First Dark Matter, Then Dark Energy, Now…Dark Gravity?

Siegfrield continues: It would be an apt title, for gravity is at the heart of the universe’s darkest mystery of all – what it is made of. For decades astronomers have known that ordinary matter makes up only a small fraction of the cosmos – at most a few percent of its total quantity of mass and energy. Something else is out there, dark enough to be invisible from Earth, betraying its presence only by the gravitational pull it exerts on the shining stars and galaxies.

More recently, astronomers have learned that dark matter isn’t the whole story. As the universe expands, the dark matter’s gravity should resist, reducing the universe’s growth rate the way the Earth’s gravity slows the rising speed of a baseball thrown upward.

But light from distant exploding stars argues otherwise, dimming in a way that suggests the universe is accelerating – expanding at a faster and faster rate. Most experts have concluded, therefore, that dark matter must be accompanied by dark energy, an invisible field of repulsive force residing in the vacuum of otherwise entirely empty space.

Such dark energy would exert negative pressure (sucking in, not pushing out, just what you’d expect from a vacuum). But in Einstein’s theory of gravity, negative pressure exerts an antigravitational effect. Ordinary pressure adds to gravity, pulling things together. Negative pressure does the opposite, blowing space apart, just the thing you need to make the universe accelerate.

However, some astrophysicists say, suppose dark energy is cosmology’s version of the Loch Ness Monster. Maybe there’s no such thing. Rather than adding another ingredient to the cosmic soup, perhaps the universe’s accelerating expansion can be explained simply by devising a more complicated theory of gravity.

“It is worthwhile considering the possibility that cosmic acceleration is not due to stuff, but rather arises from new gravitational physics,” scientists from the University of Chicago and Syracuse University write in a recent paper.

After all, a century ago everybody thought that Isaac Newton had gravity’s formula all figured out. But then Einstein identified subtleties beyond Newton’s grasp. Einstein replaced Newton’s idea of featureless space with a dynamic space warped by the presence of matter. As a consequence, gravity dictates slightly different behavior to the cosmos than Newton would have predicted.

In a similar way, new ideas about space today could change the equations describing gravity. It’s plausible, for instance, that space has more than three dimensions; if so, particles transmitting the force of gravity might wander outside the 3D-space of the visible universe, requiring modifications to gravity math. Several proposals along these lines have appeared in recent years, as astrophysicists Glenn Starkman and Arthur Lue (of Case Western Reserve University) and Roman Scoccimarro (New York University) point out in a new paper.

Even if there are no extra dimensions, though, maybe Einstein’s equations simply don’t include aspects of gravity that show up only at very large distances. To put it another way, his equations might need changes that would have a noticeable effect only after the universe had aged enough to reach a certain size.

Just such a possible “fix” to Einstein’s equations is reported by Sean Carroll, Vikram Duvvuri and Michael Turner (Chicago) and Mark Trodden (Syracuse) in their [recent]paper. They tweak Einstein’s formula in a way that would make little difference in the universe’s expansion until recently. And that might explain why the universe is accelerating today, but was not accelerating for the first few billion years of its life.

“We have shown that the current epoch of cosmic acceleration can arise through purely gravitational effects, eliminating the need for dark energy,” the scientists write.

It’s much too soon to say whether any of these ideas for editing Einstein will turn out to be right. But Dr. Starkman and colleagues say that some of the proposals could be tested. A change in gravity on universe-sized scales might also cause small effects on shorter scales, perhaps even on the orbits of planets in the solar system. Measurements to detect such effects might be possible within the next decade.

Those tests may indicate that Einstein was right after all, and dark energy is causing the universe to speed up. Or they might show that Einstein’s theory needs changes, there is no dark energy, and the universe is simply doing what gravity is telling it to do. If so, physicists face the challenge of learning to comprehend gravity’s true language, and then rewriting the script for the end of the dark universe story.