Two diverging theories have sprung up in an attempt to explain why the laws of gravity appear to be under the influence of an unexplained fifth force outside our own galaxy. One theory, based on the concept of dark energy, posits that the vacuum of space has a unique energy associated with it and that energy causes stars and galaxies in deep space to move at an accelerated pace. The other is characterized as a "scalar-tensor" gravity theory, which suggests a fifth force (supplemental to gravity, electromagnetism, and the strong and weak nuclear forces) that can alter gravity on cosmic-length scales.
University of Pennsylvania astrophysicist Bhuvnesh Jain, is focused on the scalar-tensor theory. "These two possibilities are both radical in their own way," Jain said. "One is saying that general relativity is correct, but we have this strange new form of energy. The other is saying we don't have a new form of energy, but gravity is not described by general relativity everywhere."
By analyzing a well-studied class of stars in nearby galaxies, Jain and his colleagues have produced new findings that narrow down the possibilities of what this force could be and vindicate Einstein's theory of gravity. If the laws of gravity acted the same everywhere, stars and galaxies propelled outward by the Big Bang should continuously slow down, like objects thrown from an explosion act here on Earth. In 1998, scientists used distant supernovae to show that the expansion of the universe is speeding up rather than slowing down.
"We've been able to perform a powerful test using just 25 nearby galaxies that is more than a hundred times more stringent than standard cosmological tests," Jain said. These galaxies are important because they contain stars called cepheids that are bright enough to be seen individually.
"You can measure the brightness of a light bulb at some distance and know that, if you move it twice as far, it will be four times as faint. So you can tell just by the difference in its observed brightness how much further you moved it," Jain said. "But you need to know how intrinsically bright the bulb is first to determine its actual distance from us."
Cepheids have a luminosity that oscillates in a precise and predictable manner over the course of days and weeks. This unique trait serves as the method of calculating its distance from Earth, which aids in the determination of the distance of other celestial objects.
The 1998 observation relied on cepheid data for scale. "Now that we understand a little bit more about what makes the cepheids pulsate – a balance of gravity and pressure – we can use them to learn about gravity, not just distance," Jain said. "If the fifth force enhances gravity even a little bit, it will make them pulsate faster."
Using Hubble Space Telescope data from more than a decade of research, Jain and his colleagues were able to compare nearly a thousand stars in 25 galaxies. They determined that some galaxies are theoretically "screened" or protected from the effects of the hypothetical fifth force, while others are not. Larger galaxies and ones that belong to galaxy clusters are screened; smaller, isolated galaxies are not.
"If we compare galaxies that don't permit this extra force, like our own galaxy, with others that do, then we should see a difference in the way those galaxies' cepheids behave," Jain said. "Because this new force would increase the speed of their oscillations and because we can use the rate of the oscillations to their measure [sic] distance from us, the measurement we get from cepheids in unscreened galaxies should be smaller than distance measurements made with different techniques."
Jain and his colleagues did not find variation between their control sample of screened galaxies and the unscreened ones. These results line up with the prediction of Einstein's general relativity, meaning that the potential range and strength of the fifth force is constrained.
"We find consistency with Einstein's theory of gravity and we sharply narrow the space available to these other theories. Many of these theories are now ruled out by the data," Jain said. Jain believes that an entire set of gravity theories could be wiped out by future data collection. However, there remains the possibility that better data will indicate the possibility for small deviations from Einstein's theory of gravity, which remains one of the most famous scientific theories of all time.
Image by NASAblueshift, courtesy of Creative Commons licensing.