Time dilation -- when time slows in extreme conditions as Albert Einstein predicted -- might offer an alternate theory of dark energy, the invisible and unknown form of energy thought to drive the accelerated expansion of the universe, one researcher suggests.
Einstein's general theory of relativity holds that slowing of time in response to gravity is directional -- time for an object experiences high gravity will be slower time than an object experiencing low gravity.
However, his theory of special relativity holds that time dilation as experienced by two moving objects is reciprocal, meaning time as experienced by each moving object appears to be slowed relative to the other.
However, a study by University of Georgia professor Edward Kipreos suggests that instead of being reciprocal, time dilation created by movement is also directional, with time dilation affecting only the object that is moving.
Kipreos uses how the GPS system works as an example.
"The satellites, which travel in free-fall reference frames, are moving fast enough, in relation to the Earth, that you have to correct for their time being slowed down, based on their speed," he says. "If we didn't correct for that, then the satellites' GPS measurement would be off by a factor of two kilometers per day."
The effect is the result of special relativity plus a theory known as the Lorentz Transformation, a mathematical description of the relation between measurements by two different observers of time and space.
However, an alternative theory, the Absolute Lorentz Transformation, takes directional time dilation into account.
"Special relativity is supposed to be reciprocal, where both parties will experience the same time dilation, but all the examples that we have right now can be interpreted as directional time dilation," Kipreos said, citing the example of the GPS system, where time is slowing down for the satellites but from their point of view our time on Earth is not slowing down.
How does this relate to dark energy?
As the universe expands, objects such as galaxies fly more rapidly away from each other in a phenomenon known as Hubble expansion.
Applying the Absolute Lorentz Transformation to the velocities observed in Hubble expansion in the present universe suggests that the present experiences time dilation relative to the past, and time's passage would as a result be slower in the present and faster in the past, Kipreos says.
Astronomers use exploding supernovas as "standard candles" to determine distances in the cosmos, but recent observation of distant supernovas that were fainter than expected, and deviate from an expected "plot line" of brightness/distance, have suggested the expansion of the universe is somehow accelerating.
"The accelerated expansion of the universe has been attributed to the effects of dark energy," Kipreos said. "However, there is no understanding of what dark energy is or why it has manifested only recently.
"The predicted [Lorentz] effects of time being faster in the past would have the effect of making the plot of supernovas become linear at all distances, which would imply that there is no acceleration in the expansion of the universe," he says.
"In this scenario there would be no necessity to invoke the existence of dark energy," he says.