At the centre of the Milky Way, Einstein鈥檚 laws rule. Measurements of light from a star that orbits close to our galaxy鈥檚 central supermassive black hole cannot be explained by classical views of gravity and instead require Einstein鈥檚 general relativity.
General relativity predicts that starlight should lose some energy as it travels through the powerful gravitational field of a black hole. That loss in energy is expected to听stretch the wavelength听of light from stars near huge black holes, making them look more red. Standard, or Newtonian, gravitational theory doesn鈥檛 predict this.听No one expects Einstein鈥檚 theory to be wrong at this scale, but if its predictions were off by even a little, it might point towards new physics.
To test between the two theories, Tuan Do at the University of California, Los Angeles, and his colleagues used 24 years of observations of a star called S0-2, which orbits relatively close to our galaxy鈥檚 central supermassive black hole, Sagittarius A*. The data included measurements of the star鈥檚 colour and position. The team calculated its velocity from measurements听of its position over time.
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鈥淲hen the star becomes redder, it also looks like it鈥檚 moving away from us faster than it would without this gravitational redshift,鈥� says Do. 鈥淭hat is why it took 24 years to do this.鈥� The researchers needed enough data to nail down the star鈥檚 orbit and make sure this wasn鈥檛 tainting the colour measurements.
There have been similar studies in the past, but this work includes new measurements of the star鈥檚 closest approach to the black hole in 2018, which increased their confidence in its orbit. Do and his colleagues found that S0-2 appeared significantly redder than we鈥檇 expect if its light wasn鈥檛 being stretched by relativistic effects. 鈥淓instein was right, for now,鈥� says Do.
S0-2 isn鈥檛 the closest star to Sagittarius A*, but it is the only one that was bright enough to observe with the equipment available when the observations began in 1995. Do and his team, as well as other research groups, have started observing dimmer stars that are even closer to the black hole, but don鈥檛 yet have enough data to test general relativity more precisely with them.
Journal reference:听Science,听
