How did the universe come to be?

It is perhaps the greatest Great Mystery, and the root of all the others. Humanity's grandest questions — How did life begin? What is consciousness? What is dark matter, dark energy, gravity? — stem from it.

"All other mysteries lie downstream of this question," said Ann Druyan, the author and widow of astronomer Carl Sagan. "It matters to me because I am human and do not like not knowing."

Even as the theories attempting to solve this mystery grow increasingly complex, scientists are haunted by the possibility that some of the most critical links in their chain of reasoning are wrong.

Fundamental mysteries

According to the standard Big Bang model, the universe was born during a period of inflation that began about 13.8 billion years ago. Like a rapidly expanding balloon, it swelled from a size smaller than an electron to nearly its current size within a tiny fraction of a second.

Initially, the universe was permeated only by energy. Some of this energy congealed into particles, which assembled into light atoms like hydrogen and helium. These atoms clumped first into galaxies, then stars, inside whose fiery furnaces all the other elements were forged.

This is the generally agreed-upon picture of our universe's origins as depicted by scientists. It is a powerful model that explains many of the things scientists see when they look up in the sky, such as the remarkable smoothness of space-time on large scales and the even distribution of galaxies on opposite sides of the universe.

But there are things about this story that make some scientists uneasy. For starters, the idea that the universe underwent a period of rapid inflation early in its history cannot be directly tested, and it relies on the existence of a mysterious form of energy in the universe's beginning that has long since disappeared.

"Inflation is an extremely powerful theory, and yet we still have no idea what caused inflation or whether it is even the correct theory, although it works extremely well," said Eric Agol, an astrophysicist at the University of Washington.

For some scientists, inflation is a clunky addition to the Big Bang model, a necessary complexity appended to make it fit with observations. This wouldn't be the last addition.

"We've also learned there has to be dark matter in the universe, and now dark energy," said Paul Steinhardt, a theoretical physicist at Princeton University. "So the way the model works today is you say, 'OK, you take some Big Bang, you take some inflation, you tune that to have the following properties, then you add a certain amount of dark matter and dark energy.' These things aren't connected in a coherent theory."

Steinhardt worries cosmologists are acting more as engineers than scientists. If an observation doesn't match the current model, they attach another component or tinker with existing ones to fit. The components aren't connected and there's no reason to add them except to match observations. It's like trying to fix an old car by adding new parts from newer but different models. Those parts may work in the short term, but eventually, you need a new car.

An ageless universe

In recent years, Steinhardt has been working with Anna Ijjas, a theoretical physicist at Harvard University, on a radical alternative to the standard Big Bang model.

According to their idea, called bouncing cosmology, the universe was born not just once, but possibly multiple times in endless cycles of contraction and expansion. The theory replaces the "big bang" with a "big bounce", which smoothly connects periods of contraction and expansion of the universe and solves many of the issues that plague the inflation theory.

The pair claims that their ekpyrotic, or "cyclic," theory would explain not only inflation, but other cosmic mysteries as well, including dark matter, dark energy and why the universe appears to be expanding at an ever-accelerating clip. [The 18 Biggest Unsolved Mysteries in Physics]

While controversial, bouncing cosmology raises the possibility that the universe is ageless and self-renewing. It is a prospect perhaps even more awe-inspiring than a universe with a definite beginning and end, for it would mean that the stars in the sky, even the oldest ones, are like short-lived fireflies in the grand scheme of things.

"I'd like to hope that the effort society pours into scientific research is getting us closer to fundamental truths, and not just a way to make useful tools," said California Institute of Technology astronomer Richard Massey. "But I'm equally terrified of finding out that everything I know is wrong, and secretly hope that I don't."

Additional resources:

Read the transcript of Stephen Hawking's lecture on "The Beginning of Time."Watch physicist Tom Whyntie explain the beginning of the universe for beginners.Check out this video from National Geographic about the origins of the universe.This article was updated on June 27, 2019, by Live Science Contributor Tim Childers.