Big Bang Cosmology: A Q&A Journey into the Universe’s Past, Present, and Future

Imagine what it must have been like to look up at the stars at night without knowing what you were seeing or where any of it came from. This was the case for a very long time in human history and prehistory. You could only see these bright spots in the sky: the Moon, the planets, the stars, a few deep-sky objects (called nebulae), and the Milky Way. You had no idea what they were made of, where they came from, or what any of it meant.

Today, the story is very different. When we look up at the night sky, almost everything we can see is something in the Milky Way galaxy. Some of those objects in the deep sky turn out to be galaxies. Better tools can see trillions more galaxies, even ones that are very small, faint, or very far away. All of these galaxies are moving away from each other, with farther away objects moving away faster than closer ones.

The expanding Universe swiftly led to the idea of the Big Bang, which was then confirmed and validated. Then, an even earlier stage called “cosmic inflation” was added to the Big Bang. This stage came before the Big Bang and set up its initial conditions. That’s the current status of our understanding of the beginning as of today, in early 2024. Here are some of the most important questions we still have about the early stages of our Universe, some of which have been answered and some of which have not.

Most of us have heard of the “Big Bang” theory, which says that the Universe started out very hot, very dense, and very uniform. It then grew, cooled, and gravitated, which led to:

where within individual galaxies, things like heavy elements, rocky planets, and even life can eventually form. That being said, the Big Bang couldn’t have been the very start of the story because it leaves out a lot of physical puzzles.

In the late 1970s and early 1980s, Alan Guth famously brought up the idea that the Big Bang might not have been the beginning, but rather the start of a state of exponentially expanding empty space that set the stage for the hot Big Bang. Other scientists, like Robert Brout, Alexei Starobinsky, Rocky Kolb, and Stephen Wolfram, also had similar thoughts. There were many people who worked out more details, including Guth, Andrei Linde, Paul Steinhardt, Andreas Albrecht, and others. They came to the following conclusion:

During an inflationary Universe, space was full of an energy that was unique to it, maybe a field energy like dark energy today. This energy made space expand not only quickly, but continuously and without limits. As soon as inflation ends, all or most of that energy is turned into particles and antiparticles. This starts the phase of the universe we call the “hot Big Bang.” But now, because of inflation:

To put it another way, inflation not only repeats the Big Bang’s successes, but it also solves all three of the big physical problems that it caused. The beginning of this beautiful story was very interesting. It explained the issues with flatness, uniform temperature, and the lack of high-energy remnants that the standard hot Big Bang scenario had. But for a new scientific theory to replace an old one, it needs to make new predictions that are different from those of the old theory.

To best understand inflation, think of it as a field, where you start as a ball on top of a hill and roll down it. Your Universe expands as long as you’re high up on the hill. When you roll all the way down the hill and into a valley below, inflation stops and the energy from it is turned into quanta, which starts the hot Big Bang. But since all fields in nature should be quantum by nature, this leads to two kinds of quantum processes:

Today, only the first type of these fluctuations was found. However, both types lead to a set of predictions that let us test and limit different inflationary scenarios and see what they mean for our Universe.

Try to imagine the Universe as it was during this inflationary state. As time goes on, the distance between any two nearby points grows exponentially, doubling after a certain amount of time, quadrupling after twice that time, octupling after three times that time, and so on. Eventually, space is empty. When 1000 times that initial time period has passed, even two points that were initially separated by the smallest amount of space that makes sense (the Planck length) will be farther apart than the two ends of the observable Universe today.

Still, even though there is inflation, there are still quantum fluctuations happening on all scales and in all of space. The changes that happen on the smallest scales get stretched out to bigger ones, and as the next moment goes by, new changes show up on the smallest ones. As “older” fluctuations get stretched to bigger sizes, “newer” fluctuations on smaller sizes join them. Each set of smaller fluctuations is then put on top of the older, bigger fluctuations. Quantum fluctuations of both scalar and tensor types will only turn into density fluctuations (for scalar types) and gravitational wave fluctuations (for tensor types) when inflation ends. These are the changes that start the Universe when the hot Big Bang happens for the first time.

There are a number of predictions about how these changes will affect the Universe we see today. These predictions are either different from the non-inflationary hot Big Bang predictions or are based on facts, while the hot Big Bang predictions are not based on facts at all. Some of these predictions have already been backed up by data that is very close to what inflation tells us will happen.

#1: Super-horizon fluctuations. The speed of light, the rate of cosmic expansion, and the amount of time that has passed since the beginning of the Universe all set a limit on how big anything in the Universe can get. These things would set the largest size that could be that big if there wasn’t a period of cosmic inflation. This is called the classical horizon. If we observe coherent structures in the Universe that are larger than that size, e. g. , super-horizon fluctuations, then we’ve found evidence for cosmic inflation. The first time this was measured was in polarization data by WMAP. Since then, Planck has confirmed this and measured it even more accurately.

#2: Almost, but not perfectly, scale-invariant fluctuations. Most of the time during cosmic inflation, the changes that happen on smaller scales have the same properties as the changes that happen on larger scales. However, there is a small change from this at a very important point in time: in the very last seconds of cosmic inflation, just before it ends and a hot Big Bang starts.

The smallest cosmic scales in the universe right now are matched by that critical set of moments. The bigger cosmic scales are matched by the earlier moments. We normally characterize this spectrum of fluctuations by a parameter called ns, or the scalar spectral index. If the changes didn’t depend on the scale at all, then ns would be exactly 1. But we see that it’s not quite that big: ns = 0. 97. There is no doubt that this is proof of inflation, and it may be the best way to limit the types of inflation that could have happened.

To the best of our observations, these predictions have all been confirmed. The Universe is flat to within a factor of 400, which is the best we’ve ever been able to measure. The changes in density are at least 98 3% adiabatic and at most 1. 7% isocurvature (the best we’ve ever measured it), and the fluctuations follow Gaussian statistics as far as we can tell, with no non-Gaussianity being found to date.

This seems like a remarkable success story for cosmic inflation, and in many ways, it truly is. We were sure that the hot Big Bang accurately described the early stages of our Universe 50 years ago, but it didn’t explain a set of conditions that must have existed at that time and had many problems that couldn’t be solved. When cosmic inflation came along, it was known that it could solve these issues, but that it would need to be used to make new predictions that could be tested.

Now is a great time for cosmology because new experiments are being planned and built to look into the changes and polarization of light left behind by the cosmic microwave background (CMB). A lot of the predictions that inflation makes have come true. For example, the hot Big Bang without inflation is no longer a possibility, and many inflationary models that don’t fit the data are also ruled out.

But if we had better data, we could imagine even more stringent tests of inflation that could:

Here are 5 questions about inflation that may still be answered with better data in the future, even though we don’t have the answers yet.

#1: Are there tensor fluctuations, or primordial gravitational waves, present within our Universe? The graph above is remarkable: it shows the spectrum of gravitational waves predicted to be generated by inflation. The only problem? The spectrum is easy to determine, but the amplitude of the spectrum is highly model-dependent. The ratio of the tensor spectral index (nt) to the scalar spectral index (ns) will be big if the tensor spectrum has a big amplitude. We’ll be able to see 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.

#2: Does the scalar spectral index, ns, have a constant value, or does it change with scale (i. e. inflationary models say that the scalar spectral index, ns, will “run” by a small amount, around 0. This is similar to how the slope of a hill can change the speed of a ball rolling down it. 1%, according to most inflationary models. Will we be able to measure this running? If so, will it match what inflation says it will be, or will it be too big or too small?

#3: Is the Universe’s geometry exactly flat, or, as inflation predicts, are there tiny departures from perfect flatness? Although inflation stretches the fabric of the Universe to be indistinguishable from flat, the quantum fluctuations imprinted during inflation can also imprint a non-zero amount of spatial curvature to it. Different models of inflation say that the amount of that curve can range from 1 part in 10,000 to 1 part in 1,000,000 If there’s more or less curvature than that, it could be bad for inflation. On the other hand, measuring curvature exactly in that range would be a huge sign that inflation is real.

#4: 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 non-Gaussianity amount match what inflation says it will be, or will it be too small or too big?

#5: 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.

It’s a big goal to make measurements sensitive enough to test these five predictions that haven’t been tested yet. But when it comes to a question as important as “where our Universe came from,” not even trying to find the answer may be the biggest mistake of all. Tags.

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