UNIVERSITY PARK, Pa. — All it takes is a look out at the horizon to remind us that there is a limit to what we can see.

Likewise, not everything that’s ever existed in the universe is visible to us. There is a point in the cosmos beyond which, even with our most powerful telescopes, we cannot see. Given that the universe is expanding, and faster all the time, some day (in the very distant future) we won’t even be able to see those stars and galaxies, as they recede to a distance too far for their light to reach us — forever lost beyond the horizon of space. On this scale, though, the limit of our observations is not imposed by the Earth’s curvature, but rather by time and the physics of light.

“If you just think about how far can light have traveled since the beginning, it's not an infinite distance,” said Penn State Associate Professor of Physics Sarah Shandera. “So even if we could see light further back, we can't see information from infinitely far away.”

In fact, there was a time in the history of the cosmos when light couldn’t really travel at all. For the first 400,000-or-so years following the big bang, all of the photons were trapped in an opaque plasma, something like a hot particle soup, until space cooled enough for matter to coalesce, the universe became transparent, and the photons could finally escape. We can see this moment in what’s known as the cosmic microwave background (CMB) — the so-called afterglow of the big bang, the first visible light in the universe, and the farthest point in the cosmos we’re able to observe.

Since the CMB was formed in a process that happened everywhere in the universe at virtually the same time, the entire cosmos is suffused with its light. So — unlike stars and galaxies, which will eventually recede beyond the cosmic horizon — the CMB will never disappear entirely from our view. But it nonetheless constitutes the ultimate limit of our observations. Thanks, though, to data we have from the CMB and other observations of the more-recent early universe, theoretical physicists like Shandera are pushing the boundaries of what we can reasonably infer about our universe’s birth beyond the veil of the visible.

“The early universe is a really interesting and, in some ways, observationally accessible place,” Shandera said. “I like to do theoretical work on what might have happened at those early times, but what I really like most about this is that there is data, and so you can't get too caught up in your magnificent theory, because at some point you have to really connect with the data.”

So what do we really know about the early universe?

“Well, there's the part that we know more for sure,” Shandera explained, “and people have often called this the 'hot big bang' universe. We know that if you go back in time, the universe became hotter and denser and smoother. Gravity hadn't had time to clump things together into galaxies. But we think even before that time something else happened, and the leading idea for what that was is the theory of inflation.”

Proposed in 1981 by Alan Guth, the theory of inflation — in a nutshell — posits that during the first fraction of a second after the big bang, the universe went through a phase of exponential expansion, faster than the speed of light. Almost instantaneously, this expansion stretched tiny, quantum-scale fluctuations across vast distances, creating regions of differing densities — “seeds” that under the force of gravity would eventually form the large-scale structure of stars, galaxies, and clusters we observe today.

“We don't know that for sure,” Shandera said, “but it's our best guess of what we think happened, our very best idea, and it's been tested and pulled every which way. We can’t really know what happened, since that era is beyond the access of our observations.”

Inflation does, however, solve certain problems with other theories — including the big bang theory — as well as with astronomers’ observations, particularly those of the cosmic microwave background, which shows the imprint of those aforementioned fluctuations, or seeds, as minute differences in temperature.

“The cosmic microwave background is pretty uniform on very, very large scales,” Shandera explained, “and there's no reason why you would expect not just a similar temperature but also the same pattern of fluctuations at different points in the background. So there are things happening on very large scales that you don't really have a good explanation for unless you posit something else, like inflation, that's the origin of these fluctuations.”

With data from observations of the CMB through to the present-day universe, inflation accurately describes the cosmos we see, and it allows theorists like Shandera to further develop plausible descriptions of the cosmos we can’t see — beyond the CMB.

“In principle, we model a lot of things that happen before that, like the formation of the elements,” Shandera said, “and we're pretty confident that modeling is right, because it's exactly what you'd expect using standard model physics, running everything backwards, and it gives you exactly what you see today. So we're pretty confident that we understand at least some of the physics before the CMB.”

Meanwhile, data from large-scale astronomical surveys like the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) could help to further fill in this picture of inflation and the early universe. As it searches for the signature of dark energy, HETDEX will also map the three-dimensional positions of around a million galaxies between 9 million and 11 million light years away and look for patterns in their distribution.

“That pattern,” Shandera explained, “is an imprint of the pattern of quantum fluctuations during inflation, run through to the present day. So by looking at how galaxies are organized or arranged in the universe, you're getting a picture of the initial conditions. And those initial conditions, we think, tell us something about inflation.”

What HETDEX discovers about dark energy may also inform the work of early universe theorists, although Shandera says that connection is more difficult to make. “The dark energy question is, in some ways, a little bit different,” she said, “because we don't know enough yet to know how to connect dark energy to the inflationary era.”

For Shandera it’s all like a cosmic puzzle, fitting theory and data together, to describe what’s otherwise unknowable.

“That there can be these really interesting ways to test theoretical physics with data, I just find that idea to be super fascinating,” she said. “And whatever the answers to these puzzles are, a lot of the observational data that we have, presumably, comes from the early universe. So that’s one of the very few ways we can have any hope of constraining these theories observationally — by understanding cosmological data.”

Editor's note: This story first appeared in the Winter 2021 issue of the Science Journal, the Eberly College of Science magazine.