Black holes and pulsars could reveal extra dimensions

We discuss the observable effects of enhanced black hole mass loss in a black hole-neutron star (BH-NS) binary, due to the presence of a warped extra spatial dimension of curvature radius L in the braneworld scenario. For some masses and orbital parameters in the expected ranges the binary components would outspiral—the opposite of the behavior due to energy loss from gravitational radiation alone. If the NS is a pulsar, observations of the rate of change of the orbital period with a precision obtained for the binary pulsar B1913+16 could easily detect the effect of mass loss. For M BH = 7 M sunM NS = 1.4 M sun, eccentricity e = 0.1, andL = 10 μm, the critical orbital period dividing systems that inspiral from systems that outspiral is P ≈ 6.5 hr, which is within the range of expected orbital periods; this value drops to P ≈ 4.2 hr for M BH = 5 M sun. Observations of a BH-pulsar system could set considerably better limits on L in these braneworld models than could be determined by torsion-balance gravity experiments in the foreseeable future….. arxiv.org

MAKING a black hole let go of anything is a tall order. But their grip may slowly weaken if the universe has extra dimensions, something that pulsars could help us to test.

String theory, which attempts to unify all the known forces, calls for extra spatial dimensions beyond the three we experience. Testing the theory has proved difficult, however.

Now John Simonetti of Virginia Tech in Blacksburg and colleagues say black holes orbited by neutron stars called pulsars could do just that – if cosmic surveys can locate such pairings. “The universe contains ‘experimental’ setups we cannot produce on Earth,” he says.

Black holes are predicted to fritter away their mass over time by emitting particles, a phenomenon called Hawking radiation. Without extra dimensions, this process is predicted to be agonisingly slow for run-of-the-mill black holes weighing a few times as much as the sun, making it impossible to measure.

Extra dimensions would give the particles more ways to escape, speeding up the process. This rapid weight loss would loosen a black hole’s gravitational grip on any orbiting objects, causing them to spiral outwards by a few metres per year, the team calculates (The Astrophysical Journal, DOI: 10.1088/2041-8205/737/2/l28 and arxiv.org).
A pulsar orbiting a black hole could reveal this distancing. That’s because the lighthouse-like pulses of radiation they emit would vary slightly depending on the size of the star’s orbit.
http://www.newscientist.com
Read also: This Week’s Hype, Part II

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