The simultaneous detection of gravitational waves and light from a cosmic collision has left a few theories of dark matter and dark energy dead in its wake. These theories require gravitational waves – ripples in the fabric of space-time – to travel slower or even faster than the speed of light. But recent observations have proved otherwise.
On 17 August, the LIGO collaboration saw gravitational waves from two cosmic objects that were spiralling towards each other. About 1.7 seconds later, NASA’s Fermi satellite detected a gamma-ray burst (GRB). The instruments had seen the collision of two neutron stars, about 130 million light years from Earth.
Other telescopes too observed the smash, catching the electromagnetic waves that travelled alongside the gravitational waves – the first such observation.
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“This allows us to measure the speed of gravitation relative to the speed of light,” says Miguel Zumalacárregui at the University of California, Berkeley. The signals from the smash-up, now named GW170817, show that gravitational waves do indeed travel at the speed of light, to an accuracy of about one in 1 million billion.
This seriously undermines some theories that modify Einstein’s general relativity to explain the mysterious dark energy thought to be driving the accelerated expansion of our universe, and the invisible dark matter that we detect only through its gravitational pull on ordinary matter.
The theories that died
The standard model of cosmology suggests that dark matter makes up about 27 per cent of the universe, and dark energy about 68 per cent. But this has some unanswered questions: why, for instance, is dark energy’s value almost 120 orders of magnitude smaller than what theories of particle physics would suggest? If the value of dark energy had been as large as those theories predict, our universe’s acceleration would have been dramatically greater, possibly ripping it apart by now.
One way to make sense of our observations of the cosmos without recourse to dark matter or dark energy is to modify our theories. Some attempts to do away with dark matter tweak the behaviour of gravity, using what’s known as modified Newtonian dynamics (MOND). A theory developed by Jacob Bekenstein called tensor-vector-scalar (TeVeS) gravity has been the most successful at achieving MOND-like behaviour, explaining the motion of stars and galaxies without requiring dark matter.
Alternative theories that attempt to explain the universe’s accelerated expansion that’s attributed to dark energy usually modify general relativity by adding new fields, either to weaken the acceleration even if dark energy is very large or to accelerate the universe’s expansion without requiring dark energy. For example, in so-called Galileon models of gravity, a new “Galileon” field could have been what kicked off this acceleration in the last few billion years.
The simplest ones survive
Now, four teams have ruled out many such theories – including TeVeS and Galileon gravity – because they all predict that gravitational waves travel don’t travel at the speed of light. “For those of us working on dark energy and modified gravity, we are going to need to seriously think about what we are doing,” says Zumalacárregui.
Pedro Ferreira at the University of Oxford agrees. “We can eliminate a huge number of [modified theories],” he says. “The simplest ones survive, as always.”
Still in the running is general relativity, of course. It includes dark energy as a cosmological constant, representing the constant energy density of the vacuum of space-time. Other simple models, such as quintessence, introduce a new field that lets vacuum energy density change over time, and these also survive.
“It is remarkable how successful Einstein’s theory of general relativity is, and how difficult it is to come up with viable alternatives,” says Avi Loeb at Harvard University. Explaining dark matter and dark energy may eventually require reconciling general relativity with quantum mechanics into a theory of quantum gravity.
Einstein’s theory breaks down when applied to the big bang and black hole singularities, Loeb says, “so it must be replaced by a theory that incorporates quantum mechanics to cure those pathological solutions.” While cosmologists got lucky with GW170817, in that it helped them weed out untenable theories, they may need a similar stroke of luck to guide them towards a theory of quantum gravity.
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