“Orbiting Earth in spaceship, I saw how beautiful our planet is. People, let us preserve and increase beauty, not destroy it!” –

Yuri Gagarin

Fifty-two years ago today, the first human being left Earth, and we began our journey into outer space. But back in 1961, we didn’t really know how far outer space stretched, or where all the matter and energy in the Universe came from.

That all changed with the discovery of the Cosmic Microwave Background (by Penzias and Wilson, with the Horn Antenna, above), and subsequent measurements that led us to the Big Bang picture of the Universe. Beginning from a hot, dense state the Universe expanded and cooled, forming baryons, light nuclei, neutral atoms, and finally stars, galaxies, clusters and superclusters of matter, where finally human beings formed on our little world, and looked out — and back — into the Universe.

The Big Bang gave us a way for all of this to make sense, in the context of relativity. We now know, thanks to a slew of measurements, including the latest from Planck, that the Universe is currently made up of about 4.9% normal matter, about 26% dark matter, and 69% dark energy.

It’s a remarkable success story for the Big Bang.

But this doesn’t explain everything, at least, not on its own. For example:

- The Universe could have had any amount of spatial curvature, positive or negative, of any magnitude. But it appears to be
*completely*, arbitrarily spatially flat. When the Universe was a few minutes old, that means it was flat to one part in 10^{51}. - Different regions of space in opposite directions, given a finite age of the Universe, haven’t had
*sufficient time*to exchange photons or any other form of information. How, then, is it that every different direction on the sky — every causally disconnected region — has the same average temperature and the same average energy density? - If we brought the Universe back in time,
*arbitrarily*far, then according to the Big Bang, it should have been arbitrarily high in energy. Where, then, are all the high-energy relics of the early Universe, like magnetic monopoles?

All of these problems *could* simply just be the way the Universe is, of course. We get one Universe that we can see and access, and — as bewildering as it might seem — not every question we have about it is going to be answerable. There are a finite number of particles, and hence a finite amount of information, in the Universe, and the clues to answer all our questions might not be accessible to us.

But there was a brilliant idea thrown out there by Alan Guth in late 1979/early 1980, which is that you *can’t* extrapolate arbitrarily far back in the history of the Universe. Not to arbitrarily early times, not to arbitrarily high energies. Instead, *before* you could describe the Universe as hot, dense, expanding and cooling — i.e., *before* the Universe could be described by the Big Bang model — there was a period where it was dominated by the energy **inherent to spacetime itself**, and it expanded exponentially.

This period of exponential expansion — known as cosmic inflation — would basically *force* the Universe to be flat. Not necessarily *truly* flat, as it could either be positively curved (like a hypersphere) or negatively curved (like a hyper-saddle), but flat enough so that, from our perspective, it’s indistinguishable from flat. Just as panel D above (or the Earth, when you look out your window) appears flat, so the entire observable Universe would appear flat to us.

This also allows the Universe to be the same temperature and energy density everywhere, as well, since a tiny region that expanded exponentially became the spacetime that contains the entire Universe! And — so long as the exponential expansion lasted *at least* some 10^{-30something} seconds — every direction in our Universe would have the same average energy and temperature properties.

And then, some 13.8 billion years ago from our perspective, this period of exponential expansion had to end!

That means we had to go from a “false vacuum” state, where there was lots of energy inherent in space itself (which is what would cause the exponential expansion), to a state where the energy of empty space was much lower.

Of course, energy is *conserved* in this Universe, as best as we can tell, so it has to go somewhere. And where did all of that energy inherent to spacetime go?

Into matter and radiation, of course! So all of that field energy gets dumped into the particles we know (in a process called cosmic reheating), at a temperature that’s *low enough* that no magnetic monopoles get created. In fact, we can place an upper limit on the temperature of the Universe after inflation has ended, and it’s something like 0.1% of the Planck energy, which may well be below both the String and Grand-Unified-Theory energy scales, even if they are relevant to our Universe.

But wait, **there’s more**!

This is still a Universe governed by quantum laws, and that means quantum fluctuations happened **even during inflation**. But rather than being confined to one region of spacetime, because it’s expanding exponentially, these fluctuations get stretched across the entire observable Universe!

This means that today, we should see a spectrum of fluctuations that’s nearly scale invariant, but *slightly* tilted (e.g., slightly less than n_{s} = 1), that has a very tiny roll (on the order of 0.008), and that should be of a magnitude that’s a few parts in a hundred-thousand.

And, lo and behold, that’s exactly what we’re seeing! That’s right, inflation has met *all* the tests of a good scientific theory:

- The new theory must be consistent with everything that came before,
- The new theory must explain this new observation, and
- It must lead to a
*new prediction*of an observable phenomena which can go out and be tested.

And there it is. Inflation happened, gave the seeds for structure in an otherwise uniform Universe, and then created a bath of matter and radiation in *almost* perfect (but, *importantly*, not quite), almost isotropic, almost homogeneous way. And it sets up everything our Universe needs for the Big Bang.

Once that happens, your Universe begins *cooling* as it expands. Now the radiation is free to have its wavelengths stretched as the Universe expands, the volume of the Universe increases even though the number of matter particles stays constant, and, eventually, gravity does its thing. Over time, the great cosmic structures we’ve come to discover form, and that’s our Universe!

That’s the story of how we started from nothing and made it to today. But today, we *also* know that the energy inherent to spacetime *isn’t* zero, but rather is some small-but-finite value!

That’s what we see when we hold two plate apart in a vacuum (the Casimir effect); that’s what we see happening all through the Universe with distant supernovae (dark energy).

So yes, the Universe was once, in the distant past, dominated by energy inherent to spacetime itself. When this period ended, the Universe could *then* (and *only* then) be described by the Big Bang, which is where all the matter and energy in our entire Universe as we know it comes into being. And now, that the Universe has diluted — or expanded and cooled — so severely, we can finally see that there’s still a little bit of energy inherent to spacetime itself left: that’s dark energy!

We don’t understand all the caveats of inflation, or of dark energy, for that matter, including whether or not they’re related. But just a generation ago, we didn’t know anything at all about the energy inherent to spacetime, and now we know it to be an integral part of our Universe’s history! So when we say the Universe “Started With A Bang,” that’s just *our* *observable* Universe, and all the matter and energy in it. But something was before: empty spacetime, expanding exponentially. In physics, that’s also known as nothing, and it’s where everything came from, and where everything will return to in the future. Let’s never stop working to understand it a little better.