The biggest questions about the Universe's beginning

# The biggest questions about the Universe’s beginning

Imagine what it must have been like, as it was for so long throughout human history and prehistory, to look up at the wonders of the night sky in ignorance: not knowing what you were seeing or where any of it came from. All you could behold with your eyes were those glittering points of light in the sky: the Moon, the planets, the stars, a few deep-sky objects (or nebulae), and the tapestry of the Milky Way, with no way of knowing what they were made of, where they came from, or what any of it meant.

Today, the story is very different. Nearly all of the night sky objects we can see with our naked eye are objects present within the Milky Way galaxy. A few of those deep-sky objects turn out to be galaxies, with trillions of more galaxies — including small, faint, and ultra-distant ones — observable with superior tools. These galaxies all expand away from one another, with more distant objects expanding at greater speeds than nearer ones.

The expanding Universe swiftly led to the idea of the Big Bang, which was then confirmed and validated. The Big Bang was then modified to include an even earlier stage known as cosmic inflation, which preceded and set up the Big Bang’s initial conditions. That’s the current status of our understanding of the beginning as of today, in early 2024. Here are the biggest questions, both answered and unanswered, that we still have about the earliest phases of our Universe.

From a pre-existing state, inflation predicts that a series of universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these “bubbles,” where inflation ended, gave birth to our Universe some 13.8 billion years ago, with a very low entropy density, but without ever violating the 2nd law of thermodynamics.

## The inflationary hot Big Bang

Most of us have heard of the Big Bang: the notion that the Universe began from a very hot, very dense, and very uniform state, and then expanded, cooled, and gravitated, eventually giving rise to: protons and neutrons, atomic nuclei, neutral atoms, stars, galaxies, and a vast cosmic web of structure, where within individual galaxies, things like heavy elements, rocky planets, and even life can eventually form. However, the Big Bang couldn’t have been the very beginning of the story, as a number of physical puzzles simply go unexplained if we insist on it.

Why does the leftover glow from the Big Bang, the CMB (or cosmic microwave background), have the same properties (e.g., temperature) in all directions, especially if these distant, disconnected regions have never had time to exchange information with one another? Why does our Universe, where the combination of energy density and the expansion rate determines its curvature, appear perfectly spatially flat, rather than either positively or negatively curved? And why are there no leftover high-energy relics from this supposedly “arbitrarily hot” phase that the Universe would have achieved, early on, if the hot Big Bang truly represented the beginning of everything?

The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. The fluctuations are only tens to hundreds of microkelvin in magnitude.

One possibility, famously noted by Alan Guth in the late 1970s/early 1980s, is that the Big Bang wasn’t the beginning, but was preceded by a state of exponentially expanding empty space, which precedes and sets up the hot Big Bang. Further details were worked out by many, including Guth himself, Andrei Linde, Paul Steinhardt and Andreas Albrecht, as well as several others, concluding the following.

In an inflationary Universe, space was filled with a type of energy inherent to itself — perhaps some type of field energy, similar to today’s dark energy — causing it to expand not just rapidly, but relentlessly, and without bound. When inflation comes to an end, all (or at least, most) of that energy gets converted into particles and antiparticles, initiating the phase of the Universe we identify with the hot Big Bang. But now, because of inflation: Different regions all have the same temperature and density because they all emerged from the same inflationary state. The Universe that emerges appears spatially flat, because the inflationary process stretched it so that it would be indistinguishable from a perfectly flat appearance, just as your own backyard appears “flat” on the surface of the Earth. And there are no leftover high-energy relics because any pre-existing ones were inflated away, and the maximum temperature that the Universe achieves when the hot Big Bang begins is now insufficient to create them again.

# Inflation’s unanswered questions

This seems like a remarkable success story for cosmic inflation, and in many ways, it truly is. 50 years ago, we had cemented the hot Big Bang as accurately describing the early phases of our Universe, but it failed at explaining a set of conditions that must have existed back then, and contained numerous pathologies (or puzzles) that had no solution. When cosmic inflation came along, it was recognized that it could resolve these problems, but that new, testable predictions needed to be extracted from them.

We have now entered a golden era for cosmology, where next-generation experiments that probe the fluctuations and polarization of light imprinted in the cosmic microwave background (CMB) are being designed and constructed. We have confirmed a number of predictions that inflation makes, ruling out the hot Big Bang without inflation and also ruling out a number of inflationary models that fail to match the data.

But if we had better data, we could imagine even more stringent tests of inflation that could: confirm its predictions still further, teach us which models agree with the data and which are ruled out, or could surprise us, and show us that certain predictions are not, in fact, borne out by nature. Although we do not have the data yet, here are 5 unanswered questions about inflation that may still be answered with superior future data.

If one wants to investigate the signals within the observable Universe for unambiguous evidence of super-horizon fluctuations, one needs to look at super-horizon scales at the TE cross-correlation spectrum of the CMB. With the final (2018) Planck data now in hand, the evidence is overwhelming in favor of their existence.

## Are there tensor fluctuations, or primordial gravitational waves, present within our Universe?

The only problem? The spectrum is easy to determine, but the amplitude of the spectrum is highly model-dependent. If the tensor spectrum has a large amplitude, then the ratio of the tensor spectral index (nt) to the scalar spectral index (ns) will be large, and we’ll be able to observe it. Right now, our best constraints on that ratio tell us that it’s less than 0.036, as determined by the Bicep-Keck collaboration.

# Does the scalar spectral index, ns, have a constant value, or does it change with scale (i.e., “run”) as predicted by inflationary models?

Just as a ball rolling down a hill can change its acceleration if the slope of the hill changes, the expectation is that the scalar spectral index, ns, will “run” by a small amount: around 0.1%, according to most inflationary models. Will we be able to measure this running, and if so, will it be consistent with inflation’s predictions, or will it be either too large or too small?

## Is the Universe’s geometry exactly flat, or, as inflation predicts, are there tiny departures from perfect flatness?

According to various inflationary models, the amount of that curvature can vary from 1 part in 10,000 down to 1 part in 1,000,000. If there’s more or less curvature than that, it could spell trouble for inflation, while measuring curvature in precisely that range would be another spectacular confirmation for inflation.

# Are there any scalar fluctuations that exhibit any amount of non-Gaussianity to their statistics?

Again, we do expect that if we get all the way down into the weeds, eventually we’ll find a small, non-zero departure from a perfect Bell curve to the temperature fluctuations that we see. Will the amount of non-Gaussianity agree with inflation’s predictions, or will it be either too small or too large?

## And finally, are there any resonant features in the spectrum of scalar fluctuations?

We expect the answer will be “no,” as inflation predicts, but you have to look for the unexpected if you want to give nature a chance to surprise you.

Making measurements sensitive enough to test these five hitherto untested predictions is an ambitious goal, but when it comes to a question as important as “where did our Universe come from,” not even making the attempt to find the answer may be the greatest folly of all.