The Big Bang at 75

Penn theoretical physicist Vijay Balasubramanian discusses the 75th anniversary of the alpha-beta-gamma paper, what we know—and don’t know—about the universe and the ‘very big gaps’ left to discover.

A child stands in front of a flatted image with specks of blue, green, yellow and orange.
A child stops by an image of the cosmic microwave background at Shanghai Astronomy Museum in Shanghai, China on July 18, 2021. (Image: FeatureChina via AP Images)

There was a time before time when the universe was tiny, dense, and hot. In this world, time didn’t even exist. Space didn’t exist. That’s what current theories about the Big Bang posit, says Vijay Balasubramanian, the Cathy and Marc Lasry Professor of Physics. But what does this mean? What did the beginning of the universe look like? “I don’t know, maybe there was a timeless, spaceless soup,” Balasubramanian says. When we try to describe the beginning of everything, “our words fail us,” he says. 

Yet, for thousands of years, humans have been trying to do just that. One attempt came 75 years ago from physicists George Gamow and Ralph Alpher. In a paper published on April 1, 1948, Alpher and Gamow imagined the universe starts in a hot, dense state that cools as it expands. After some time, they argued, there should have been a gas of neutrons, protons, electrons, and neutrinos reacting with each other and congealing into atomic nuclei as the universe aged and cooled. As the universe changed, so did the rates of decay and the ratios of protons to neutrons. Alpher and Gamow were able to mathematically calculate how this process might have occurred.

Now known as the alpha-beta-gamma theory, the paper predicted the surprisingly large fraction of helium and hydrogen in the universe. (By weight, hydrogen comprises 74% of nuclear matter, helium 24%, and heavier elements less than 1%.)

The findings of Gamow and Alpher hold up today, Balasubramanian says, part of an increasingly complex picture of matter, time and space. Penn Today spoke with Balasubramanian about the paper, the Big Bang, and the origin of the universe.

When did we first start to think about the Big Bang theory as it is known today?

There’s actually a question of whether it’s even possible to talk about the origin of the universe. But across cultures, humans seem to have an innate drive to try to discuss this sort of question. In India, there was this idea of an infinite cyclic universe that went in gigantic cycles from origin to destruction, origin to destruction, over long lengths of time. The Aztecs had a cosmology that involves gigantic cycles of creation and construction, too. In the Christian West, people had the idea that the horizon of all of time was smaller, a few thousand years, although the Bible doesn’t actually say anything specific about that. 

In the 19th century, the first scientific inkling of the age of the world was given by Charles Lyell, a geologist, who wrote about the stratification of rocks. Charles Lyell basically gave Darwin the gift of time. Realizing that the earth was actually much older than a few thousand years gave room for the theory of evolution and expanded the horizon in time. That’s a prerequisite for being able to even conceive of the origin of the universe. 

Then in 1914, Einstein comes up with the modern theory of gravity. This led scientists to try to understand whether you could use this theory to think about the cosmos as a whole. One of the striking things that comes out of that kind of reasoning is that you get forced into a picture where the universe has to be dynamic, basically because gravity is constantly trying to squeeze it together.

To start with, if you look around the sky, it looks reasonably stable and static. It doesn’t look like it’s going anywhere, right? So, people initially tried various ways to construct cosmologies in which they can be kind of stable and static. To do that, you’ve got to poise the universe exactly between an expanding phase and a shrinking phase. You need balance these tendencies. For example, you can give the universal an initial outward push, like a Big Bang, but gravity will try to pull everything back together. How the push and pull compete depends on the amount of kind of energy distributed in the cosmos: regular matter like the stuff that makes stars, pure energy like light, dark matter which does not make stars, and so-called dark energy which can either push the fabric of spacetime apart or try to pull it together. So theoretical physicists tried to figure out whether the laws of gravity, along with these kinds of energy, could explain the apparently static structure of observed universe.

And then a series of astronomical measurements, notably by Edwin Hubble, showed definitively that despite initial appearances, the universe on large scales is not stable and static. Rather, all the stars and galaxies, as observed now, seem to be spreading apart from each other, as if they are embedded in a space-time fabric that is stretching wider as time passes. 

This was a revelation, because physicists realized that if the universe is expanding now, if you run the movie backward, it had to be smaller earlier. In fact, some 13 billion years ago all the matter and energy in the universe had to be crammed together at incredible densities that have never been seen on Earth. You can also conclude that the universe would have been a lot hotter in this compressed phase. This is just like what happens if you compress a bicycle pump; he air inside gets hotter because you are cramming more energy into a smaller space. And when things get that hot, the microscopic processes of nuclear physics and even quantum gravity play an important role because of the enormous energies involved.

So, to summarize, the idea of the modern Big Bang comes about because General Relativity makes a prediction: Given the current expansion of the universe, if you run time backwards, you have to start from a very highly compressed phase. At some point, time begins. This didn’t have to be. It could have been very compressed forever, and time could have been infinite. But Einstein’s theory of gravity predicts a beginning for time from which the universe explodes out. That’s the Big Bang.

What are the weaknesses of the Big Bang theory and our current conception of the origin of the universe?

It involves an extrapolation of the things we know and can measure in the lab, along with rather uncertain measurements of the expansion rate of the universe. People like Hubble measured distant stars and galaxies and realized that they look as they’re moving away from us, as an expansion. You put that expansion together with the equations of general relativity. Physics can predict forward in time and can predict backward in time. The equations tell you, given the current state, what the future will look like. But they can also tell you about the past. You know, take your pick. 

If you assume Einstein’s theory of relativity and you run the movie backward, time begins some 13 or 14 billion years ago. The question is, should you believe such a wild prediction?

While there are excellent reasons to believe the general theory of relativity—there’s lots of evidence about many things that it gets right—in the history of science, it’s been often the case that a well-tested theory, extrapolated to regimes very far from the region where it was tested, will need corrections of some kind. 

We’re extrapolating into regions that have been out of the reach of laboratory experiments to date, for which we do not have direct observational evidence. We should keep in mind that this theory may need corrections, and things like string theory attempt to correct it. Then there are unknown factors that the theory didn’t include, new forms of energy that could prevent the expansion or shrinking or could stabilize the universe. 

I’m laying out here the many uncertainties of the theory, but that’s partly because that’s where the opportunities are. If everything was already done, we wouldn’t have to think about it anymore.

Physicists can imagine stuff that makes the world work. That’s what we do for a trade. We imagine stuff that would be necessary for the logical consistency of the world around us. The alpha-beta-gamma paper took Einstein’s theory for granted. They predicted the abundances of the primordial elements, the hydrogen-helium ratio, which turns out to be right. They said, ‘Okay, well, if the universe was very hot, it had to have cooled down over time. So if it cooled down, I’m going put all I know about nuclear physics in the lab to represent the expansion of the universe. As it cools, the primordial soup will freeze out into quarks and gluons and electrons, and those things will freeze out some more, and eventually, when it’s done freezing out, based on what I know about nuclear reaction rates, I predict the following ratio of hydrogen to helium.’ That’s what they did.

The theory then proceeded to predict that you will see a glow in the distant sky as the Big Bang cooled down to a few degrees Kelvin. The discovery of that glow, the cosmic microwave background, in the 1960s, really nailed it.

How do you predict this theory will evolve, or be adjusted, with time?

The hydrogen-helium ratio and the cosmic microwave background are two primary reasons to support the Big Bang theory. Those are certainties that we are seeing now. But what does Hamlet say? ‘There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.’

We keep discovering that our assumptions about the nature of the universe are incorrect or approximate. 

The laws of physics are full of laws that turn out not to be laws. They turn out to be approximations. So, Newton’s laws, which we still call Newton’s laws out of respect for Newton, are approximations to the more general laws of general relativity and quantum mechanics. There’s a progression in science where we devise rules and descriptions of nature that work extremely well in some regime, and then, as you push outside the regime, you have to be able to edit them. I try to remain aware that, while the default conclusion is there was a big bang, understood as a singularity in space and time, about 13, 14 billion years ago. There may be escape routes from that conclusion, if our understanding of the laws of nature or something in the data has not been fully correct.

Questioning where the cosmos came from has long been part of human speculation, in philosophy and religion. Ancient peoples drew pictures in caves involving their cosmologies. There’s clearly a human need to talk about origins and causation of the universe. It is kind of amazing and remarkable that we live in a time when there’s a scientific approach to such questions, which we can use with any kind of confidence.

We’re just little people sitting on this irrelevant little planet of a very medium-sized solar system on the edge of a no-account galaxy that is part of a local cluster. We’re sort of just tiny things, right? And yet, we’re claiming to be able to say something about the actual origin of everything. It’s amazing that we have a hope of doing that. But there’s pretty good evidence, that at least in the rough, that this picture is correct: There was a hot, dense space about 13 some billion years ago, and it’s expanded since then. 

The core description fits beautifully. The ballpark version seems correct. But the detailed version has gaps, so there is a lot left to do in this process of discovery to understand how the universe is organized and what is in it, Today the most important questions involve dark matter, a form of matter that does not form stars, and dark energy, a form of energy that appears to be forcing the universe apart at an ever faster rate. Together, these substances appear to constitute about 96% of the energy in the universe and have huge consequences for the large-scale organization of the cosmos, its past history, and its future. The race is on to figure out what dark matter and dark energy are.