We’ve got to come up with a better name that ‘Milkomeda’ to describe what’s going to eventually happen when the Milky Way and Andromeda merge. Remember that Andromeda is one of the galaxies with a blueshift, showing that it is moving toward us. That the merger will probably happen — in about five billion years — appears inevitable, and it’s fascinating to speculate on the evolution of the elliptical galaxy that should result from all this. In fact, Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and colleague T.J. Cox have run computer simulations showing a faint possibility that our Solar System will be pulled into a ‘tidal tail’ of orphan stars and eventually, before the final merger, wind up in the Andromeda galaxy.
But after a series of close passes, the galaxies will most likely begin to intermingle. Loeb is the one behind the Milkomeda coinage, but I’ve also heard the even worse ‘Milkymeda’ and the at least acceptable ‘Andromeda Way.’ There’s plenty of time to work this out, so I put it to Centauri Dreams readers to ponder a poetic and inspirational name for the ultimate elliptical galaxy. By the time it has fully merged, our Sun will be entering its red giant stage, and our Solar System most likely pushed out to 100,000 light years from the new galactic center. That’s four times the current 25,000 light year distance, a move way out beyond the familiar galactic suburbs.
Image: A near galactic-collision between NGC 2207 (left) and IC 2163 captured by the Hubble Space Telescope. Scientists predict the Milky Way will merge with its neighbor Andromeda in about 5 billion years. Credit: NASA and The Hubble Heritage Team (STScI).
Will the descendants of the human race, however constituted, still be around to study the stars? It’s impossible to know, but it’s clear that any astronomers living in the merged galaxy era will have a far different night sky than ours to work with. And that view will hardly be static. Let’s run the process forward as if we had an H.G. Wells-style time machine (or a Loeb-style computer). 100 billion years from now the Sun and many of the stars we are familiar with will have burned out. Moreover, the accelerated expansion of the universe will have pushed many galaxies out past our cosmic horizon, while many of those that can be seen will only grow dimmer.
Loeb talked about that scenario back in 2001, noting that in this era, 100 billion years from now, an astronomer’s view will be reduced to about a thousand members of the local Virgo Cluster and surrounding areas. When a remote galaxy crosses our ‘horizon,’ the light it emits after that point will not be able to reach us. The galaxy will simply be moving too fast for us to see it. As Loeb says, “This process is analogous to what you see if you watch a light source fall into a black hole. As an object crosses the black hole’s event horizon, its image seems to freeze and fade away because you can’t see the light it emits after that point.”
Trillion Year Spree
Galaxies will slowly disappear, their image frozen and fading. It’s a chilling prospect, but Loeb’s latest paper takes us into an even more remote scenario, fully one trillion years from now, when the universe is 100 times older than it is today. By then the photons of the Cosmic Microwave Background will have a wavelength longer than the visible universe, and all other galaxies will be lost to our view. But Loeb believes that the astronomers of this era will still be able to figure out the Big Bang and the existence of dark matter by studying hypervelocity stars flung out from the galaxy.
Flung out, that is, from the center of the inelegantly named Milkomeda. Here’s the scenario: When a binary star system gets too close to the black hole at galactic center, one star falls into the black hole while the other is thrown outward at speeds high enough to cause it to be ejected from the galaxy. This occurs roughly every 100,000 years, and sharp-eyed future astronomers will be able to use these hypervelocity stars to infer the accelerated expansion of the universe as the stars move beyond the galaxy’s gravitational pull. Advanced technologies measuring that acceleration should make it possible to infer an expanding universe and, in Loeb’s view, calculate the age of the universe and key parameters like the cosmological constant.
“We used to think that observational cosmology wouldn’t be feasible a trillion years from now,” says Loeb. “Now we know this won’t be the case. Hypervelocity stars will allow Milkomeda residents to learn about the cosmic expansion and reconstruct the past… Astronomers of the future won’t have to take the Big Bang on faith. With careful measurements and clever analysis, they can find the subtle evidence outlining the history of the universe.”
Beyond the evidence afforded by hypervelocity stars, other possibilities come to mind, as Loeb outlines in his new paper. Consider all the possible sources of information:
The existence of an early radiation-dominated epoch could be inferred by measuring the abundance of light elements in metal-poor stars and interpreting it with a theory of Big Bang nucleosynthesis. The mass fraction of baryons within Milkomeda could be assumed to be representative of the mean cosmic value at early times. The nucleosynthesis theory can then be used to find the necessary radiation temperature Tγ ∝ a−1 , such that the correct light element abundances would be produced. This would lead to an estimate of the time when matter and radiation had the same energy densities. Since density perturbations grew mainly after that time, it will be possible to estimate the amplitude of the initial density fluctuation on the mass scale of Mtot that was required for making Milkomeda at a time (t − t⋆ ) ∼ Hv-1 after the Big Bang. Without a radiation-dominated epoch, this amplitude could have been arbitrarily low at arbitrarily early times. Future astronomers may already have cosmology texts available to them, but even if they do not, we have outlined a methodology by which they will be able to arrive at, and empirically verify, the standard cosmological model.
Loeb sketches out a remote futurity indeed, but it’s a comforting thought that science will still be able to unlock cosmological mysteries even when the compelling view of other galaxies is long gone. You can follow this up in Loeb’s paper “Cosmology with Hypervelocity Stars,” accepted by the Journal of Cosmology and Astroparticle Physics and available as a preprint. And please, for the good of our remote descendants, give some thought to a name more elegant than ‘Milkomeda.’