Scientists plan to pin down the date of a galactic collision billions of years in our future by first reaching back to the Big Bang, a feat that will be made possible by the extreme precision of NASA's planned SIM PlanetQuest mission.
By recreating some of the earliest moments of our portion of the universe, a team led by the University of Maryland's Ed Shaya will establish for the first time the masses and orbits of galaxies ranging from one million to 15 million light years from Earth. The study will reveal when the Andromeda galaxy will collide with our galaxy, the Milky Way, and will also strip away some of the mystery surrounding dark matter, the unknown substance whose gravity is thought to hold galaxies together.
SIM PlanetQuest will enable Shaya and his team to pinpoint the locations of from five to 20 stars in each of up to 29 galaxies (that's how many galaxies outside the Milky Way and its satellites have individual stars bright enough for SIM to see). It will do so several times over the course of the ultra-precise telescope's five-year primary mission. The change in location for each star over this period will establish its "proper motion" -- that is, the sideways motion as seen from Earth -- and thereby enable Shaya's team to derive a proper motion for each galaxy in which the stars reside.
The portion of each galaxy's motion that moves it closer to or further away from Earth, which is called its "radial motion," has already been well-established using the Doppler-shift technique. That's the observation of how light shifts toward the blue end of the spectrum for approaching galaxies, and toward the red for receding galaxies. But proper motion has been elusive because of the exceedingly slow rate at which galaxies millions of light-years away appear to move across the sky, even if they're actually traveling at very high speeds.
A slug in no hurry on Alpha Centauri
SIM PlanetQuest will be able to detect movement of just four microarcseconds per year. According to Shaya, that's equivalent to the motion one would detect if observing "a slug in no hurry" at the distance of Alpha Centauri, a star about 4.3 light years away.
"I was certainly not planning that in my lifetime we would have the technology to measure that," Shaya said. "Before I heard of SIM, it was something for several centuries out, I thought."
Proper motion, combined with the values established for radial motion and distance, will enable Shaya and his colleagues to calculate the three-dimensional velocity and direction of motion for each galaxy in the study. Then, after the observation phase of the study is completed, they will run a series of computer models to calculate masses and orbits for each galaxy.
Back to the Big Bang
They'll begin by feeding the model the current location of each galaxy, its velocity at the time of the Big Bang, an estimate of its mass, and a starting position based on the mass estimate. "You just think about following the center of mass of all the atoms that are in a galaxy. You can follow that center of mass all the way back to the Big Bang," Shaya explained. "It becomes totally arbitrary where you pick the center (of the universe, where everything would converge). So I just take the center of mass of the galaxies that I'm considering, and in the calculation everything will collapse to that point. And I know their velocity at T=0 (the beginning of time) because it's just the Hubble flow." Hubble flow is the velocity at which the universe expanded.
The mass estimate for each galaxy will include not only total mass but also how that mass is distributed within and between the galaxies, including the majority of mass that is presumed to be dark matter.
Each run of the model will yield a value for the current velocity of each galaxy in the study. Shaya's team will compare that with the actual velocity based on observations, and then refine the mass estimates and run the model again, repeating the process as many times as needed. When the model's predicted present-day velocities match those that were observed, they will know that the mass estimates in that model are likely to be accurate. As an intriguing bonus, the distribution of dark matter in this accurate model may contain hints of the properties of the substance.
The successful model will also produce an orbital path taking each galaxy from the Big Bang to its present location. Running the model forward into the future will enable Shaya to determine, among other things, when the Milky Way is destined to collide with the Andromeda galaxy.
"You look at the radial velocity of Andromeda and you see that it's heading towards us," Shaya said, "but you don't know the tangential motion. So you don't know whether it's headed straight towards us or is in an elliptical orbit in which it will pass around us and go back out again. Ultimately the question is, will it merge on the first pass or will it take many passes before it merges?"
Shaya's study will answer the question. Until then, you might want to make a note to update your homeowner's policy in about 6 billion years.
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