It all started with the Big Bang — and then what happened?
Scientists explain what really puzzles them about how the universe became what it is
The Big Bang is the name we use to describe the instant our universe exploded into existence.
gremlin/E+/Getty Images Plus; adapted by L. Steenblik Hwang
Our universe started with a bang. The Big Bang! Energy, mass and space flashed into existence — all within a fleeting instant. But what exactly happened during this event remains one of the toughest puzzles facing science.
This question was sparked nearly a century ago by a discovery made by astronomer Edwin Hubble. In 1929, Hubble found that distant galaxies were moving away from Earth. Importantly, galaxies farther away were moving away faster. This was true no matter in which direction he looked.
That pattern came to be known as Hubble’s law. Since then, images taken by telescopes gazing across the cosmos have confirmed it. And it seems to point to one mind-boggling conclusion: The universe is expanding.
This expansion is a primary piece of evidence for the Big Bang. After all, if everything in the universe is expanding away from everything else, it’s easy to imagine “rewinding” that motion. That rewind video might show everything coming closer and closer together as time runs backward to the beginning — until the entire cosmos squishes into a single point.
The term Big Bang is cosmologists’ nickname for the almost unimaginable process by which the entire universe expanded from a single point. It marks the beginning of everything that we now see, feel and know. It describes how all matter was created and how our most fundamental laws of nature evolved. It may even mark the beginning of time itself. And it’s thought to have started when the early universe was infinitely dense.
To many scientists who are trying to understand the Big Bang, the first hint of trouble is that phrase: “infinitely dense.”
“Any time you get infinity as an answer, you know that something is wrong,” says Marc Kamionkowski. He’s a physicist at Johns Hopkins University in Baltimore, Md. Coming to infinity “means that we either did something wrong, or we don’t understand something well enough,” he says. “Or our theory is wrong.”
Scientific theories can describe with incredible accuracy how the universe evolved over time after the Big Bang. Telescope observations have confirmed those theories. But every one of those theories breaks apart at a certain point. That point is within a tiny fraction of the first second after the Big Bang.
Most scientists believe that our laws of physics are leading us in the right direction to understand the universe’s first moments. We’re just not there yet. Cosmologists are still struggling to understand the early infancy — and perhaps conception of — our universe and everything in it.
Evidence for the Big Bang
One of the strongest pieces of evidence for the Big Bang also presents one of its biggest challenges: cosmic background radiation. This faint glow fills the cosmos. It is leftover heat from the explosive Big Bang.
Everywhere astronomers gaze, they can measure the temperature of that background radiation. And everywhere, it’s almost exactly the same. This condition is known as a homogeneity (Hoh-moh-jeh-NAY-ih-tee). The universe does, of course, have big differences in temperature here and there. Those places are where stars, planets and other celestial objects exist. But between them, the background temperature in all directions appears the same: a very frigid 2.7 kelvins (–455 degrees Fahrenheit).
The big question is why, says Eva Silverstein. This physicist works at the Stanford Institute for Theoretical Physics in California. There, she investigates how certain structures appear to have formed after the Big Bang. Summarizing the sense of mystery she sees in current theories, she says, “Nobody promised us that we would understand everything.”
The seemingly even spread of cosmic background heat suggests that everything that burst out of the Big Bang should have cooled off the same way. But when we look across the universe now, Silverstein says, we see distinct structures everywhere. We see stars and planets and galaxies. How did they start to form if everything had originally started out as one uniform thing?
“Think about mixing liquids, and how they will come to the same temperature,” Silverstein says. “If you pour cold water into hot water, it will just become warm water.” It won’t become beads of cold water that persist within a pot of otherwise hot water. Likewise, one would expect the universe today to look like a fairly even spread of matter and energy. But instead, there are cold stretches of space dotted with hot stars and galaxies.
Over the last few decades, astronomers think they may have found an answer to this question. They have measured tiny differences in the cosmic background’s temperature. These differences are on the scale of one hundred-thousandth of a degree kelvin (0.00001 K). But if such tiny variations existed right after the Big Bang, they might have grown over time into what we see now as structures.
It’s like blowing up a balloon. Draw a tiny dot onto an empty balloon. Now inflate it. That dot will end up looking a lot bigger once the balloon is full.
Scientists have named this period after the Big Bang inflation. It’s when the newborn universe expanded so tremendously that it’s truly hard to comprehend.
Explosively fast inflation
Inflation appears to have been fast — far faster than any expansion before or since. It also took place during a stretch of time so tiny it’s hard to imagine. The idea of inflation is well-supported by telescope observations. Scientists have not, however, fully proven it. Inflation is also extremely difficult to physically describe.
“The Big Bang was not an explosion of matter into space. It is an explosion of space,” explains astronomer Adrienne Erickcek. Her work at the University of North Carolina at Chapel Hill focuses on how the universe expanded within the first few seconds and minutes after the Big Bang.
Lots of astronomers use the idea of raisin bread to illustrate this. If you leave a ball of fresh raisin-bread dough on a countertop, that dough will rise. The raisins will spread apart from each other as the dough expands. In this analogy, raisins represent stars, galaxies and everything else in space. Dough represents space itself.
Erickcek offers a more mathematical way to think about the expansion of the universe. “It’s like laying down an image of a grid across all of space, with galaxies at all of the points where the lines meet.” Now imagine that the expansion of the cosmos is like the gridlines themselves expanding. “Everything stays at their places on the grid,” she says. “But the spacing between the gridlines is expanding.”
This part of the Big Bang theory is extremely well-proven. But when we imagine a grid, it’s hard not to wonder about the edges of that grid.
“There’s no edge,” Erickcek points out. “The grid goes infinitely in all directions. So, every point seems like the center of the expansion.”
She emphasizes this because people so often ask if the universe has an edge. Or a center. In fact, she says, there is neither. On that imaginary grid, “every point is getting farther away from all of the others,” she notes. “And the farther away two points are, the faster they seem to be moving away from each other.”
This may be hard to wrap your head around, she admits. But this is what we see in the data. Space itself is what’s expanding. “That grid,” she reminds us, “is infinite. It’s not expanding into anything. There’s no empty space we’re expanding into.”
So where did the Big Bang happen? “Everywhere,” says Erickcek. “By definition, the Big Bang is that moment when the infinite number of gridlines were infinitely close together. The Big Bang was dense — and hot. But there was still no edge. And everywhere was the center.”
Erickcek works to bring theories together with observations. There’s a lot of evidence to support the universe’s inflation. But what caused that inflation? (To go back to the raisin bread analogy, what is the yeast of the universe?) To answer that, a new source of data may be needed.
Hints of the Big Bang in dark matter and gravity waves
To learn what spurred inflation, we may need to look in unexpected places. The invisible, unidentified substance known as dark matter, for instance. Or ripples in spacetime called gravity waves. Or weird new particle physics. Any of these scientific curiosities may hold the secrets to inflation.
Let’s start with dark matter. In the late 1970s, astronomer Vera Rubin discovered that galaxies were rotating far faster than their mass should allow. She proposed the existence of unseen matter — dark matter — as the missing mass. Since then, dark matter has become an important part of cosmology.
Physicists estimate that more than one-fourth of the universe is composed of dark matter. (Only 4 to 5 percent is the “regular” matter that fills our everyday lives and also includes all stars, planets and galaxies. The rest of the universe — almost two-thirds of it — is made of dark energy.) Alas, we still don’t know what dark matter is.
Historically, scientists have looked for clues about the Big Bang among the regular matter we can see. But dark matter is a huge blind spot in the universe. If scientists understood it better, maybe they’d uncover how it — and ordinary matter — came to be.
Until we know for sure how the universe works, it’s good to ask lots of questions and come up with new ideas, says Katelin Schutz. This astronomer works at McGill University in Montreal, Canada. There, she studies dark matter and gravitational waves. Her specialty is studying how these things might have interacted in the early universe to form stars and the other structures we see today.
“Right now, we’re thinking about dark matter as if it’s only one kind of particle,” Schutz says. In fact, dark matter could be as complex as visible matter.
“It would be weird if we only have complexity on our side — with normal matter, which is what allows us to have people and ice cream and planets,” Schutz says. But “maybe dark matter is similar, in the sense that it is multiple particles.” Teasing out those details could help reveal how the Big Bang created ordinary and dark matter.
Schutz’s other research focus, gravitational waves, could also offer clues about the Big Bang’s aftermath. As more sensitive telescopes look farther out into space — and therefore further back in time — scientists hope to spot gravitational waves created shortly after the Big Bang.
Such wrinkles in spacetime could have formed while the evolving universe changed quickly, like a growth spurt — as would have happened during inflation. Gravitational waves aren’t a form of light, so they might offer scientists an unfiltered glimpse of the Big Bang. These gravitational waves could offer “a really interesting window on that time, when we don’t have a lot of other data,” Schutz points out.
Dealing with uncertainties to our origins
So how did stars, galaxies and other cosmic structures come into being? Cosmologists have some idea, but the precise processes remain blurry.
“Honestly, we may never know,” says Schutz. “And I’m okay with that.” She remains excited about the vast frontiers of questions she can investigate. “My favorite theory is one that I know how to test.” And there’s no way to test ideas about the Big Bang in the lab without starting another universe.
“It’s kind of remarkable to me how successful physics has managed to be,” with this huge gap in knowledge about the beginning of time, says Adrienne Erickcek at UNC. New theories and observations are helping shrink that gap. But unanswered questions still abound. And that’s okay. In our search for the answers to fundamental questions, many cosmologists, like Schutz, are comfortable concluding, “I don’t know — at least not yet.”
