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Pulsar Test Limits Existence of "Fifth Force".

SCIENTISTS RECENTLY STUDIED a pulsar binary system to constrain the existence of a hypothetical fifth fundamental force of nature.

We already know about four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. Some scientists attempting to explain anomalous experimental results have speculated about the existence of a fifth force of nature, one that could work on dark matter.

Relativity predicts that normal matter should fall freely toward dark matter. But a fifth force that has the ability to interact with both normal and dark matter could strengthen or diminish dark matter's gravitational pull. Lijing Shao (Max Planck Institute for Radio Astronomy, Germany) and colleagues tested for this effect using the binary system PSR J1713+0747.

This pulsar and its white dwarf companion, which are in a relatively wide 68-day orbit, lie 3,800 light-years from Earth in the direction of the galactic center. The pulsar, whose atoms have been compacted into neutrons, is so dense that its extreme gravitational field could enhance any possible interactions with dark matter. The white dwarf isn't nearly so compact. If a fifth force did exist, the Milky Way's dark matter halo, whose density peaks in the galactic center, would pull on the neutron star and the white dwarf in different ways, slightly altering their mutual orbit.

Drawing from more than 20 years of radio observations of this system, the researchers conclude that if a fifth force does exist, it must have less than 1% of gravity's strength--and gravity is already the weakest of the four known forces. The results appear in the June 15th Physical Review Letters.

Caption: Artist's concept of pulsar-white dwarf pair

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Title Annotation:COSMOLOGY
Author:Howell, Elizabeth
Publication:Sky & Telescope
Article Type:Brief article
Date:Oct 1, 2018
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