When we think of the Big Bang, we think loud, hot, and bright. But was it followed by a colder, darker shadow?
Five-sixths of the universe seems to be made up of a mysterious substance called “dark matter.” Until now, physicists have assumed dark matter was created in the same flash of heat that jump-started the universe. But dark matter lives in a kind of parallel, shadow world, barely interacting with the regular matter that makes up everything we can see, feel, and observe. Maybe it was created in a kind of “shadow” Big Bang, days after the universe began.
That’s what UT Austin physicists Katherine Freese and Martin Winkler propose in a new theory they’re calling the “Dark Big Bang,” which may explain why dark matter has avoided detection thus far and how scientists might be able to finally change that.
“In our scenario there are two big bangs,” Winkler said of the result, which appeared in preprint last month. “The Hot Big Bang, as in the standard picture, creates the hot plasma of visible matter and radiation.” The dark matter, however, is created in a later, ‘darker’ Big Bang.
In the Big Bang story we’re all familiar with, regular matter like atoms was created in the early universe, when space was hot and crowded. As the universe cooled and expanded, atoms began coalescing into structures like galaxies and stars. That’s where dark matter came in.
We know dark matter exists because nothing else can supply enough gravity to hold stars within their galaxies—at least, not unless our understanding of gravity is somehow wrong. Dark matter has played this crucial role since right after the Big Bang. Like a kind of cosmic usher, dark matter guided the atoms to their seats with its gravitational pull. The dark matter decided where today’s galaxies would sit, and to this day every galaxy, including ours, is held together by a cloud of these invisible particles.
But where did dark matter come from? Most theories say it was made in the same Big Bang as everything else, but Winkler and Freese started to question that assumption. “It is often forgotten that we have zero evidence for dark matter before the times relevant for structure formation,” Winkler said. So maybe dark matter wasn’t around yet when regular matter formed. “Indeed, we found that dark matter formation could have occurred as long as one month after,” he said, “which is almost an eternity by cosmological standards.”
It’s a wild new theory, with major implications for scientists trying to measure dark matter. If the Dark Big Bang really happened, the effects of its seismic blast would still be rippling across the universe today. When massive objects undergo dramatic changes, they can produce “gravitational waves” that cause space to wobble like the surface of a pond after a stone is dropped in. In 2017, an experiment called LIGO won the Nobel Prize in Physics for measuring the gravitational waves produced by two colliding black holes. But the gravitational waves produced by the Dark Big Bang would be too slow-moving and faint for LIGO to detect.
Instead, the authors suggest looking for the gravity waves’ impact on faraway blinking stars called pulsars. Pulsars are spinning dead stars that beam light toward Earth at regular intervals, sort of like a lighthouse. These periodic beams of light should be impacted as waves of gravity course through the universe. According to Freese and Winkler, upcoming studies combining information from lots of these beams, such as the International Pulsar Timing Array and the Square Kilometer Array, might find hard evidence of the Dark Big Bang.
The new idea comes at a critical moment, when physicists’ search for dark matter is nearing a breaking point. What makes the stuff such a mystery is how it seems to pass right through regular matter. Scientists have built giant underground detectors to try to catch dark matter particles as they sail through the Earth, but the elusive ghosts seem to leave no mark. Physicists are becoming frightened that only gravity couples the dark world to our own, and this tiny tug is too small to measure in a lab.
But the new theory addresses this scary possibility. “Once we allow for the idea of purely gravitationally-coupled dark matter, the Dark Big Bang is perhaps the most plausible production mechanism for the dark matter,” said Winkler. “And the good news from our article is that even if dark matter only couples gravitationally, there are still great chances to test such a scenario.”
Indeed, the coming years could reveal the Dark Big Bang’s aftershocks and finally fill this giant gap in our knowledge of the cosmos.
Joseph Howlett is a freelance science writer and physicist at Stanford University. You can find him at josephjhowlett.github.io.
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