Cosmology in the Year 1 Trillion

[/caption]Much of what is known today about the birth of the cosmos comes from astronomical observations at high redshifts. Due to the accelerated expansion of the Universe, however, astronomers of the future will be unable to use the same methods. In a trillion years or so, our own Milky Way galaxy will have merged with the Andromeda galaxy, creating a new galaxy that has been quaintly termed “Milkomeda.” All of our other galactic neighbors will have long disappeared beyond our cosmological horizon. Even the CMB will have been stretched into invisibility. So how will future Milkomedans study cosmology? How will they figure out where the Universe came from?

According to a paper published by the Harvard-Smithsonan Center for Astrophysics, these astronomers will be able to decode the secrets of the cosmos by studying stellar runaways from their own galaxy: so-called hypervelocity stars (HVSs). HVSs originate in binary or triple-star systems that wander just a hair too close to their galaxy’s central supermassive black hole. Astronomers believe that one star from the system is captured by the black hole, while the others are sent careening out of the galaxy at colossally high speeds. HVS ejections occur relatively rarely (approximately once every 10,000-100,000 years) and should continue to occur for trillions of years, given the large density of stars in the galactic center.

So how would HVSs help future astronomers study the origins of the Universe? First, these scientists would have to locate an ejected star beyond the gravitational boundary of Milkomeda. Once beyond this boundary (after about 2 billion years of travel), the acceleration of a HVS could be attributed entirely to the Hubble flow. With advanced technology, future astronomers could use the Doppler shift of its spectral lines and thus deduce Einstein’s cosmological constant and the acceleration of the Universe at large. Next, scientists could use mathematical models of galaxy formation and collapse to determine the Universe’s mass density and age at the time that Milkomeda formed. From their knowledge of the galaxy’s age, they would be able to tell when the Big Bang occurred.