; all we have access to are the stars and galaxies which are close enough to have their light crawl slowly, painfully across the ever-expanding fabric of spacetime to be received by our telescopes within the 13.7 billion year lifetime of the universe. Anything emitting radiation which takes more than 13.7 billion years to get here will forever be locked behind a causal event horizon that we’re unlikely to ever penetrate, and not having access to an unknown proportion of the universe makes it pretty difficult to estimate the overall size of the thing. However, even the bits of it we can see raise some pretty interesting questions.

For starters, the  universe — everything we can see — appears to be about 93 billion light years across. A common misconception is that since the universe is 13.7 billion years old, and light travels one light year per year, then the observable universe must have a 13.7 billion light year radius. This is not true; the fact that the fabric of spacetime is stretching and expanding underneath the light as it wends its way through the cosmos means that by the time it reaches us the distance between us and the source has grown to several times what it was when the light set out on its journey. This also has the effect of stretching the light itself, spreading it out into longer wavelengths and shifting it into the red part of the spectrum; this redshifted light is what tells us space is expanding in the first place, and is distinct from the Doppler effect caused by a simple moving body which is used to explain shifting wavelengths to schoolchildren.


So that’s why the observable universe is so big. Thinking about it, though, this just presents us with more problems. Consider the case of two galaxies situated a long way away from us, one on each side of the Milky Way at opposite ends of the universe, whose light is only just now starting to be detected by our telescopes. Logic dictates that since the speed of light is an absolute limit and since the light has twice as far to go until it reaches the other galaxy, neither galaxy can see the other, and that the only reason we can see both is that we happen to be conveniently situated at the midpoint between the two. From the perspective of each of these galaxies the other is locked behind that causal event horizon I mentioned earlier. No information can travel faster than light, and so these galaxies are effectively out of causal contact — they cannot influence each other in any way, and never have been able to influence each other.

(Everyone with me so far?)

This gives rise to something called the horizon problem: if these two galaxies lie beyond each other’s cosmic horizons, and are completely isolated from each other and unable communicate information to each other in any way whatsoever, then why are they so similar? Everywhere we look in the universe we see two things: homogeneity, and isotropy. This is a fancy way of saying that forces act uniformly throughout the universe, and that when viewed on a large scale the distribution of matter and energy is astonishingly even (this is one of the reasons the increasingly accurate measurements of the cosmic microwave background are such a big deal, which we’ll get to later). Which would be fine, except the fact that large portions of the universe are (apparently) out of casual contact with each other means this should be impossible. Think about molecules of gas that have been pumped into a container; initially different parts of the gas will have different kinetic energies, and thus different temperatures, but eventually the gas will reach thermal equilibrium as the gas molecules bounce off of each other and communicate their energy to all the other gas molecules in the container. If two parts of the gas were isolated from each other in the same way that the portions of the universe containing these galaxies are, though, we would expect them to have different thermal energies and different temperatures since there’s no way for one part of the gas to communicate its thermal energy to the other. So it is with the universe in general: if these two galaxies are out of causal contact then they should have evolved along dissimilar lines and look at least broadly different to us, and each different region of space should have its own unique character. Instead, the whole damn thing is the same to us no matter which direction we look in.


This should be impossible — hence our calling it the horizon problem rather than the horizon curiosity or the horizon discrepancy or whatever — but obviously it’s happened somehow, and so we’ve had to come up with a mechanism by which different regions of the universe which have been out of communication with each other since the dawn of time have somehow acquired such similar physical properties. The theory goes that very shortly after the beginning of the universe — we’re talking 10-36 seconds here — it underwent a phase of rapid and massive expansion, growing to 10^78 times the size in just 10-33 seconds. Much like a scrunched up sheet suddenly being stretched out to its full flat length this period inflation had the effect of smoothing out irregularities in the structure of the universe, removing nearly all of the inhomogeneity and (eventually) resulting in the uniform structure we observe around us today. This explains why those two galaxies have evolved like the regions of space that contained them were once in causal contact with each other: they were, since the entire universe originates from this one small causally connected part of space. After this period of dramatic inflation the expansion of the universe slows down1, achieving its current, relatively sedate character.

Inflation theory explains why the universe is homogenous, why it is flat (see the sheet analogy above) and why there are no magnetic monopoles when particle physics says there should be (too complicated to go into here). However it always seemed to me like a classic case of post hoc ergo propter hoc; writing the theory solely to fit observed phenomena without making any falsifiable predictions is bad science (hello string theory). It’s a good thing the cosmic microwave background is a thing that exists, then, since it provides actual experimental evidence that inflation theory might have something concrete to it. The CMB is residual thermal radiation left over from the formation of the universe, and it is very nearly uniform throughout the entire sky. When the CMB was first observed by the COBE satellite back in 1993, though, it was observed to contain tiny irregularities on the scale of one part in  104; these irregularities just happen to match those we’d expect to see if you took a relatively small region of hot gas and suddenly expanded it to the size of a universe. Quantum fluctuations in the hot gas would suddenly be magnified to the point where they have a significant effect on the macro scale of things, and despite their tiny scale the resulting irregularities were sufficient to provide a starting point for the clumping together of matter to form dust clouds, to form stars, to form galaxies. This is one of the reasons the CMB is such an intense target of study for astrophysicists, with the far more accurate WMAP probe being launched in 2001 and the Planck spacecraft following that up in 2009; it basically explains why we even have a universe in the first place, and better measurements of those irregularities (called anisotropies) give us a much improved understanding of how things all began, and incidentally provide a compelling reason why inflation theory might not be such a bit of flim-flam after all.

(Of course exactly how and why inflation happened is still a bit of a mystery. But then science wouldn’t be any fun if we knew everything, would it?)


  1. The inflation is supposed to be driven by a high initial cosmological constant, which is another way of representing dark/vacuum energy. Since we know almost nothing about dark energy, theories outlining the exact mechanism behind inflation are speculative at best.
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20 thoughts on “Inflation Theory.

  1. Gap Gen says:

    Granted, there is no direct observational evidence for The Colossal Fudge, but theory predicts it and I don’t know of any other way to explain the Missing Caramel Problem.

    • Hentzau says:

      I don’t think there’s any way to get direct observational evidence for anything that happened immediately after the Big Bang thanks to photon decoupling. It’s a bit frustrating, since it’s (apparently) had profound influence on how our universe eventually turned out, and also incidentally explains why we spend so much money on particle accelerators; it’s the only way to even approximate those sorts of conditions.

  2. Smurf says:

    So with the whole speed of light thing, does that mean that there are galaxies and stars that are effectively popping into existence from our perspective as the light reaches us?

    Like, are there blank areas of space that tomorrow may well have a galaxy in them as the light from them reaches us?

    Space confuses me.

    • Hentzau says:

      That’s exactly what it means. With a constant rate of expansion of space which the speed of light can overcome to eventually reach us, our cosmic horizon should be gradually moving outwards as we recieve light from more and more distant galaxies. Thanks to dark energy and the apparent accelerating expansion of the universe, though, it may not stay that way forever

      • Gap Gen says:

        I was under the impression that the reverse was true – that the expansion of the universe is already accelerating and so the cosmic horizon is shrinking, but I honestly can’t remember if this is true or not, it’s been a while since I looked at the Friedmann equations

        In practice, improvements in telescope technology are revealing fainter galaxies, so yeah, we’re continually finding more distant galaxies as new telescopes come online. That said, often it’s a case of “maybe this fuzzy pixel here is the most distant galaxy known, but the errors are huge so who knows”.

        • Hentzau says:

          I think this is part of the whole uncertainty around dark energy, where nobody really has any idea what the fuck is going on. It’s generally agreed that the expansion of the universe is accelerating, but it’s not clear whether this is a) slowing the rate at which the horizon moves outwards or b) reversing it entirely, largely because (as you say) it’s difficult to disentangle shifts in the horizon with improvements/errors in technology, and anyway these things don’t quite happen overnight. I imagine it’d take hundreds or thousands of years for a shrinking horizon to be noticeable by us.

          • Gap Gen says:

            Plus the horizon would be shrinking quite slowly if it is at all. But I think most people mean the absolute distance light can travel as the horizon, rather than the furthest detectable galaxy. And as you point out, we already detect the surface of last scattering in the CMB, which is older than any galaxy. But hell, if we manage to build gravity wave telescopes, things could get very interesting indeed.

          • Gap Gen says:

            Shit, I meant gravitational wave. We already detect plenty of gravity waves, given that the atmospheric physics community nabbed the term before we could get our hands on it.

        • Hentzau says:

          (That being said if the horizon is shrinking then the universe could be amusingly screwed — look up the Wikipedia page on the Big Rip for deets.)

  3. I feel stupid asking this, but here we are: if the universe is 13 billion years old and the speed of light is a maximum speed, how could exist any 2 objects that are more than 13 billion years apart? Yet it’s obvious they exist since we get something from opposite sides.

    You’ve mentioned space expanding on universe creation. So all of those big long distances are due to space-time expanding rapidly in a first second of creation?

    • Hentzau says:

      This might end up being even more confusing, but: the speed of light is a hard limit for things travelling inside space, but it does not limit the speed of the expansion of space itself. Given the right conditions is theoretically possible for space to expand much faster than the speed of the light travelling within it, which is what inflation theory posits happened in the first second after the Big Bang and is the reason why we have a universe with a radius that’s far larger than 13.7 billion light years.

      So yes, basically.

      • Janek says:

        I would be interested in a further post detailing roughly how we’ve calculated the size of the observable universe.

      • Gap Gen says:

        And even then space doesn’t need to expand faster than the speed of light to get this effect – since the universe is expanding, if light from a distant galaxy left 10Bn years ago, that galaxy will now be further away than it was when the light left it. Imagine someone firing rolled-up bits of paper with their current GPS coordinates at you from a moving car, if you want a completely useless analogy. This is why we think the universe is around 50Bn light years wide, not 13.7 (or whatever new number the Planck satellite gives us).

        Inflation is largely there to explain why the CMB is so uniform in temperature when parts of it 2 degrees apart shouldn’t be connected if you assumed a smoother expansion rate, as Hentzau says. You can get an expanding universe without it, provided your CMB isn’t so silky smooth.

  4. Darren says:

    All of this stuff is WAAAAAAAY over my head.

    However, I’m not sure I understand why people would be baffled by the fact that galaxies look the same throughout the galaxy. I guess I’ve always thought of it like this:

    Water behaves the exact same on our planet, regardless of where it is. The exact composition might be different (salt levels, etc.) but pour it in a bathtub and it will slosh around the same way. The different natural laws which exist ensure this is always the case. Zoom way out, and it really shouldn’t be surprising that there exists some grand force that exerts the same kind of effects upon entire galaxies, causing them to behave in largely similar ways even though they may differ in size and composition.

    Don’t know if I explained that well, but it seems strange for scientists to frame this as a problem rather than simple evidence that a current theory doesn’t suffice for the given scale.

    • Janek says:

      To borrow your sloshing example, imagine the universe is a really big bath. If you look back at very distant (i.e. dating back to just after running the bath) water, you would expect the water to still be sloshing about, with peaks and troughs such that it looks very different depending on which bit of the bath you’re looking at.

      The problem is that as far as we can see, the water is flat and calm except for barely detectable rippling. Waves from the two points you’re looking at shouldn’t have reached each other yet, and the system shouldn’t have had time to settle down. Hence, there must be something else going on.

      (I’m sure Hent and Gap can explain it better, it’s not something I’ve had much exposure to)

      • Hentzau says:

        Janek’s analogy is pretty good, as far as analogies go.

      • Gap Gen says:

        Some people try to explain dark energy by saying that the universe is denser just outside the horizon (without much success so far, granted, but then dark energy is just a number people added to make the equations work, so eh). So to use your analogy, if this is true (which it probably isn’t) all the water we can see is sloshing outwards due to a difference in the height of the water in the bath. As far as I can remember, the main reason people invoke inflation is because the universe just after the Big Bang was the same temperature all over (as measured by the COBE, WMAP and Planck satellites), which shouldn’t happen unless the universe was much smaller so that the gas could interact, which then means that at some point it needed to expand very fast to catch up with the size we observe today.

        As for the laws of physics themselves, I don’t know much about fundamental physics or why they seem to be the same across the entire universe or why they exist at all, but then I suspect no-one else really knows either.

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