Gigantic deserts of almost complete nothingness that make up most of the universe may be causing the expansion of the universe to speed up, new research suggests. That means these vast tracts of nothingness could explain dark energy, the mysterious force that seems to be flinging the universe apart.
Welcome to the desert
Zoom all the way out from the solar system and the Milky Way galaxy, and an interesting pattern emerges: the cosmic web, the largest pattern found in nature. At these scales, where entire galaxies appear as little dots of lights, astronomers observe long, thin ropes of galaxies called filaments, dense clumps called clusters, and between them all vast regions of almost total emptiness. These barren regions are the great cosmic voids, the smallest of which are 20 million light-years across, while the largest can be more than 160 million light-years across.
Like the gaps in a spider web, the voids make up the vast majority of the volume of the universe, despite hosting almost none of the matter. Indeed, aside from the cosmic web itself, which stretches from one end of the observable universe to the other, the cosmic voids are the single largest things in the cosmos.
The power of nothing
Astronomers first detected cosmic voids in the late 1970s, but since then, they've largely been ignored. Astronomers and cosmologists have instead focused on the brightly lit structures of the universe, such as galaxies and clusters. Through those studies, astronomers detected a surprise in the 1990s: dark energy.
Dark energy is the name given to the observed accelerated expansion of the universe. This means that not only is the universe expanding every day; it's expanding ever faster with every passing moment.
Astronomers have no clue what's powering this period of accelerated expansion, which appears to have started about 5 billion years ago. Hence the term dark energy — it's a cool name for a massive cosmological conundrum.
What do the voids have to do with dark energy? For one thing, the effects of accelerated expansion aren't felt inside of star systems or galaxies; there, the gravitational attraction of matter is more than strong enough to completely overwhelm it. For example, neither our own solar system nor the Milky Way is getting bigger because of dark energy. But because the voids are almost completely empty, they feel the effects of dark energy far more readily. So it makes sense to investigate the nature of this accelerated expansion where its influence is strongest.
And a new research paper, led by a team of Iranian theoretical physicists, takes this line of thinking one step further. In their paper, published in July to the preprint database arXiv (opens in new tab) and accepted for publication in the journal Monthly Notices of the Royal Astronomical Society: Letters, the authors claim that dark energy isn't just found in the voids, but is caused by them.
From the darkness
How can these gigantic regions of emptiness cause accelerated expansion? The answer, according to the authors, isn't to look at only the existence of cosmic voids but also their dynamics.
Cosmic voids don't simply exist. Like all other large structures in the universe, they grew from humble beginnings into their present enormous stature. Billions of years ago, all the matter in the universe was spread out pretty evenly; there were no big density differences from place to place. But over time, any place that had a little more matter than average started to attract more matter onto it. With more matter, that region had even more attraction, which fueled even more growth. Over billions of years, matter accumulated to form galaxies, groups and clusters.
And as those structures grew, the voids emptied and enlarged. But instead of seeing it as a passive process, we can view the growth of voids as exerting pressure on the structures around them. For example, as voids grow, the walls of galaxies between them steadily thin out and eventually dissolve, allowing voids to merge. In the next few billion years, the voids will end up dissolving the cosmic web, forcing all matter into isolated clumps separated by hundreds of millions of light-years of emptiness.
This pressure distorts space-time around the voids, just like any other source of matter or energy in the universe. The space-time distortion means that as the voids expand, they push on the galaxies at their borders, causing them to separate despite the gravitational attraction between them.
The authors found that the cumulative effects of all the large voids in the universe working together to dissolve the cosmic web leads to an accelerated expansion. The strength of this void-driven accelerated expansion matches the current estimates of dark energy.
Astronomers will need further studies to test this idea. For one, we need more measurements of voids to get a better calculation of their combined pressure. Also, we need more information about dark energy itself, especially whether its strength has changed in the past few billion years. Still, it's an intriguing idea: Maybe dark energy isn't caused by some exotic force or process in the universe but is simply a byproduct of the normal evolution of emptiness.
Originally published on Live Science.