The gravitational-wave signal traveled to Earth from a distance of approximately how many light-years?

• A. Over a billion light-years
• B. Over a million light-years
• C. Over a trillion light-years
• D. Under a thousand light-years

Answer: (A)

Gravitational waves come from extremely energetic events far beyond our galaxy, and they travel through the universe at the speed of light, so Earth receives them from very distant sources. The first detection, GW150914, is estimated to have originated about 410 megaparsecs away. Since 1 megaparsec is roughly 3.26 million light-years, 410 Mpc corresponds to about 1.3 billion light-years. That means the signal traveled over a billion light-years to reach us. It’s far farther than within our galaxy and well short of any trillion-light-year distance, so the best answer is that it came from over a billion light-years away.